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Three Phase / Single Phase Motors &Circuits

NRE 8

Student Resource Package No: NRE8.

Delivery: Competence in this training program can be achievedthrough either a formal education setting or in theworkplace environment.

Recognition of Prior

Learning: The student/candidate may be granted recognition of

prior learning if the evidence presented is authentic andvalid which covers the content as laid out in thispackage.

Package Purpose: This package provides the student with theunderpinning knowledge and skills to identify and testthree / single phase motors and their associated power /control circuits. In addition the operation of three phasemotors, contactors, thermal overloads, single phasemotors and their starting devices.

Suggested Resources: Australian Refrigeration and Air Conditioning Vol1&2.Electrical Principles for the Electrical Trades, J.R.Jenneson

Assessment Strategy: The assessment of this package is holistic in nature andrequires the demonstration of the knowledge and skillsidentified in the student package content summary. Tobe successful in this package the student must showevidence of achievement in accordance with the

package.

NRE 8 Three Phase/Single Phase Motors & CircuitsCompiled by Greg Riach and Robert Baker Ultimo 2005

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Assessment:Grade Code: 72

GRADE CLASS MARK (%)

DISTINCTION >=83CREDIT >=70PASS >=50

Assessment Events:

1. Practical Test 20/40 50%2. Theory Test 40/40 50%

Practical Test: Simulated practical exercises in regards to three phase motors, single

phase motors, DOL contactors and associated circuits..

Theory Test 1: Short answer Questions, multiple choice questions.This assessment covers the contents from sections 1 to 7 in the studentresource package.

Summary of Contents: Page No

Delivery & Assessment Details: 1/2 Section No 1 Three Phase Electrical Connections 4 Section No: 2 Three Phase Induction Motors: 9 Section No: 3 Three Phase D.O. L. Motor Starter 21 Section No: 4 Three Phase Motor Protection 29 Section No: 5 Split Phase, Single Phase motors & Starters: 35 Section No: 6 Capacitor, Shaded Pole & Single Phase Motors

& Starters: 46

Section No: 7 Series Universal single Phase Motors: 55 Answers to Review Questions: 57 End of Package Assessment: 64

This package is designed as a self paced package with all relevant information and

references contained. At the end of each section there are a number of review questions

which require responses. (Answers to Review Questions page 57).


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NRE 8

Student Resource Package NRE 8

Delivery: Competence in this training program can be achieved through either a formaleducation setting by completing the attached student resource package or inthe workplace environment.

Prerequisite:The following resource package or equivalent is a prerequisite:

Occupational Health & Safety (7793T) Refrigeration Electrical 1 & 2.(NRE 1 & 2) Refrigeration Electrical Wiring (NREW)

Recognition of Prior Learning:

The student/ candidate may be granted recognition of prior learning if theevidence presented is authentic and valid which covers the content as laid outin this package.

Competency: Upon successful completion of the Occupational Health & Safety (7793T),Refrigeration the: Electrical 1& 2 (NRE 1&2), Refrigeration Electrical Wiring (NREW) andThree Phase / Single Phase Motors & Circuits training programs you will be assessed againstthe following competencies:

UTE NES208A: Disconnect & Reconnect fixed wired electricalequipment (up to 1000Volts).

UTE NES 209A: Attach flexible cords and plugs to electrical equipmentconnected to 240 volt supply

UTE NES 210A: Attach flexible cords and plugs to electrical equipmentconnected to a supply up 1000 volts AC ( Three phase 415)

Suggested Resources: Jenneson, JR., 1995, Electrical Principles for Electrical Trades, McGraw Hill,

Sydney.

Australian Refrigeration and Air Conditioning Volume 1 & 2.


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NRE 8

Three Phase Electrical Connections:

Section No: 1

Purpose: The purpose of this section is to provide you with the underpinning knowledge andskills to identify and test three phase voltages, currents and three phase electricalconnections.

At this stage you should be familiar with:

Single phase alternating current. Ohms Law. Impedance. Resistive, Inductive and Capacitive Circuits. Power in Single & Three Phase Alternating Circuits.

Note: reference to Refrigeration Electrical 1 & 2 may be necessary to revise single and

three phase alternating circuits and their characteristics, before the

commencement of this student resource package.

Three Phase Electrical Connections:Three phase voltages are produced by three sets of windings, mechanically fixed to eachother. Because there are separate voltages, each one can be used as a single phase power

source provided a neutral connection is available, however in practice the three windings areinterconnected to form a three phase ac power source.

Types of three phase connections:References:* ARAC page 14.7.* J.R. Jenneson chapter: 11, Page 218 221. Electrical Principles For The Electrical

Trades

There are two ways in which three phase systems can be connected are they are:

Star (Y) Delta ( )

Star connection (Y)

The star connection is formed, by connecting three similar ends of the windings together asshown in Fig.1 (a & b). Either, the start or the finish ends of the windings can be used butnot dissimilar ends. Note: the common connecting point is called the star point.


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Fig. 1 Three Phase Star Connections

Three actives (lines) are connected to the motor phase windings. The voltage between these three actives is called the Line Voltage and the current

flowing through the lines is referred to as the Line Current.

The neutral is connected at the Star Point.The Line Voltage is not equal to the Phase Voltage because two windings are connected inseries with two Line Voltages.

Line Voltage (V line) = 415volts Formula: VVline 415=

Phase Voltage (Vp) = 240volts3

VlineVp =

Note: for calculation purposes 732.13 = 732.1

415=Vp

voltsVp 240=

The line current equals the phase current. Formula: IpIline =

Where: Iline = line currentIp = phase current

Star point

L1

L2

L3

Vp

VLine

VLine

VLineL1 L2 L3

A1

A2

Star point

C2B2

C1B1


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Delta connection ( )The windings of a three phase alternator can also be connected in delta as shown in Fig 2.The delta connection forms a closed loop with dissimilar ends joined together with the linevoltage cables joined at these junctions.

Fig. 2 Delta Connection

L1

L2

L3

V Line

V Line

V Line

Vp

A2

A1

C2

C1

B2

B1

L1 L2 L3

In a Delta connection, each phase winding is connected across two line voltages andtherefore the line voltage equals the phase voltage. VpVline =

The line current in a delta connection is a combination of the phase currents. If the phasecurrents are all equal, then the line current is equal to:

3 x phase current or simply put: IpIline = 3


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Electrical Power:The amount of power consumed by a three phase load is determine by using the followingformula: PFIVP = 3

Where: P = power in watts or kilowatts (w or kW)

3 = 1.732V = voltage in voltsI = current in amperesPF = power factor (0 to 1)

Example: A 415 volt three phase motor draws 6.5 amperes with a power factor of 0.84.Determine the total power consumed.

