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DIRECT ON LINE MOTOR STARTERS

Although these terms are used interchangeably, they describe two different devices.

A motor starter is a device designed to:

Start a motor

Accelerate the motor rated speed in the shortest time

Provide protection from overload conditions

A motor starter is therefore a switch with an overload relay.

A motor controller is a device designed to control the operation of a motor. As such it can be

used to control things such as:

Control when the motor starts and stops

How long it runs for

How often the motor run in a given period

Direction of rotation

Hence the motor controller usually contains a starter but the converse is technically not true.

Several devices have been used to energize electric motors many of which do not meet the

requirements of a starter. Many of these devices are permissible depending on the type and

function of the motor. For example, household appliances typically use a simple switch which

only makes or breaks the circuit. Our study will be limited to three-phase starters.

Several types of three-phase starters have been manufactured to start motors. These can be

classified as:

Manual Starters

o Manual starting switch

o Manual starter

Magnetic Starters

o Manually operated magnetic starters

o Automatic magnetic starters

MANUAL MOTOR STARTERS

A manual starter is an ON-OFF switch whose contact mechanism is operated by a mechanical

linkage from a toggle handle or push button that is operated by hand. These starters are

Designed for infrequent starting of AC motors

Used where undervoltage protection is not required

Used where remote or automatic operation is not required

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Manual Starter

o ON-OFF switch with overload relays.

o Use is limited to small motors

o Moving the handle or pushing the start button closes the contacts which

remain closed until

the handle is moved to off

the stop button is pushed

the overload relay trips

o If power fails, the contacts remain closed and the motor restarts when power

returns.

o Suitable for use with fans, pumps, compressors, oil burners

o Unsuitable for tools and may damage work or personnel

Manual Starting switch

o Their use is usually limited to small motors where overload protection is not

required or provided separately.

Construction of manual starters

Several constructed the most popular of which were:

The knife switch

The rotary blade switch

The drum switch

The manual contactor

The manual contactor

A contactor is a device designed to repeatedly establish and interrupt an electrical power circuit.

The have double-break contacts that opens and closes contacts in an electrical circuit. This

device is the basic building block for most motor controllers. The manual contactor is a contactor

that is manually operated.

The contactor consists of

Double break power contact assembly

o Three sets of normally open NO double break contacts – one in each phase

Two sets of fixed contacts mounted on insulators and having terminals

or lugs marked L for the supply side and T for the motor connections.

Movable contacts mounted on a conductive shorting bar that is

mounted on an insulated T-frame plunger (having no lugs)

Start-stop buttons

Insulated frame

Arc hoods to

insulate each set of contacts from the others

contain and quench the arcs drawn across the contacts as they open

and close

Insulated enclosure

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The double-break contacts allow for a higher contact rating in a smaller space than single-break

contacts. When the set of moveable contacts are forced against the fixed contacts the NO are

closed and the motor power circuit is completed and the motor is energized. When the stop

button is pushed, the contacts are forced apart to open the circuit.

Contacts closed Contacts open

Contact Construction

The diagram below illustrates the construction of a typical contact.

Most contacts today are made of low-resistance silver alloy (silver mixed with cadmium or

cadmium oxide) to give the following characteristics:

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MAGNETIC MOTOR STARTERS

The contactor is the base of the magnetic starter but the heart of this device is an electromagnet.

This electromagnet makes it possible to control the contactor to provide remote or automatic

operations.

Construction of the Magnetic Starter

This type of starter consist of:

Double break power contact assembly as with the manual starter consisting of

o Three sets of normally open (NO) double break contacts – one in each phase

Two sets of fixed contacts mounted on insulators and having terminals

or lugs

Movable contacts mounted on a conductive shorting bar that is

mounted on an insulated T-frame plunger (having no lugs)

Insulated mechanical linkage between the armature and the T-frame

Magnetic circuit consisting of

o the magnetic circuit assembly – stationary magnetic core

o a coil – supported by and surrounds part of the core and magnetizes it

o an armature – magnetic arm into which the stationary core induces flux and

attracts it to the core

Start-stop buttons that operate the coil circuit

Insulated frame

Arc hoods

Insulated enclosure

Principle of Operation of the Magnetic Starter

The figure below illustrates the physical structure and principle of operation of the magnetic

starter.

