Induction motors draw a very high starting current—up to 6 to 8 times their rated current. To avoid damage to the motor and associated equipment, various types of starters are used. In this blog post, we will explore the different types of starters used for induction motors, their working principles, advantages, and applications.
📚 Table of Contents
- Why Starter is Needed for Induction Motor?
- Types of Starters for Induction Motors
- Comparison Table
- Frequently Asked Questions (FAQs)
⚠️ Why is a Starter Required for an Induction Motor?
When an induction motor starts, it behaves like a short-circuited transformer. This leads to a large inrush current which can:
- Damage the winding insulation
- Cause voltage dip in the supply system
- Trigger circuit breakers or fuses
- Induce mechanical stress on motor shaft
To address these issues, a suitable starter is used.
⚙️ Types of Starters for Induction Motors
1. Direct-On-Line (DOL) Starter
This is the simplest type of starter. The motor is connected directly to the supply. It includes an overload relay and contactor.
- Used for: Motors up to 5 HP
- Advantage: Simple, low cost
- Disadvantage: High starting current
2. Star-Delta Starter
Initially connects the motor winding in star, then switches to delta once the motor gains speed. Starting current is reduced to about one-third.
- Used for: Motors above 5 HP
- Advantage: Lower starting current
- Disadvantage: Sudden torque change during switching
3. Autotransformer Starter
Uses an autotransformer to reduce the initial voltage. Offers multiple taps (50%, 65%, 80%) for different starting torque requirements.
- Used for: Large motors (above 15 HP)
- Advantage: Smooth start
- Disadvantage: Higher cost and size
4. Rotor Resistance Starter
Used for slip ring induction motors. External resistors are added to the rotor circuit to control torque and current.
- Used for: Slip ring motors
- Advantage: High starting torque
- Disadvantage: Not suitable for squirrel cage motors
5. Soft Starter
A solid-state device that gradually increases voltage using thyristors, offering smooth acceleration and protection.
- Used for: Industrial automation
- Advantage: Less mechanical stress
- Disadvantage: Expensive
6. VFD (Variable Frequency Drive)
Controls both voltage and frequency for smooth speed and torque control. Also acts as a starter with ramp-up functionality.
- Used for: Pumps, fans, conveyors
- Advantage: Full motor control
- Disadvantage: High cost, complex setup
📊 Comparison Table of Starters
| Starter Type | Starting Current | Torque Control | Application |
|---|---|---|---|
| DOL Starter | High | Low | Up to 5 HP motors |
| Star-Delta | Medium | Medium | 5 HP to 20 HP |
| Autotransformer | Low | High | Large motors |
| Rotor Resistance | Controlled | High | Slip ring motors |
| Soft Starter | Low | Smooth | Automation, Industries |
| VFD | Variable | Excellent | Process control systems |
❓ Frequently Asked Questions (FAQs)
Q1. Why is a starter required for an induction motor?
Ans: A starter is required to limit the high starting current of an induction motor and to provide protection during start-up.
Q2. What is the most commonly used starter for a 3-phase induction motor?
Ans: The Direct-On-Line (DOL) starter is the most commonly used for motors up to 5 HP.
Q3. What is the advantage of a star-delta starter?
Ans: A star-delta starter reduces the starting current to about one-third of the DOL starter, making it suitable for larger motors.
Q4. Which starter is used for slip ring induction motors?
Ans: Rotor Resistance Starter is used for slip ring induction motors to insert external resistance in the rotor circuit.
Q5. What is a soft starter?
Ans: A soft starter is a solid-state device that gradually increases the voltage to the motor, reducing mechanical stress and current spikes.