Induction motors and synchronous motors are the two main categories of AC motors used in electrical engineering. While both operate on the principle of a rotating magnetic field, they differ significantly in speed behavior, starting method, power factor, and applications.
In this article, we will compare induction motor and synchronous motor across all important parameters — helping you understand when to use which motor.
Table of Contents
Basic Difference
The fundamental difference lies in how the rotor interacts with the rotating magnetic field:
- Synchronous motor: The rotor rotates at exactly the synchronous speed — in perfect sync with the stator's rotating field. There is zero slip.
- Induction motor: The rotor always rotates slightly slower than the synchronous speed. This speed difference (called slip) is essential for torque production.
Broadly, AC motors are categorized into two types:
- Synchronous motors — doubly excited (AC on stator + DC on rotor)
- Asynchronous motors (Induction motors) — singly excited (AC on stator only)
Detailed Comparison Table
Speed and Slip
The synchronous speed of any AC motor is given by:
A synchronous motor runs at exactly Ns. Its speed is completely independent of load — whether at no load or full load, the rotor maintains synchronous speed. If the load exceeds the pull-out torque, the motor falls out of synchronism and stops.
An induction motor always runs at a speed slightly less than Ns. The difference is called slip:
Slip is essential for an induction motor — without it, no relative motion exists between the rotor conductors and the rotating field, so no EMF is induced and no torque is produced. Typical full-load slip is 2–5%.
Excitation
A synchronous motor is a doubly excited machine:
- The stator (armature) winding is energized from a three-phase AC supply
- The rotor (field) winding is energized from a separate DC source (exciter)
An induction motor is a singly excited machine:
- Only the stator receives AC supply
- The rotor current is induced by electromagnetic induction (no external connection needed)
This is why induction motors are simpler and cheaper — they don't need slip rings, brushes, or a DC exciter for the rotor.
Self-Starting Capability
An induction motor is self-starting. When three-phase supply is connected, the rotating magnetic field immediately induces current in the rotor, producing starting torque.
A synchronous motor is NOT self-starting. The stator field rotates at synchronous speed instantly, but the rotor cannot accelerate from zero to synchronous speed due to inertia. External starting methods are needed:
- Damper windings: Short-circuited bars on the rotor produce induction-motor-like starting torque
- Pony motor: A small auxiliary motor brings the rotor near synchronous speed
- VFD starting: Gradually increasing the supply frequency from zero
Power Factor
This is one of the biggest advantages of a synchronous motor. By adjusting the field excitation (DC current), the power factor can be controlled:
- Under-excitation: Motor operates at lagging power factor
- Normal excitation: Motor operates at unity power factor
- Over-excitation: Motor operates at leading power factor
This behavior is shown by the V curve of synchronous motor. An over-excited synchronous motor can supply reactive power to the system, acting as a synchronous condenser for power factor improvement.
An induction motor always operates at a lagging power factor because it draws magnetizing current from the supply. At light loads, the power factor can be as poor as 0.3–0.4. At full load, it improves to about 0.8–0.85.
Applications
Synchronous Motor Applications
- Power factor correction (synchronous condenser)
- Constant-speed drives — cement mills, compressors, pumps
- High-power, low-speed applications (ball mills, crushers)
- Precision speed applications in textile and paper industries
Induction Motor Applications
- General industrial drives — fans, blowers, conveyors
- Domestic appliances — washing machines, refrigerators
- Traction and electric vehicles
- Cranes, hoists, and elevators
- For single-phase applications, see our guide on single phase induction motor types and working
Which Motor to Choose?
In practice, induction motors dominate industrial applications (over 90% of motors in use) because of their simplicity, ruggedness, and low cost. Synchronous motors are used in specific situations where constant speed or power factor correction justifies the higher cost and complexity. Both motor types are also used in electric vehicles — with induction motors in Tesla Model S and synchronous (PMSM) in most modern EVs.
FAQs
Why is an induction motor called asynchronous?
Because its rotor never runs at synchronous speed — it always lags behind the rotating field. The word "asynchronous" means "not in sync." The slip between rotor speed and field speed is what produces torque.
Can a synchronous motor run below synchronous speed?
No. A synchronous motor either runs at exactly synchronous speed or it doesn't run at all. If the load exceeds the pull-out torque, the motor loses synchronism and stops. There is no stable operation below synchronous speed.
Why is the induction motor more popular than the synchronous motor?
Induction motors are self-starting, cheaper, simpler in construction, require less maintenance (no brushes or exciter), and are available in a wide range of sizes. For most applications, the slight speed variation is acceptable.
Can an induction motor improve power factor?
No. An induction motor always draws lagging reactive current. It cannot supply reactive power to the system. External capacitor banks or synchronous condensers are needed for power factor correction.
What is the main advantage of a synchronous motor over an induction motor?
The ability to control power factor. By adjusting field excitation, a synchronous motor can operate at unity or leading power factor, reducing reactive power demand from the supply and improving overall system efficiency.
No comments:
Post a Comment