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Figure A shows the starter in its normal de-energized condition.

When the push button switch is operated, figure B, the coils circuit is closed and

current flows through the coil

The coil current sets up a flux in the core.

The core flux magnetizes and attracts the armature

As the armature moves towards the core it carries the insulated plunger with the

movable contacts unto the fixed contacts thereby closing the power contacts.

When the coil has been energized and moved to the closed position the

controller is said to be “picked up” and the armature “Seated or sealed-in”.

A small built-in air gap between the armature and assembly when the armature has

sealed-in

When the push button is released, the coil circuit is opened and de-energized, thereby

removing the flux from the magnetic pieces.

The armature is forced from the core by the spring ensuring that the power contacts

open.

The air gap ensures that the residual flux is insufficient to hold the armature in the

sealed-in position.

Coil Data

o Given in voltamperes VA

o Inrush current – current at energization

o Sealed current – current when sealed-in

o Inrush current is approximately 6 to 10 times Sealed current

The minimum control voltage that will cause the armature to start to move is called

the pick-up voltage

The minimum control voltage that will cause the armature to seat is called the seal-in

voltage

Effects of coil voltage variation

o If the control voltage is reduced sufficiently the controller will open. The

voltage at which this happens is called the dropout voltage and is always

lower than the seal-in voltage.

o If the voltage is too high the coil will draw higher than rated current, overheat

and become damaged. The magnetic pull will also be too high causing the

armature to move with excessive force and rapidly wear the magnet faces. The

contact bounce may also be excessive resulting in reduced contact life.

o Low coil voltage produces low coil current and reduce magnetic pull. If the

voltage is greater than pick-up but less than seal-in the controller may pick up

but not properly seal. The current will not fall to the sealed value and may

burn out. The armature will chatter becoming noisy and wear the magnet-

faces.

o If the armature does not seal properly, there will be inadequate pressure to

close the main (power) contacts resulting in excessive heat, arcing and the

possibility of welding of the contacts.

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6.3 OVERLOAD PROTECTION

The overload relay is a device that is designed to protect the motor against abnormal current flow

through the windings. These devices are designed to:

Have a time delay to allow harmless temporary overloads without

disrupting the circuit

Have a trip capability to open the circuit if mildly dangerous currents

which could result in motor damage continue over a period of time

Have some means of resetting the circuit once the overload is removed

Protection is accomplished by utilizing the effects of the electric current flow namely:

Heating

Magnetic

Hence there are two classes of overload devices i.e.

Thermal overload relays

Magnetic overload relays

Although the principles of operation of both types of devices differ, they are constructed to be

connected and function in a similar manner.

Each has power contacts that are connected in series with the motor

windings so that the motor current flows through them.

These power contacts are mechanically linked to switches that are

installed in the control circuit of the starter

Protection standards require that an overload device be placed in each phase of a motor winding.

Therefore the three phase motor starter will have an overload relay with three coils, one in each

phase. The device may be constructed to have one or three switches in the control circuit.

A relay with one control switch is so constructed that any of the three

power switches will activate it

A relay with three switches will need to have these switches either

o Mechanically ganged or

o Electrically wired in series with each other

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THERMAL OVERLOAD DEVICES

Several Thermal Overload Relays devices are presently in use. The more popular types are:

The Bi-metallic relay

The melting alloy relay

Each uses a device called a heater element in combination with a sensing element to detect the

overload condition and activate the trip mechanism. The heater is wired in series with the motor

windings in the power circuit. The heater transmits the heat developed as a result of current flow

to the sensing element. The sensing element is calibrated to respond to the amount of heat

developed. If excessive heat is developed due to excessive current flow over a period of time,

then it respond by mechanically activating the trip mechanism.

