eet272 worksheet week 8 questions question 1 - giriş...

EET272 Worksheet Week 8

answer questions 1-5 in preparation for discussion for the quiz on Monday. Finish the rest of thequestions for discussion in class on Wednesday.

Questions

Question 1

We will now look at a variety of circuits to control motors. This is not an exclusive list. The goal isto learn to read ladder diagrams and diagnose what the function of the various parts of the circuit and thecomplete function is.

In our text read and outline the section onMotor Control Circuits, Mutliple Start/Stop Controls, MultipleMotor Starter Control, Sequential Starting Control, Various starting methods, and Braking in Ch 20 of ourtext (pgs 476-498)

file q0060

Question 2

A very common form of latch circuit is the simple “start-stop” relay circuit used for motor controls,whereby a pair of momentary-contact pushbutton switches control the operation of an electric motor. Inthis particular case, I show a low-voltage control circuit and a 3-phase, higher voltage motor:

M1

M1

StartStop

M1

motor

To 3-phasepower source

F1

F2480/120 V

OL

OL

Explain the operation of this circuit, from the time the “Start” switch is actuated to the time the “Stop”switch is actuated. The normally-open M1 contact shown in the low-voltage control circuit is commonlycalled a seal-in contact. Explain what this contact does, and why it might be called a “seal-in” contact.

Kuphaldt file i02304

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Question 3

Examine this motor control circuit for a start/stop/jog control:

StartStop

M1

CR1

CR1

CR1

Jog

L1 L2

Explain in your own words what distinguishes the “Start” function from the “Jog” function, and thinkof a practical application where this might be useful.


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Question 4

The most common method of starting up a three-phase induction motor is to simply apply full powerall at once by closing the three contacts of a large “contactor” relay. This is called across-the-line starting:

motor3-θ power

"Across the line" motor starter

Across-the-line starting is simple, but results in huge “inrush” currents at the moment of contactorclosure, and also places a lot of mechanical and thermal stress on the motor as it rushes to attain full speed.

A “gentler” method of starting an induction motor is to place impedances in series with the three-phasepower, using two contactors (one “start” and one “run”) to sequence the motor from start-up to full-speedrun. The impedances ideally take the form of inductors (“reactors”):

motor3-θ power

Start

Run

Explain how and why this method of starting is gentler than across-the-line starting.

Suggestions for Socratic discussion

• Would large (high-power) resistors work instead of inductors?• Would large capacitors work instead of inductors?


3

Question 5

Examine this control circuit diagram for an air compressor, where a pair of pressure switches controlsthe starting and stopping of the electric motor turning the air compressor:

L1 L2

Cut-out switch OLMHand

Off

Auto

Control switch

Receiver tank

Condensate drain valve

Compressor

Intake filter

PS PS

Control circuit schematic

Compressed air

Pictorial diagram of system

Cut-out switch Control switch

Explain what the “Hand-Off-Auto” switch does in this circuit, and also describe the functions of eachpressure switch.


• Which of these two pressure switches should have the greater trip setting, and why?• Why do you think operations personnel might find it useful to have a “Hand” position as well as an

“Auto” position on the switch in this air compressor system?• Some “Hand-Off-Auto” switches place the “Auto” position in the middle, between the “Hand” and the“Off” settings – explain why this might be a better way to arrange the three-position switch.


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Question 6

Suppose we wish to have three separate pushbutton start/stop stations for operators to use in controllinga single three-phase electric motor. The control circuit wiring schematic shows how this will work:

Start

Start

Start

Stop Stop Stop

L2

M1

L3Fuse

M1

Sketch the necessary connecting wires to build this control circuit:

Motor

Contactor

H1

H2

H3

H4

X1

X2

Transformer

L1 L2 L3

T1 T2 T3

Start Start Start

Stop Stop Stop

Fuse


• An overload contact has been omitted from this motor control system for simplicity’s sake. Identifywhere one would be properly inserted into the schematic diagram, and also in the pictorial diagram.


