Top 25 Electrical Machine Interview Questions and Answers in 2022

 The electrical field is a growing one, and with this comes the need for qualified individuals. If you are looking to gain entry into this industry, or are already in and want to further your chances of landing an interview, then you’ve come to the right place.

The interview is the most crucial part of the job application process. If you do well in the interview, your chance of getting the job is higher. The more practice you can get with your interview skills, the better your chance of passing your interview. This article will cover some basic questions you will most likely meet during the process.

1. How Is Eddy Current LAoss Minimized?

The iron core of an equipment’s variable flux cathode generates eddy currents, which lowers efficiency and increases power usage. Laminating the core, obstructing the eddy current’s course, or raising the core’s resistance can minimize eddy current loss.  And the center is laminated and insulated with paper or varnish. Thus, using thin wire or wire with high resistance is an effective way to reduce Eddy current losses.

2. What Is A Circuit Breaker? Explain The Working Principle Of A Circuit Breaker.

A circuit breaker is an electrical safety device that impedes the flow of current in an electric circuit.  A circuit breaker has two contacts— Fixed and moving contacts.

When the circuit is closed, the contacts engage and carry current around the circuit.  Once the circuit detects a fault, the circuit breaker opens its connections to interrupt the course.

During this condition, the circuit opens, producing an arc between the contacts.  The circuit breaker contacts must withstand the arc heat produced without excessive heating.

The arc is contained, cooled, and extinguished by an arc-quenching medium.  Examples of arc quenching media are oil, air, and vacuum.

3. What Are The Components Of A Transistor?

Transistors comprise combinations of n-type and p-type semiconductor materials, with at least three terminals for connection to an electric current.  The transistor has three electric leads: the emitter (or source), the base (or gate), and the collector (or drain).  The transistor controls a large amount of current using a small amount of current.

4. What Is The Role Of A Transistor In A Circuit?

The transistor is a semiconductor device that amplifies or switches electronic signals and electric power.  The transistor allows you to control the current flow through one channel by varying the intensity of a much smaller current flowing through a second channel.

 It can function as an amplifier, switch, voltage stabilizer, signal modulation, and oscillator.  As a switch or gate, it opens the circuit when the current is flowing and closes it when it isn’t.

5. What Is An R-L-C Circuit?

An R-L-C circuit is a circuit that consists of a resistor (R), an inductor (L), and a capacitor (C) linked in series or parallel across an alternating voltage supply.  These three components are passive.  They consume energy rather than produce it.  They also have a linear relationship between voltage and current.

The R-L-C circuit exhibits the property of resonance, but the oscillation dies out quickly as compared to the L-C circuit.  When these components are connected in series, the current in each element remains the same while the total voltage is a vector sum.  In parallel, the applied voltage remains the same across all components, while the total current is a vector sum of all the current flowing in the components.

6. What Is The Difference Between A Magnetic Circuit And A Magnetic Field?

A magnetic circuit is a complete closed path followed by magnetic lines of force called magnetic flux.  The lines of force represent the magnetic field in the magnetic circuit.

The working principle of electrical machines like DC machines, transformers, synchronous machines, induction motors, and generators depends on the magnetism produced by their magnetic circuit.

On the other hand, a magnetic field is a region around a magnet where its poles feel the force of attraction or repulsion.

7. Why Is It Important To Keep Air Gaps In Magnetic Circuits As Small As Possible?

Air gaps are small energy stores in a magnetic circuit.  Magnetic saturation causes loss of inductance, increase in current, and power loss in the circuit.

The air gaps influence the parameters of the magnetic inductor.  They increase the reluctance of the magnetic circuit and prevent core saturation.

They are designed to be as small as possible to avoid an undesirable drop in energy density (losing too much energy).

8. What Type Of DC Machine Employs Wave And Lap Winding?

There are two types of winding—Wave and lap.  D.C machines with low current and high voltage ratings use wave windings.  The wave winding has two parallel paths (the number of parallel paths is definite), there is less sparking at commutation, and it doesn’t require an equalizer ring.

DC machines with high current and low voltage ratings lap winding.  In a lap-connected winding, the number of parallel paths equals the number of poles in a machine. Equalizer rings are used in Iap winding to allow for even distribution.

