Electric motors are the backbone of industrial and domestic electrical applications. Two major types of AC motors—Induction Motor and Synchronous Motor—are commonly used for different purposes. Understanding their key differences is crucial for electrical engineering students and competitive exam aspirants (SSC JE, RRB JE, UPPCL JE, etc.).
⚙️ Basic Definition
- Induction Motor: A motor in which the rotor receives power by electromagnetic induction from the stator's magnetic field. It always runs at a speed less than synchronous speed.
- Synchronous Motor: A motor that runs at a constant speed equal to the synchronous speed, regardless of load. Rotor is excited with a DC supply and rotates in synchronism with the stator field.
🛠️ Construction
➤ Stator
- Common to Both: Three-phase laminated core with distributed windings connected to an AC supply.
- Function: Produces a rotating magnetic field (RMF).
➤ Rotor
Induction Motor:
- Squirrel Cage Rotor: Consists of aluminum/copper bars shorted by end rings.
- Wound Rotor: Has windings connected to external resistances via slip rings.
Synchronous Motor:
- Salient Pole Rotor: Poles project out from the rotor surface.
- Non-salient (Cylindrical) Rotor: Smooth cylindrical shape used in turbo alternators.
➤ Slip Rings (if any)
- Induction Motor: Present in wound rotor motors.
- Synchronous Motor: Present in most synchronous motors to supply DC excitation to the rotor field.
⚡ Working Principle
🔹 Induction Motor
Works on Faraday's law of electromagnetic induction. When stator is energized, a rotating magnetic field is produced which induces a current in the rotor. This current generates its own magnetic field and causes the rotor to rotate.
🔹 Synchronous Motor
Works on the principle of magnetic locking. Rotor is fed with DC to produce a fixed magnetic field. When stator’s RMF matches the rotor’s magnetic poles, the rotor locks with the field and rotates at synchronous speed.
🔄 Type of Motors
🔹 Types of Induction Motors
- Based on Rotor:
- Squirrel Cage Induction Motor
- Slip Ring (Wound Rotor) Induction Motor
- Based on Phases:
- Single-phase
- Three-phase
🔹 Types of Synchronous Motors
- Non-Excited Synchronous Motor (Permanent Magnet)
- DC Excited Synchronous Motor
- Hysteresis Motor
- Reluctance Motor
📉 Slip: Zero vs Non-Zero
- Induction Motor: Has non-zero slip (typically 2–6%) to induce rotor current.
- Synchronous Motor: Slip is zero because rotor speed equals stator’s RMF speed.
Slip Formula:
Slip (S) = ((Ns - Nr) / Ns) × 100%
🧮 Speed Equation and Formula
➤ Synchronous Speed
Ns = (120 × f) / P
Where:
f = Supply frequency
P = Number of poles
➤ Induction Motor Speed
Nr = Ns(1 - S)
- In synchronous motor: Nr = Ns
- In induction motor: Nr < Ns
📊 Efficiency and Power Factor Comparison
| Parameter | Induction Motor | Synchronous Motor |
|---|---|---|
| Efficiency | Moderate to High | High |
| Power Factor | Lagging, poor at low loads | Can be controlled (lagging, leading, or unity) |
🔌 Starting Methods
➤ Induction Motor
- Direct-On-Line (DOL)
- Star-Delta Starter
- Auto-transformer Starter
- Rotor Resistance Starter (for wound rotor)
➤ Synchronous Motor
- Pony Motor
- Damper Windings
- Variable Frequency Drives (VFDs)
🔁 Torque Characteristics
- Induction Motor: Torque depends on slip. High starting torque in slip ring motors.
- Synchronous Motor: No starting torque inherently. Constant torque at synchronous speed.
⚡ Applications
🔹 Induction Motor
- Fans, pumps, blowers
- Industrial drives
- Domestic appliances
🔹 Synchronous Motor
- Constant-speed industrial loads
- Power factor correction
- High-precision machines
- Paper and textile mills
📋 Key Differences in Tabular Format
| Feature | Induction Motor | Synchronous Motor |
|---|---|---|
| Rotor Speed | Less than synchronous speed | Equal to synchronous speed |
| Starting Method | Self-starting | Not self-starting |
| Slip | Non-zero | Zero |
| Excitation | Not required | DC excitation required |
| Power Factor | Lagging | Adjustable |
| Efficiency | Moderate to high | Higher |
| Application | General-purpose | Constant-speed and power factor correction loads |
❓ FAQs
- Q1. Why is synchronous motor not self-starting?
Because there is no torque at standstill as the rotor needs to catch up with the rotating magnetic field. - Q2. Can induction motor run at synchronous speed?
No, due to the requirement of slip for induction of current, it can never reach synchronous speed. - Q3. Which motor is preferred for power factor correction?
Synchronous motors are preferred as they can operate at leading power factor. - Q4. Why is squirrel cage rotor preferred in induction motors?
It is simple, rugged, and maintenance-free.
📝 MCQs for Practice (SSC JE, RRB JE, UPPCL JE)
- Which of the following motors has zero slip?
A) DC motor
B) Synchronous motor
C) Induction motor
D) Universal motor
✅ Answer: B - In a 4-pole, 50 Hz induction motor, if the slip is 4%, the rotor speed is:
A) 1500 RPM
B) 1440 RPM
C) 1380 RPM
D) 1300 RPM
✅ Answer: C - Why are synchronous motors used for power factor correction?
A) High torque
B) Adjustable speed
C) Ability to operate at leading power factor
D) None of the above
✅ Answer: C - Which motor is not self-starting?
A) Induction motor
B) Synchronous motor
C) Universal motor
D) Stepper motor
✅ Answer: B