Important Short Questions and Answers: Drive Motor Characteristics

Drive Motor Characteristics

Why a single phase induction motor does not self start?

When a single phase supply is fed to the single phase induction motor. Its stator winding produces a flux which only alternates along one space axis. It is not a synchronously revolving field, as in the case of a 2 or 3phase stator winding, fed from 2 or 3 phase supply.

What is meant by plugging?

The plugging operation can be achieved by changing the polarity of the motor there by reversing the direction of rotation of the motor. This can be achieved in ac motors by changing the phase sequence and in dc motors by changing the polarity.

Give some applications of DC motor?

Shunt: driving constant speed, lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating pumps

Series: electric locomotives, rapid transit systems, trolley cars, cranes and hoists, conveyors Compound: elevators, air compressors, rolling mills, heavy planners.

What are the different types of electric braking?

Dynamic or Rheostatic braking,

Counter current or plugging and

Regenerative braking

What is the effect of variation of armature voltage on N-T curve and how it can be achieved?

The N-T curve moves towards the right when the voltage is increased. This can be achieved by means of additional resistance in the armature circuit or by using thyristor power converter.

Compare electrical and mechanical braking?

Mechanical

Brakes require frequent maintenance      

Not smooth       

Can be applied to hold the system at any position    

Electrical

very little maintenance

smooth

cannot produce holding torque.

When does an induction motor behave to run off as a generator?

When the rotor of an induction motor runs faster than the stator field, the slip becomes negative. Regenerative braking occurs and the K.E. of the rotating parts is return back to the supply as electrical energy and thus the machine generates power.

Define slip?

Define synchronous speed?

It is given by N_s = 120f / p rpm.

Where      N_s = synchronous speed, p = no. of stator poles, f = supply frequency in Hz

What is meant by regenerative braking?

In the regenerative braking operation, the motor operates as a generator, while it is still connected to the supply here, the motor speed is grater that the synchronous speed. Mechanical energy is converter into electrical energy, part of which is returned to the supply and rest as heat in the winding and bearing.

What are the disadvantages of inserting resistance in the rotor circuit in slip ring induction motor?

Disadvantages:

Losses (I²R) is increasing and efficiency of the motor is decreased.

Since speed is dependent on both resistance and load. We can change speed for short periods only.

under what condition, the slip in an induction motor is

Negative

Greater than one

Slip of an induction motor is negative when the induction motor is operating in generating mode.

Slip of an induction motor is greater than one when the induction motor is operating in the braking mode (its direction is opposite to the direction of rotating magnetic field).

Differentiate cumulative and differential compound motors?

Cumulative

The orientation of the series flux aids the shunt flux  

differential

series flux opposes shunt flux

Draw the speed torque characteristics of DC shunt motor?

Draw the speed torque characteristics of DC series motor?

Draw the speed torque characteristics of compound motor? 

What is meant by mechanical braking?

In mechanical braking, the stored energy of the rotating parts is dissipated in the form of heat by a brake shoe (or) a break drum. in electric braking, the stored energy of rotating parts is converted into electrical energy and dissipated in the resistance in the form of heat or returned to the supply.

What is back emf?

If the voltage is applied and current is passing in the armature, the motor starts rotating. At that time armature conductors also rotates and hence cuts the flux. Due to that an emf is induced in the armature and the direction is found to be opposite to that of the supply this is known as back emf.

What are all the types of electrical machines?

Electrical machines are classified as AC machines and DC machines.

Types of DC machines

DC Generator

DC Motor

Types of AC machines

Transformers

Single phase (b) three phase

Alternators

Synchronous motor

Induction motor

(a)Single phase (b) three phase

What are the two types of 3 phase induction motor?

There are two types of 3-phase induction motor based on the type of rotor used:

Squirrel cage induction motor.

Slip ring induction motor.

What is the principle used in induction motor?

Conversion of electrical power into mechanical power takes place in the rotating part of an Electric motor. In D.C. motor, the electrical power is conducted directly to the armature (i.e. rotating part) through the brushes and Commutator. Hence, in this sense, a D.C. motor can be called a conduction motor. However, in A.C. motors, the rotor does not receive electric power by conduction but by induction in exactly the same way as the secondary of a 2-winding transformer receives its power from primary. That is why such motors are called induction motors. In fact, induction motors can be treated as a rotating transformer i.e. in one which primary winding is stationary but the secondary is free to rotate.

What are the advantages of the slip-ring induction motor over squirrel cage Induction motor?

Advantages:

It is possible to speed control by regulating rotor resistance.

High starting torque of 200 to 250% of full load voltage.

Low starting current of the order of 250 to 300% of the full load current.

Hence slip ring induction motors are used where one or more of the above requirements are to be met.

What are the types of single phase induction motors?

The types of single phase induction motors are:

Split phase induction motor.

Capacitor start induction motor.

Capacitor start and capacitor run motor.

Shaded pole induction motor.

Why regenerative braking not possible in DC series motor?

In DC series motor, regenerative braking is not possible without necessary modifications, because reversal of Ia would result in reversal of field and hence of E_b.

What is meant by dynamic braking?

In this method of breaking, the motor is disconnected from the supply, the field connections are reversed and motor is connected in series with a variable resistance R