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

Answer.3. 210 V

Explanation:-

Terminal voltage of DC shunt generator is

V = E – IaRa

Where E is generated EMFI

Ia is armature current

Ra Armature resistance

V = 230 – 0.1 x 200

V = 230 – 20

V = 210 V

Answer.2. A rectifier

Explanation:-

• The commutator is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature.
• The commutator converts the alternating current generated in armature into the unidirectional current.

Answer.2. DC motor

Explanation:-

Fleming’s left-hand rule is applicable to DC motor because the direction of rotation of motor or direction of force experienced i.e motoring action can be determined by Fleming’s left-hand rule

Answer.2. The voltage drop due to armature reaction, the voltage drop due to decreased field current and voltage drop in armature resistance

Explanation:

The above figure shows the circuit diagram of DC shunt generator where

I_A = Armature current

I_L = Load current

I_F = Field current

R_A = Armature Resistance

In the shunt generator, the field winding is connected parallel to the armature winding. When the load is connected across the armature, then there exists a voltage drop in field as well as armature winding. The voltage drop appears to IaRa armature reaction and weakened flux.

• The load characteristics of DC shunt generator are also called the external characteristics and It will only be slightly shifted from the internal characteristic as I_L = I_a — I_f where I_f (field current) is usually very small.

Answer.2. 40 Hz & 0.025 sec

Explanation:

f = 1/ t

t = 1 /40

= 0.025 sec

Answer.1. Plugging condition

Explanation:

Plugging in DC Motor

• Plugging of Dc motor is the method of reconnecting the motor to the line with reverse polarity hence now the motor will produce torque in the opposite direction to that of rotation.
• Plugging in DC motor means the reversing of either field or armature current. So either Eb or V gets reversed. Therefore, the voltage across the armature will be V+Eb which is almost twice the supply voltage.
• The rotor speed decreases until it becomes zero and then the rotor accelerates in the opposite direction. Therefore, the plugging is used to get a quick reversal and a rapid stop or Braking.

Answer.3. 837 RPM

Explanation:

Here

V = 400 V , I₁ = 20 A, N₁ = 1100

R_a = 0.06 ohm, R_se = 0.08 ohm

I₂ = 50 A, N₂ =?

Φ₂ = 130 % of Φ₁ = 1.3 Φ₁

While drawing line current of 20 A

E_b1 = V – I₁(R_a + R_se)

= 400 – 20(0.06 + 0.08)

= 397. 2 V

While drawing line current of 50 A

E_b2 = V – I₂(R_a + R_se)

= 400 – 50(0.06 + 0.08)

= 393 V

We know that

Answer.4. Either (A) or (B)

Explanation:

• The commutator is a mechanical rectifier, so the commutator collects induced EMF or current developed in the armature.
• In a motor, the commutator applies electric current to the windings. By reversing the current direction in the rotating windings each half turn, a steady rotating force (torque) is produced.

Therefore commutator work as the mechanical rectifier and current controller.

Answer.3. Bring mechanical balance of the 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.1. 2

Explanation:

In wave winding the number of brushes is always 2 irrespective of the number of poles