**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

T_{a} α I_{a}

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

T_{a} α I_{a}^{2}

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

**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.

**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 ascends
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.

**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

**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

**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.

**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.

**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

**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 ( I^{2 }r ). The direction of the generating
current is opposite therefore it produces torque in the opposite direction
resulting slow down the motor.

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

Plugging or reverse current braking

**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.

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

Regenerative
braking

**Why starter is required to start DC motor?**

Necessary
of starter

The
armature current in the DC motor is given by

I_{a} = ( V – E_{b}
) / R_{a}

R_{a} = 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

I_{a} = V/ R_{a}

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

I_{a} = V / R_{a} +
R

Where R = starting resistance.

As the
starting resistance gradually cut out, the DC motor gather up speed and
accordingly back emf is developed.

**Why 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 I_{sh}
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.

T_{a }= ФI_{a}

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

The
fraction HP DC motor does not require 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 speeds 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.

**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.

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

Three-point
starter resistance

**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.

Lower value
of field current de – magnetize the electromagnet resulting starting arm pull
back from ON position to OFF position and motor will be shut down.

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

Face plate
starter

**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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