How and why current is limited on starting a large industrial induction motor?

To limit the starting current surge, large induction motors are started at reduced voltage and then have full supply voltage reconnected when they run up to near rotated speed. … If the current is above the rated current for the motor, the contactor will be tripped automatically to disconnect the motor from the supply.

Why does induction motor have high starting current?

Why High Starting Currents:

Magnetizing component of current flowing through induction motor is proportional to the applied voltage and is independent of load on the motor similar to transformer . … A load resistance (variable) is connected in series to the fixed rotor and stator impedance.

In which method of starting a motor the starting current is maximum?

The autotransformer is used to limit the starting current of the induction motor. The above three starters are used for the cage rotor induction motor.

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How can we reduce the starting current of an induction motor?

Some of the techniques used to reduce the starting current of a squirrel induction motor:

  1. Soft starting (voltage control)
  2. Variable Frequency Drives (Voltage and frequency control)
  3. Star/Delta Starting.
  4. Stator impedance and/or resistance starting.
  5. Autotransformer Starting.


On what factor does the starting current of the induction motor depend?

It is based upon the consideration that when the motor is fed by a distorted voltage waveform, in no-load steady-state operating conditions, the slips corresponding to harmonics are very near to the unity.

Why is DC motor current so high?

Answer: The starting current is high as there is no back EMF or counter EMF present in the armature circuit because atstarting counter EMF is zero. … It’s armature has very less resistance due to this it need more current at starting time. Hence DC starters are used to limit the starting current ofmotor.

What are the starting methods of induction motor?

Starting methods of Induction motor include:

  • Direct –On– line (DOL) starters for less than 10 Kw motors.
  • Star–Delta starters for large motors. The stator winding is initially connected in a star configuration and later on changed over to a Delta connection, when the motor reaches rated speed.
  • Auto transformer.


Why can’t an induction motor run at synchronous speed?

The Induction Motor cannot run at Synchronous speed because the Slip is never zero in an Induction Motor.

How do you calculate motor starting current?

When the actual load current is equal to the full load current rating of the motor then the starting current can be estimated as FLA x 7.5.

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What are the starting methods of 3 phase induction motor?

The starting methods of three phase induction motor generally are direct-on-line starting, reduced-voltage starting and soft starter.

Reduced-voltage starting

  • Stator circuit series resistance starting. …
  • Star-delta starting. …
  • Autotransformer starting.


Why Motor takes high current?

Electrical overload or over-current is caused by an excessive current flow within the motor windings, exceeding the design current which the motor is able to carry efficiently and safely. This can be caused by a low supply voltage, resulting in the motor drawing in more current in an attempt to maintain its torque.

Why is DOL starter used up 5 hp?

For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points and hence increases the life of starter. DOL starter consists of MCCB, contactor, and overload relay.

What is the difference between inrush current and starting current?

The inrush current is the current transient that flows in the first cycle or so that establishes the magnetic field in the iron. … The inrush current can be significantly higher than the locked rotor current with 2 – 2.5 times the start current being quoted by some.

What are the factors on which torque produced depends on an induction motor?

1. The part of rotating magnetic field which reacts with rotor and is responsible for producing emf. 2. The magnitude of rotor current in running condition.

Why does induction motor lag?

In induction motor we have windings over its stator and windings has its own resistance and inductance. Therefore theoretically induction motor will always run at lagging power factor.

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Why no load current of induction motor is high?

Magnetic circuit of Induction motor has large reluctance than transformer due to air gap between stator and rotor, more mmf(magneto motive force is equal to product of current and number of turns) or more current is required to set up working flux in Induction motor, this is the reason of high no load current in …