21
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The DC shunt motor is not capable to
start against heavy load. – True or False
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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.
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22
|
Describe the effect on speed when ( a )
DC shunt motor and ( b ) DC series motor start against heavy load.
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|
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.
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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.
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24
|
State the different application of DC
shunt motor, DC series motor, DC cumulative compound motor and DC
differential compound motor
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|
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
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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
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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
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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.
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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.
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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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