PFIVP = 3 84.05.6415732.1 =P

wattsP 5.3924= or kW925.3

Note: the power consumed is measured and charged as a kilowatt hour kWHr

Example: If the above motor operates for 24 hours and the cost per kWHr is 12 centsdetermine the total cost of operation over 24 hours.

timekWHrtkWtTotal = /coscos 2412925.3 =

100

4.1130 cents=

30.11$=

Therefore the total cost over a period of 24 hours equals: $11.30

Remember electrical safety at all times.When working with electrical circuits you should always analyse and assess any possible

risks that may affect you or any other person within your working environment.

Electricity does kill!


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Review Questions Section No: 1

Q.1 There are two methods of connecting a three phase induction motor. What are they?

Q.2 Can a single phase motor be connected into a three phase electrical supply system and

are there any additional requirements:

Q.3 What is the line and phase voltage of a three phase delta connected motor?

Q.4 A three phase delta supply draws a phase current of 5 amperes from a 415, volt 50HZ power supply. Determine following in the space provided:

line voltage phase voltage line current phase current:

Q.5 A star connected 415 volt three phase motor draws a phase current of 3.6 amperes.Determine the following: in the space provided:

line voltage phase voltage line current phase current:


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NRE 8

Three Phase Induction Motors

Section No: 2

Purpose: The purpose of this section is to provide you with the underpinning knowledge andskills to identify, disconnect, diagnose and service various types of three phase motors whichare used in the refrigeration / air conditioning industry.

Production of a rotating magnetic field: (flux)The production of a rotating magnetic field (RMF) is essential for the operation ofalternating current (a.c.) induction motors.

Synchronous speed (RMF speed)The speed of rotation of an RMF is known as the synchronous speed and this will begoverned by the value of the supply frequency and the number of poles wound within eachphase winding. Note: each of the phase windings are 120 electrical degrees apart.

Synchronous speed can be calculated using the following equation:

nsyn = 120 x f Where: nsyn = synchronous speed of the motor in Revs/min.P f = supply frequency in hertz (HZ)

120 = converted number from seconds to minutes.p = number of poles per phase winding.

Note: The synchronous speed is magnetic flux speed within the windings (stator).

Induction Motor Action:

The two major components of a three phase induction motor are the (Stator) whichhousesthe phase windings and the (Rotor) the rotating part which reacts to the RMF.

Energising the stator winding causes the following to occur:

The RMF travels at synchronous speed. Stator flux induces an emf into the rotor bars. Rotor current flows in the rotor bars producing an opposing magnetic flux to the

stator.

Rotor flux reacts with the stator flux which in turn causes motor rotation (torque). Torque is maintained because the rotor tuns at a slower speed than the RMF.This is referred to as the slip speed usually around 4%. In other words the stator isdragging the rotor along in a rotational force.

Note: the stator core is laminated to reduce eddy current loss.


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Three Phase Induction Motor:Reference: J.R. Jenneson pages, 253 260.

The operation of a three phase motor relies on:

Electromagnetic induction Force on a current carrying conductor when under the influence of a magnetic field. The stator which houses three phase windings and produces the RMF. The rotor which has conductor bars that react to the RMF.

Slip:is the term used to describe the differential between the synchronous speed (RMF) andthe actual rotor speed. The larger the slip the larger the amount of torque developed.

Slip is determined By: nslip = nsyn _ nrotor (rpm)

Where: nslip = slip speed

nsyn = Synchronous speednrotor = rotor speed.

The actual rotor speed is expressed as the difference between the synchronous speed and theslip speed.

Example: A 415 volt three phase, 4 pole, 50 HZ induction motor travels at a 4% slip.Determine the actual rotor speed:

nrotor = 120 x f x 0.96

Pnrotor = 120 x 50 x 0.96

P

nrotor = 1440 rpm

Reversal of three phase induction motors:Since the rotor follows the rotational magnetic force (RMF). To reverse the direction of athree phase induction motor you must reverse the direction of the RMF. Directional changecan be achieved by changing the phase sequence (swapping two supply lead connections).Note: Fig. 3 three phase induction motor reversal.

Fig. 3 Three phase delta connected induction motor reversal

L1

L2

L3

A1

B1

C1

B2

C2

A2

A1 B2

L1

L2

L3

C2B1

A2C1


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Construction of a Three Phase Induction Motor:There are two main kinds of three phase induction motors:

Squirrel cage motor Slip ring motor.

Both of these motors consist of two major parts.1. Stator, (stationary part)

2. Rotor, (rotating part).Both of these motors have no electrical connectional or electrical differences between thestators and as well there are no electrical connection between the stators and rotors. Howeverthe wound rotor has a set of slip rings.

Motor frames:The frame consists of cast or fabricated steel and the stator core is pressed directly into theframe.The type of frame used will be governed by the environment that it operates within:

Open type which allows free ventilation through the motor. Drip proof that has a closed upper half, while allowing ventilation through the lower

half.

Totally enclosed, that prevents air or moisture entry.Squirrel cage motor:

The squirrel cage rotor consists of solid copper or aluminium bars embedded in the rotor

slots. Each bar is short circuited by an end ring.Note motor construction Fig. 4.

Advantages of a squirrel cage motor include:

simplicity and rugged construction No sparking contacts in the motor so they can be used in explosive atmosphere

(provided; the appropriate enclosure is used).

Wide range of speed control (when used with electronic frequency controllers).Disadvantages of squirrel cage motor include:

Relatively poor starting torque Fixed characteristics.


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Fig. 4 Exploded view squirrel cage induction motor.

Slip ring or wound rotor type: (Limited use in HVAC)

This type of rotor has three phase windings similar to its stator winding and wound withthe same number of poles as the stator. The rotor winding is star connected with the openends of the winding bought out to a set of slip rings which are mounted on the rotor shaft.Brushes ride on the slip rings and during the starting period they are connected to externalresistor banks. This type of motor can have its operation varied by these external resistors.Refer: JR Jenneson page: 254.

Advantages of a wound rotor induction motor include:

High starting torque Relatively small starting current Smooth speed control over a wide range.

Disadvantages of a wound rotor induction motor include:

High production cost Brushs mean more maintenance Inefficient speed control Note: Fig 5

Fig. 5 Three phase wound motor stator and rotor.

ElectricalTerminals

Stator

Rotor

Frame

Bearings

ElectricalTerminals

Rotor BarsStator Frame

Bearing

Bearing


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Electrical terminal markings:The three phase electrical supply voltage is connected to the stator windings at the motorterminal junction (block). The windings can be connected internally in delta or star. Theconnection method will appear on the name plate of the motor. In most cases there will besix leads brought out of the motor terminal block. Note: Fig. 6 & 7.

