DMRC JE Electrical Question Paper With Explained Solution – 2016

Ques 41. In a 3-phase voltage source inverter used for speed control of induction motor, antiparallel diodes are used across each switching device. The main purpose of diodes is to:

  1. Protect the switching devices against overvoltage
  2. Provide the path for freewheeling current
  3. Allow the motor to return energy during regeneration
  4. Help in switching off the devices

In regenerative braking, the motor, instead of being disconnected from the supply, remains connected to the supply and returns the braking energy to the supply line.

VSI fed IM drive has a voltage source inverter at the machine end and a six-pulse converter at the source end. When the induction machine is motoring, the source end converter operates as a rectifier, and the machine end converter operates as an inverter.

With a single 6 pulse converter at the input side, regeneration is not possible since the electrolytic capacitor does not allow reversal of the voltage Polarity and the SCRs do not allow the reversal of currents.

Regeneration is possible by having an antiparallel 6 pulse converter at the input side. When regeneration takes place, firing pulses to the converter 1 are stopped and firing pulses are given to machine end converter such that it operates as an inverter. The motor operates as an induction generator since it converts the mechanical power into electrical power. The diode rectifier part of VSI converts AC to DC. Converter II converts DC power into AC power and is fed to the AC mains. The disadvantage being that an additional six-pulse converter is required for regeneration.

 

Ques 42. Skewing of rotor bars eliminate the:

  1. Effect of space harmonics
  2. The entire effect of crawling
  3. Magnetic noise
  4. Vibration due to unequal force developed on the rotor

Rotor conductors are skewed because of these two main reasons
Cogging:- Cogging is magnetic locking. When an induction motor refuses to start even if the full voltage is applied to it, this is called cogging. This happens when the rotor slots and stator slots are the same in number or they are integer multiples of each other. due to this the opposite poles of stator and rotor come in front of each other and get locked.

Skewing the rotor bars prevents locking thus preventing Cogging.

Crawling:– Crawling is a phenomenon where harmonic components introduce oscillations in torque. With the bar skewed, the amount of the bar cutting the field line grows continuously and the next bar starts cutting the field lines as the first finishes. Due to this, we get Uniform Torque.

Ques 43. Out of the following, which one is correct in case of a circuit breaker?

  1. Making capacity = 2.55 × braking capacity
  2. Braking capacity = 2.55 × making capacity
  3. Making capacity = 1.8 × braking capacity
  4. Braking capacity = 1.8 × making capacity

Making capacity used the peak value of current while breaking capacity work on the RMS value of current that’s why making capacity is more than breaking capacity.

Making capacity = 2.55 × braking capacity

 

Ques 44. The steady-state fault current during a 3 phase terminal fault on a generator is determined by:

  1. The transient reactance of the generator
  2. The Sub-transient reactance of the generator
  3. The synchronous reactance of the generator
  4. DC offset during the instant of the fault

  1. Sub-transient (xd”). Subtransient reactance is a value used to determine the short circuit current during the first few cycles after a short-circuit occurs. This is the short-circuit current value to be used in all short-circuit studies.
  2. Transient reactance (Xd’). Transient reactance is a value used to determine the short-circuit
    current from the first few cycles up to about 30 cycles after the short-circuit occurs (depending
    upon the design of the machine). This value is often used in voltage regulation studies.
  3. Synchronous reactance (Xd). Synchronous reactance is a value used to determine the short-circuit current when the steady-state condition has been reached. Steady-state is reached several seconds afier the shortcircuit occurs. This value is often used to determine the setting of the generator backup overcurrent relays.

 

Ques 45. A 3-phase delta connected squirrel cage induction motor when started with a DOL starter has a starting torque of 600 NM. Its starting torque when star-delta starter is used:

  1. 600 NM.
  2. 200 NM
  3. 300 NM
  4. 1200 NM

Starting torque of star delta starter is equal to 1/3 of its delta value

hence starting torque = 600/3 = 200 NM

 

Ques 48. Making current of a circuit breaker refers to:

  1. Steady-state rated current
  2. Average value of transient short circuit current
  3. R.M.S. value of the short-circuit current at the time of major peak
  4. Equal to the fault current

Rated short-circuit Making current:- It is the maximum r.m.s. current (which includes both ac and dc currents) against which the breaker is capable of closing and immediately opening Without welding of contacts (due to intense heat) or any other damage. The making current may also be expressed in terms of an instantaneous value of current in which case it is equal to the first peak of the short-circuit current wave.

The making current is measured at the first peak of the short-circuit current wave While the braking current is measured at the instant of separation of contacts.

when the breaker is going to be closed and at that time fault occurs then it will close sub-transient state and the current-carrying capacity at this instant contains RMS value of current (the symmetrical component in the transient state) and the DC offset current in the sub-transient state which is also called Making capacity of the breaker at the sub-transient period amount of arc produce will be very high. Thus making capacity is very high.i.e

Making Capacity=DC offset current+rms value of breaking current in the transient state

Making capacity used the peak value of current while breaking capacity work on the RMS value of current that’s why making capacity is more than breaking capacity.

Thus Making capacity > Breaking Capacity

 

Ques 49. In case of a ‘High Voltage DC’ transmission, how many minimum conductors are required?

  1. One
  2. Two
  3. Three
  4. Four

In HVDC ( DC) transmission, only two conductors are needed for a single line. A single conductor line with the ground as a return in HVDC can be compared with the 3-phase single circuit HVAC line.

Only one conductor is enough using earth return and by using two conductors and earth return, the capacity of the line is doubled. But, in case of AC transmission, at least three conductors are needed and six conductors would be needed for double circuit line.

 

Ques 50. Which of the following motor has squirrel-cage winding on the stator:

  1. 3 phase squirrel cage induction motor
  2. Single phase squirrel cage pump motor
  3. Single phase ceiling fan induction motor
  4. Single phase table fan induction motor

 

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