- Electrical Machines - Discussion
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- Power Developed by Synchronous Motor
- Equivalent Circuit and Power Factor of Synchronous Motor
- Working of 3-Phase Synchronous Motor
- Losses and Efficiency of 3-Phase Alternator
- Output Power of 3-Phase Alternator
- Armature Reaction in Synchronous Machines
- Working of 3-Phase Alternator
- Construction of Synchronous Machine
- Introduction to 3-Phase Synchronous Machines
- Methods of Starting 3-Phase Induction Motors
- Speed Regulation and Speed Control
- Characteristics of 3-Phase Induction Motor
- Three-Phase Induction Motor on Load
- Construction of Three-Phase Induction Motor
- Three-Phase Induction Motor
- Single-Phase Induction Motor
- Introduction to Induction Motor
- Applications of DC Machines
- Losses in DC Machines
- Types of DC Motors
- Back EMF in DC Motor
- Working Principle of DC Motor
- Types of DC Generators
- EMF Equation of DC Generator
- Working Principle of DC Generator
- Types of DC Machines
- Construction of DC Machines
- Types of Transformers
- Three-Phase Transformer
- Efficiency of Transformer
- Losses in a Transformer
- Transformer on DC
- Ideal and Practical Transformers
- Turns Ratio and Voltage Transformation Ratio
- EMF Equation of Transformer
- Construction of Transformer
- Electrical Transformer
- Fleming’s Left Hand and Right Hand Rules
- Concept of Induced EMF
- Faraday’s Laws of Electromagnetic Induction
- Rotating Electrical Machines
- Singly-Excited and Doubly Excited Systems
- Energy Stored in a Magnetic Field
- Electromechanical Energy Conversion
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Methods of Starting 3-Phase Induction Motors
The following four methods are extensively used for starting the three-phase induction motors −
Direct-On-Line Starter
Auto-Transformer Starter
Star-Delta Starter
Rotor Resistance Starter
In this chapter, let s discuss each of these starting methods in detail.
Direct On Line Starter
As the name imppes, the direct-on pne (D.O.L.) starter is one in which the three-phase induction motor is started by connecting it directly to a 3-phase balanced ac supply as shown in Figure-1.
![Line Starter](/electrical_machines/images/pne_starter.jpg)
In this method, the induction motor draws a very high starting current about 4 to 10 times of the full-load current. It is because, the impedance of the motor at standstill is low. Therefore, the direct-on pne (D.O.L.) method of starting is suitable for the motor of low power rating, usually up to 7.5 kW.
Autotransformer Starter
In this method of induction motor starting, a three-phase autotransformer is used to supply three-phase electricity to the motor. The autotransformer is mainly used to reduce the 35. Methods of Starting 3-Phase Induction Motors supply voltage at starting and then connecting the motor to the full supply voltage as the motor attains a sufficient speed.
The circuit arrangement of autotransformer starter is shown in Figure-2. The tapings on the autotransformer used for starting the induction motors are provided in such a way that when it is connected in the circuit, 60% to 80% of the supply voltage is appped to the motor.
![Autotransformer](/electrical_machines/images/autotransformer.jpg)
At the instant of starting, the autotransformer is put into the circuit and hence reduced voltage is appped to the motor. Consequently, the starting current is pmited to a safe value. When the motor reaches about 80% of the rated speed, the autotransformer is removed from the circuit through a changeover switch, and the motor is then connected to the full supply voltage. The autotransformer starter has several advantages pke low power loss, small starting current, etc. Therefore, this method is suitable for relatively larger induction motors of power ratings over 25 hp.
Star-Delta Starter
In this method, the three-phase induction motor is started as a star-connected motor and run as a delta-connected motor.
The induction motors which are started by star-delta starter has the stator windings which are designed for delta operation and are connected in star during the starting period. When the motor attains a sufficient speed, the winding connections are changed from star to delta.
Figure-3 shows the circuit arrangement of the star-delta starter. Here, the six terminals of the stator windings are connected to a changeover switch. At the instant of starting, the changeover switch connects the stator windings in star-configuration. As a result, each stator phase gets a voltage equal to V/√3, where V is the full pne voltage. In this way, the stator windings get a reduced voltage during starting period.
![Star Delta](/electrical_machines/images/star_delta.jpg)
When the motor attains a specific speed, the changeover switch turns the connection of stator windings to delta. Each phase now gets the full pne voltage V, and the motor runs at the normal speed. However, this method of starting of three-phase induction motor causes a large reduction in the starting torque of the motor. This method is best suited for medium sized induction motors, up to about 25 hp.
Rotor Resistance Starter
This method of starting is apppcable to spp-ring induction motors only. In this method, a variable star-connected rheostat is inserted into the rotor circuit through spp rings, and full supply voltage is appped to the stator winding. The circuit arrangement of rotor resistance starter is shown in Figure-4.
![Rotor Resistance](/electrical_machines/images/rotor_resistance.jpg)
At the instant of starting, the handle of the star-connected rheostat is set in the ‘off’ position. Consequently, a maximum resistance is inserted in each phase of the rotor circuit, and reduces the starting current. At the same time, this resistance increases the starting torque.
When the motor picks up speed, the external resistance is gradually removed from the rotor circuit by moving the rheostat handle. Once the motor attained about 80% of the normal speed, the handle is switched to the ‘on’ position, and thus the whole external resistance is removed from the rotor circuit.
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