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MOTOR STARTERS

A.C. Induction motors are traditionally started and stopped by applying and removingthe A.C. supply. In some cases, a full voltage start is acceptable, but in many situations,

the start current must be reduced, and so a reduced voltage starter is employed.

1. Direct On Line

The simplest form of motor starter for the induction motor is the Direct On Line starter.

The DOL starter comprises a switch and an overload protection relay.

The switch may be a manually operated load brea switch, but more commonly it wouldbe an electromagnetic contactor which can be opened by the thermal overload relay.

Typically, the contactor will be controlled by separate start and stop buttons, and anau!iliary contact is used as a hold in contact i.e. the contactor is electrically latched

closed while the motor is operating. To start, the contactor is closed, applying full linevoltage to the motor windings. The motor will draw a very high inrush current for a very

short time, to establish the magnetic field in the iron, and then the current will be

limited to the Loced Rotor Current of the motor. The motor will develop Loced RotorTor"ue and begin to accelerate towards full speed. As the motor accelerates, the current

will begin to drop, but will not drop significantly until the motor is at a high speed,typically about #$% of synchronous speed. The actual starting current curve is afunction of the motor design, and the terminal voltage, and is totally independent of the

motor load.

The motor load will affect the time taen for the motor to accelerate to full speed and

therefore the duration of the high starting current, but not the magnitude of the starting

current. &rovided the tor"ue developed by the motor e!ceeds the load tor"ue at allspeeds during the start cycle, the motor will reach full speed.

If the tor"ue delivered by the motor is less than the tor"ue of the load at any speed

during the start cycle, the motor will cease accelerating. If the starting tor"ue with a

DOL starter is insufficient for the load, the motor must be replaced with a motor whichcan develop a higher starting tor"ue. The acceleration tor"ue is the tor"ue developed bythe motor minus the load tor"ue, and will change as the motor accelerates due to the

motor speed tor"ue curve and the load speed tor"ue curve. The start time is dependant

on the acceleration tor"ue and the load inertia. DOL starting results in ma!imum startcurrent and ma!imum start tor"ue. This may cause an electrical problem with the

supply, or it may cause a mechanical problem with the driven load.

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2. Primary Resistance.The &rimary 'esistance starter will have one or more sets of resistors which, during

start, are connected in series with the supply to the motor. The series resistors limit thestarting current drawn by the motor, and thus reduce the starting tor"ue of the motor.

Once the motor is up to full speed (or after a period of time) the resistors are bridged by

a contactor to apply full voltage to the motor. If the full details of the motor startingcharacteristics are nown, and the starting characteristics of the load are also nown, it

is practical to determine the correct value of the resistors to provide enough start tor"ue

for the load while minimi*ing the starting current.

A primary resistance starter correctly designed and constructed will cause the motor to

accelerate the load to almost full speed with the resistors in circuit before they arebridged out. In this case, the transition to full voltage only occurs once the impedance of

the motor has risen, and the resulting current is much less than the L'C of the motor.

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In a poorly designed system, the transition to full voltage will occur at less than #+% fullspeed, and the current will then step up to almost DOL current, resulting in little gain

from the use of the primary resistance starter other than the increased cost of thestarter. (Advantageous to the starter supplier, not to the end user) Improved starting

characteristics with some loads can be achieved by the use of several stages ofresistance and bridging out increasing amounts of resistance as the motor accelerates.

ith the primary resistance starter, it is not easy to alter the resistance and hence thestarting characteristics once the starter is built. Therefore, it is important that thecorrect resistors are selected in the first place.

The primary resistance starter reduces the voltage applied to the motor terminals whilepassing the full starting current to the motor. Conse"uently, there is very high power

dissipation in the resistors, resulting in the re"uirement for very high power ratedresistors. Typically, the resistors will dissipate as much as -$+% /++% the power

rating of the motor for the duration of the start.

The resistors may be either metallic resistors, or li"uid resistors. 0etallic resistors have apositive temperature coefficient and as a result, as they heat up, their resistance

increases. Li"uid resistors, such as saline solution, have a negative temperature

coefficient and so conse"uently, as they heat up, their resistance reduces. The heat buildup in the resistors during start, and their temperature dependant resistancecharacteristics, mae it essential the resistors are allowed to fully cool between starts.

This restricts the starting fre"uency and the minimum time between the starts.

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3. Primary ReactanceA Primary reactance starter is similar to a primary resistance starter except

that the resistors are replaced by a three phase reactor to limit the startingcurrent. The operation of the primary reactance starter is essentially the same

as that of the primary resistance starter, but the use of a three phase reactor inplace of the resistors offers the adantage of reduced heat loss and greater

ease of start current setting due to the ability to change taps on the reactor.

4. Auto transformerAn Auto transformer starter uses an auto transformer to reduce the oltageapplied to a motor during start. The auto transformer may hae a number of

output taps and be set!up to proide a single stage starter, or a multistagestarter. Typically, the auto transformer "ould hae taps at #$%, % and '$%

oltage, enabling the motor to be started at one or more of these settings.