Star connection:

In figure 6 the supply voltage is connected from L1, L2 and L3; to A1, B1 and C1 on themotor terminals, while the terminals marked A2, B2 and C2 are joined together to form a starpoint.

Fig 6 Star Connection

Phase Windings

A1 A2

B1 B2

C1 C2

L1

L2

L3

Delta connection:In figure seven the winding connections are connected from the start and finish of eachwinding ie: A1-B2, B1-C2 and C1- A2 with the supply L1, L2 and L3 connected to eachphase winding.

A1 A2

B1 B2

C1 C2

L1

L2

L3

Phase Windings

Fig. 7 Delta connection


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Motor terminal arrangements and reversal:

Three phase motor terminals are arranged for ease of connection in either star or delta byconnecting the terminal block links as detailed below in figure 8.

B2 C2 A2

A1 B1 C1 A1 B1 C1

B2 C2 A2

L1 L2 L3

A1 B1 C1

B2 C2 A2

L1 L2 L3

Standard Terminals Star Connection Delta Connections

Fig.8

The direction of rotation of a three-phase motor bridged for either star or delta operation canbe reversed by interchanging any two phases at the motor or main contactor. Some motors donot have these bridge connections and are best suited for a starting method where both starand delta connections are used.To reverse the direction of a motor with this type of starting arrangement you must

interchange opposing pairs of supply conductors at the motor terminal block as in figure.9.

B2 C2 A2

A1 B1 C1 A1 B1 C1

B2 C2 A2

L1 L2 L3

L1 L2 L3

A1 B1 C1

B2 C2 A2

L1 L2 L3

L1 L2 L3

Fig.9


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Electrical testing of three phase induction motors:

The stator windings represent a balanced load that is each phase winding should have asimilar resistance.

An ohmmeter is used to measure the resistance of each winding against themanufacturers, requirements. Winding resistance readings can be between 10 to 100ohms depending on the motor output capacity in kW, (more kW less resistance).

The windings that are in good condition will have similar resistance of several ohmsor more.

Damaged windings will have open circuit resulting in an infinite reading of resistanceand a short circuit will result a much lower resistance.

Insulation resistance must be checked with an insulation resistance tester (Megger). The insulation resistance between each phase winding and from the phase winding

terminals to earth (frame), must be in excess of 1 M Ohm.

When connected to the supply the frame of the motor must be solidly earthed at themarked earth terminal.

Cable protection such as flexible conduit must be secured at the motor. Check the motor line currents with a clamp on ammeter for correct operation. There should be three similar line currents that do not exceed rated current on the

nameplate.

Motor nameplate details:

The following details are usually listed on the nameplate of a three phase motor:

Manufacturer Type of motor Output rating in mechanical power (kilowatts) Ampere rating when operating at full load (FLA) Voltage rating Frequency rating Number of phases RPM Winding connection method.

Motor rating: It is important to realise that electric motors convert electric energy intomechanical energy. Because of this they are rated in terms of their output power in watts (W)or kilowatts (kW) which is the torque at the shaft.


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Practical Exercise

Three Phase Motor Connections

Aim: To draw a complete wiring diagram for a three phase motor for star and then delta in

accordance with the following details:

Electrical training boards in workshop. 415 volt power supply Isolation switch Fuses or circuit breaker D.O.L. motor contactor 240 volt control circuit ON/OFF control switch Overload Three phase motor Note: drawing to be done on attached page.

Procedure: On completion of circuit diagram check with you teacher/mentor and proceedto:

Isolate the power supply and proceed to connect the motor circuit(STAR).

Test the motor windings resistances and record. Switch the power supply on and measure the operating voltages,

current and record results.

Reverse the direction of the motor.Electrical meters:

Megohm-meter Multi-meter Tong-ammeter

Record Results:

Star:

Resistances across each winding: 1 2 3 Line voltage: Phase voltage: Current:

Delta:

Resistances across each winding: 1 2 3 Line voltage: Phase voltage: Current:


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Three Phase Electrical Wiring Diagram


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Q.1 When testing a three phase motor the tong tester is placed:

(a) across the active or neutral cables(b) between any phase and earth cables(c) around any two phase cables(d) around any one phase cables.

Q.2 The stator core is laminated to:

(a) improve starting torque(b) provide silent running(c) reduce eddy current loss(d) reduce hysteresis loss.

Q.3 Rotor bars are usually made from:

(a) steel or copper(b) copper or aluminium(c) carbon or copper.(d) steel or aluminium.

Q.4 Slip speed is the difference between:

(a) standstill and rotor speed(b) rotor and stalling speed

(c) Synchronous speed and rotor speed(d) Synchronous speed and standstill speed.

Q.5 A typical three phase stator winding would be expected to have a resistance of:(a) 10 to 100 ohms(b) 100 to 1000 ohms(c) greater than 1 megohm(d) almost zero ohms.

Q.6 On completion of an insulation test between each of the phase windings and earth a

three phase induction motor would pass if all test results were:

(a) less than 2 ohms(b) between 10 kilohms and 1 megohm(c) greater than 1 megohm(d) between 2 ohms and 100 ohms.

Q.7 The rotor windings of a wound rotor induction motor is usually connected in:

(a) delta(b) series(c) parallel(d) star.


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Q.8 To reverse the direction of rotation of a three phase squirrel cage motor youwould:

(a) interchange any two supply lead connections(b) replace the squirrel cage rotor with a wound rotor

(c) change the delta connected stator winding to star(d) remove the rotor and reverse end for end.

Q.9 At what speed is the RMF produced by the stator winding rotate at?

Q.10 How would you change the direction of a three phase motor?

Q.11 Briefly explain why the rotor speed of an induction motor is always less that

the speed of the stator RMF:

Q.12 A three phase 415 volt, 50 HZ, 4 pole induction motor operates with a 4%

slip. Determine the synchronous speed and the actual rotor speed:

Q.13 From the details below determine the voltages for a three phase power supply

between:

Any two lines: =

Any line and earth: = Any line and neutral: = Earth and neutral: =

Q.14 The diagrams below (fig 1 & 2) represent a schematic layout of the motor windingsfor a six terminal three phase motor. Complete the diagram from the three phasesupply (L1, L2, and L3) to include the phase windings for both a star and delta

connections.


