Question

A brief theory about Synchrony Motors. (Computer text only. 3 paragraphs minimum)

Answer 1

Electrical motors in general convert electrical energy into mechanical energy.

Synchronous motors:

A electric motor rotating at a synchronous speed (the rotor speed is equals to the speed of rotating magnetic field), is known a synchronous motor. It have fixed speed in operation or it rotates at synchronous speed for a fixed frequency.

Working principle of synchronous motor:

When 3-phase electric conductors are placed in certain geometrical positions with certain angle from one another, then an electrical field is generated. Now the rotating magnetic field rotates at a certain speed, that speed is called synchronous speed. Now if an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates with the same speed of rotating field.

The speed of synchronous motor is given as

Ns=120f/P

Where f= supply frequency and P= number of poles

Operating principle:

Synchronous motor is a doubly excited machine (two electrical inputs are provided to it). We provide a Three phase AC supply to the stator winding and a DC supply to the rotor winding. The three phase field windings carrying a 3 phase current by which it produce a 3 phase rotating megnatic field. The rotor carrying DC supply also produces a constant flux. Considering 60 Hz power frequency, from the above relation we can see that the 3 phase rotating flux rotates about 3600 revolutions in 1 min or 60 revolutions in 1 sec. At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing a repulsive force on the rotor and the very next instant it will be N-S causing attractive force. But due to the inertia of the rotor, it is unable to rotate in any direction due to that attractive or repulsive forces, and the rotor remains in standstill condition. Hence a synchronous motor is not self-starting. Here we use some mechanical means which initially rotates the rotor in the same direction as the magnetic field to speed very close to synchronous speed. On achieving synchronous speed, magnetic locking occurs, and the synchronous motor continues to rotate even after removal of external mechanical means.

Starting of synchronous motor:

There are mainly two modes available to start a synchronous motor I) By using external prime mover or ii) By damper windings.

1. External prime mover: in this mode we use a mechanically coupled induction or DC motor, which rotate the synchronous motor near to the synchronous speed at starting. After that the prime mover is removed.

2. Damper winding:

In this mode of starting, the synchronous motor is of salient pole type, additional winding is placed in rotor pole face. Initially, when the rotor is not rotating, the relative speed between damper winding and rotating air gap flux is large and an emf is induced in it which produces the required starting torque.

Features of synchronous motor:

They are not self starting hence required a external means to start.

At constant frequency they rotate at a constant speed/ synchronous speed

It can be used in electric system to improve power factor when runs at no load.