The Bi-metallic relay

Several types of bi-metallic relay exist. Their physical construction may vary but they all have

the following features in common:

They operate on the principle of the bi-metallic strip

The bi-metallic strips are connected in the power circuit

The wrapping effect of the bi-metallic is utilized as a trip lever or a

means of separating the contacts

Once the tripping has taken place, the bi-metallic strip will cool and

reshape itself

Depending on the needs, the trip lever may be reset manually or

automatically

The diagram below illustrates the principle of operation of the bi-metallic relay.

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The contacts are forced close by the bi-metallic strip, which at the same time extends the spring.

When the strip bends, it releases the force against the moveable contact bar and the spring pulls

the bar so that the contacts open. The degree of wrapping of the element is proportional to the

amount of heat current that flows through the heater and The bi-metallic now rests against the

reset lever. When the bi-metallic cools and reshapes, manually pressing the reset lever forces the

bi-metallic back against the moveable contact bar so that the contacts forced close against the

force of the extended spring.

Melting Alloy Overload Relay

There are several constructions of these types of relays. Most of these use a eutectic alloy in

conjunction with a mechanism to activate a tripping device when overload occurs. The most

commonly type uses a eutectic alloy to sense overloads and a ratchet wheel to activate a trip

mechanism. The eutectic alloy will respond to the rise of a definite temperature level. It is a

liquid at a definite temperature and a solid at a definite temperature and has constant

characteristics regardless of the number of melting and resetting that occur.

Figure below illustrates one type of overload relay. The heart of this melting alloy relay is the

melting solder tube. The tube consists of a ratchet wheel with a spindle attached, an outer tube,

and the solder between the inner spindle and outer tube. The heater element, which fits outside

the tube, conducts the current drawn by the motor and developed heat is transmitted to the tube.

Figure a Samples of heater elements Figure b Sample of a melting alloy tube

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Figure c When the solder is cold it is solid Figure d If excessive current flows

and holds the and shaft firmly into the tube. through the heater coil, the heat

The ratchet cannot move. developed will melt the solder thereby

freeing the shaft so that the wheel is free

to rotate.

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Figure e The normal position for the ratchet Figure f When an overload occurs, the

and pawl. The ratchet jams the pawl and stops ratchet is freed and the spring can

the spring from moving the contacts upward now force the contacts open.

and open.

The solder must be cooled before the overload can be rest. To reset the overload, the button is

pressed until the spring is fully compressed. The pawl moves downward across the face of the

wheel and jams into one of the ratchets. The contacts are now closed and the spring reloaded.

The Magnetic Overload Relay

These devices used a current coil wired in series with the motor power circuit as the a part of the

current sensing element. The coil develops magnetic flux proportional to the current flow

through it. At a specified overcurrent value, the soil acts as a solenoid and attracts a piston and

plunger assembly which in turn hits against the tripping element causing the contacts to open and

de-energize the control circuit. Figure below illustrates the operation of a Magnetic Overload

Relay

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The Magnetic Overload Relay has the following advantages over the thermal relay:

It is extremely quick on reset since it does not have to cool

It can be set for instantaneous of inverse time-tripping characteristics

Selecting and Adjustable Overload Settings

Since the overload relay is intended to protect the motor, if is essential that it gives close

protection. At the same time the device should not react to cause nuisance tripping and disrupt

the operational flow of a process. Modern philosophy is that overload settings should be within

125% of the full load current rating of the motor. This will avoid tripping on the starting current

and for moderate temporary overloads. The type of device used will depend on the type of motor

and process involved and on the speed of operation required.

