IES ESE BASIC INTERVIEW QUESTIONS AND ANSWER(ELECTRICAL MACHINES)

Classifications of amplifiers

It is useful classify the amplifiers into four types.they are,

1.Voltage amplifier

2.Current amplifier

3. Transconductance amplifier

4. Transresistance amplifier

This classification based on the magnitudes of the input and output impedance of an amplifier relative to the source and load impedance.

1.Voltage amplifier :

The figure given in below shows the thevenin's equivalent circuit of a two port network which is represents the voltage amplifier.

from above voltage amplifier circuit consists of 

Ri = Input resistance Rs = source resistance Vs = supply voltage 

Ro = output resistance RL = load resistance .

• If the amplifier input resistance Ri is large as compared with source resistance Rs. 

Ri >> Rs Then 

Vi = Vs

• If the external load  resistance RL is large as compared with the output resistance Ro of the amplifier.
• The amplifier provides a voltage output proportional to the voltage input,so the proportionality factor is independent of the magnitudes of the source and the load resistance.such a circuit is called a voltage amplifier.
• An ideal voltage amplifier must have infinite input resistance Ri and zero output resistance Ro. 

Synchronous induction motor

Introduction :

In the applications where high starting torque and constant speed are desired then synchronous induction motors can be used. It has the advantages of  both synchronous and induction motors. The synchronous motor gives constant speed whereas induction motors can be started against full load torque.

connections and working principles :

Consider a normal slip ring induction motor having three phase winding on the rotor as shown in the Fig.

The motor is connected to the exciter which gives d.c. supply to the motor through slip rings. One phase carries full d.c. current while the other two carries half of the full d.c. current as they are in

parallel. Due to this d.c. excitation, permanent poles (N and S) are formed on the rotor.

Initially it is run as an slip ring induction motor with the help of starting resistances. When the resistance is cut out the motor runs with a slip. Now the connections are changed and the exciter is

connected in series with the rotor winding's which will remain in the circuit permanently.

 As the motor is running as induction motor initially high starting

torque (upto twice full load value) can be developed. When d.c.

excitation is provided it is pulled into synchronism and starts running at constant speed. The synchronous induction motor provides constant speed, large starting torque, low starting current and power factor correction.