Mostafa Ahmed IbrahimMostafa Ahmed Ibrahim

Three-phase asynchronous induction Three-phase asynchronous induction motors are the most commonly used type in motors are the most commonly used type in industry. Connected directly to the supply, industry. Connected directly to the supply, these motors have typical starting currents these motors have typical starting currents of 5 to 8 x FLC. Often, starting currents need of 5 to 8 x FLC. Often, starting currents need to be reduced, and various forms of starter to be reduced, and various forms of starter have been developed to make this possible.have been developed to make this possible.

AbstractAbstract

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Starting of cage-rotor induction motors may Starting of cage-rotor induction motors may be performed by: be performed by: 1 - Direct connection to power grid 1 - Direct connection to power grid 2 - Star-delta switch connection 2 - Star-delta switch connection 3 - Low voltage auto-transformer 3 - Low voltage auto-transformer 4 - Additional resistance (reactance) in the 4 - Additional resistance (reactance) in the stator stator 5 - Soft starting (through static variacs) 5 - Soft starting (through static variacs)

STARTING OF CAGE-ROTOR INDUCTION MOTORS

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Direct-on-line (DOL) starters

- The stator windings are connected directly The stator windings are connected directly to the three phase mains supply.to the three phase mains supply.- The peak Starting current is between 5 and - The peak Starting current is between 5 and 8 times normal full-load current.8 times normal full-load current.- The peak starting torque is between 0.5 and The peak starting torque is between 0.5 and 1.5 times the motors nominal operating 1.5 times the motors nominal operating torque.torque.

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Advantages of DOL starters :-Advantages of DOL starters :-

- SimplicitySimplicity

- Low cost Low cost

- High starting torqueHigh starting torque

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Drawbacks of DOL startersDrawbacks of DOL starters- The peak starting current would result in a - The peak starting current would result in a

serious voltage drop on the supply systemserious voltage drop on the supply system

- The equipment being driven cannot tolerate the - The equipment being driven cannot tolerate the

effects of very high peak torque loadingseffects of very high peak torque loadings

- The safety or comfort of those using the - The safety or comfort of those using the

equipment may be compromised by sudden equipment may be compromised by sudden

starting as, for example, with escalators and liftsstarting as, for example, with escalators and lifts

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Star-delta starters

-On starting, the supply is first applied to the motor On starting, the supply is first applied to the motor

with its stator windings star-connected.with its stator windings star-connected.- As the motor accelerates, its speed stabilizes when As the motor accelerates, its speed stabilizes when

its developed torque become equal to its load its developed torque become equal to its load

torque. This usually happens at about 75% - 80% of torque. This usually happens at about 75% - 80% of

nominal speed. nominal speed. -The star contactor is then de-energised, and the The star contactor is then de-energised, and the

delta contactor energised to deltaconnect the stator delta contactor energised to deltaconnect the stator

windingswindings

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Advantages of Star-delta starters :-Advantages of Star-delta starters :-

During star connection, the phase voltage Vs During star connection, the phase voltage Vs becomes becomes VVss = V = VLL / √3 / √3so the phase current, for same slip, IsY, is reduced so the phase current, for same slip, IsY, is reduced √ √ 3 times 3 times IIsy sy = I= ISdSd / √3 / √3

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Drawbacks of Star-delta startersDrawbacks of Star-delta startersTTeyey/T/Teded = (V = (Vsysy/V/VLL))^2^2 =1/3 =1/3-Star-delta starters are particularly suited to -Star-delta starters are particularly suited to machines which do not present a high load torque machines which do not present a high load torque at start-up, or which normally start off-load.at start-up, or which normally start off-load.- Although the transient produced at the star-delta - Although the transient produced at the star-delta transition is very brief, the current can be quite transition is very brief, the current can be quite large and, particularly for larger motors, some form large and, particularly for larger motors, some form of current limiting may be necessary. of current limiting may be necessary. - This method can only be used with low-inertia - This method can only be used with low-inertia loads.loads.

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Auto-transformer starters

- In auto-transformer starters, the motor is started In auto-transformer starters, the motor is started

at reduced voltage which is supplied from an auto-at reduced voltage which is supplied from an auto-

transformer.transformer.