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A motor starter is usually designed to operate over a range of currents and voltages. It can

therefore be used for a number of motors with differing full load currents. The appropriate

overload element must therefore be selected to operate with the motor.

Overload elements may come with fixed or variable setting. If the device has a fixed setting then

careful calculations must be done to select the appropriate device. Devices with variable settings

must be calibrated to the appropriate setting.

Mechanisms for adjusting overload settings

Thermal Overload Devices

These usually have a set-screw, knob or a dial labeled with current values or as a percentage of a

maximum value. The adjusting mechanism usually adjusts the tension on a spring or distance of

travel of the tripping mechanism.

Magnetic Overload Devices

Magnetic devices are usually adjusted by adjusting the dashpot – changing the distance of travel

of the piston

Resetting the Overload Device

Before the overload is reset, the cause of tripping should be determined and corrected. Adequate

time should be allowed for the motor windings to cool also. Recall that

motor starting develops a significant amount of heat.

overloads generate abnormal levels of heat.

The combination of both of these events cause dangerous heating levels to develop in the motor

windings that can

cause immediate burnout

deterioration which lead to burnout over a period of time.

Some devices are designed to reset automatically. The use of these devices should be limited to:

automatic operations in which damage to the motor is highly unlikely

human interfacing will not result in injury caused by a motor automatically restarting

6.4 DIRECT ON LINE CONTROLLERS

The motor controller has two distinct circuits namely:

The power circuit

The control circuit

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The Power Circuit

The power circuit is consists of a number of circuit elements connected in series through which

the motor is supplied with electrical power. It consists of the following elements:

The supply main disconnect switch and overload protection

o These may be two devices such as a disconnecting switch and a fuse or one

device such as a circuit breaker

o The disconnect switch provides a means of disconnecting the supply voltage from

the motor circuit for purposes of maintenance work

o The overload device provides overload and short circuit protection for the line

conductors

The circuit conductors carry current to the motor through the disconnect switch and the

starter

The starter

o The NO main contacts – the means by which the power to the motor is controlled

o The overload relay - protects the motor against overloads

The motor the object of control

The figure below illustrates the wiring diagram for a motor circuit.

The figure below shows the schematic diagram of the same motor circuit.

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Several models of the DOL have been used over the years. Three popular models are:

Open starter - Contactor and overload alone

Enclosed starter - Contactor, overload, and stop-start station in an enclosure

Combination starter - Supply main disconnect switch and overload protection,

Contactor, overload, and stop-start station in an enclosure

Where the starter is remote from the main disconnect switch, it is the practice to install another

disconnect switch before the starter to allow disconnection for maintenance purposes.

The Basic Control Circuit

The coil circuit is generally referred to as the control circuit. The control circuit is always

a single-phase circuit. The basic elements of the circuit are:

The magnet coil

The contacts of the overload assembly

The momentary pilot devices (start and stop push buttons)

The auxiliary contact (holding-in contacts)

Figure a below illustrates the wiring diagram of a three-wire control circuit of a DOL. Figure b

shows the schematic diagram of the same starter.

The start button is a normally open, spring loaded, momentary pushbutton

o Normally open so that the circuit remains open if it is de-energized

The stop button is a normally closed, spring loaded, momentary pushbutton

o Normally closed so that the circuit can be energized under normal conditions

The auxiliary contacts may be on the same insulator as the main contacts or may be a

part of a removable side mounted device attached in such a manner that it is

mechanically engaged by the insulator on the movable plate.

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Figure a Figure b

Operation of the control circuit

When the start button is pressed, the coil circuit is closed and energizes the coil.

The armature closes the main and auxiliary contacts

Because the magnetic force holds the auxiliary contacts in place, when the start button is

released they are still closed and maintain the coil circuit closed hence the starter remains

energized.

The main contacts transfer power from L1, L2, L3 to T1, T2, T3 respectively. This

connects the motor directly to the full line voltage.

