Chapter 3, Problem 3.1Q

What is the principal difference between a synchronous machine and an induction machine?

To determine

To discuss: The principal difference between a synchronous machine and an induction machine.

Explanation of Solution

The principal difference between the synchronous machine and the induction machine are as follows:

Constructional difference:

The stator of both the machines are similar in construction except the rotor and the slip ring arrangement.The rotor of the synchronous machine has either salient pole or cylindrical poleconstruction.Among these two, the salient pole rotor is most common. The windings of the rotor are provided with dc supply with the help of the slip rings. The cylindrical rotor of the synchronous motor is shown below.

  

The rotor of the induction motor has either squirrel cage or wound rotor type construction. In squirrel cage induction motor the conductors are shorted with the help of the end ring. While in the wound rotor the windings are permanently shorted. The squirrel cage rotor of the induction motor is shown below.

  

Difference in the working principle:

Synchronous machine: The synchronous machine always runs at the synchronous speed.

The synchronous speed of the Synchronous machine is given by then equation:

  $\begin{array}{l}{N}_s=\frac{120\times f}{P}\\ \text{Where,}\\ N_s\text{ is the synchronous speed}\text{.}\\ P\text{ is the number of the poles of the machine}\text{.}\\ f\text{ is the supply frequency}\text{.}\end{array}$

Induction machine:

The relative difference between the speed of the stator and the rotor m.m.f causes the induction machine to rotate.

Induction machine always runs at the speed which is lower than synchronous speed.

  $N<N_S$