9. What Is A Transformer?

A transformer is an electrical device that transfers electrical energy from one electrical circuit to another while maintaining a stable frequency.  The working principle of a transformer relies on “Faraday’s law of induction.”

When the current in the primary winding is changed, the magnetic flux changes and induces an EMF in the secondary coil.  A transformer has core and copper losses caused by Eddy and hysteresis.

Copper loss occurs due to heat losses during energy transfer from one electrical circuit to another. Core losses or magnetic losses are the total energy lost during heat generation.

10. What Is An Ideal Transformer?

An ideal transformer is a type of transformer in which no losses occur.  Its windings have no ohmic resistance, leakage flux, and hence no copper or core losses.

The primary and secondary windings of an ideal transformer have negligible resistance, and the magnetic core has unlimited permeability since there is no loss of power within it.

The output power of an ideal transformer is equal to the input power.  And its efficiency is 100%.  It’s impossible to have such transformers in practice.

11. What Is The Difference Between A Stator And A Rotor?

A stator is the stationary part of the system in an AC/DC motor or generator.  It comprises all the non-rotating electrical components of a generator or motor.

The parts of the stator include the frame, core, and winding.  The frame supports the core and protects the three-phase winding.  The core converts the rotating magnetic field to electric current.

The rotor is the rotating part of the AC/DC motor or generator.  It comprises all of the rotating electrical components of a generator or motor.  The rotor is inside the core of the stator.

12. What Is An Autotransformer?

An autotransformer is an electrical device with only one winding wound on a laminated magnetic core.  The same winding acts as the primary and secondary winding sides of the transformer.

Autotransformer automatically regulates the voltage of transmission lines.  Also, autotransformers are used to transform voltages depending on the load.

Autotransformers are more advantageous than the usual dual-winding transformers.  They are lighter, small, and cheaper.  In Autotransformers, ohmic and core losses are minimal.  And they have high efficiency.

13. What Are The Applications Of Autotransformers?

Autotransformers have a vast application.  In the power transmission system, they are used to interconnect the power system at different voltage classes like 132 kV and 230 kV.  In distribution systems, they are used to boost the supply voltage. Autotransformers can also be used for starting induction and synchronous motors and as variable a.c.

14. What Is A Zener Diode?

A Zener diode is a semiconductor device that operates in the reverse direction.  It allows current flow from the anode to the cathode and backward when it reaches Zener voltage.

A Zener diode is used as a shunt voltage regulator to regulate the voltage across small loads.  Other applications include surge protectors to limit transient voltage spikes.  They are used to protect circuits from overvoltage and electrostatic discharge.

15. What Is The General System Requirement Of Alternators?

For the alternator to generate emf, two systems are required: The magnetic field system and the Armature system.  The magnetic system produces the magnetic field, whereas the armature system houses the conductors on which the emf induces.

16. Why Is A Stationary Armature Better Than A Rotating Field System?

There are several benefits a stationary armature rotating field has over a rotating field system.  With a stationary armature, slip rings and brushes are not required to carry the output current from the terminals.

Also, it is easier to insulate the coils of a stationary armature for high voltage.  It is not subject to vibration or centrifugal forces because it is motionless; hence there are extra spaces for more armature windings.

17. Compare Single-Phase And Three-Phase Supplies.

  • A three-phase power supply transmits three times as much power as a single-phase power supply.  
  • The three-phase system requires less conductor material to transmit power over a distance, as compared to single-phase power supplies.  
  • The size of a three-phase system-operated machine is less than the size of a device operated with a single-phase voltage having the same output rating.  
  • The three-phase system is more straightforward in construction, small in size, and can be started automatically with smooth operation.

18. What Are The Advantages And Disadvantages Of Short- Pitched Winding?

The advantages of short-pitched winding include: it requires a small amount of copper for end connections, improves the waveform of generated emf, and reduces or eliminates high-frequency harmonics.  All these help reduce eddy current and hysteresis loss that will occur and increases efficiency.

A significant disadvantage of the short-pitched winding is that it reduces the total voltage around the coil by a factor of KP.

