Electrical Interview question and answers

1) What are the most typical reasons for transformer humming?

Stray magnetic fields force the enclosure and accessories to vibrate, generating electric hum around transformers. magnetostriction is a secondary source of vibration that arises when the core iron is subjected to magnetic fields and undergoes minute form changes.

The core generates transformer noise. The amount of noise is generally determined by the transformer's design.

2) What is the voltage regulation of the transformer & why is it important?

A transformer's voltage regulation is the percentage change in secondary voltage from no load to full load. The voltage regulation is 0 if the secondary voltage remains constant throughout the load. However, in practice, it changes with the power factor of the load.

The voltage regulation value determines the transformer's efficiency, thus it is ideal to use a transformer with low voltage regulation.

3) There is a Transformer and an Induction Machine. Those two have the same supply. For Which Device the load current will be maximum and why?

For same rating and same loading, the losses occurred in both devices will be different because of its construction and application.

1) Because the transformer, unlike an induction motor, has no moving parts, less magnetizing current is required for the same load action. Induction motors, on the other hand, have an air gap between their primary (stator) and secondary (rotor) windings, which requires greater magnetizing current due to higher leakage reactance than a transformer.

2) Windage losses will have to be compensated for by the induction motor. Because of the rotation of the rotor to generate the same output. As a result, the induction motor consumes more load current than the transformer.

4) How many types of cooling system are there in Transformers?

The types are

1. ONAN (oil natural,air natural).

2. ONAF (oil natural,air forced).

3. OFAF (oil forced,air forced).

4. ODWF (oil direct,water forced).

5. OFAN (oil forced,air forced).

 

5) What is an Ideal Transformer?

An ideal transformer is a fictitious transformer in which no losses occur. To put it another way, the transformer The input power is equal to the output power of the transformer, implying that they are 100% efficient. Because a real transformer must have some losses, it is only a theoretical transformer. Transformer power input = Transformer

Output Power.PIN = POUT. 

6) What is Plugging Breaking?

The supply connections are reversed in this approach, which causes the generator torque to reverse, which resists the usual rotation of the motor and causes the speed to drop. External resistance is also put into the circuit during plugging to reduce the flowing current. The biggest downside of this technology is that it wastes power.

7) What is dynamic breaking?

In this method of braking, the running motor is separated from the source and linked across a resistance. When the motor is unplugged from the power supply, the rotor continues to rotate owing to inertia, acting as a self-excited generator. When the motor is used as a generator, the current and torque flow reverse. Sectional resistances are cut out one by one during braking to maintain constant torque.

8) What Is Meant By Armature Reaction?

Armature reaction refers to the influence of armature flux on main flux. The armature flux influences the main field flux in two ways:The main field flux is distorted by the armature reaction.The magnitude of the primary field flux is reduced.

9) Which Motor Has High Starting Torque And Staring Current Dc Motor, Induction Motor Or Synchronous Motor?

The DC series motor has the highest beginning torque of any motor, which is why it is employed in electrical apparatus requiring strong starting torque, such as cranes and hoists.

10) What is a Universal Motor?

A universal motor can be powered by a direct current (DC) or a single phase alternating current (AC) supply. When a DC supply is connected to the universal motor, it operates as a DC series motor.An electromagnetic field is created when electricity travels through the field winding. The same current runs through the armature conductors as well. A mechanical force is experienced by a current carrying conductor when it is put in an electromagnetic field. The rotor begins to revolve as a result of this mechanical force, or torque. The direction of this force is determined by fleming's left hand rule.

It still creates unidirectional torque when powered by an alternating current supply. Because the armature and field windings are linked in series, they are in the same phase.As a result, as the polarity of the alternating current changes on a regular basis, the current direction in the armature and field winding reverses at the same time.

Thus, direction of magnetic field and the direction of armature current reverse in such a way that the direction of force experienced by armature conductors remains same. Thus, regardless of whether the power source is AC or DC, Universal Motors operate on the same principles as DC series motors.