starters for induction motors

7/30/2019 Starters for Induction Motors

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Induction motor is similar in action to a Poly-phase

Transformer with a Short Circuited rotating Secondary.

When normal Voltage is applied to the stationary motor,

it takes larger current like transformer for short time.

At the time of starting there is no back emf to block the

Initial Inrush Current.

Starters are used to:

Reduce heavy starting currents and prevent motor

from overheating &

Provide overload and no-voltage protection.


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Stator Resistance Starter

Direct On-Line (DOL) Starter

Star-Delta Starter

Auto Transformer Starter

Rotor Resistance Starter

Variable Frequency Drive (VFD)

Soft Starter


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Resistance was added in series with each phase ofthe stator winding.

Initially the resistances are kept maximum in the

stator circuit. Hence reduced voltage is applied to the stator, which

reduces the high starting current.

Resistances are gradually cut-off from the statorcircuit, when motor starts running.

Simple in construction and cheap.

More Power loss and Less Starting Torque.


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Simplest and most in-expensive to all startingmethods.

Directly connects the contacts of the motor to the

full supply voltage. The starting current is very large, normally 6 to 8

times the rated current & the starting torque is likelyto be 0.75 to 2 times the full load torque.

To avoid excessive voltage drops in the supply linedue to high starting currents, the DOL starter is usedonly for motors with a rating of less than 5KW.


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The Star-Delta starting is very common and extensively

used, compared to the other types of the starters.

Stator windings connected first in Star and then in Delta,

Since it is connected in star, at starting each phase of thestator winding gets 1/3 times of line voltage.

Line current drawn by the motor at starting is reduced to

one-third as compared to starting current with the

windings connected in delta. Star- Delta starting reduce the starting torque to one

third that obtainable by direct delta starting.


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The principal of operation is same like Star-Deltastarter , but more expensive, Complicated andBulkier.

The starting current is limited by applying thereduced stator voltage using three phase AutoTransformer.

Suitable for both Star and Delta connected motors

Starting current and torque can be adjusted to adesirable value by taking the correct tapping fromAuto Transformer. Whereas in Star-Delta, we canadjust the voltage only to a factor of 1/3 .


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Applicable for Slip ring Induction Motor and it allowsexternal resistance to be connected to the rotorthrough slip rings and brushes.

Initially, the rotor resistance is set to maximum and

is then gradually decreased as the motor speedincreases, until it becomes zero.

The rotor impedance method allows the motor to bestarted while on load.

Very bulky, expensive and limited starting frequency

Considerable amount of heat is generated throughthe resistors when current runs through them.


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The drive consists primarily of two parts, one whichconverts AC (50 or 60 Hz) to DC and the second partwhich converts the DC back to AC, but now with avariable frequency of 0-250 Hz.

It controls the speed of the motor by changing theoutput frequency and this is a big advantage if there is aneed for speed regulation during a continuous run.

The rated motor torque is available at a low speed andthe starting current is low, between 0.5 and 1.0 timesthe rated motor current, maximum 1.5 x In.

Another feature of VFD is the soft stop function, whichis very useful when stopping pumps (to avoid waterhammering) and Conveyor belts (to avoid materialdamage)


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It has thyristors in the main circuit, and the motorvoltage is regulated with a printed circuit board.

Principle - When the motor voltage is low during start,the starting current and starting torque is also low.

During the first part of the start , the voltage to themotor is so low, it eliminates the unnecessary jerksduring the start. Gradually, the voltage and the torqueincrease so that the machinery starts to accelerate.

We can adjust the torque to the exact need, whether theapplication is loaded or not.

Another feature of the soft starter is the soft stopfunction, which is very useful when stopping pumps (toavoid water hammering) and Conveyor belts (to avoidmaterial damage)


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starters for induction motors

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