Electrical Interview Questions

Electrical Interview Question 1

Why should the secondary winding of the C.T not be open-circuited?

In a power transformer, the primary current depends on its secondary current. If secondary is open-circuited, the primary current also reduces to the no-load current value. Whereas in the current transformer, the primary winding is connected in series with the power circuit, so the primary current is constant irrespective of the condition of the secondary winding.

During normal operation, the secondary M.M.F will oppose the primary M.M.F as per Lenz’s law. If we suddenly disconnect the C.T secondary, there is no counter force for the primary M.M.F, which is constant. This primary M.M.F will increase the core flux and causes it to saturate, or if the core is still capable, it will produce a huge flux linkage at the C.T secondary, which has a large number of turns. This flux linkage will produce a high voltage across the secondary, damaging the insulation and harming the operator responsible for the secondary winding opening.

Electrical Interview Question 2

Why cannot we perform a back-to-back test on Induction Machine?

To answer this, one must have a basic idea of the components involved in the back-to-back test. The back-to-back test needs two identical machines, of which one is the generator and the other acts as a motor. Both these machines are coupled on the same shaft to run at the same speed.

But in an induction machine, it is impossible to operate the generator and motor at the same speed as the motor can only operate at less than the synchronous speed. In contrast, the generator should always be run at speed greater than the synchronous speed. So, running both machines on the same shaft is impossible.

Electrical Interview Question 3

Why the capacitor blocks DC while allowing AC?

A capacitor blocks DC, but it can pass the AC through it. To understand this, we must have the basic idea of capacitor construction. The capacitor contains two conducting plates separated by a dielectric material. When we apply DC to the capacitor, it will charge up to its maximum capacity after the DC cannot flow through it as the dielectric material would not allow the flow of current through it as it is a non-conducting material made up of the materials like mica.

But the passage of AC is possible because of the ability of the dielectric material to polarise. But a displacement current, not conducted current, flows in a conductor. After the dielectric gets polarised, the oscillating current will produce an oscillating magnetic field; according to maxwell’s theory of electromagnetism, this can be considered equivalent to the current flow. So, even though the dielectric is an insulator, it is acceptable that the alternating current flows through it as the charging and discharging are repeating in an alternative way.

According to maxwell’s equation

×H=JC+JD

Here JC is the conducting current density, and JD is the displacement current density. Then the displacement current is

ID=AJD

Here A is the surface area of the capacitor plate. 

Electrical Interview Question 4

An ammeter is always connected in series with the circuit in which the current is to be measured. Why?

The ammeter is an instrument designed with low internal resistance to allow the maximum flow of current to be measured. Ideally, the value of internal its internal resistance is zero. If we connect it in parallel with the circuit in which the current is to be measured, then the majority of the load current can be diverted through the ammeter as it has low resistance, and there is less or no current through the load. So, this action is disturbing the load or circuit current.

Hence the ammeter is always connected in series with the load/ circuit as the same current will flow in the elements connected in series. In this case, the circuit current can be measured without disturbing the current flow through the circuit or load. So, while designing the ammeter, it is designed by considering that the voltage across it will be very small, and the insulation is designed for those low voltages only. If we connect it across the load, it may not withstand high voltages and damage.

Electrical Interview Question 5

The induction motor is self-starting. Then why do we have to use the starter while starting the induction motor?

Let us assume we have applied the rated voltage to a three-phase induction motor of a large rating. Initially, there is no slip as the rotor is at a standstill; the rotor will draw a significantly high current. In addition to that, the rotor is short-circuited at both ends means the rotor will draw a current that is equal to the short circuit current of the rotor bars (ISC), which is 7 to 10 times higher than the rated operating current of the induction motor, these large currents may not damage the induction motor that boasts the robust and rugged construction and that too these currents are of inrush type.

 But the problem is the voltage dip produced by these high currents can damage the domestic loads nearby.  Hence it demands the control of these high currents during the starting of an induction motor. As the motor attains rated speed, the slip will come into play, and the rotor current will be reduced.

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