Electrical Interview Questions: Series-3

Electrical Interview Questions-Series-3

Electrical Interview Questions

In this series of Interview questions we will see some Electrical DC related problems in Electrical
Engineering.

DC Electrical Questions:

The following are the important DC Electrical Questions asked in technical interviews.

1. Why doesn’t electrical tester glow for DC?

To understand this first of all we have to know the DC Grounding Methods which are

  1. Floating from ground system
  2. DC Ground detection system

1.Floating from Ground System:

Stationary battery and dc power systems used in switchgear and control applications are typically designed and operated as a floating from ground system which means that there is no intentional low resistance or solid connection to ground from either the positive polarity or negative polarity of the dc system.

In this case with no unintentional ground paths should have zero potential between the positive bus to ground and the negative bus to ground.

This type of floated from ground dc system is dangerous to personnel. A low-resistance ground on the positive bus can result in an electrical shock if personnel were to accidentally touch the negative bus. Likewise, a low-resistance ground on the negative bus can result in an electrical shock if personnel were to accidentally touch the positive bus.

Why Doesn’t Tester Glow?

In this case if we touch the tester to either positive or negative poles of MCB, no current flows as it is grounded at only one point, through the human body and there is no return path.

2. DC Ground Detection System:

The most common types of ground detection systems are balanced resistor ground detectors. It has a medium resistance connected between the positive polarity and ground, and an equal resistance connected between the negative polarity and ground.

This arrangement is made inside the Earth fault Relay. Here we will get equal amount of potentials for positive and negative poles with respect to earth.

Now if we place the tester on either positive or negative poles of MCB the unbalance in earthing causes it should glow.

Why Doesn’t Tester Glow?

In any case if we bypass the earth fault relay or disconnect its terminals then the grounding no more exists and it is same as floating ground system, so tester will not glow.

2. Why DC MCCBS are having less breaking capacity than AC MCCBS for the same voltage and current ratings?

Switching of DC currents is much severe phenomenon than switching AC currents due to non occurrence of natural zero.

3. Why the size of DC motor is larger compared to the AC motor for same capacity?

I have noticed that for running Jock Oil Pump two motors are using in my plant one is A.C for normal operation and the other one is D.C in case of emergency.

The ratings of these motors are as follows.

  1. A.C motor

Rating (KW): 55, Full load current (Amps):88

  1. D.C motor

Rating (KW): 55, Full load current (Amps):289

From the above we can observe that to develop the same torque D.C motor needs more current but why the size of the D.C motor is larger than A.C motor is explained here.

There are two factors which decide the size of the motor as follows.

  1. Area of the winding.
  2. The torque produced by the motor.

1. Area of the winding

The cross sectional area of the conductor is more in d.c motor than a.c motor.

As resistance is inversely proportional to the area

one

To decrease the resistance of the winding, hence to increase the current carrying capacity of d.c motor need to use more cross-sectional area of conductor.

whereas in a.c  motor it is not possible to increase the current carrying capacity to a great extent like in case of d.c motor by increasing the cross-sectional area due to skin-effect.

This is the reason for limit the value of current in a.c motor.

2. The torque produced by the motor

In DC motor torque is proportional to flux and current as

two

Here to increase the torque, the only alternative is to increase the current as the flux become constant after saturation of the magnetic field. To increase the current we have to use low resistance conductor means more cross sectional area so its winding occupies more space and hence more size.

Whereas in AC three phase induction motor torque depends on rotor resistance, square of the induced e.m.f and inversely proportional to square of the rotor impedance as

three

Hence to obtain more torque we can design the rotor with low impedance by using less no. of turns so that winding requirement is less.

Also by increasing the rotor resistance we can get the more torque. If the rotor resistance is high it means the cross section of the conductor is less. If conductor cross section is less obviously A.C motor winding occupies less volume and hence less size.

The other factors which effect the size of DC motor are

  1. Presence of commutator and brushes.
  2. High thickness of insulation

4. What are Boost Charging and Float Charging of Battery?

To keep a battery in healthy condition and to achieve optimum life of the battery, two modes of Charging Boost charging and Trickle/Float charging are required.

i) Boost Charging:

Boost charging means, charging of a battery from discharged condition to fully charged condition. In this mode constant current process is followed for efficient charging.

The Boost mode is basically meant for quick charging the battery, after a heavy discharge, so as to restore the capacity of the battery within minimum time.

ii) Trickle / Float Charging:

Trickle charging means, charging a battery at constant voltage to compensate local discharge.

The float mode is meant for supplying the DC continuous load at constant voltage mode and at the same time, trickle charging the battery to keep it in fully charged condition.

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