Starting of Three-phase induction motors/ study of star-delta starter and slip ring motor starter.

Experiment No.: 01

Aim of the Experiment: –

Starting of Three-phase induction motors/ study of star-delta starter and slip ring motor starter.

Apparatus Required: –

Sl. No.NameSpecificationQuantity
1Star-Delta Starter1 nos.
2DOL Starter1 nos.
3Auto-Transformer Starter1 nos.
4Rotor Resistance Starter1 nos.
Table: 12.1

Circuit Diagram: –

Fig. 12.1
Fig. 12.2
Fig. 12.3
Fig. 12.4

Theory:

The induction motor draws a huge amount of current at the start-up. This starting current can damage the motor windings. In order to avoid any damage, we use different techniques to reduce the starting current using Motor Starter. These techniques depend on the motor ratings and the load connected to the motor. Apart from this, the motor starter also protects the motor from overloading and over current.

Types of Starter: –     

  1. Direct-On-Line(DOL) Starter
  2. Auto Transformer Starter
  3. Star Delta Starter
  4. Rotor Resistance Starter  (for Slip-ring Induction Motor)
  1. Direct-On-Line (DOL) Starter:

DOL starter is a device consisting of the main contactor, protective devices, and overload relay, which is used for motor starting operations. It is used for low ratings, usually below 5HP motors. In the direct online starter method of motor starting, the motor stator windings are directly connected to the main supply where the DOL protects the motor circuit from high inrush current which may damage the overall circuit as the initial current is much higher than the full rated current. The circuit diagram of the DOL method is shown in Fig. 12.1

Advantages: –

  1. Simple and most economical starter.
  2. More comfortable to design, operate and control.
  3. Provides nearly full starting torque at starting.
  4. Easy to understand and troubleshoot.
  5. DOL starter connects the supply to the delta winding of the motor.

Disadvantages: –

  1. High starting current (5-8 times of full load current).
  2. DOL Starter causes a significant dip in voltage, hence suitable only for small motors.
  3. DOL Starter reduces the lifespan of the machine.
  4. Mechanically tough.
  5. Unnecessary high starting torque.

2. Auto Transformer Starter:

In this method, we connect the auto-transformer in between the three-phase power supply and the induction motor as shown in Fig. 12.2. The auto-transformer is a step-down transformer hence it reduces the per phase supply voltage and hence starting current reduces. In the starting, we start the supply from 0 volt to gradually increase it to full rated voltage.  This method is used for the high rating of squirrel cage induction motors. So, this method is most suitable for motors having ratings of more than 10 HP.

3. Star-Delta Starter:

A star-delta starter is the most commonly used method for the starting of a 3-phase induction motor. In star-delta starting an induction motor is connected through a star connection throughout the starting period. Then once the motor reaches the required speed, the motor is connected through a delta connection as shown in Fig. 12.3.

A star-delta starter will start a motor with a star-connected stator winding. When the motor reaches about 80% of its full load speed, it will begin to run in a delta-connected stator winding.

Advantages:-

  1. Inexpensive
  2. No heat is produced, or tap changing device needs to be used, hence efficiency increases.
  3. Starting current is reduced to 1/3 of direct online starting current.
  4. Produce high torque per ampere of line current.

Disadvantages:-

  1. Starting torque is reduced to 1/3 of full load torque.
  2. A particular set of motors is required.

4. Rotor Resistance Starter (for Slip-ring Induction Motor):

In a rotor resistance starter, a star-connected variable resistance is connected in the rotor circuit through slip-rings as shown in Fig. 12.4. The full voltage is applied to the stator windings.

At the instant of starting, the handle of variable resistance (rheostat) is set to the ‘OFF’ position. This inserts maximum resistance in series with each phase of the rotor circuit. This reduces the starting current and at the same time starting torque is increased due to external rotor resistance.

As the motor accelerates, the external resistance is gradually removed from the rotor circuit. When the motor attains rated speed, the handle is switched in the ‘ON’ position, this removes the whole external resistance from the rotor circuit.

Slip Ring induction motors are mainly used for driving high inertia loads or, the loads which require a starting torque across a full speed range.

Advantages:-

  1. High starting torque may be obtained with the rotor resistance starter.
  2. It improves the power factor of the motor.
  3. Absence of line current harmonics.
  4. Reduced starting current.
  5. Smooth and wide range of speed control is possible.

Disadvantages:-

  1. Lower efficiency due to the I2 R losses in the external resistance connected to the rotor circuit.
  2. This starter can only be used for starting a slip ring induction motor.
  3. It needs frequent maintenance due to presence of slip rings and carbon brushes.

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