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A1 B1 C1

A2C2B2

L1 L2 L3Star Y

Fig. 1

L2

A1 B1 C1

A2C2B2

L1 L2L2 L3Delta

Fig. 2

Q.15 Indicate on the diagram below the main components of the squirrel cage inductionmotor:


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NRE 8

Three Phase Direct On Line (D.O.L.) Motors Starters

Section No: 3

Purpose: the purpose of this section is to provide you with the underpinning knowledge andskills required to identify a number of three phase motor starters and in particular theconstruction and function of D.O.L. motor starters utilising the latch and interlocking controlmethods for three phase motor circuits.

AC Motor Starters (Contactors)

Motor starters (contactors) are devices that are connected between the supply lines and motorin a functional system, for example:

Starter MotorSupply Load

Energy Control Drive Production

Functional system

The operating characteristics of motors are that the current and torque

conditions are considerably different during the starting sequence and the

running conditions. For example: a squirrel cage induction motor has a starting

current up to eight times the full rated load current and a starting torque three

times the running torque.

Large increases in current and torque can be harmful to the motor the load

being driven and the supply (electrical energy).


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Three Phase D.O.L. Motor Starter

Types of Three Phase Motor Staters (Contactors)There are a number of three phase motor starters used within the refrigeration , airconditioning and electrical industries and they are as follows:

Note: further reference: ARAC, page 14.11 14.19 D.O.L. (Direct on Line) Reduce Voltage

- star- delta starter- primary resistance starter- secondary resistance starter- auto-transformer starter- part winding starters.

Electronic (soft- start) starter.

Note: reduced voltage and electronic soft starters will be covered in more detail in the

next student resource package.

Direct on Line Starter (D.O.L.)The basic D.O.L. motor starter, consist of three contacts with a thermal overload device thatwill trip a switch in the event of an overload condition. The D.O.L. starter has the addedfeature in that it operate by manual stop start switches or automatically by special controldevices.

Note: the three main components are: coil, armature and electrical contacts.

M3~ N

O/L

95 96 K1

K1-1

K1-2

K1-3

A1 A2

Thermal overload heaters

L1

L2

L3

D.O.L. above motor starter has three line contacts K1-1, K1-2 andK1-3, these contacts when closed supply the electrical power tothree phase motor windings. An ON/OFF control switch whenclosed energises the contactor coil KI through the overload andthus creating an electromagnet causing the three contacts close.

Note: No: 95 & 96, A1 and A2 are the electrical terminals for the

overload and contactor coil respectively for the majority of three

phase contactors.


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Control Devices:A number of control devices can be used to stop and start a three phase motor circuitutilising a D.O.L. starter. These devices can be used as part of the following circuits:

Latching circuit Interlocking circuit.

Latching circuits:

Latching is the use or auxiliary contacts on contactors to hold (latch) circuits andcomponents for as long as they are required. A typical example of this is a three phasepedestal drill with a stop start switching arrangement as in Fig. 3.

K1

O/L

Stop Switch Start switch

Auxiliary LatchingContact (K1-4)

Fig. 3 Latching; circuit using a stop / start switching arrangement

AN


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Interlocking circuits:

Interlocking is the use of auxiliary contacts on contactors to automatically bring in othercomponents. The diagram in fig. 4 illustrates the use of interlocking.

C1

O/L

K1

t LP HP O/L

C1-4

A N

Fig. 4 Interlocking compressor drive motor circuit

Refrigerated Coolroom

E1

O/L

The interlocking circuit in Fig. 4 represents the control circuit for a refrigerated coolroomoperating with a three phase evaporator and condenser fan motors (connected in star) withcompressor drive motor connected in delta.

Note: the diagram interlocking compressor drive motor in a refrigerated coolroom.

The compressor drive motor contactor will be energised when the condenser fan

motor contactor is energised.

Advantages of D.O.L. motor starters:

Fail safe operation of the control circuit Simple operation Low initial cost Remote stop and start features of control.

Disadvantages of D.O.L. motor starters:

No current limiting ability, therefore maximum current occurs at start.


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Practical Exercise


Aim: To construct an electrical wiring circuit diagram for a three phase exhaust fan motor

in accordance with the following details:

415 volt power circuit Isolation switch / fuses etc D.O.L. contactor Star connection. Control circuit consists of: STOP/START switching and overload protection

for the fan motor.

Note: complete circuit diagram on the page provided below.

Procedure: On completion of your circuit diagram proceed to electrical workshop andhard wire on three phase training boards, using the correct meters tools andsafety procedures.

Test run on completion of connection and check current and voltages undersupervision of your teacher or mentor.

Remember: always work safely working on electrical circuits.

An Electric shock can be Fatal!


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Circuit Diagram


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Q.1 Describe the operation of a DOL motor starter:

Q.2 What are the three main components of a three phase contactor?

Q.3 What is the major advantage of a D.O.L. contactor over an isolation control switch?

Q.4 List three types of motor starters used to operate three phase induction motors:

Q.5 Draw an electrical control circuit for D.O.L. contactor controlled by a start / stopswitch. (space provided on the next page).


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Three phase motor controlled by a start / stop switch.

Q.6 With the aid of the above circuit diagram, explain how the control circuit operates:


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NRE 8

Three Phase Motor ProtectionSection No: 4

Purpose: The purpose of this section is to provide you with the underpinning knowledgeand skills to identify various motor protection devices and how they operate within threephase motor circuits.

Motor Protection:Motor protection is required to prevent motor windings being damaged or burnt out.Three phase motor breakdown is typically due to electrical or mechanical damage.

Irrespective of the cause of motor breakdown the effect is often a rise in motor temperaturedue to an increase in motor current.Excessive rise in motor phase current is due to one or more of the following:

Mechanical load is greater than the electrical rating (kW) Rotor shaft becomes locked. Part loss of supply voltage. Reduction in supply phase voltages Winding insulation breakdown.

Motor Protection Devices:Protection devices installed in the electrical circuit must be capable of discriminatingbetween normal operating conditions and conditions which may cause damage.

Protection devices are designed to either monitor short duration high current overloadconditions or short circuits or long duration medium to low current overload conditions.

Types of Protection Devices:

1. Fuses and circuit breakers:In addition to providing protection for motors against excessive current overloads, fusesand circuit breakers also protect the motor, control circuit equipment and wiring fromdamage due to short circuit.

The most common type of fuse used on three phase motors is the (HRC) this of fuseoperates with a high rupturing capacity, A combination of the HRC fuse and circuitbreaker are used in most cases. On some older installations you may come acrossrewirable fuses (outlawed under AS3000).


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2. Thermal overload:There are two common types of over temperature devices which detect temperaturechange.

Devices that detect excessive line current by using a bi-metal strip which ismechanically connected to the overload control switch. Three sets of heaters areconnected in series with the motor windings, as shown below, Fig 1. (reference:ARAC 14.11).

