Electrical Interview Questions - Part 3

Electrical Interview Questions

1.What is the purpose of motor thermal protection?

The purpose of motor thermal protection is to allow the motor to start and run within the given manufacturer specified limits but trip if the motor heat energy exceeds those ratings because of overloads, negative-sequence current, or locked rotor starting.

2. How many times more the induction motor will be heated during starting than running?

An induction motor usually draws six times the full load current for which it is rated when starting. This high stator current induces a comparably high current in the rotor. The rotor resistance at zero speed is three times the rotor resistance when the motor is at rated speed. Thus, the I2r heating in the rotor is approximately 62*3=108 times the I2r heating when the motor runs normally. So the manufacturer has to consider the maximum locked rotor time and locked rotor amps while designing the motor.

3. What is the purpose of transformer oil?

[ Transformer Oil is a highly refined mineral oil that is stable at high temperatures and has
excellent electrical insulating properties. Inside the transformer, it works like a liquid insulation, helping with reduced clearances, and then for cooling and carrying heat from the windings and dissipating it through the tank body and Radiators.

4. Why transformer oil filtration is required?

Being a mineral oil, even fresh oil has certain level of acidity, which when it comes in contact with the air, reacts, creating sludge. At higher temperatures, the activity is faster. Secondly, being mineral oil, it has a tendency to absorb moisture, which in turns reduces the dielectric strength of transformer oil.

The most important job of filtration is

  1. To improve the Insulation Resistance of the Transformer oil.
  2. To remove moisture, solid contamination or sludge& gases.
  3. Filtering process can reduce the acidity of oil to a great extent, thereby improving viscosity, specific resistivity, ageing characteristics, etc., hence extending the life-span of the oil.