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Difference between induction motor and synchronous motor

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This article covers the key differences between the induction motor and synchronous motor on the basis of several important factors such as Construction, Starting Mechanism, Excitation, Speed Control, Power Factor, Load Change, Cost, Slip, Efficiency, and Applications.

The induction motor is the most common type of AC motor. It is relatively inexpensive to build, is very rugged, and requires little maintenance. Single-phase induction motors are used for residential and commercial applications, but industry relies on the three-phase induction motor for its smooth operation and higher efficiency.

The synchronous motor converts electrical power into mechanical power. Synchronous motor delivers torque and power when it is running at synchronous speed. It is a true, constant-speed motor, providing the electrical frequency is constant. The disadvantage of the synchronous speed is that it requires an excitation system, which adds to the initial and recurring costs of the motor.

Characteristics Synchronous motor Induction motor
Construction Design, structure, and construction is complicated Construction is simpler, particularly in case of cage rotor design.
Starting Mechanism Starting mechanism is required to initially rotate the rotor near to the synchronous speed. No starting mechanism is required.  
Excitation Separate DC source is required for rotor excitation. Rotor gets excited by an induced emf so separate source is not necessary.
Speed control Speed control is not feasible. Speed control is possible through different methods.
Speed Vs Load As load increases, load angle increases, keeping speed constant. (independent of load change) As load increases, the speed keeps on decreasing. (depends on load change)
Power factor By changing excitation, the motor power factor can be adjusted. (lagging/leading) It always operates at lagging power factor.
Power factor improvement It can be used as the synchronous condenser for power factor improvement. It cannot be used as the synchronous condenser.
Load change Motor is sensitive to sudden load changes and hunting results. The phenomenon of hunting is absent.
Cost Motor is costly and requires frequent maintenance Motor is cheap, especially cage rotor and requires less maintenance.
Slip Zero 0<S<1
Efficiency They are more efficient. They are less efficient.
Applications They are used in synchronous clocks, precision servomechanism, and tape recorders.

Three-phase induction motors are widely used in various industrial applications whereas single-phase induction motors are used in different household appliances.

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About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.