What is the need of a starter in 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.

What is the importance of direct online starting an induction motor?

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.

What is the best starting method of induction motor?

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.

What are the applications of slip-ring induction motors?

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.

What is the factor of reduction in rotor current, in the star-delta method of starting an induction motor?

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.

Starting Methods of Induction Motor

By BYJU'S Exam Prep

Updated on: September 25th, 2023

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.

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Table of content

1. What are the Starting Methods of Induction Motor?
2. Starting Methods of Induction Motor – Direct Online Starting
3. Starting Methods of Induction Motor – Stator Resistance/ Reactance Starting
4. Starting Methods of Induction Motor – Auto Transformer Starter
5. Starting Methods of Induction Motor – Star Delta Starting
6. Starting Methods of Induction Motor – Rotor Resistance Control

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

orque

Where

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

otor

Where

I_2.Stis 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 I_SC.

⇒I_2.St= I_SC

⇒T_st∝ I²_SC

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

T_f= (3×60/2πN_s)×(I²_fR₂/s_f)

here

s_f is the slip at full load,
I_fis full load rotor current.

⇒T_f∝ I²_f/s_f

⇒T_st/T_f= (I_SC/I_f)²s_f

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 I_SCcan be reduced by a factor of x to xI_SC.

⇒T_st∝ (xI_SC)²

⇒T_st∝ x²(I_SC)²

⇒T_st/T_f= x²(I_SC/I_f)²s_f

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

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

⇒T_st/T_f= x²(ISC/If)²s_f

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

⇒V₁I₁= xI_SCxV₁

I₁= x²I_SC (x<1)

Here

I₁is 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.

⇒T_st/T_f= 1/3(I_SC/I_f)²s_f

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 I_SCby 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.