K1

O/L

N

M

L1

L2

L3

Bi- metal strips:

Thermal heaters:Fig.1 Three phase D.O.L. contactor

incorporating a thermal overload.

Operation: The heaters sense a rise in current on either of the phases, which causes a risein temperature resulting in the bi-metal strips warping and through a levermechanism tripping the overload switch open.

Note: the rise in temperature is caused by a rise in current flow.

Devices that directly detect motor winding temperature using positivetemperature coefficient (PTC) thermistors are physically located in the motor

windings.Thermistor type overloads are commonly used in large air conditioningcompressor motors which sense a temperature rise in the windings.

Inverse time characteristics of overloads:

Overload devices have a design feature called inverse time characteristic whichreduces the operating time when there is an increase in overload conditions andprevents tripping under normal starting currents.Overloads are designed to absorb start up current without tripping; but willheat up during an overload condition.


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Setting and re-setting overloads:

Only in-line overloads can be set and the correct setting must be made to themotor specifications. Set at motor full load amperes.

If the setting is to low nuisance tripping may occur. A motor may burn out due to a high setting. This is particularly relevant for

modern motors because of their design efficiency has increased and so has theiroperating temperatures.

3. Magnetic overloads:Magnetic overloads detect an increase in the magnetic field surrounding the motorsupply conductors. If there is an increase in current flow the magnetic field increasesand if above the set point the overload opens and drops the motor circuit out. Thesedevices are much quicker in response compared to the thermal overload and featureinstantaneous trip characteristics.

4. Under Voltage and over Voltage Relays:The under voltage relay will respond to low voltage on control and the motor circuitand the effects of under voltage are as follows:

Relays and contactors will not operate correctly.Sensing devices do not respond as required.Increase in temperature of motor windings (high current).

The over voltage relay responds to over voltage on control and the motor circuit andthe effects of over voltage are as follows:

Increase power consumption in the control circuit. Sensing and controlling devices operating incorrectly. Increase in motor temperature.

5. Electronic motor protection relays:

The electronic motor protection relays offer the ultimate protection functions byincorporating thermal, magnetic and voltage protection.

Isolation Switch:

The circuit between the motor supply and its associated control circuit must becapable of being isolated, through the installation of an approved isolation switch inthe event of electrical and mechanical hazards.


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Motor Timer Protection:Timers can be used to protect three phase motors against the problems associated from shortcycling. These problems include:

Increased heating of protective devices. Increase in temperature of the motor Extra stress placed on the motor. Control contacts, both power and control, have increased operation which will

result in lowering the life span

Coupling between motor and load is subjected to increased stress.Fault finding in three phase motor installations:

An electric motor may not operate due to a fault within the motor or in the circuit supplyingthe motor. A fault in the load the motor is intended to drive, can also affect the motorsperformance. Remember a motor is a component that converts electrical energy tomechanical energy and therefore a mechanical fault will be reflected in the electrical circuit.

The general areas where faults in motors ane their associated circuits occur are:

The electrical supply and controls to the motor Within the motor itself In the load being driven by the motor.

Note: to become competent in motor fault finding it is essential that you have on the job

training guided by a licensed mentor in the workplace.


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Q.1 List four problems that are associated with excessive current flow in a three phase

induction motor:

Q.2 List three types of circuit protection devices:

Q.3 List and describe two types of thermal overloads:

Q.4 List two main considerations that need to be taken into account when setting

overloads:

Q.5 Describe the operation and advantage of a thermal overload:


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Q.6 List two problems associated with under voltage in a three phase motor circuit:

Q.7 List two problems associated with over voltage in a three phase motor circuit:

Q.8 Electronic motor protection relays offer three protective functions. What are they?

Q.9 What is the function of an isolation switch?

Q.10 Timers can be used to protect three phase motors against the problems associated

from short cycling. List three of these problems:


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NRE 8

Split Phase, Single Phase Motors & Starters

Section No: 5

Purpose: The purpose of this section is to provide you with the underpinning knowledgeand skills required in the construction, operation, directional change applications and toidentify the motor terminals for various types of split phase motors and starters.

Split Phase Motors: (ARAC, 14.1)Split phase motors like all a. c. induction motors requires a rotating magnetic field to beproduced in the windings so that it can act on the rotor and cause it to rotate.

Construction:The following basic components make up a split phase motor:

Stator or windings (two windings: Run & Start) Rotor Bearings and end shields Starting switch or relay.

Development of starting torque in split phase induction motors:A split phase motor is designed to produce a directional, non-rotating magnetic field. Thistype of field is developed by having two separate windings located on the core which isdisplaced by 90 electrical degrees, Refer: Fig. 1.

Fig. 1


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Run & Start Windings:

The run and start windings are electrically dissimilar in that they are wound using differentgauge wire, with a different number of turns and are physically positioned at different depthsin the stator slots which affects their inductance.

Winding Characteristics:

Run Winding (Main Winding)

Physical:

Heavy gauge wire Large number of turns Located deep in the stator slots.

Electrical:

Low resistance High inductance Lagging current of approximately 40 electrical degrees.

Start winding (Auxiliary winding)

Physical:

Fine gauge wire Low number of turns Located towards the top of the stator slots.

Electrical:

High resistance Low inductance Lagging current of approximately 15 electrical degrees.

Operation:

Since the winding currents are out of phase (split) their resultant magnetic fields will also beout of phase, hence the time delay between the field strengths of the two windings creates theRMF. The RMF intersects the conductors on the rotor. This induces an EMF in the rotor barscausing a current and a magnetic field around the rotor bars that then interacts with the statorfield causing motor action.

Once the motor reaches approximately 75% of its full speed the start winding isdisconnected. The motor action continues due to the alternating flux in the stator and theinertia of the rotor.

Note: The starting switch is normally closed and opens once the motor reachesproximately 75% of its full speed, disconnecting the start winding. Refer Fig.2.


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R

CS

AN

O/L

Split phase motor circuit diagram Fig.2

Note: the electrical motor terminal connections are asfollows:

start winding connects between S and C run winding between R and C C is common to both windings.

Construction details:Split phase motors are constructed very similar to those of a three phase squirrel cage motor. Themain differences are in the stator winding where by the split phase stator houses two dissimilarwindings.

Sealed compressor motors:

The sealed unit or hermetic motor compressor is used in both refrigeration and air

conditioning systems and consists of: a compressor with a drive shaft connected to the rotor

of the split phase stator which is attached to the internal frame. The whole unit is fitted into a

two piece pressed metal housing, called the dome and hermetically sealed by welding the

two parts together. Note: refer to Fig.3.

