SESSION:18

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1)       What is meant by cogging in the Induction motor? How to prevent the cogging?

When the number of teeth in stator and rotor are equal, the stator and rotor teeth have a tendency to align themselves exactly opposite to each other, since this corresponds to minimum reluctance position. In such case the rotor may refuse to accelerate. This phenomenon is called as magnetic locking or cogging. 

This problem can be prevented by proper choice of stator and rotor slots and also by skewing the rotor slots by one slot pitch.

2)       What are the various methods of measuring slip?

The methods are as follows:

·        By actual measurement of rotor speed

·        By measurement of rotor frequency

·        Stroboscopic method

3)       What are the various methods of speed control in three phase induction motors?

·         Control from stator side

a)    By changing the supply frequency

b)    By changing the number of stator poles

c)    By changing the supply voltage

·         Control from rotor side

a)    By inserting resistance in rotor circuit

b)    By various ways of cascade connection

c)    By injecting EMFs in the rotor circuit.

4)       What is meant by crawing in the induction motor?

In induction motors, particularly squirrel cage type induction motors, sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their synchronous speed Ns. This phenomenon is known as crawling of an induction motor and the speed is called as crawling speed.

5)       What is meant by synchronous condenser?

An over excited synchronous motor running on no load is known as synchronous condenser. It is used to improve the power factor of the system.

6)       What is meant by Sag?

The difference in level between points of supports and the lowest point on the conductor is called sag.

7)       How can we reduce the effect of corona?

·         By increasing conductor size:

By increasing conductor size, the voltage at which corona occurs is raised and hence corona effects are considerably reduced.

·         By increasing conductor spacing:

By increasing the spacing between conductors, the voltage at which corona occurs is raised and hence corona effects can be eliminated.

8)       What is tariff?

The rate at which electrical energy is supplied to a consumer is known as tariff.

The tariff should include the following items:

·         Recovery of cost of producing electrical energy at the power station

·         Recovery of cost on the capital investment in transmission and distribution systems.

·         Recovery of cost of operation and maintenance of supply

·         A suitable profit on the capital investment.

9)       What are the various types of tariff?

The various types of Tariff are:

·        Simple tariff

·        Flat rate tariff

·        Block rate tariff

·        Two-part tariff

·        Maximum demand tariff

·        Power factor tariff

·        Three-part tariff

10)    What are the advantages and disadvantages of nuclear power plant?

Advantages:

·        The amount of fuel required is quite small.

·        It requires less space so it can be located near to the load centre.

·        Huge deposits of nuclear fuels available all over the world. So supply of electricity can be ensured.

·        It ensures reliability of operation.

Disadvantages:

·        The fuel used is expensive and is difficult to recover.

·        The capital cost on a nuclear plant is very high as compared to other types of plants

·        The disposal of the by-products is a big problem. They are radioactive in nature.

·        The maintenance charges are high.

11)    What are the general system requirements of alternator?

For the generation of emf, there should be two basic systems.

·         Magnetic field system to produce the magnetic field

·         Armature system which houses the conductors on which the EMF is to be induced.

12)    Will the alternators have rotating armature system or stationary armature system?

Generally in alternators, the armature is stationary and the field rotates. Small low-voltage alternators often have a rotating armature and a stationary field winding. But in large alternators rotating armature field type is used.

13)    What are the advantages of stationary armature and rotating field system?

The advantages are:

·         The stationary armature coils can be insulated easily.

·         Higher peripheral speed can be achieved in the rotor.
Cooling of the winding is more efficient.

·         Only two slip rings are required to give DC supply to the field system.

·         Output current can be easily supplied to the load circuit. Slip-rings and brushes are not necessary.

14)    What is meant by stator? What is meant by rotor?

In any electrical machine (AC/DC motor or generator) the stationary member is called as stator. Similarly in all machines the rotating member is known as rotor.

15)    What are the advantages of three phase motor over single phase motor?

Three phase motors are having following advantages over single phase motor:

·         Higher starting torques

·         Improved speed regulation

·         Less vibration

·         Quieter operation