5

Question 7

This motor control circuit commands three motors to start and stop together:

L1 L2

M1

motorTo 3-phasepower source

M1

M2

motor

M2

motor

M3

OL1

OL2

OL3

OL1

OL2

OL3

M3

StartStop

M3

M1

M2

Control circuit

Power circuit

Examine the control circuit and then explain how starting one motor starts up the others. Also,determine what will happen if motor #3 suffers an overload (i.e. OL3 warms up enough to trip).


• Explain why inrush current could be a problem in this three-motor control system, and identify at leastone practical solution for it.


6

Question 8

The direction of rotation for a three-phase AC electric motor may be reverse by reversing any two ofthe three power conductor connections. With this in mind, explain how this reversing motor control circuitworks:

L1 L2

M1


M1

Forward

Reverse M2

M2

M1

M2

In particular, what it the function of the two normally-closed “M” contacts (called interlock contacts)in the control circuit? What do you think might happen if those contacts were not there?


• Explain why reversing any two phase conductors supplying AC power to an induction motor will causeit to reverse direction.

• Explain what arc flash is, and how to protect yourself from it while working on high-voltage motorcontrol circuits such as this one.


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Question 9

Large electric motors are often equipped with some form of soft-start control, which applies powergradually instead of all at once (as in “across the line” starting). Here is an example of a simple “reducedvoltage start” control system using a time-delay relay (TD1):

L1 L2

StartStop CR1 OL

CR1 TD1

M1

TD1 M2

M2


M1 OL

M2

R

Analyze this ladder logic diagram, and explain the function of the time-delay relay, particularly how tointerpret its switch symbol (with arrowhead). How does the time-delay relay cause a soft start?


8

Question 10

The following ladder logic diagram is for a reversing motor control circuit:

L1 L2

M1


M1 OL

OLForward

Reverse M2

M2

M1

M2

M1

M2

Stop

Study this diagram, then explain how motor reversal is accomplished. Also, identify the function ofeach ”M” contact in the control circuit, especially those normally-closed contacts in series with the motorstarter coils.

9

Now consider the following modification made to the reversing motor control circuit (motor and powercontacts not shown here):

L1 L2

M1 OLForward

Reverse M2

M2

M1

M1

M2

Stop

TD1

TD2

TD2

TD1

What extra functionality do the time-delay relays contribute to this motor control circuit?Kuphaldt file i02496

10

Question 11

This motor-control “bucket” has a problem: the motor refuses to start when the “Start” pushbutton ispressed. A voltmeter connected to test points C and E indicates 118 volts AC with no pushbuttons pressed:

M1

M1

StartStop

M1

motor

To 3-phasepower source

F1

F2480/120 V

OL

OL

A

B

CD E

F

Identify the likelihood of each specified fault for this circuit. Consider each fault one at a time (i.e. nomultiple faults), determining whether or not each fault could independently account for all measurementsand symptoms in this circuit.

Fault Possible Impossible

Fuse F1 blownFuse F2 blown

Start switch failed openStop switch failed openM1 coil failed open

M1 auxiliary contact failed openM1 power contact(s) failed open

OL contact failed openStart switch failed shortedStop switch failed shorted

Transformer secondary failed shorted

Finally, identify the next diagnostic test or measurement you would make on this system. Explain howthe result(s) of this next test or measurement help further identify the location and/or nature of the fault.


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Question 12

An interesting way to achieve reduced-voltage starting for a three-phase motor is to use a 6-leadmotor where the three stator winding sets are individually wired so as to allow either wye (start) or deltaconfigurations:

1 2 3

4 5 6

1

4

2

5

3

6

1

425

3

6

Wye connection

Delta connection

6-lead, 3-phase motor

A “Start” contactor sends power to the stator windings in a wye configuration for a short start-up time(perhaps 10 seconds), then that starter disengages and a “Run” starter energizes to send power to the statorwindings in a delta configuration. In the “wye” configuration, each winding receives 1

√

3of the line voltage.

In the “delta” configuration, each winding receives the full line voltage.