Reciprocating hermetic compressor Fig.3


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Methods of starting split phase motors: (ARAC, 14.4 14.7)

There are two methods that are commonly used to disconnect the start winding from thesupply and they are as follows:

Speed sensitive switching, eg Centrifugal Switch Current sensing relay, eg Current Coil Relay and Solid State Relay.

Centrifugal Switch:

The centrifugal switch is a speed sensitive switch, for example at standstill the switch isclosed and when the motor is energised the switch will open when it reaches approximately75% of its full speed Refer to Fig. 4

Rotor at standstill Centrifugal

switch closed

Rotor Shaft

Rotor at normal speed, Centrifugal

force set up in switch mechanism

which causes the switch contacts to

open.

Centrifugal switching, Fig. 4.

Current sensing relays:There are two types of current sensing relays that operate due to a variation in current andthey are:

1. Current coil relay2. Solid state relay.


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1. Current coil relay:

Normally used on hp split phase motor compressors and smaller, it is an electromagnetictype relay which operates on current flow. For example: when the motor is energised a rushof current through the coil causes the relay contacts to close bringing in both the start and runwindings and as the motor reaches approximately 75% of its full speed. The current flowfalls due to the lack of electromagnetism on the relay armature, the contacts the start nowopen and thus the start winding drops out leaving the run winding in circuit. Note: Fig. 5.

R RC

SO/L

AN

Relay coil

Armature

Contacts normallyo en. N / O .

Current coil relay Fig. 5

2. Solid state relay:

These types of relays are now available as a direct replacement for current type relays. Thesolid state relay uses a thermistor-type semiconductor of a PTC type (positive temperaturecoefficient) and relies on a change in temperature which in turn changes its electricalresistance.

Operation: As the motor starts the resistance in the PTC is low, allowing current to flow tothe start winding. In approximately 3 seconds the current will have heated the PTC and itsresistance increases considerably and reduces the current flow to just a few milliamps. This

then drops the start winding out of circuit and leaves the run winding in circuit.


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Characteristics of a split phase motor: The run winding by itself does not develop starting torque. The start winding, working with the run winding, is needed to develop starting

Starting currents are around five to seven times the rated motor current. The starting current is not damaging because it decreases as soon as the motor

The starting torque developed is around 1.5 to 2 times the rated full load torque.dvantage: operates on a single phase supply.

isadvantage: low starting torque and high slip on heavy load.

pplications:

ic refrigerator and freezer compressor motors

Rat are: 250 watts 1.5kW.

peed: synchronous speed can be calculated by:

N = 120 f

torque.

accelerates.

A

D

A

Domest Washing machines Bench Grindersing: output ratings

S

- slip speed Where: n = speed in RPM

rtzP f = frequency in heP = number of poles.

RC

S

A N

O/L

PTC

Fig 6. Solid State Relay

Solid state relays should not be used on motors fitted with a runca acitor and an off c cle of 1 minute should be a minimum.


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Revinduction motor will rotate in a direction governed by the instantaneous

start or run winding

esting and fault finding:the motor frame and all windings and active conductors

amage to start windings could be due to one of the following:

Starting device not opening because of a sticking centrifugal switch or a malfunction

Frequent stop / start

Excessive load on the motor Incorrect winding connections Low supply voltage.

ossible causes of non-starting are as follows:

Loss of supply voltageeds to be reset

d on compressor motor (Trips out on overload)

orrect electrical terminal identification:

etically sealed motor compressor and are

een the start and run terminals

.

ersal:A split phasedirection of the current flow in the start and run windings.To reverse a split phase induction motor: reverse either the

connections but not both.

TThe insulation resistance betweenmust be maintained to not less than 1 megohm.

D

of the current sensing device (current coil / solid state relay).

P

Overload rest button ne Open circuit in either winding Locked rotor Faulty relay Excessive loa

C

There are three terminal connections on a herm

as follows: Common (C), Start (S) and Run (R). To identify the correct terminal

connection the following procedure applies.

The highest resistance reading is betw The middle resistance reading is between the start and common terminals The lowest resistance reading is between the run and common terminals.


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Practical Exercise

Aim: with the aid of the ohms s gohm meter determine the

r

rocedure: After obtaining the results place the appropriate resistance readings acrossR)

Split Phase Motor

cale of a multi-meter and me

motor winding resistances and check for earth leakage on a split phase sealed motocompressor.

P

each winding connection and indicate the Common (C), Start (S) and Run (terminals on the diagram below.

Hermetically sealed motor compressor motor windings

Recorded results:


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Q.1 Which of the wi switch?ndings is open circuited by the current sensing

.2 Explain how you would reverse the direction of rotation of a split phase motor:Q

.3 Explain the operation of the following switching mechanisms on a split phase motor:Q

Current coil relay:

Solid state relay:

.4 When energised, a split phase induction motor fails to start. List in point form theQ

steps you would take to locate the fault:

.5 Describe the operation of a centrifugal switch when used on a split phase motor:Q


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Q.6 When energised a split phase induction motor continually trips out on its thermal

overload. List a least four possible faults that would cause this condition:

Q.7 A 50 HZ two pole split phase induction motor operates on full speed with a 4% slip.

Calculate the full load rotor speed:

Q.8 In the space below draw an electrical wiring diagram for a split phase compressor

motor controlled by a thermostat, current coil relay and overload protection. Include

on your drawing the motor windings and terminal connections.

Q.9 You are to replace the current coil relay with a solid state relay within the circuit

above. Draw an appropriate electrical circuit diagram to meet the requirements for a

solid state relay.


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Q10. After checking a hermetically sealed motor compressor the following resistances

were measured between the terminals. Indicate the common (C), start (S) and run (R)

terminals on the diagram below:

22 ohms 25 ohms

47 ohms



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NRE 8

Capacitor, Shaded Pole & Single Phase Motors & Starters

Section No: 6Purpose: the purpose of this section is to provide you with the under pinningknowledge and skills required in the principle operation, construction and applicationsfor capacitor , shaded pole, single phase motors and starters.In addition you will learn to use the appropriate test equipment to test motors and startersfor safe and correct operation.

Capacitor start motors:(ARAC, 14.3 14.4)The magnetic field winding phase displacement of a basic split phase motor is due to the

electrical phase difference of the windings. To increase (torque) the phase displacement,a capacitor is connected in series with the start winding. If the correct capacitor isselected a phase displacement can be up to 90 electrical degrees.

Methods of starting a capacitor start motor:There are three methods that are commonly used to disconnect the auxiliary winding(start) from the supply and they are as follows:

Speed sensitive switching, eg Centrifugal Switch (refer section No:5 Fig.4) Current sensing relay, eg Current Coil Relay and Solid State Relay. Potential coil relay or voltage coil relay. Solid state relays are suitable for capacitor start motors. Note there are two windings are known as the auxiliary and the main windings or

start and run.

