Electrical Interview Questions
In this series of Interview questions, we will see some DC electrical-related problems in the Electrical
Engineering.
DC Electrical Interview Questions:
The following are the important DC Electrical Interview Questions asked in technical interviews.
- Why doesn’t an electrical tester glow for DC?
- Why does DC MCCBS have less breaking capacity than AC MCCBS for the same voltage and current ratings?
- Why size of the DC motor is large compared to the AC motor for the same capacity?
- What are Boost Charging and Float Charging of Battery?
1. Why doesn’t an electrical tester glow for DC?
To understand this first of all we have to know the DC Grounding Methods which are
- Floating from the ground system
- DC Ground detection system
- Floating from the 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 the 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 the ground and the negative bus to the 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 accidentally touch the negative bus. Likewise, a low-resistance ground on the negative bus can result in an electrical shock if personnel 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 connection between the positive polarity and the ground and an equal resistance connection between the negative polarity and the ground.
This arrangement is made inside the Earth fault Relay. Here we will get an equal amount of potential for positive and negative poles to Earth.
Now if we place the tester on either positive or negative poles of MCB the unbalance in earthing causes it will 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 the same as the floating ground system, so the tester will not glow.
2. Why does DC MCCBS have less breaking capacity than AC MCCBS for the same voltage and current ratings?
Switching of DC currents is a much more severe phenomenon than switching AC currents due to the non-occurrence of natural zero.
3. Why the size of the DC motor is large compared to the AC motor for the same capacity?
I have noticed that for running Jock Oil Pump two motors are used 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.
- A.C motor
Rating (KW): 55, Full load current (Amps):88
- 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 the A.C motor is explained here.
There are two factors which decide the size of the motor as follows.
- Area of the winding.
- The torque produced by the motor.
1. Area of the winding
The cross-sectional area of the conductor is more in the d.c motor than the a.c motor.
As resistance is inversely proportional to the area
To decrease the resistance of the winding, hence to increase the current carrying capacity of the d.c motor needs to use a more cross-sectional area of the conductor.
whereas in an a.c motor, it is not possible to increase the current carrying capacity to a great extent like in the case of a d.c motor by increasing the cross-sectional area due to the skin-effect.
This is the reason for limiting the value of current in the a.c motor.
2. The torque produced by the motor
In DC motor torque is proportional to flux and current as
Here to increase the torque, the only alternative is to increase the current as the flux becomes constant after the saturation of the magnetic field. To increase the current we have to use a 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, the square of the induced e.m.f, and inversely proportional to the square of the rotor impedance as
Hence to obtain more torque we can design the rotor with low impedance by using fewer turns so that the winding requirement is less.
Also by increasing the rotor resistance, we can get more torque. If the rotor resistance is high it means the cross-section of the conductor is less. If the conductor cross-section is less, it is obvious that A.C motor winding occupies less volume and hence less size.
The other factors which affect the size of the DC motor are
- Presence of commutator and brushes.
- The high thickness of the 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 a battery from a discharged condition to a fully charged condition. In this mode, a constant current process is followed for efficient charging.
The Boost mode is basically meant for quickly charging the battery, after a heavy discharge, so as to restore the capacity of the battery within a minimum time.
ii) Trickle / Float Charging:
Trickle charging means, charging a battery at a constant voltage to compensate for 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.