If the stop button is pressed, the coil circuit opens and the magnets de-energize and

dropout causing the auxiliary contacts to open. The main contacts open also.

The starter remains de-energized as the start button holds the coil circuit open.

If the motor experiences an overload the trip mechanism of the overload relay opens the

coil circuit thereby de-energizing the starter. When the overload is reset, the coil circuit is

still open so that the coil cannot be automatically re-energized and hence the motor

cannot restart automatically.

Circuit Protection

SHORT CIRCUIT PROTECTION - provided in the distribution system

OVERLOAD PROTECTION - provided by the three overload relays

NO-VOLTAGE PROTECTION - Uses three-wire control protection

o Also called low voltage - the starter contacts drop out when there is a

protection voltage failure but do not automatically close

when voltage returns.

NO-VOLTAGE RELEASE - Uses two wire control

o Also called low voltage the starter contacts drop out when there is a

Release voltage failure by picks up as soon as the voltage

returns.

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Motor Control and Indicating Elements

The basic magnetic motor starter can be converter into a motor controller with endless functions

by the simply adding other devices capable of controlling the coil circuit. The controller can be

used in a manual, semi-automatic or fully automatic mode. A few of the more commonly used

devices are presented below.

Pushbutton Control Stations

A pushbutton control station is an assembly of components that contain pushbuttons and may

contain other accessories.

A pushbutton is an assembly that contains a master switch, manually operable plunger or button

for actuating device. Pushbuttons can control a motor by the engaging the appropriate button.

Using pushbutton control station the motor can be controlled from several locations and used to

perform a variety of operations. The pushbutton consists of two basic components:

The contact block - the switches that make or break the electrical circuits

o Contacts are of the double break type

o Contacts are assembled in a number of contact arrangements

o Contact blocks are constructed for single and double circuit control

o Contain NC and NO contacts

o Several contact blocks may be stacked together and operated by one operator

The operator - the mechanism that operates the switches

Some of the basic building blocks of pushbutton stations are:

Start-Stop Station

o The most common pushbutton station

o used to manually start and stop a motor

o consists of two switches, one normally closed (NC) and one normally open (NO),

in one block. These switches may be electrically linked or may be separated.

o Each switch is operated separately

Multiple Start-Stop Station o used to control a motor starting and stopping from several remote locations

o contains a number of NC and NO switches

o to function

stop buttons are wired in series

start buttons are wired in parallel

Forward-Stop-Reverse o used to reverse the direction of rotation of a motor

o consists of three switches, one normally closed (NC) and two normally open

(NO), in one block

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o each switch is operated separately

o switches can be mechanically linked (interlocked) so that only one operation can

occur at any given time

Run-Stop-Jog

o used to control the motor running continuously once started (run) or intermittently

starting and stopping (jogging). Jogging operation can be performed manually or

automatically (inching).

o made with a number of different configurations

o typical switch consists of start, stop and ganged NC and NO jog switch. The NC

contact is wired in series with the start and auxiliary circuit. Pressing the start

button will close and latch in the control circuit. When the jog switch is pressed it

opens the start circuit so that the control circuit cannot latch in.

Selector Switches

o a master or manually operated switch with rotating motion for the actuating

device or assembly

o a mechanical switch made with a variety of poles and configurations and positions

o the switch selects only one connection at any given time

o the position of the switch may be maintained or spring returned

o may or may not have an off position

Illuminated Pushbutton Switches o pushbutton switches with a lamp that can be wired to indicate the condition of the

switch, control circuit and motor.

