Different Types of Motor Starters Explained

Confusion regarding the varying types of motor starters is common in the electrical industry. We are here to help you get rid of this and keep yourself informed about all the types of motor starters. Start reading below to learn about them in detail. 

What is Motor Starter?

A motor starter is a basic electrical appliance machinery used for the controlling, starting, and stopping of a motor. While a relay just switches the power ON or OFF, a motor starter also safeguards the gadget from low voltage and overcurrent. 

Key Functions of a Motor Starter

• Safe Motor Start: Makes sure that the motor starts safely to reduce instances of damage. 
• Safe Motor Stop: Enables safe shutting down or stopping off the motor. 
• Direction Control: Allows the motor to function forward and reversibly in case it is needed.
• Protection: Protects the motor from low voltage and over-current situations. 

Induction Motor Starter Types Based on Starting Mechanism

Several methods are applied in industries to ensure that AC induction motors start properly. Based on these methods, there are the following types of motor starter. 

Full Voltage or Across-the-Line Starter

• Closely connects the motor to the actual full power line voltage.
• Recommended for applications where the motor power is low since high voltage drops would be observed. 
• Suitable when motors revolve in a singular direction, that is, forwards. 

Full Voltage Reversing Starter

• This permits the reversing of the direction of a three-phase motor by the interchanging of two out of the three phases. 
• The motor switches between the forward and reverse directions using two mechanically operated magnetic contacts. 
• It is suitable for use in applications where rotation in both directions is possible. 

Multi-Speed Starter

• It works by either varying the supply frequency of AC or the pole number of the windings involved in the motor. 
• It offers multiple pre-selected speeds for different operations and comes with a numeric keypad for the selection of the preferred speed. 

Reduced Voltage Starter 

• For a given voltage supply, it adjusts the voltage at the beginning to lower the inrush current, which is destructive to the motor windings. 
• Frequent applications are found in high-power motors, which help reduce voltage fluctuations and safeguard the motor. 

AC Motor Starter Types

Motor starters can be classified into various types based on their starting methods and features: 

Direct Online (DOL) Starter 

• Translates the motor directly with the supply without the reduction of the voltage. 
• It has pull-in and holding characteristics for the protection purpose. 
• The ordinary type of water pump is good for use only with motors of not more than five horsepower (HP). 
• Simple operation with two push buttons. As a result, they usually use the keys, one for starting and one for stopping. 

Stator Resistance Starter

• Brings some resistance in order to minimise the start current. 
• Delivers a progressive torque to the motor to prevent exploitation of a motor with increasing and steady acceleration measures.

Rotor Resistance or Slip Ring Motor Starter

• It uses external impedance in the rotor circuit to start current regulation and speed. 
• Enables easier starting of the accelerator pedal and less mechanical strain. 

Autotransformer Starter

• Utilises an autotransformer to bring down the amount of voltage that is utilised in the motivation of the motor during its starting phase. 
• Play a role in inrush current control and mitigation of voltage sags. 

Star-Delta Starter

• Connect it to the star for starting, thereby decreasing the starting current and switching to the delta connection for normal operation. 
• Cuts down the starting torque and current for a large motor. 

Soft Starter

• Achieves smooth acceleration since the voltage that is supplied to the motor is gradually raised. 
• The mechanical stress of the motor is decreased, and its life is extended. 

Variable Frequency Drive (VFD)

• The speed and torque of the motor are regulated by the change in frequency of the power supply voltage. 
• It provides accurate speed control and saves energy. 

Read More -  Variable Frequency Drives - Everything You Need to Know About VFDs

Why is a Motor Starter Essential for Induction Motors?

A motor starter is essential for the proper and safe functioning of an induction motor, particularly because of the motor’s low rotor impedance. This impedance is in proportion to the reciprocal of the motor’s slip, which is a measure of the difference between the speed of the rotating magnetic field created by the stator and the actual speed of the rotor. 

The slip is maximum at a standstill when the motor is not running or when the motor runs at a very slow speed. This leads to the minimum impedance of the rotor. This causes the motor to draw a large current, which is referred to as the inrush current at the beginning. This high inrush current is needed in the first moments when the motor is switched on to overcome the inductance of the motor and magnetise the air gap between the rotor and stator. 

Magnetization causes an electromotive force to be induced in the rotor windings, which then produces an electrical current and, hence, a magnetic field in the rotor. This magnetic field can generate the required torque to commence the rotation of the rotor. By the time the rotor accelerates to near synchronous speed, its slip becomes small, and consequently, the current the motor takes from the supply system is reduced. 

A motor starter assists in reducing this high inrush current by applying a stated and slow current to the motor. This provides additional protection to the motor as well as the electrical systems against any damages brought by the starting current and allows for efficient and smooth running of the motor.

Summing Up

Understanding the different types of motor starters is crucial for optimising motor performance and ensuring safety. Each starter type, from Direct On-Line to Variable Frequency Drives, offers distinct advantages tailored to specific applications. Whether it’s managing inrush current, providing protection against faults, or enabling precise speed control, selecting the appropriate motor starter can enhance efficiency, prolong motor life, and safeguard your electrical system. By carefully evaluating the needs of your motor and application, you can choose the right starter to meet performance requirements and ensure reliable operation.

Read More - What's the difference between a Soft Starter and a VFD?

Frequently Asked Questions

Q1. What are the types of DC motor starters?

Ans: Types of DC motor starter include:

• Direct-On-Line (DOL) Starter: Connects the motor directly to the power supply for simple applications.
• Series Field Starter: Uses a series winding to limit the starting current.
• Resistance Starter: Incorporates external resistors to control the starting current.
• Autotransformer Starter: Reduces voltage to limit starting current.

Q2. What are the types of 3 phase motor starters?

Ans: Types of 3 phase motor starter are as follows.

• Direct On-Line (DOL) Starter: Connects the motor directly to the power supply.
• Star-Delta Starter: Reduces starting current by initially connecting in a star configuration and then switching to delta.
• Autotransformer Starter: Uses an autotransformer to reduce voltage during startup.
• Soft Starter: Gradually increase the voltage to the motor for a smooth start.

Q3. What are the types of starter in AC motors?

Ans: DOL, soft, autotransformer and star starters are the types of starter in AC motors.