Question

1. Explain the no-load saturation curve and the paragraph curve of the synchronous generator and then explain how to obtain the paragraph ratio (including the reason for the short-circuit curve becoming a straight line when the field current increases) Please describe the characteristics of a large synchronous generator.

2. Explain the phenomena that occurs when the number of revolutions of one of the two synchronous generators is running in parallel and the number of revolution is unknown, disperse the load between the two generators in parallel with the generator Please explain how to do.

3. Explain the exploration of synchronous motors and explain what kind of principle the braking winding prevents exploration.

4. Describe the speed control (V / f constant control) of the synchronous motor.

Answer 1

4ans) Speed Control of Three Phase Induction Motor
A three phase induction motor is basically a constant speed motor so it’s somewhat difficult to control its speed. The speed control of induction motor is done at the cost of decrease in efficiency and low electrical power factor. Before discussing the methods to control the speed of three phase induction motor one should know the basic formulas of speed and torque of three phase induction motor as the methods of speed control depends upon these formulas.
Synchronous Speed

Where,
N is the speed of the rotor of an induction motor,
Ns is the synchronous speed,
S is the slip.
The torque produced by three phase induction motor is given by,

The Speed of Induction Motor is changed from Both Stator and Rotor Side. The speed control of three phase induction motor from stator side are further classified as :

V / f control or frequency control.

Changing the number of stator poles.

Controlling supply voltage.

Adding rheostat in the stator circuit.
The speed controls of three phase induction motor from rotor side are further classified as:

Adding external resistance on rotor side.

Cascade control method.

Injecting slip frequency emf into rotor side.

Speed Control from Stator Side

V / f Control or Frequency Control
Whenever three phase supply is given to three phase induction motor rotating magnetic field is produced which rotates at synchronous speed given by

Where, K is the winding constant, T is the number of turns per phase and f is frequency. Now if we change frequency synchronous speed changes but with decrease in frequency flux will increase and this change in value of flux causes saturation of rotor and stator cores which will further cause increase in no load current of the motor . So, its important to maintain flux , ? constant and it is only possible if we change voltage. i.e if we decrease frequency flux increases but at the same time if we decrease voltage flux will also decease causing no change in flux and hence it remains constant. So, here we are keeping the ratio of V/f as constant. Hence its name is V/ f method. For controlling the speed of three phase induction motor by V/f method we have to supply variable voltage and frequency which is easily obtained by using converter and inverter set.

Controlling Supply Voltage
The torque produced by running three phase induction motor is given by

We know that rotor induced emf E2 ? V. So, T ? sV2.
The equation above clears that if we decrease supply voltage torque will also decrease. But for supplying the same load, the torque must remain the same, and it is only possible if we increase the slip and if the slip increases the motor will run at reduced speed. This method of speed control is rarely used because a small change in speed requires a large reduction in voltage, and hence the current drawn by motor increases, which cause overheating of the induction motor.

Changing the number of stator poles:
The stator poles can be changed by two methods
Multiple stator winding method.

Pole amplitude modulation method (PAM)

Multiple Stator Winding Method
In this method of speed control of three phase induction motor, we provide two separate windings in the stator. These two stator windings are electrically isolated from each other and are wound for two different numbers of poles. Using switching arrangement, at a time, supply is given to one winding only and hence speed control is possible. Disadvantages of this method are that the smooth speed control is not possible. This method is more costly and less efficient as two different stator windings are required. This method of speed control can only be applied to squirrel cage motor.