Pushbutton switches are used in a variety of situations in a variety of environments and under

a variety of conditions. Therefore switches must designed to function in the environment for

which it is intended. To accomplish this the National Electrical Manufacturers Association,

NEMA, specified several classes of switches according to:

o current carrying capabilities - eg. Standard or heavy duty

o enclosures - eg. General purpose, watertight

Pushbutton Accessories

Pushbuttons may be supplied with a variety of fittings to enhance their operating and

indicating functions such as:

o padlocking attachment – to lock the stop button in the depressed position

o pilot lights - lamps used to indicate the status of the switch, control

circuit and motor.

o legend plates - metal plates that indicate the function of the switch or the

- status of the switch in a particular position

- fit over the locknut or operator

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Relays

A relay is an electrically controlled device that opens and closes electrical contacts to effect the

operation of other devices in the same or other circuits. Relays are used primarily as switching

devices in control circuits. Except for small motors and solenoid, relays are not used in power

circuits. Typical applications include:

switching starting coils

controlling other relays

turning on small devices such as

o pilot lights

o audible alarms

o signals

Classification

There are two major types of relays

electromechanical relays - electrical switches actuated by electromagnetic or

mechanical means

solid state relays - electrical switches actuated by electronic circuitry

Relays are also classified as light, medium and heavy-duty; Industrial, Commercial and

Military. Another classification is by Electrical Control, Performance, Mechanical Action,

Enclosure.

The general purpose relay is a mechanical switch operated by a coil. Hence the relay has two

A Typical Electromagnetic General Purpose Relay

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sections the relay power (energizing) circuit and the switching contacts. The power circuit must

be specified in terms of the circuit from which the signal will come i.e. in terms of supply

voltage. Typical information given is:

voltage system AC or DC

voltage value numerical value

The contacts are specified in terms of

type of circuit in which they should function AC or DC

power value usually given in terms of current

types of contacts provides information about possible

switching configurations

Relay contact operation

Three words are commonly used to describe relays:

Poles the number of completely isolated circuits that can pass

through the switch at one time

eg. Double pole switch carries current through two

simultaneously with each circuit isolated from each

other.

Throws the number of different closed contact positions per pole

that are available on the switch i.e. the number of

independent circuits that the switch can control.

Eg. Single throw = one circuit,

double throw = two circuits.

Break the number of separate contacts the switch uses to open or

close each individual circuit

Single break = electrical contacts broken in one

place

Double break = contacts broken in two places

NARM use number codes to specify each possible contact arrangement.

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The tables below illustrate the types of contact arrangements giving contact names and

designator.

The arrangement and types of relay contacts NARM Switching Identification

System

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Solid State Relays

Solid state relays use electronic components such as TRIACS to perform switching. They are

faster and provide arcless switching.

Relays are manufactured to be connected in one of two ways in a circuit:

o Contacts are wired to pins that protrude out of the casing and fit into separate

sockets called bases.

o Contacts are wired to terminals to which conductor are connected either by:

Soldering

Push on terminals

Screwed terminals

Whether electromechanical or solid state relays are used the specifications must be carefully

considered so that the correct relay is chosen for the given application.

Time Delay Relay

A time delay relay TDR is one in which the operation of the contact occur some time after the

relay has been energized. This is achieved by having some built-in device or mechanism to delay

the contact operation. TDRs have been designed to perform a number of switching functions.

The same relay may have different contacts that perform a variety of switching functions.

Typical examples are:

Contacts NC delay to open - TDO

Contacts NO delay to close - TDC

Some contacts operate delayed while others operate instantaneously

Some TDRs have fixed delay time while it is possible to adjust the delay time in others. One type

of TDR uses a motor and cam to produce a repeated cycle of operations.

Control Mechanisms for TDR

A typical Time Delay Relay Wiring Diagram

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A Typical Pneumatic TDR The Pneumatic Timing Unit

The Repeat Cycle Timer

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Special Control Switches

Switches designed to control motors base on predetermined stimuli. Some of these switches

convert mechanical stimuli into mechanical action that is used to operate contacts. Others

convert stimuli into electrical signals, typically voltage and current. These are used to turn on or

off other control elements or vary existing circuit conditions. The diagram below shows a

typical float switch which is used to control a circuit based on rising or falling liquid levels.