Top 10 Differences: Difference Between Induction and Synchronous Motors

Both Induction and Synchronous motors are widely used in electrical industry. This article presents top 10 differences between induction and synchronous motors.

Induction motor Synchronous motor
Speed Runs lower than synchronous speed. Runs at synchronous speed.
DC Field No DC field current is required DC field current is required
Construction Squirrel cage doesn’t contain

slip ring and brushes

wound rotor has slip ring and brushes.

Slip rings and brushes
Rotor type Squirrel cage or wound Salient pole rotor
P.F Lagging Leading, lagging or unity
Cost Cheap Expensive
Design Simple Complicated
Maintenance Requires little maintenance Required more maintenance
Slip 0 < Slip < 1 Required more maintenance
Rotor construction Rotor bars shortcircuited to end rings Windings over the rotor

Construction

Rotor

  • Synchronous motors: Usually they contains a salient pole type rotor.
  • Induction motors: Its rotor can either be squirrel cage rotor or a wound rotor. Its rotor comprises of copper or aluminum bars that are mounted near the surface of rotor. The bars are short circuited at ends using rings.

Stator

The typical three phase stator has three phase windings that are mounted in slots of laminated steel core. The entire construction involves three single phase windins that are electrically spaced at an angle of 120 degrees. Both synchronous and induction motors are similar in terms of stator construction.

Working

  • Synchronous motor: A stead state magnetic field is produced in rotor of motor using the field current. The three phase current flow is produced in motor by applying voltage to stator as a result of which a rotating magnetic field is produced. The rotor field chases the magnetic field of stator but never reaches it.
  • Induction motor: It doesn’t requires a seprate dc field current. Instead of this the rotor voltage in induced instead of physical connections.

Synchronous speed

The synchronous speed of a motor is related to its pole number and the operating frequency by the following relationship:

Speed = (120 * f) / P

  • Synchronous motor: Always operates at syncrhonous speed.
  • Induction motor: Always operates at speed lower than the synchrnous speed.

Slip

Slip represents relationspeed of motor as difference of speed of magnetic fields and shaft speed.

Mathematically,

s = {(η(sync) – η(m)) /η(sync) } * 100

where

η(sync) = Speed of magnetic fields

η(m) = Mechanical shaft speed

s = slip

  • Synchronous motor: Always turns at synchronous speed and its slip = 0
  • Induction motor: Its slip is always higher than zero but lower than 1

Other Differences

  1. On the basis of design a sycnhronous motor is complex, it requires more maintenance and handling than induction motor.
  2. Induction motor is smaller in size for same rating and is a rugged and simple to handle motor.

So that was all about difference between Synchronous and Induction Motors.

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