SSC JE DC Motor & DC Generator Solved Questions (2020)

Answer.2. To increase the speed of rotation

Explanation:-

The voltage generated by the DC generator is given as

E_g = PφZN/60A

where

P – Number of poles of the machine

ϕ – Flux per pole in Weber.

Z – Total number of armature conductors.

N – Speed of armature in revolution per minute (r.p.m).

A – Number of parallel paths in the armature winding.

Hence from the above relation, the voltage generated is directly proportional to the speed of the rotation flux per pole, number of armature conductor, Number of poles

E_g ∝ N

Hence Generated voltage can be increased by increasing the speed of rotation.

Answer.4. Cumulative Compound motor

Explanation:-

The characteristics of cumulative compound motor lie between those of shunt and series motors. The series field provides a high starting torque and the shunt field prevents overrunning in the no-load condition.

Class D motors are characterized by high starting torque, low starting current, and high operating slip. The rotor cage bars are made of high-resistance material such as brass instead of copper. The torque-speed characteristic is similar to that of a wound-rotor motor with some external resistance connected to the rotor circuit. The maximum torque occurs at a slip of 0.5 or higher. The full-load operating slip is high (8 to 15 percent), and therefore the running efficiency is low. The high losses in the rotor circuit require that the machine be large (and hence expensive) for a given power. These motors are suitable for driving intermittent loads requiring rapid acceleration and high-impact loads such as punch presses or shears. In the case of impact loads, a flywheel is fitted to the system. As the motor speed falls appreciably with load impact, the flywheel delivers some of its Kinetic energy during the impact.

As we know that the in wound Induction motor we can achieve high starting torque by adding some external resistance. There it can be used for press and punches.

These motors are used for drives where intermittent high load torque is required with the probability of the load being totally removed such as punch, press, shears, planning machine, conveyors, crushers, bulldozers, lid haulage gears, mine hoist, power fans, rolling mills, stamping press, and the large printing press.

Answer.3. Bring about the mechanical balance of armature

Explanation:-

• Dummy coil is used with wave winding when the requirement of the winding is not met by the standard armature.
• Dummy coil  is not connected to the commutator so they do not influence the electrical characteristics of the winding
• Dummy coil ends are cut short and taped.
• Their main use is to provide mechanical balance for the rotor because the rotor having some slots without winding would be out of balance mechanically. 

Answer.2. Interchanging the armature or field connection

Explanation:-

The direction of rotation of a DC shunt motor can be reversed by interchanging the leads of either the field winding or the Armature Winding.

Generally changing the direction of the field is easier, because it carries lesser current as compared to armature current. However, the reversal should not be done while the armature is excited.

Answer.2. Speed

Explanation:-

The voltage generated by the DC generator is given as

E_g = PφZN/60A

where

P – Number of poles of the machine

ϕ – Flux per pole in Weber.

Z – Total number of armature conductors.

N – Speed of armature in revolution per minute (r.p.m).

A – Number of parallel paths in the armature winding.

Hence from the above relation, the voltage generated is directly proportional to the speed of the rotation flux per pole, number of armature conductor, Number of poles

E_g ∝ N

Hence Generated voltage can be increased by increasing the speed of rotation.

Answer.2. 25% of the previous value

Explanation:-

In DC series Motor, the torque is directly proportional to the square of the armature current

T ∝ Ia²

So if the armature current is reduced to 5% then torque is

T = 5²

T = 25%

Answer.1. Directly proportional to back e.m.f. and inversely proportional to the flux

Explanation:-

Voltage Equation of DC Motor

The voltage equation of the DC Motor is given as

V = E_b + I_aR_a

The voltage V applied across the motor armature has to
(i) overcome the back e.m.f. E_b and
(ii) supply the armature ohmic drop I_aR_a

The above equation can also be written as

E_b = V − I_aR_a

Back EMF of DC Motor

When the armature of a d.c. the motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator. The induced e.m.f. acts in the opposite direction to the applied voltage  V (Lenz’s law) and is known as back or counter e.m.f. Eb. 

