Starting Methods of Induction Motor

By Aina Parasher|Updated : August 26th, 2022

A spinning magnetic field is created when an induction motor is powered by a three-phase AC source. This field forms a connection with the conductors of the rotor, giving it torque. An induction motor starts in this manner. A starter is used in each of the starting methods of induction motor starting to keep the starting current within acceptable limits. Here, we will read the appropriate starting methods of induction motor in the upcoming sections.

Further, we will discuss various starting methods of induction motor, along with this, we will discuss how the starting torque of the motor varies in these methods. Candidates can go through these study notes on different starting methods of induction motor to prepare for GATE and other related examinations.

Table of Content

What are the Starting Methods of Induction Motor?

There is a necessity to control the high currents during the starting of an induction motor as they will harm the nearby loads, hence we have to adopt suitable starting methods of induction motor to reduce the high starting currents. But while the motor runs at the rated speed, there is no issue with these inrush currents, as the motor attains the rated speed, the slip will come into play and the rotor current will be reduced. The most commonly used starting methods of induction motor are given below.

  • Direct Online Starting
  • Stator resistance/reactance starting
  • Auto-Transformer starter
  • Star-Delta starter
  • Rotor resistance control

Along with this, we will discuss how the starting torque of the motor varies in these methods, to understand these we must have basic knowledge regarding the torque and the current relation of induction motors.

Starting Methods of Induction Motor - Direct Online Starting

In this method of starting the induction motor, we would not bother about the voltage dip as we employ this method of starting the low power rating motors (up to 5 HP). But it has some basic features like no-volt protection, over-current protection, and an isolator to attend to the basic maintenance of the induction motor. We can use this method for fairly rating machinery by using recently developed devices like quick-acting A.V.R (Automated voltage regulator).

During starting, the torque is

torque

Where

Pcu is the rotor copper loss, at starting slip is equal to 1 and during starting rotor copper losses are

rotor copper

Where

I2.St is the per phase current through the rotor, but while the rotor is at standstill condition it is equal to the per phase short circuit current ISC.

⇒I2.St = ISC

⇒Tst ∝ I2SC

During the induction motor running at full load, the torque is

T= (3×60/2πNs)×(I2fR2/sf)

here

  • sf is the slip at full load,
  • Iis full load rotor current.

⇒T∝ I2f/sf

⇒Tst/T= (ISC/If)2sf

Starting Methods of Induction Motor - Stator Resistance/ Reactance Starting

In this method of starting of induction motor, we connect an external rheostat in series with each phase of the stator to reduce the voltage across the stator by a factor x(x<1). Consequently, the stator current is also reduced by the same factor. Using reactors may affect the operating power factor, which may also affect the starting torque of the motor. Hence the reactors are less preferred. This starting method of induction motor is not suitable for large rating machines.

stator

By using the stator reactance or resistance starting the current ISCcan be reduced by a factor of x to xISC.

⇒Tst ∝ (xISC)2

⇒Tst ∝ x2(ISC)2

⇒Tst/T= x2(ISC/If)2sf

Starting Methods of Induction Motor - Auto Transformer Starter

This method of starting of induction motor will be preferred for large-rating induction motors. By adjusting the tapings, the autotransformer can reduce the voltage in the range of 50 to 70 percent. This method of starting is efficient but expensive.

auto transformer

During starting the current in the rotor winding is xISC, So, it is reduced by a factor x.

⇒Tst/T= x2(ISC/If)2sf

If the no-load current of the autotransformer is neglected, then per phase input and output power of the autotransformer are equal.

⇒V1I= xISCxV1

I= x2ISC (x<1)

Here

Iis the current from the supply mains to the autotransformer.

Starting Methods of Induction Motor - Star Delta Starting

This starting method of induction motor is suitable for the induction motor stator with its stator winding designed for delta connection under running conditions. This is the simple starter that requires a triple-pole double-throw (T.P.D.T) switch. Initially, the stator winding is converted into a star connection by using the TPDT switch, hence the voltage across each phase of the stator is reduced by a factor 1/√3, Hence the rotor short circuit current will also be reduced by the same factor.

star delta

⇒Tst/T= 1/3(ISC/If)2sf

This method of starting can only offer single-step control. Generally, induction motors with ratings greater than 3.3 kV are designed with a star-connected stator to reduce the insulation requirements. Hence this method will not be applicable for such designs.

Starting Methods of Induction Motor - Rotor Resistance Control

Among the starting methods of induction motor, this is the best method of starting but is very rarely used in regular applications as it involves an expansive design that is prone to regular repairs and maintenance. In this method, the rated value of voltage can be applied across the stator but still, we can control the ISCby using the rheostats connected to the rotor winding of the slip ring induction motor. The added advantage is we can improve the starting torque of the motor.

rotor resistance

Important Topics for Gate Exam
Types Of SlabsGear Terminologies
Thermal StressRolling Contact Bearing
Shear ForceSliding Contact Bearing
Mohr's CircleSurface Tension
Resonance In RLC CircuitTurning Moment Diagram

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FAQs on Starting Methods of Induction Motor

  • A starter is a tool that lowers the motor's initial high current by lowering the supply voltage that is applied to the motor. To prevent harm to the engine and any nearby equipment, a starter is required to restrict the starting current.

  • This starting method of induction motor comes with some basic protecting features like no-volt protection and over current protection. As we employ this method of starting to the low power polyphase induction motors, in the low power segment we may not care about the high rotor currents during starting. 

  • The rotor resistance control method is the best way to start an induction motor as it offers high starting torque along with the rotor current control, but we rarely prefer this method in practice as it involves a complex design of slip rings and fragile rotor construction that is very expensive.  

  • Due to their unique design, slip-ring induction motors can be employed for loads that demand high starting torque like traction, cranes, elevators, hoists, crushers, plunger pumps, compressors, and conveyors. 

  • The rotor current during starting will be reduced by a factor of 1/√3 or 0.577. This method of starting can be applied to induction motors with delta-connected stators. This method of starting can only offer single-step control. 

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