19. What Conditions Need To Be Satisfied For The Parallel Operation Of DC Generators?

The following are the requirements for parallel operation:

  • The polarity of the generator must be the same.  Otherwise, swap the connections until the polarity is the same.
  • Both generators must have the same voltage.
  • The voltage change should have the same character as the change in load.
  • The positive and negative terminals of the bus bar must be connected to the positive and negative terminals of the generator.
  • The generator’s prime movers must have stable and similar rotational speed characteristics.

20. What Are The Reasons For The Parallel Operation Of Transformers?

For large loads, it’s not ideal to have a single large transformer, so many small transformers are arranged in parallel to meet the load demand.  This parallel transformer operation allows us to add extra load if required.

If required, one can shut down any transformer systems connected in parallel without affecting other system performance.  This feature is advantageous, as it allows for damaged transformers to be removed for repair and maintenance without interrupting the power supply.

21. What Is The Working Principle Of Electric Motors?

An electric motor has a rectangular coil between two magnets.  When an electric current flows through it, a force acts in the coil, producing torque.  The torque, in turn, creates a rotational motion and causes the coil to rotate continuously until the electricity passes through it.

Thus, an electric motor functions on the working principle of the interaction between the electric current and magnetic field.  When a current-carrying conductor is placed in an external magnetic field perpendicular to the conductor, the conductor experiences a perpendicular force on itself and the magnetic field.  The right-hand rule can determine the direction of the force experienced by the conductor.

22. What Are The Advantages And Disadvantages Of The DC Motor?

DC motors come in different sizes and dimensions; hence they are more suited to meet different needs. The smaller ones are used in home appliances, while the larger ones are useful in industrial machines.

Other advantages include:

  • They have high efficiency and a strong starting torque.
  • The speed of a DC motor is adjustable.
  • They have a longer lifespan.
  • DC motors are more efficient.
  • They make better use of their input energy.
  • Simpler Installation and maintenance.
  • Faster response to starting, stopping, and acceleration.
  • They are available in standard voltages.

Disadvantages of a DC motor include:

  • The initial cost of acquisition is high.
  • It requires a controller to manipulate its position, speed, and torque.

23. What Are The Advantages And Disadvantages Of AC motors?

Advantages of AC motors include:

  • They have a long lifespan.
  • AC motors are available in standardized types to suit different applications.
  • Low cost of acquisition.
  • High efficiency.
  • They generate higher torques using a more powerful current.

The disadvantages of an AC motor include:

  • A variable frequency source is required to control its speed.
  • It has a high starting current inrush.

24. Why Is The Efficiency Of A Three-Phase Induction Motor Less Than That Of A Three-Phase Transformer?

A three-phase induction motor is like a rotating transformer.  The stator induces current and voltage in the rotor conductors the same way the primary windings of a transformer will induce current and voltage in the secondary winding.

The only difference between a three-phase induction and a rotating transformer is that the secondary windings of a rotating transformer are stationary, unlike in the three-phase induction motor, where it is free to move.

Thus, the efficiency of a three-phase induction motor is less because its rotating nature allows for friction, winding, and rotational losses.

25. Why Is The Air Gap Between The Pole Pieces, And The Armature Kept Very Small?

The small air gap between the pole pieces and the armature is minimal because of the following reasons:

  • To keep the field strength at a high value.
  • To generate a high voltage.
  • Because a large air gap will require a more substantial air gap to produce the required voltage, hence a need to keep the air gap as small as possible.
  • To allow the flux lines to pass through iron faster than the air gap.

Conclusion

Here are some extra tips to help make your following electrical machine interview a success:

  • Prepare for a variety of situations.
  • Practice answering technical and behavioral questions.  Do not limit yourself to a few questions. The electrical field is very vast and practical.
  • Learn about different types of jobs and careers in the industry.
  • Practice with sample questions.  Sample questions will help you prepare for the actual interview.  You will learn how to answer questions correctly and adequately and without hesitation.
  • Use your experience.  If you have experience with machines, explain what types of devices you’ve worked with and how long you’ve been working with them.

After preparing for an electrical machine interview, what’s next?  Attend the interview, ace it, and land that dream job.  Good luck!