Sketch the proper wire connections to create just such a “wye-delta” motor starter. Hint: terminals 1,2, and 3 of the motor always connect to the three-phase power lines!

Start

Run

To three-phasepower source

Fuses

1

2

3

4

5

6

Motor


• Explain the purpose of using reduced-voltage starting for a large electric motor.


12

Question 13

Synchronous AC motors by their nature rotate at precisely the same speed as the rotating magneticfield produced by the stator windings. The practical problem with this is how to get a synchronous motorstarted, since it is physically impossible for the rotor to jump from a stand-still to 100% speed in zero time.

Therefore, synchronous motors are usually started as regular induction motors at first, and then theyare switched to synchronous mode when their speed is very near 100%. The following control circuit showsone scheme for this dual-mode start-up. The rotor on this synchronous motor has its own winding:

Rotor winding

Three-phase synchronous motor M

Stator windings

Line power

+−

125 VDCRun

Start

L1 L2

M

OL

OL

M

StartStop

F

Start

Run

Control circuit wiring

Explain how this start-up circuit functions, and what goes on with the switching of the rotor windingto make the motor start up and then run in two different modes.


• What practical applications might warrant the use of a synchronous AC motor instead of an inductionAC motor?


13

Question 14

There are three basic types of braking control circuits. They are coasting, plugging, and dynamicbraking. Explain each one in your own words. What are some advantages and disadvantages of each typeof braking circuit?


• Many electric cars are able to recharge their batteries when the brakes are applied. They call thisregenerative braking. Explain how this works.

file q0061

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Question 15

Expalin how this control circuit works. What is the function of TD1?

L1 L2

M1


M1 OL

OLStart

TD1M1

M2

M1

TD1

Stop M2

M1 M2


• If you are unsure how long the delay of TD1 should be. Decide if you would rather error on the side oftoo short or too long. Explain why you chose the way you did.

file q0062

15

Question 16

Examine this three-phase motor control circuit (sometimes referred to as a “bucket”), where fuses protectagainst overcurrent faults, a three-pole relay (called a contactor) turns power on and off to the motor, anda set of overload heaters detect mild overcurrent conditions. Control circuit wiring has been omitted forsimplicity’s sake. Only the power wiring is shown:

Fuse

Motor

Shaft

Fuse Fuse

1 2 3

1 2 3

Contactor

123

motor

Schematicdiagram

1 2 3

1 2 3

Fuses

Reset

Overloadblock

OL’s

Line 1 Line 2 Line 3

After years of faithful service, one day this motor refuses to start. It makes a “humming” sound when thecontactor is energized (relay contacts close), but it does not turn. A mechanic checks it out and determinesthat the shaft is not seized, but is free to turn. The problem must be electrical in nature!

16

You are called to investigate. Using a clamp-on ammeter, you measure the current through each ofthe lines (immediately after each fuse) as another start is once again attempted. You then record the threecurrent measurements:

Line Current1 52.7 amps2 51.9 amps3 0 amps

Determine at least two possible faults, either one fully capable of causing the motor’s refusal to startand the three current measurements taken. Then, decide what your next measurement(s) will be to isolatethe exact location and nature of the fault.


• Is there a way we could have determined a lack of current in line 3 without the use of a clamp-onammeter, using a multimeter incapable of directly measuring current over 10 amps?


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Question 17

This “lift station” pump control circuit has a problem. The sump pump is supposed to come on whenthe high level is reached, and turn off when the water pumps down to the low level point. Instead, however,the motor “cycles” on and off at the low-level point. Using an AC voltmeter, you measure a voltage frompoint B to point D that switches back and forth between 120 volts and 0 volts:

LSH LSL

M1

M1 OL

motor

OL

To 3-phaseAC power

M1

H1 H2 H3 H4

F1 F2

F3

A B

C

D E

F G

(480 V)

120 VAC

Identify the likelihood of each specified fault for this circuit. Consider each fault one at a time (i.e. nomultiple faults), determining whether or not each fault could independently account for all measurementsand symptoms in this circuit.


