08/03/2020

DC Motor Interview Question Answer - 2


21
The DC shunt motor is not capable to start against heavy load. – True or False


The DC shunt motor has lower starting torque as compared to DC series motor and DC compound motor. It does not mean that DC shunt motor does not capable to start against heavy load. The DC
Shunt Motor capable to take heavy load with excess of starting input current as compared to DC Series Motor and DC Compound Motor.
DC shunt motor
  • The torque in the DC shunt motor

                  Ta α Ia
  • If the starting torque is twice that of full load torque, DC shunt motor draws two times full load current.

DC series motor
  • The torque in the DC series motor

                    Ta α Ia2
  • If the starting torque is twice that of full load torque, DC series motor draws only approximately one and half times full load current.


22
Describe the effect on speed when ( a ) DC shunt motor and ( b ) DC series motor start against heavy load.


DC series motor
  • The DC series motor developed high starting torque for heavy load condition with decreasing its speed and taking small increase in the input current.

DC shunt motor        
  • The DC shunt motor developed high starting torque for heavy load condition with keeping its speed nearly constant with large increasing in the input current.


23
Which type of DC motor has small rating that of other one for same loading condition? Explain with suitable example.


The rating of the DC series motor would be less than that of DC shunt motor for the same loading condition.
  • When a car ascend a grade, the DC shunt motor developing high starting torque by keeping its speed constant with increasing its input current.
  • Whereas the DC series motor developing high starting torque by slows down its speed with slight increase in input current for ascending a car against grade. 
  • The drop in the speed permits to develop large torque with slight increase in the input current.


24
State the different application of DC shunt motor, DC series motor, DC cumulative compound motor and DC differential compound motor


DC shunt motor
  • Fans, centrifugal pumps, reciprocating pumps, machine tools etc.

DC series motor
  • Trolley, cranes, hoists, conveyors, electric locomotives etc.

DC cumulative compound motor
  • Elevators, conveyors, heavy planners, rolling mills, printing press machine, air compressors, shear and punching machine

DC differential compound motor
  • Research work


25
What are the different methods to control the speed of DC shunt motor?


The following are the methods to control the speed of DC shunt motor
  • Flux control
  • Armature control
  • Voltage control


26
Describe the limitation of flux control for the speed control of the DC shunt motor?


Limitation of flux control
  • The speed in the DC shunt motor is given by

           N α ( 1 / Ф )        
  • Therefore speed of the DC shunt motor is increased by decreasing flux and vice versa.
  • If the field flux weakens beyond its limit, it will affect the commutation condition. 
  • Therefore there is limitation of increasing in the speed of the DC shunt motor.


27
Describe the applications of Ward – Leonard speed control method.


The Ward – Leonard speed control is applicable to the wide and sensitive speed control of the electric excavators, elevators, main drives in steel machines and in paper mills.


28
What is electrical braking? What are the different methods of electrical braking?


Electrical braking
  • It is a method by an electrical motor may be bought to rest or stop quickly.

Methods of electrical braking
  • Rheostatic braking
  • Plugging and
  • Regenerative braking


29
How the dynamic braking is achieved in the DC shunt motor?


Dynamic braking
  • The armature of the DC shunt motor is disconnected from the supply and it is connected to the variable resistance.
  • The braking effect is controlled by changing the varying the variable resistance.
  • When armature is disconnected from the supply, the DC motor acts as a generator resulting feed current to the variable resistance which produces heat ( I2 r ).
  • The direction of the generating current is opposite therefore it produces torque in the opposite direction resulting slow down the motor.


30
In which method of braking power is drawn from the supply?


Plugging or reverse current braking

31
Explain the term : Regenerative braking


Regenerative braking
  • The load acts as prime mover in this method therefore the DC shunt motor drives as a generator.
  • The direction of the armature current and resulting direction of the torque is also reversed.
  • The speed of the DC shunt motor falls until the back emf becomes lower than the supply voltage. The power returning to the line during slowing down the motor.


32
In which method of braking load acts as a prime mover?


Regenerative braking

33
Why starter is required to start DC motor?


Necessary of starter
  • The armature current in the DC motor is given by

            Ia = ( V – Eb ) /  Ra
            Ra = armature resistance
  • When motor is at standstill condition, back emf is equal to zero therefore armature current is only due to effect of armature resistance

            Ia = V/ Ra
  • The DC motor armature draws very large current particularly at starting because of armature resistance of the winding is very low.
  • The starting current is in the range of 10 to 15 times that of full load current. 
  • This excessive current blow out fuses and also it may damage the armature winding and also commutator.
  • The starting resistance must be inserted is in series with the armature winding in order to limit the starting current to safe value

            Ia = V / Ra + R
            Where R = starting resistance.
  • As the starting resistance gradually cut out, the DC motor gather up speed and accordingly back emf is developed.


34
Why the starting resistance is connected is in series with armature winding with the motor circuit?


Function of starting resistance is in series with armature winding
  • If the starting resistance is connected in the motor circuit, field current Ish decreases particularly at start.
  • Therefore the starting torque developed by the DC motor will be small resulting the DC motor experienced some difficult to start motor at starting period.

              Ta = ФIa       

35
Why starter is not required in the small HP (Fractional HP) DC motor?


The fraction HP DC motor does not requires starter due to following reasons.
  • As the armature resistance of the motor is large as compared to large HP DC motor, the starting current is comparatively low.
  • Such motor has low moment of inertia due to small size, therefore it speed up very quickly.
  • The DC motor takes momentary large starting current which is not sufficient to disturb in the voltage regulation in the supply lines.


36
Describe the function of Hold on coil in the three point starter. Where it is connected?


Function of Hold on coil
  • The function of the hold on coil is to hold the starting arm when the DC motor is in normal operation.
  • However if the field is disconnected due to some reason the starting arm is disconnected (pull off) from ON condition to OFF condition by spring tension due to de-magnetizing of electromagnet.
  • Therefore the hold on coil protects the motor against field failure. The Hold on coil is connected is in series with the field winding.


37
Which resistance has smallest value ( a ) Armature resistance      ( b ) Field resistance ( c ) Three point starter resistance.


Three point starter resistance

38
How to control the speed of DC shunt motor by three point starter?


Speed control by three point starter
  • The speed of the DC shunt motor is controlled by inserting a variable resistance is in series with the field winding.
  • The speed of the DC motor can be increased by weakening of the field flux.
  • However there is one limitation of this method of speed control
  • If the field resistance is cut out too much by the field rheostat, field current is reduced very much.
  • If the field current decreases too much, it will result in de – magnetize the electromagnet and therefore the starting arm pull back from ON position to OFF position and motor will be shut down.


39
Which type of starter is used to start the DC series motor?


Face plate starter

40
Why brake test is applicable only for small rating DC motor?


Brake test
  • The brake test is applicable only for small rating DC motor because it is very difficult to arrange brake for higher capacity DC motor or it is difficult to dissipate large amount of heat generated to braking action.




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