In this series of Interview questions, we will cover the following basic Electrical interview questions.
1️⃣ Why does an induction motor draw high current during starting?
At starting, the rotor is stationary, so slip s = 1. The motor has low effective impedance, causing high starting current.
2️⃣ Why does an induction motor never run at synchronous speed?
At synchronous speed, the relative speed between the rotating magnetic field and the rotor becomes zero. Hence, rotor EMF, rotor current, and torque become zero. Therefore, the rotor must run below synchronous speed.
3️⃣ What happens if the field winding of a DC motor gets disconnected while running?
The flux decreases sharply. Since N ∝ 1/Φ, the motor speed can rise dangerously, causing overspeeding and possible damage.
4️⃣ Why does a synchronous motor operate at constant speed even when the load changes?
Once synchronized, it runs at synchronous speed, Ns = 120f/P. When the load changes, the power angle changes to develop the required torque while speed remains constant.
5️⃣ Why does a transformer make a humming sound?
Alternating magnetic flux causes the core to undergo tiny dimensional changes called magnetostriction. These vibrations produce the humming sound.
6️⃣ What happens if one phase of a running 3-phase induction motor is disconnected?
This is called single-phasing. The motor may continue running but draws excessive current in the remaining phases, causing overheating and possible damage.
7️⃣ Why should a CT secondary never be open-circuited while the primary is energized?
Because an open CT secondary produces a dangerously high voltage across the secondary terminals, which can damage insulation and cause electric shock.
8️⃣ Why is reactive power required in an AC power system?
Reactive power is required to establish magnetic and electric fields in AC equipment like motors, transformers, and reactors. It is essential for their operation, but does not perform useful mechanical work directly.
9️⃣ Why is a capacitor used for power-factor correction instead of a resistor?
A capacitor is used because it supplies leading reactive power, which cancels the lagging reactive power of inductive loads and improves power factor.
🔟 What happens to rotor current frequency when an induction motor is at standstill?
At standstill, rotor speed = 0, so slip s = 1.Therefore: Rotor frequency,
fr = s × fs = fs
So, the rotor current frequency equals the supply frequency.
