106 110

 106. What is stepper motor? What is its uses?

Stepper motor is the electrical machine which act upon input pulse applied to it.it is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle.so, in automation parts it is used.

107. Tell me in detail about c.t. and p.t.?

The term C.T means current transformer, and the term P.T means potential transformer.

In circuit where measurements of high voltage and high current is involved, they are used there.

Particularly when a measuring device like voltmeter or ammeter is not able to measure such high

value of quantity because of large value of torque due to such high value it can damage the

measuring device.so, CT and PT are introduced in the circuits. They work on the same principle

of transformer, which is based on linkage of electromagnetic flux produced by primary with

secondary. They work on the ratio to they are designed. E.g. if CT is of ratio 5000\5A and it has

to measure secondary current of 8000A.then ANS=8000*5\5000=8Aand this result will be

given to ammeter .and after measuring 8A we can calculate the primary current. Same is the operation of PT but measuring voltage.

108. There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why?

The motor has max load current compare to that of transformer because the motor

consumes real power and the transformer is only producing the working flux and it’s not

consuming. Hence the load current in the transformer is because of core loss so it is minimum.

109. What is power factor? Whether it should be high or low? Why?

Power factor should be high in order to get smooth operation of the system.

Low power factor means losses will be more.it is the ratio of true power to apparent

power. It has to be ideally 1. If it is too low then cable over heating &equipment

overloading will occur. If it is greater than 1 then load will act as capacitor and starts

feeding the source and will cause tripping. (If pf is poor ex: 0.17 to meet actual power

load has to draw more current (V constant), result in more losses if pf is good ex: 0.95 to

meet actual power load has to draw less current (V constant), result in less losses)

110. What is the difference between Isolator and Circuit Breaker?

Isolator is an off load device which is used for isolating the downstream circuits

from upstream circuits for the reason of any maintenance on downstream circuits. It is

manually operated and does not contain any solenoid unlike circuit breaker. It should not

be operated while it is having load. First the load on it must be made zero and then it can

safely operate. Its specification only rated current is given. But circuit breaker is on load

automatic device used for breaking the circuit in case of abnormal conditions like short

circuit, overload etc., it is having three specification 1 is rated current and 2 is short circuit

breaking capacity and 3 is instantaneous tripping current .