Motor Starter, Applications and Advantages

If you an electrical engineering student then you must hear about the motor starter. As an electrical engineer, you should know all types of starter and their applications, advantages.

What is Motor Starter

An electrical motor starter is a device or can be called a circuit which is to be connected in series with the motor to reduce the starting current and maintain the starting speed of the motor and also to protect the motor from overload, short circuit faults.

An electrical motor starter can be used alone or with the Contactor, circuit breaker according to requirement.

We also can say a motor starter is a device which is used to start and stop a motor safely.

The main principle of a motor starter is that it reduces the terminal voltage and current of the motor during starting time.

DC Motor Starter Types, Applications and Advantages

The starter is used for DC motors is called DC Motor Starter. To safely start and stop the DC Motors DC Motor starters are used. DC Motor starter does not only start and stop the motor safely they also protect the motor from over voltage and short circuit faults. DC motor starters very simple in construction and they are cheaper than AC Motor Starter.

Why we need a Starter for starting of DC Motor

In DC motor there is no principle of induction exists. Back EMF is very important for running of a DC Motor. As the polarity of back emf is opposite to the supply voltage, therefore, the back emf can control the armature current.

When the motor is in off condition there is no back emf in the armature winding. And we know that the resistance of the armature winding also very low. Now if we connect the motor to the full supply voltage a huge amount of current will flow through the armature winding. It can be shown by the equation

Ia=(V-Eb)/Ra

You can see the back emf (Eb) is Zero and the armature resistance (Ra) is also very low so the value of armature current (Ia) will be very high.

This huge amount of armature current may causes,

1. Burning of the armature winding
2. Damage the commutator and brushes
3. A large voltage fluctuation in supply voltage which can damage other equipment.

So for these reasons to reduce the starting current Starter is used with DC Motors.

Types DC Motor Starter

There are three types of DC Motor Starter,

1. Two-point Starter
2. Three-point Starter
3. Four-point Starter

Applications of Two-point Starter

1. Two-point starters are used with DC series motors.
2. Two-point starters are used in railway to start or stop the rail.
3. Two-point starters are used in cranes.

Advantages of Two-point Starter

1. It protects the motor from drawing high starting current.
2. It can give protection against overload and short circuit faults.
3. It automatically gets off when the power supply is off.

Applications of Three-point Starter

1. The three-point starter is used with DC Shunt motors
2. Three-point starters are used in lathe factories
3. Three-point starters are used in spinning mills

Advantages of Three-point Starter

1. Three-point starter gives the protection to the motor from overload and short circuit faults.
2. The three-point starter helps to speed control of the motor by adding some extra equipment.

Disadvantages of Three-point starter

The main disadvantage of the three-point starter is that it is not suitable for variable speed motors.

Application of Four-point Starter

1. The four-point starter is used with mainly DC compound motors but it can be used with DC shunt motors.
2. Four-point starters are used in press industries.
3. Four-point starters are used in rolling mills

Advantages of Four-point Starter

1. The four-point starter is suitable for variable speed motors.

AC Motor Starter Types, Applications and Advantages

The starter used for AC motors is called AC Motor Starter. Like DC motor AC motor also need the starter for safely starting. Induction motors are widely used in any industry. Induction motor need starter for safely staring. So we will discuss the starter of induction motors.

Why we need Starter for starting of Induction Motor

We know that the Induction Motor is acting as a transformer with the secondary winding short-circuited during off condition.

We know that, I₁N₁=I₂N₂

This equation is the same for the transformer and the Induction motor at standstill condition

Where I₁= primary winding current or stator winding current

           N₁ = no. of turns in primary winding or stator winding

           I₂ = Secondary winding current or rotor winding current

           N₂= no. of turns in secondary winding or rotor winding

So at standstill condition of the motor if we connect the motor directly to the full supply voltage a high-value emf will be induced in the rotor of the induction motor for the transformer action. This high-value emf will cause to flow a high current(I₂) in the rotor winding. A huge amount of current also will be flow in the stator winding to balance the equation, I₁N₁=I₂N₂

This high current in stator winding may,

1. Damage the stator winding.
2. Create a large fluctuation in supply voltage which may damage others equipment connected in the same line.

For these reasons, to reduce the magnitude of supply voltage and current at starting period Starter is used with the Induction motor.

Types of AC Motor Starter

1. Direct-on-line Starter or DOL Starter
2. Stator resistance starter
3. Autotransformer Starter
4. Star-Delta Starter
5. Rotor Resistance Starter

Applications of Direct-on-line Starter or DOL Starter

1. DOL starters are used with up to 7.5 KW Induction Motors.
2. DOL starter acts as normally ON/OFF switch so it is used where frequently start and stop of the motor is required.
3. DOL starters are used in conveyor belt and compressor.

Advantages of Direct-on-line Starter or DOL Starter

1. It is very simple in construction.
2. It is cheaper than other starters
3. It can give overload and short-circuit protection.

Disadvantages of Direct-on-line Starter or DOL Starter

1. It can not be used with more than 7.5 kW motors.
2. It can not reduce the magnitude of starting current of the motor acts only normal ON/OFF switch.

Applications of Stator Resistance Starter

1. This types of starter are suitable for more than 7.5 KW rating motors.

Advantages of Stator Resistance Starter

1. It is very simple in construction.
2. It reduces the terminal voltage of the motor during the starting time hence the starting current also decreases.
3. It is also a low-cost starter.

Disadvantages of Stator Resistance Starter

1. It lowers the starting torque of the motor.
2. It increases the accelerating time of the motor
3. A huge amount of power loss in by the starting resistance.

Advantages of Auto-Transformer Starter

1. Simple operation
2. Very low power loss as it has no resistance
3. Easy to control

Disadvantages of Auto-Transformer Starter

1.  It lowers the starting torque of the motor
2. There is a chance of sparking.

Applications of Star-Delta Starter

1. It used in heavy industries for starting high KW motors.

Advantages of Star-Delta Starter

1. Star-delta starter can be controlled automatically by using electronic circuits
2. Low power loss
3. Smooth operation
4. Starting current is reduced to one-third of full load current

Disadvantages of Star-Delta Starter

1. Low starting torque
2. Six terminals motor required.

Application of Rotor Resistance starter

1. These starters are used with slip-ring induction motors.

Advantages of Rotor Resistance starter

1. It can not decrease starting torque.
2. Smooth speed control is possible.

Solid State Starter or Soft Starter

Nowadays Solid State Starters are widely used instead of the magnetic starter. in industries. Solid state starter has no moving elements. These Starters uses the Silicon Controlled Rectifier for their operation.

Advantages of Solid State Starter or Soft Starter

1. These starters eliminate the power distribution issues in industries.
2. These starters can handle multiple motors at a time.
3. These starters decrease the acceleration time of the motor
4. Very low power loss.

Applications of Solid State Starter or Soft Starter

1. These starters are used in industry for the operation of Conveyor belt motors.
2. It used with blowers to increase the accel time.
3. These Starters are used with pumps also.