-Values for peak starting current are 1.7 to 4 times Values for peak starting current are 1.7 to 4 times

nominal full-load current and, for peak starting nominal full-load current and, for peak starting

torque, 0.5 to 0.85 times nominal operating torque.torque, 0.5 to 0.85 times nominal operating torque.

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Advantages of Auto-transformer Advantages of Auto-transformer starters :-starters :-

- The starting current and torque are reduced The starting current and torque are reduced

as a function of the reduced starting and run-as a function of the reduced starting and run-

up voltages (Usupply/Ustarting)2. up voltages (Usupply/Ustarting)2. - Auto-transformer starting is particularly - Auto-transformer starting is particularly used for large motors (above 100kW)used for large motors (above 100kW)

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Drawbacks of Auto-transformer starters Drawbacks of Auto-transformer starters :-:--Expensive solution, largely because of the cost of Expensive solution, largely because of the cost of

the autotransformer itself. the autotransformer itself.

-These starters may also produce a current peak atThese starters may also produce a current peak at

the instant when the motor is switched directly to the instant when the motor is switched directly to

the supply.the supply.

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Primary resistance starters

- Starters of this type start the motor at reduced Starters of this type start the motor at reduced

voltage by connecting a resistance bank in series voltage by connecting a resistance bank in series

with the motor windings.with the motor windings.- Once the motor has run up and its speed has Once the motor has run up and its speed has

stabilised, the resistance bank is shorted out, and stabilised, the resistance bank is shorted out, and

the motor becomes direct-connected.the motor becomes direct-connected.

- Peak starting current will be around 4.5 times - Peak starting current will be around 4.5 times

nominal full-load current, and peak starting torque nominal full-load current, and peak starting torque

willbe around 0.75 times nominal operating torquewillbe around 0.75 times nominal operating torque.

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Advantages of Primary resistance Advantages of Primary resistance starters:-starters:-

Primary resistance starters are especially Primary resistance starters are especially

suitable for applications, such as ventilator suitable for applications, such as ventilator

fans, where the load torque increases with fans, where the load torque increases with

speed.speed.

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Drawbacks of Primary resistance Drawbacks of Primary resistance starters :-starters :-

A possible disadvantage is the high peak current A possible disadvantage is the high peak current

at the instant of starting, but this can be reduced at the instant of starting, but this can be reduced

by increasing the resistor values, but this also by increasing the resistor values, but this also

reduces starting torque.reduces starting torque.

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Electronic soft starters

This relatively recently introduced form of starter This relatively recently introduced form of starter

is rapidly growing in popularity. Soft starters is rapidly growing in popularity. Soft starters

operate by gradually increasing the voltageoperate by gradually increasing the voltage

applied to the motor, so as to produce steady, applied to the motor, so as to produce steady,

smooth acceleration. smooth acceleration.

This technique eliminates sudden changes in This technique eliminates sudden changes in

voltage which could produce peaks in both voltage which could produce peaks in both

starting current and torquestarting current and torque

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Advantages of Electronic soft Advantages of Electronic soft starters :-starters :-control of the acceleration and deceleration control of the acceleration and deceleration ramps of the motor in such a way as to keep ramps of the motor in such a way as to keep within all required limits on current and within all required limits on current and torquetorque• • mechanical protection for the machine mechanical protection for the machine being driven, by eliminating sudden changes being driven, by eliminating sudden changes in current – and, therefore, torque - during in current – and, therefore, torque - during starting and stopping.starting and stopping.

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Drawbacks of Electronic soft startersDrawbacks of Electronic soft starters :-:--Expensive solution Expensive solution

-Complex control Complex control

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CONCLUSIONS

The principle objective of all methods of motor The principle objective of all methods of motor

starting is to match the torque characteristics to starting is to match the torque characteristics to

those of the mechanical load, while ensuring that those of the mechanical load, while ensuring that

the peak current requirements do not exceed thethe peak current requirements do not exceed the

capacity of the supply. Many starting methods are capacity of the supply. Many starting methods are

available, each of which has slightly different available, each of which has slightly different

characteristics. characteristics.

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