O/Lt

N

Fig. 1 A, capacitor start motor with a current coilrelay and start capacitor. Note the start capacitorand auxiliary are disconnected after the motor

reaches approximately 75% of its full speed.

A


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Current coil relay:

The current coil relay has a normally open set of contacts and used on hp split phaseand capacitor start motor compressors and smaller, it is an electromagnetic type relaywhich operates on current flow. For example: when the motor is energised a rush ofcurrent through the coil causes the relay contacts to close bringing in both the start andrun windings and as the motor reaches approximately 75% of its full speed. The currentflow falls due to the lack of electromagnetism on the relay armature, the contacts the startnow open and thus the start winding drops out leaving the run winding in circuit.

Potential coil relay:

The potential or voltage coil relay has a normally closed set of contacts. When used on acapacitor start motor the coil is designed to open the contacts only when there issufficient voltage generated by auxiliary winding. Since the voltage of back E.M.F.generated is proportional to the speed, the relay contacts will open as the motor reaches

approximately 75% of its full speed and thus disconnecting the start capacitor and theauxiliary winding. Note: Fig.2.

O/Lt

A N

Fig. 2 A, capacitor start motor with a potential coilrelay and start capacitor. Note the start capacitorand auxiliary are disconnected after the motorreaches approximately 75% of its full speed.

Reversal:To reverse a capacitor start motor: reverse either the start or run winding connections but

not both.


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Characteristics: Capacitors must be matched to the motor to obtain optimum starting torque. Manufacturers will specify the capacitor value required for a particular motor. Start capacitors can range from 120f to 180f. Start capacitors remain in the circuit for approximately three seconds and are limited

on the number of starts per hour. This depends on the motor design.

Increasing the starting torque with this type of motor is accompanied by an increasein starting current.

Applications:

Capacitor start motor, are suited for applications with a large starting torque for example:

-

Motor compressors- Washing machines, dish washers- Pump motors

Capacitor start / capacitor run motors:Capacitor start / capacitor run motors have a much higher starting and running torque incomparison to the capacitor start motor. This type of motor as shown in figure 3, has thestarting capacitor disconnected during the starting sequence.

Both the auxiliary and main windings remain in circuit during normal operation with the runcapacitor charging and discharging between both sets of windings and thus greater startingand running torque is achieved.

MC

AO/Lt

A N

Fig. 3 Capacitor start / capacitor run motor,potential coil relay, start and run capacitors.Note: winding connections: M = Main,

A = Auxiliary

C = Common


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Characteristics:

Increased operating torque. Reduced noise and vibration. Improved motor efficiency and power factor. Note: solid state relays cannot be used with capacitor start/ capacitor run motors. It should be noted that a current coil relay can be used with a capacitor start /

capacitor run motor but it must be wired as per diagram in figure.4.

MC

AO/Lt

A N

Fig. 4 Capacitor start / capacitor runmotorusing a current coil relay.

Applications:

Commercial refrigeration cabinets and ice making machines that have highstarting and running torques. (For example: refrigeration systems fitted with a TXvalve as a metering device).

Air compressors and industrial polishers.


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Permanently split capacitor motors (PSC)(ARAC, 14.4)

The PSC motor does not need any switching device to disconnect the auxiliary winding orcapacitors.

This type of motor also has two windings permanently connected across the supply, with acapacitor in series with one of them as indicated in Fig.1. Both windings are identical in wiresize and the number of turns and are referred to as the main and auxiliary windings.

MC

AO/Lt

A N

Fig.1. Permanently Split Capacitor Motor

Note: Reversal is done by switching the supplyconnection from one side of the capacitor tothe other

Because the capacitor is in series with one of the windings, the current in that winding leadsthe current in the other and thus providing the necessary phase displacement to produce arotating stator field.

This phase displacement between the two is relatively small and so is the starting torque.

These motors are designed for light applications which require low starting torque such as: Ceiling fans Blowers Sealed compressor motors for air conditioning systems up to 7kW Fan motors used in air conditioning systems.


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Capacitors:Start capacitor features:

- AC electrolytic- Connected during starting only- Limited number of starts per hour (20)- Capacitance ratings between 120 and 250f.

Run Capacitors features:- Oil bath- Permanently connected- Capacitance ratings between 4 and 35f.

Shaded Pole Induction Motors:The shaded pole motor has a squirrel cage rotor with only one winding (coil). The stator has

salient poles (extended poles) which have shading coils on diagonally opposite pole tips.

Shading coil

Main Coil

Squirrel cagerotor

Laminated iron core carriesthe magnetic flux.(referred to as the flux path)

A N

Operation:

The flux field travelling through the stator is swept across the two pole faces. This fluxtravels through the air gap and cuts the rotor conductors. This induces a rotor field and theinduced rotor field reacts with the stator field causing the rotor to turn in the same direction.


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Advantages:

Simple construction Low cost Low maintenance Simple speed control.

Disadvantages:

Low starting torque Low motor efficiency Not easy to reverse direction.

Reversal:

Reversal is done by:

Turning the whole motor end for end (where the rotor is doubled ended). Dismantling and turning the stator end for end, relative to the rotor.

Applications:

Small evaporator fan motors Pumps Appliances.


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Q.1 Name four methods used to switch the start/auxiliary winding of a split phase motor.

Q.2 Describe how would you change the direction of a shaded pole motor:

Q.3 Explain why a capacitor start motor can develop a greater starting torque than a split

phase motor.

Q.4 Explain how you would change the direction of a permanently split capacitor motor:

Q.5 Explain the operation of a potential coil relay:

Q.6 Which of the following motors is best suited for a single phase evaporator fan motor?

(a) Series universal motor

(b) Permanently split capacitor motor

(c) Split phase motor.

(d) Capacitor start motor


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Q.7 The single phase motor which is most suitable for a domestic refrigerator evaporator

fan motor is the:

(a) Capacitor start motor

(b) Series universal motor

(c) Shaded pole motor

(d) Permanently split capacitor motor.

Q.8 Which of the following motors would be best suited for a domestic refrigerator

compressor?

(a) Shade pole motor

(b) Permanently split capacitor

(c) Capacitor start / capacitor run motor

(d) Split phase motor

Q.9 List a least three desirable characteristics of a capacitor start / capacitor run motor:

Q.10 Why would you use a capacitor start capacitor run motor in preference to a split

phase motor compressor on a refrigeration system that has a TX valve as a refrigerant

metering device?


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NRE 8

Series Universal, Single Phase Motors

Section No: 7Purpose: The purpose of this section is to provide you with the underpinning knowledge andskills to identify the construction and applications for both the series universal andsynchronous motors.

