What is the difference between synchronous motor and induction motor?

 

Synchronous and induction machines both produce torque through the interaction of a rotor magnetic field and a stator magnetic field. The differences between the two types of machines arise because of the differences in the way the rotor magnetic field is generated.
Synchronous machines have a stationary (relative to the rotor) magnetic field on the rotor. This field can be generated either by permanent magnets, or by a field winding powered through slip rings. The interaction of this field with the rotating field on the stator creates torque and causes the motor to rotate. A synchronous motor always rotates at some multiple, determined by the number of poles, of the line frequency. If a synchronous motor loses lock with the line frequency, e.g., by torque overload, it will stall. A synchronous motor cannot start by itself on a fixed frequency AC source. It either needs to be fed a variable frequency source, or it needs to be brought up to speed by an auxiliary motor, sometimes called a pony motor, so that it can generate torque. Synchronous machines usually require some form of control to keep the rotor speed locked to the line frequency.
Induction machines have a rotating (relative to the rotor) magnetic field on the rotor. In a squirrel cage motor, this field is created because the motion of the stator field relative to the shorted rotor cage induces currents in the rotor. These currents generate the rotor field, which interacts with the stator field to create torque. A wound-rotor induction machine has rotor windings similar to a synchronous machine, in which currents are induced by the rotating stator field. Induction motors always rotate in some narrow speed range that is less than synchronous speed. This speed difference, which is necessary to generate the rotor field, is called the "slip." Low slip machines, which turn at very near synchronous speed, are more efficient than high slip machines, but have lower starting torque. Induction machines can produce some torque at zero speed, so they are capable of starting themselves if the load torque is low enough at zero speed. The torque-speed characteristic of induction machines at rated speed has a negative slope (as speed decreases, torque increases). As a result, induction machines do not require controls to operate - the feedback mechanism is built into the machine.
The winding resistance a wound-rotor induction machine can be varyied by connecting resistors to the rotor windings via the slip rings. This allows the torque-speed characteristics of the wound-rotor machine to be varied as needed (e.g., high resistance (= high slip) for high starting torque and then low resistance (= low slip) for high efficiency at rated speed).
The absence of a rotor winding makes squirrel cage induction machines significantly cheaper to manufacture than synchronous machines (or wound-rotor induction machines). Squirrel cage machines are extremely rugged because of the lack of a wound rotor (the cage is usually cast right into the rotor laminations), and the lack of slip rings makes them more suitable for explosive environments because there is no arcing mechanism. The circulating currents in an induction machine rotor lead to resistive losses that make induction machines less efficient that synchronous machines.
Summary of differences:
Synchronous machines:

• Wound-rotor or permanent magnet to generate the rotor magnetic field.
• Rotor magnetic field is stationary with respect to the rotor.
• Always turn at synchronous speed.
• Require some form of control to operate.
• More expensive to produce than squirrel cage induction machines.
• Not self-starting.
• More efficient than induction machines.

Induction Machines:

• Wound-rotor or squirrel cage to generate the rotor magnetic field.
• Rotor magnetic field rotates with respect to the rotor.
• Always turn at less than synchronous speed.
• Do not require control.
• Much cheaper to produce (true for squirrel cage machines).
• Self-starting.
• Less efficient than synchronous machines.
• More suitable for explosive environments.
• No maintenance (for squirrel cage machines).