[latex display=”true”]{E_b} = \dfrac{{P\Phi ZN}}{{60A}}[/latex]

Where

P – Number of poles of the machine

ϕ – Flux per pole in Weber.

Z – Total number of armature conductors.

N – Speed of armature in revolution per minute (r.p.m).

A – Number of parallel paths in the armature winding.

From the above equation, it is clear that the EMF of DC Motor is Directly proportional to the Number of poles of the machine (P), Flux per pole in Weber (ϕ), Total number of armature conductors(Z), and  Speed of armature (N)

From the voltage equation and Back EMF equation, we can conclude that

[latex]\begin{array}{l}\dfrac{{P\Phi ZN}}{{60A}} = V – {\rm{ }}{I_a}{R_a}\\\\N = \dfrac{{V – {\rm{ }}{I_a}{R_a}}}{\Phi } \times \left( {\dfrac{{60A}}{{PZ}}} \right)r.p.m\\\\{E_b} = V – {\rm{ }}{I_a}{R_a}\\\\\therefore N = K\dfrac{{{E_b}}}{\Phi }\end{array}[/latex]

The above equation shows that speed is directly proportional to back e.m.f. E b and inversely to the flux Φ 

N ∝ E_b /Φ.

[latex]N = \dfrac{{{E_b}}}{\Phi } \propto \dfrac{{V – {\rm{ }}{I_a}{R_a}}}{\Phi }[/latex]

But as the value of armature resistance R_a and series field resistance R_se is very small, the drop I_aR_a and Ia (R_a + R_se) is very small compared to applied voltage V. Hence, neglecting these voltage drops the speed equation can be modified as,

[latex]N \propto \dfrac{V}{\Phi }[/latex]

Answer.4. Reduce slightly

Explanation:-

A large majority of About 95 percent of the current drawn by the shunt motor flows in the armature circuit. Since the field current has little effect on the strength of the field, the motor speed is not affected appreciably by variations in load current. The relationship of the currents that flow through a DC shunt motor is as follows:

I_T = IA + IF where:

I_T = the total current drawn from the power source,

I_A = the armature current

I_F = the field current.

For a dc shunt motor field current is practically constant and thus the torque is directly proportional to the armature current. Hence the torque-armature current characteristic is a straight line passing through the origin. However, due to the armature reaction under the loaded conditions of the machine. the flux decreases slightly with the load. Since flux is practically constant for DC shunt motor. The speed of DC shunt motor is

N = K (V – IaRa)

The shunt-wound DC motor has very good speed regulation. The speed does decrease slightly when the load increases, as the result of the increase in 

The voltage drop across the armature Increases as the load on the motor is increased. Thus, the speed of the dc shunt motor will fall slightly with the armature current. As the speed characteristic is only slightly drooping, the dc shunt motor is normally regarded as a constant speed motor.

Answer.2. Shunt generator

Explanation:-

Battery charging requires a constant voltage supply of course DC. And the current required here is low as compared to the load current. So we could use a DC shunt generator for the use.

Shunt generators are used to charge batteries. In this application, the voltage should drop off slightly as the load increases, because the voltage of a lead battery is lower when the battery is discharged than when the battery is charged. Because of its drooping characteristic, the shunt generator is admirably suited for battery charging service. In a general way, the voltage curve of the generator has the same shape as the voltage curve of the battery itself. In both cases, as the load falls the voltage rises.

Answer.3. Reduces Slightly

Explanation:-

For a dc shunt motor. field current is practically constant and thus the torque is directly proportional to the armature current. Hence the torque-armature current characteristic is a straight line passing through the origin. However, due to the armature reaction under the loaded conditions of the machine. the flux decreases slightly with the load.

Since flux is practically constant for DC shunt motor. The speed of DC shunt motor is

N = K (V – IaRa)

Armature current increases as the load on the motor are increased. Thus, the speed of the dc shunt motor will fall slightly with the armature current. As the speed characteristic is only slightly drooping, the dc shunt motor is normally regarded as a constant speed motor.