Table of Contents
- Introduction
- Construction of 3 Point Starter
- Working Principle of 3 Point Starter
- Advantages of 3 Point Starter
- Disadvantages of 3 Point Starter
- Applications of 3 Point Starter
- SSC JE Previous Year MCQs on 3 Point Starter
- Conclusion
- FAQs
1. Introduction
DC motors require a starter to limit the high starting current and to protect the motor from damage during starting and running conditions. The 3 Point Starter is commonly used for starting DC shunt motors and some series motors. It provides overload and no-voltage protection.
2. Construction of 3 Point Starter
3 Point Starter consists of the following main components:
- Main Terminal (T): Connected to the supply.
- Armature Terminal (A): Connected to the motor armature.
- Field Terminal (F): Connected to the shunt field winding.
- Starting Resistance (R): A set of resistors connected in series with the armature to limit starting current.
- Starting Handle (S): A movable contact or lever that moves over a set of studs (notches) controlling the resistance.
- No-Volt Release Coil (NVR coil): Electromagnetic coil connected in series with the shunt field winding. It holds the handle in the 'run' position when supply voltage is present.
- Overload Release Coil (OLR coil): Connected in series with the armature to protect the motor from overload.
3. Working Principle of 3 Point Starter
When the motor is off, the starting handle rests at the 'off' position with maximum starting resistance in series with the armature.
To start the motor, the handle is moved gradually from 'off' towards 'run' position. As it moves, the starting resistance is gradually cut out, allowing more current to flow, and the motor speeds up.
The NVR coil is energized from the shunt field circuit. When supply voltage is available, it creates magnetic flux holding the starting handle in the 'run' position.
If the supply voltage fails, the NVR coil loses magnetism, and the handle returns to the 'off' position, stopping the motor and preventing automatic restart.
The OLR coil is connected in series with the armature. If the armature current exceeds the safe limit (overload condition), this coil activates the release mechanism, moving the handle to 'off' position and stopping the motor to protect it.
Thus, the 3 Point Starter provides both no-voltage protection and overload protection.
4. Advantages of 3 Point Starter
- Provides overload protection to the motor.
- Provides no-voltage protection (motor stops automatically when supply fails).
- Protects the motor from damage due to sudden starting currents.
- Simple and economical for small to medium size DC motors.
5. Disadvantages of 3 Point Starter
- Not suitable for series motors as the field current passes through the starter, which can cause problems.
- Limited to shunt motors or separately excited motors.
- Requires manual operation to start the motor.
- It cannot be used for large motors because of limitations in the overload protection mechanism.
6. Applications of 3 Point Starter
- Used mainly for starting DC shunt motors.
- Suitable for small and medium power motors.
- Commonly used in applications like lathes, machine tools, fans, and pumps where DC shunt motors are employed.
7. SSC JE Previous Year MCQs on 3 Point Starter
Q1. Which protection is NOT provided by a 3 Point Starter?
a) Overload protection
b) No voltage protection
c) Short circuit protection
d) None of the above
Answer: c) Short circuit protection
Q2. The No Volt Release coil in a 3 Point Starter is connected in series with:
a) Armature
b) Field winding
c) Supply line
d) Starting resistance
Answer: b) Field winding
Q3. The main purpose of the starting resistance in a 3 Point Starter is to:
a) Increase starting torque
b) Limit starting current
c) Increase speed
d) Decrease motor voltage
Answer: b) Limit starting current
Q4. For which type of DC motor is the 3 Point Starter mainly used?
a) Series motor
b) Shunt motor
c) Compound motor
d) Universal motor
Answer: b) Shunt motor
Q5. What happens if the supply voltage fails while the motor is running with a 3 Point Starter?
a) Motor continues to run
b) Handle returns to OFF position and motor stops
c) Motor runs at reduced speed
d) Overload coil activates
Answer: b) Handle returns to OFF position and motor stops
8. Conclusion
3 Point Starter is a simple and efficient method to start and protect DC shunt motors from high starting current, supply failure, and overload conditions. It is widely used in small and medium DC motor applications and remains a crucial topic in SSC JE Electrical exams.
9. FAQs
Q1. Can a 3 Point Starter be used for DC series motors?
No, because it provides no-voltage protection through the field winding, which is connected in series with the armature in series motors, causing operational issues.
Q2. Why is the No-Volt Release coil important?
It ensures the motor stops when supply voltage is lost, preventing unsafe motor operation or damage.
Q3. How is the overload protection achieved?
Through the overload release coil connected in series with the armature, which trips the starter if current exceeds a safe limit.