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

Answer.4. Differentially compounded generator

Explanation:-

The generator used for welding purposes should have high current and low voltage which can be obtained by the differential compound generator. Differentially compounded DC generator has drooping characteristics that mean if load current increases the net flux will decreases(i.e shunt field and series fields in opposition will increases)due to the demagnetization effect hence the Induced  EMF and terminal voltage decreases.

Answer.2. 304 V

Explanation:-

EMF Of generator is given by

E = PΦZN/60A

Φ = flux per pole in Weber

P = number of poles

Z = total number of armature conductor

N = rotation speed of the armature in revolution per minute (r.p.m)

A = number of parallel paths (Number of parallel paths in lap winding, A = P)

E = (4 x 0.01 x 1520 x 1200)/(60 x 4)

= 304 V

Answer.2. E_b = V/2

Explanation:-

Condition for maximum power

Mechanical power developed in the motor is

P = E_bI_a = VI_a – Ia²R_a

Answer.3. 8

Explanation:-

Pole pitch = slots/pole

Pole pitch = slots/pole

Number of commutator segments = 16

Number of conductors or coil side = 16 x 2 = 32

Pole pitch = 32/4 = 8

Answer.2. Same as number of poles

Explanation:-

In a lap winding, the number of parallel paths, a, is always equal to the number of poles, p, and also to the number of brushes.

Answer.1. Wave Winding

Explanation:-

• E.M.F Equation of d.c generator is
• Now for Lap wound generator  A = P
  Where is A = Number of parallel path
  And P = Number of poles
• Therefore in Lap wound D.C generator they canceled each other.
• For Wave wound Generator A = 2
• Therefore the E.M.F is greater in Wave wound D.C generator than Lap wound

Answer.4. Less than 50%

Explanation:-

Why efficiency of D.C motor is below 50%

• The DC motor develops maximum power when the Back EMF is half the applied voltage.
  E_b = V/2.
• Practically it is not possible to develop an exact 50% of maximum power because in that case, the current would be much beyond the rated current of the motor.
• Some of the energy is wasted in the form of heat and other losses.
• Therefore motor efficiency is below 50%.

Answer.1. To provide mechanical balance for the rotor

Explanation:-

What is the dummy coil?

• 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.