High level switch failed openLow level switch failed open

Broken wire between D and M1 coilContactor auxiliary contact failed open

Contactor auxiliary contact failed shortedContactor main contact(s) failed open

Broken wire between B and G

Thermal overload unit trippedHigh level switch failed shorted

Transformer secondary winding failed open

Finally, identify the next diagnostic test or measurement you would make on this system. Explain howthe result(s) of this next test or measurement help further identify the location and/or nature of the fault.


18

Answers

Answer 1

Answer 2

Even though the “Start” and “Stop” switches are momentary, the “seal-in” contact makes the circuitlatch in one of two states: either motor energized or motor de-energized.

Answer 3

The “Start” function is latching, whereas the “Jog” function is not. A common application of thisconcept is in a kitchen blender, where one button starts (and latches) the blender, while another simply“pulses” the blender.

Answer 4

The “Start” contactor must be energized first, then at a later time is de-energized as the “Run” contactoris simultaneously energized. Either timing relays or a PLC handles this sequencing of contactors.

Answer 5

Both pressure switches are normally closed and open when a specified pressure is reached.

Answer 6

Motor

H1

H2

H3

H4

X1

X2

Transformer

L1 L2 L3

T1 T2 T3

Start Start Start

Stop Stop Stop

Fuse

Answer 7

19

Answer 8

The normally-closed contacts are referred to as interlock contacts, and they prevent simultaneous forwardand reverse actuation of the motor.

Answer 9

For starting M1 should be closed and the resistors limit the motor current. During normal running M2is closed and M1 is open. This applies full voltage to the motor.

Remember that time-delay relay symbols always use an arrowhead at the switch contact to denote thedirection of timing. With this switch, the arrowhead points in the closed direction, which means the relaytakes time to close. Being normally-open, this means the delay happens upon energization of the relay coil,the implication being that the relay will return to its normal (open) state immediately upon de-energization.

Answer 10

The normally-open and normally-closed ”M” contacts provide seal-in and interlock functions,respectively. The time-delay relays prevent the motor from being immediately reversed.

Answer 11


Fuse F1 blown√

Fuse F2 blown√

Start switch failed open√

Stop switch failed open√

M1 coil failed open√

M1 auxiliary contact failed open√

M1 power contact(s) failed open√

OL contact failed open√

Start switch failed shorted√

Stop switch failed shorted√

Transformer secondary failed shorted√

20

Answer 12

Start

Run

To three-phasepower source

Fuses

1

2

3

4

5

6

Motor

Answer 13

In the start-up mode, the motor’s rotor winding is short-circuited by the “Start” contact. This makesthe motor behave like a normal squirrel-cage induction motor with its rotor bars and shorting rings.

As soon as the speed switch detects adequate rotor speed, the “Start” coil de-energizes and the “Run”coil energizes, connecting the rotor winding directly to a DC power source to magnetize it and lock it intosynchronous mode.

Answer 14

Coasting takes the longest to stop a motor. Plugging is faster but can be hard on the motor. Sometimesplugging is pulsed on an off if you have digital control. Dynamic Braking is the fastest but also the mostcomplictated.

Answer 15

TD1 is involved in a braking of the motor. Be sure to explain the type of braking

Answer 16

Here are some possibilities:

• Fuse #3 blown open• Third relay contact damaged (failed open) inside the contactor• Overload heater #3 failed open• One winding failed open inside the motor (assuming a “Y” winding configuration)

There are several valid “next steps” you could take from this point. Discuss alternatives with yourclassmates.

21

Answer 17

Partial answer:


High level switch failed openLow level switch failed open

√

Broken wire between D and M1 coil√

Contactor auxiliary contact failed openContactor auxiliary contact failed shortedContactor main contact(s) failed open

√

Broken wire between B and G√

Thermal overload unit trippedHigh level switch failed shorted

Transformer secondary winding failed open

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eet272 worksheet week 8 questions question 1 - giriş...

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