There are t"o "ays of connecting an auto transformer starter, the mostobious "ay is to apply full oltage to the transformer ia a contactor, and

connect the motor to the tap by means of a contactor. (hen the motor hasaccelerated to full speed, or has run out of acceleration tor)ue, the tap

contactor opens, disconnecting the motor from the transformer and anothercontactor closes connecting the motor to the supply. The transformer can no"

be disconnected from the supply. This format is *no"n as an open transitionstarter and is less than ideal due to the fact that the motor is disconnected for

a short period of time during the start period. (hile the motor is connected andaccelerating, there is a rotating magnetic field in the stator "hich causes flux

in the rotor and thus a rotor current to flo". At the instant the motor isdisconnected, there is a magnetic field in the rotor "hich is spinning "ith!in

the stator "inding. The motor acts as a generator until the rotor field decays.The oltage generated by the motor is not synchroni+ed to the supply, and so

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on reconnection to the supply, the oltage across the contactor at closure canbe as much as t"ice the supply oltage resulting in a ery high current and

tor)ue transient. This open transition s"itching is often *no"n as the auto!recluse effect as it yields similar characteristics to opening and closing a

brea*er on a supply to one or more motors. The conse)uences of opentransition s"itching can be as bad as bro*en shafts and stripped gears.

y a rearrangement of the po"er circuit, it is possible, at no extra cost, to builda closed transition starter and thereby eliminate the current and tor)ue

transients. The closed transition auto transformer starter is *no"n as the

-orndorffer starter. The open transition s"itching is achieed by reconnectingthe tap contactor bet"een the transformer and motor, to the star connection of

the transformer, hard "iring the motor to the tap, and altering the se)uence ofcontactor control.

To start the machine, the main contactor and the star contactors are closed

applying reduced oltage to the motor. (hen the motor has reached full speedor run out of acceleration tor)ue/ the star contactor is opened effectiely

conerting the auto transformer starter into a primary reactance starter. 0extthe primary reactance is bridged by a contactor applying full oltage to the

motor. At no time does the motor become disconnected from the supply.

The transformer is generally only intermittent rated for the starting duty, andso the fre)uency and duration of the starts is limited. (ith a transformer

starter, it is relatiely easy to change taps and thereby increase the startingoltage if a higher tor)ue is re)uired. The auto transformer starter is a

constant oltage starter, so the tor)ue is reduced by the oltage reductions)uared oer the entire speed range, unli*e the primary resistance or primary

reactance starters "hich are constant impedance starters and "here the startoltage is dependant on the ratio of the motor impedance to the motor plus

starter impedance. As the motor accelerates, its impedance rises and

conse)uently, the terminal oltage of the motor also rises, giing a smalltor)ue increase at higher speeds.

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1nli*e the primary resistance and primary reactance starter, the currentflo"ing into the motor is different from that flo"ing from the supply. The

supply current flo"s into the primary circuit of a transformer, and thesecondary current is applied to the motor. The transformer reduces the primary

current by the same ratio as the oltage reduction. 2f the motor is connected tothe #$% tap of the transformer, the oltage across the motor terminals "ill be

#$%. Assuming an LRC of &$$%, there "ill be 3$$% current flo"ing into themotor. 2f 3$$% current flo"s into the motor, then the current into the

transformer "ill be 4#$%. This "ould suggest that the lo"est starting current"ill be achieed by the use of an auto transformer starter. 2n most instances,

the load "ill re)uire an increasing tor)ue as it accelerates, and so often ahigher tap must be selected in order to accelerate the load to full speed before

the step to full oltage occurs. 2f a multistage transformer starter is employed,

then the primary current "ill certainly be lo"er than other forms of inductionmotor starter.

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5. Star DeltaThe 5tar Delta starter can only be used "ith a motor "hich is rated forconnection in delta operation at the re)uired line oltage, and has both ends

each of the three "indings aailable indiidually. At start, the line oltage isapplied to one end of each of the three "indings, "ith the other end bridged

together, effectiely connecting the "indings in a star connection. 1nder this

connection, the oltage across each "inding is 46 rt 3/ of line oltage and sothe current flo"ing in each "inding is also reduced by this amount. Theresultant current flo"ing from the supply is reduced by a factor of 463 as is the

tor)ue i.e. a motor "hich exhibits a LRC of &$$% and an LRT of 4'$% "illexhibit characteristics of7 LRC star of 8$$% and LRT star of &$%.

2n some cases, this may be enough to get the motor up to full speed, but most,

as this is a constant oltage starter, the transition to full oltage "ill occur atpart speed resulting in a irtual DOL type start to step to full oltage

accompanied by a ery high tor)ue and current transient. 2n most situations,

there "ould be less damage to the e)uipment and less interference till the starconnection is opened, effectiely open circuiting the motor, and the ends of the

"indings are then connected to the three phase supply in a fashion to create adelta connection.

The star delta is not easily conerted to a closed transition starter, and eenthe closed transition (anchop/ star delta starter still has the problem that the

start oltage can not be altered. 2f there is insufficient tor)ue aailable in star,then it "ill go DOL. The star delta starter does get around the regulations in

some countries "here there is a re)uirement for a reduced oltage starter, butin reality, in many situations results in more seere transients than DOL. The

main benefits of the star delta starter are that it puts more money in thepoc*ets of the s"itchgear supplier, and it is politically correct.

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