what are the diffences between a synchronous and induction motor?

Answer 1

Induction Motor:
An alternating-current motor in which a primary winding on one member (usually the stator) is connected to the power source, and a secondary winding on the other member (usually the rotor) carries only current induced by the magnetic field of the primary.
An alternating-current motor in which the currents in the secondary winding (usually the rotor) are created solely by induction. These currents result from voltages induced in the secondary by the magnetic field of the primary winding (usually the stator). An induction motor operates slightly below synchronous speed and is sometimes called an asynchronous (meaning not synchronous) motor.

Induction motors are the most common electric motors due to their simple construction, efficiency, good speed regulation, and low cost. Polyphase induction motors come in all sizes and find wide use where polyphase power is available. Single-phase induction motors are found mainly in fractional-horsepower (1 horsepower = 746 W) sizes, and those up to 25 hp are used where only single-phase power is available.

There are two principal types of polyphase induction motors: squirrel-cage and wound-rotor machines. The difference in these machines is in the construction of the rotor. The stator construction is the same and is also identical to the stator of a synchronous motor. Both squirrel-cage and wound-rotor machines can be designed for two- or three-phase current.

Single-phase induction motors display poorer operating characteristics than polyphase machines, but are used where polyphase voltages are not available. They are most common in small sizes (½ hp or less) in domestic and industrial applications. Their particular disadvantages are low power factor, low efficiency, and the need for special starting devices.

Synchronous Motor:
A synchronous machine that transforms alternating-current electric power into mechanical power, using field magnets excited with direct current.
An alternating-current (ac) motor which operates at a fixed synchronous speed proportional to the frequency of the applied ac power. A synchronous machine may operate as a generator, motor, or capacitor depending only on its applied shaft torque (whether positive, negative, or zero) and its excitation. There is no fundamental difference in the theory, design, or construction of a machine intended for any of these roles, although certain design features are stressed for each of them. In use, the machine may change its role from instant to instant. For these reasons it is preferable not to set up separate theories for synchronous generators, motors, and capacitors. It is better to establish a general theory which is applicable to all three and in which the distinction between them is merely a difference in the direction of the currents and the sign of the torque angles.
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Answer 2

A synchronous motor operates with a winding in its rotor which is connected to an external resistor for speed control during start-up, while an induction motor, squirrel caged type has no external resistor. The rotor windings are short-circuited at the ends and is like a squirrel. The speed of a synchronous motor has no slippage. 2,poles-3600 rpm; 4-poles-1800 rpm; 6poles-1200 rpm while an induction motor has a slip speed, 2poles- 3540 rpm; 4 poles-1750 rpm; 6poles-1150 rpm.A synchronous motor always operates with carbon brushes, while induction motors may have none.Synchronous motors are usually rated at least 100Hp, while, induction motors may be rated in fractional Hp.

Answer 3

Synchronous Motor:

1.)Speed: Starting from its name ‘synchronous’, this motor runs at synchronous speed whatever the amount of load it may be. The speed of this kind of motor is not dependent on the load.
2.)Starting Torque: This motor does not have any self starting torque, so some other auxiliary means have to be provided for starting the synchronous machine.
3.)Excitation: Synchronous motor is a doubly excited machine. The field winding that is rotor is excited using DC source and its stator that is armature winding is excited using AC source. In addition to that it can be made to operate at leading power factor from lagging power factor just by changing its excitation.
4.)Efficiency: This is comparatively more efficient than induction motor.
5.)Cost: This kind of motor is much costlier that a similar rating induction motor.

Induction Motor:

1.)Speed: The speed of induction motor is always less than the synchronous speed and it is dependent on the load since the speed decreases with the increase in load.
2.)Starting Torque: This kind of motor has its own self starting torque.
3.)Excitation: It is a singly excited machine where its stator windings are excited using an AC source. Contrary to the case of synchronous motor, induction motor works only under lagging power factor.
4.)Efficiency: It is comparatively less efficient.
5.)Cost: The cost of induction machine is less when compared with a synchronous motor of same rating.

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Answer 4

Rotor of a induction motor gets supply by means of mutual induction from stator, and in synchronous motor the rotor is fed up with DC supply.
Synchronous motor always runs at synchronous speed and induction motor always runs at a speed which is less than synchronous speed.

http://www.electricaleasy.com/2014/02/synchronous-motor-construction-working.html

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