Construction: Components parts of a series universal motor are as follows:

1. Stator consists of: Frame, rolled steel or cast iron shell Field core, laminations pressed tightly together and contains the field

windings.

Field coils, few turns of wire and are connected in series with the armature.2. Armature consists of:

Laminated core having either straight or skewed slots Windings housed in slots with the coil sides brought out to the commutator

segments

Both armature core and commutator are pressed onto the armature shaft3. End plates and brush holders

End plates house the bearings in which the armature revolves. One end plate has brush holders and can be removed. Brush holders and brushes provide the sliding electrical contact between the

field and the armature.Applications:

Portable hand held tools- drills- sanders- jig saws- routers- grinders- circular saws.

Domestic appliances:- vacuum cleaners- sewing machines- washing machines- hair dryer.


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Synchronous Motors:

The rotor of a synchronous motor rotates at a constant synchronous speed. That is at thesame speed of the rotating magnetic field and there is no slip. The speed is determined by thenumber of poles and the supply frequency.

Applications:

Clocks Timers Recording devices.

T C

A N

Typical refrigeration defrost timer with a NO andNC set of contacts.

Synchronous motor

N C

N C


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Answers to Review Questions:

Section No: 1

Q.1 Star and Delta

Q.2 Yes a single phase motor can be connected to a three phase supply as long as aneutral is present.

Q.3 Phase voltage = 415 volts Line voltage = 415 volts.

Q.4 Line Voltage: VpVline = voltsVl 415=

Phase Voltage: voltsVp 415=

Line Current: IpIline = 3 5732.1 = amperes66.8=

Phase Current:3

IlineIp =

732.1

66.8= amperes5=

Q.5 Line voltage volts415=

Phase voltage3

Vline=

732.1

415= volts240=

Line current IpIl == amperesIl 6.3=

Phase current amperes6.3=

Section No: 2

Q.1 (d)Q.2 (c)Q.3 (b)Q.4 (c)Q.5 (a)Q.6 (c)Q.7 (d)Q.8 (a)Q.9 Synchronous.Q.10 Swap any two phase connections.Q.11 because of the slip speed.Q.12 Actual speed = 120 x 50 X 0.96 = 1440 rpm.

4


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Q.13 any two lines = 415 volts.any line and earth = 240 volts.any line and neutral = 240 volts.earth and neutral = zero volts.

Q.14

A1 B1 C1

A2C2B2

L1 L2 L3Star Y

L2

A1 B1 C1

A2C2B2

L1 L2L2 L3

Delta

Fig. 2


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Q.15

Electrical terminals

Bearing

RotorFrame

Stator

Section No: 3

Q.1 The three contacts when closed supply the electrical power to three phase motor windings.An ON/OFF control switch when closed energises the contactor coil KI through the overloadand thus creating an electromagnet causing the three contacts close.

Q.2 Coil, armature, electrical contacts.

Q.3 Can switch the three phase appliance on and off automatically and provides overloadprotection.

Q.4 D.O.L. Star Delta Auto- transformer Primary Resistance Part Winding. Electronic Soft Start. Secondary resistance.

Q.5

K1

O/L

Stop Switch Start switch

Auxiliary Latching

Contact (K1-4)

N


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Q.6 Pushing the start button energises K1 and closes K1 contacts and thus when startbutton is released K1 is latch in through K1 4.

Section No: 4

Q.1 Mechanical load is greater than the electrical rating (kW)Rotor shaft lockedPart loss of supply voltageReduction in supply phase voltagesWinding insulation breakdown.

Q.2 Fuses, circuit breakers, thermal overload, magnetic overload.

Q.3 Thermal overload (bi-metal strip) Thermistors.

Q.4 Not to be set to low to prevent nuisance trippingNot to high causing motor burn out.

Q.5 A heater senses a rise in temperature on either one of the phases, this rise intemperature causes the bi-metal strips to warp and through a lever mechanism theoverload switch is opened.

Q.6 Relays and contactors will not operate correctly.Sensing devices do not respond as required.Increase in temperature of motor windings.

Q.7 Increase power consumption in the control circuit.Sensing and controlling devices operating incorrectly.Increase in motor temperature.

Q.8 ThermalMagneticVoltage

Q.9 To isolate electrical circuit between the motor supply and or its associated controlcircuit in the event of electrical or mechanical hazards.

Q.10 Increased heating of protective devices

Increase in temperature of the motorExtra stress placed on the motorControl contacts, both power and control, have increased operation which will resultin lower life span.Coupling between motor and load is subjected to increased stress.

Section No: 5

Q.1 Start

Q.2 Change either the start or run winding connections but not both.


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Q.3

Circuit energised causes a high rush or current through the relay coil. Relay contacts close and energises both start and run. As motor reaches full speed current falls and relay contacts to open. The start winding drops out and leaves the run winding in circuit.

Q.4

Ensure correct voltage is present. Isolate power supply to motor. Check relay and overload operation. With the motor terminals disconnected check the winding resistance against the

manufacturers specifications using an ohm meter or megger.

Q.5

When the motor is at standstill the centrifugal switch is closed. Both start and run are energised. Centrifugal switch will open at approximately 75% full motor speed dropping the

start winding out and leaving the run in.

Q.6 Locked rotorOpen circuit in either windingFaulty relayExcessive load on motor trips out on overload.

Q.7 N = 120 x f x slip N = 120 x 50 X 0.96 N = 2880 rpm

PQ.8

R RC

SO/L

AN

t

Split phase motor with acurrent coil relay


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RC

SO/L

PTC

A N

t

Q.9

Split phase motor with a solidstate relay

Q10

22 ohms 25 ohms

47 ohms


R C S

Section No: 6

Q.1 Speed sensing, current sensing or voltage sensing devices. Which include: centrifugalswitch, current coil relay, solid state relay and potential coil relay.

Q.2 Dismantling and turning the stator end for end, relative to the rotor.

Q.3 The capacitor start motor has a start capacitor connected in series with the startwinding.

Q.4 By switching the supply connection from one side of the capacitor to the other.


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Q.5

The contacts are normally closed energising both windings (aux & main). The contacts will open at approximately 75% full speed due the back EMF generated

in the auxiliary winding.

The auxiliary drops out and leaves the main in circuit.Q.6 (b)

Q.7 (c)

Q.8 (d)

Q.9 Reduced noise and vibration.Improved motor efficiency and power factor.Current coil relay can be used on this type of motor.

Q.10 Because the refrigeration system would require a motor with high starting andrunning torques to compensate for varying loads. Split phase motor has both lowstarting and running torques.

appliance motors

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