10 Interview Questions and Answers on Motors - Part-2

Q1: What causes a motor to turn?

A1: There are two parts to a motor: a stator, or field, and a rotor, or armature. Around one part there exists a magnetic field from the line current, and in the other part there is an induced current that causes a magnetic field of opposite polarity. These magnetic fields repel one another, thereby causing the rotating member to turn.

Q2. Why would you use a phase splitter and a three-phase motor instead of a single-phase motor?

A2: It is possible that at the moment single-phase power is all that is available, but in the future three-phase power is expected. Therefore, if you purchase the three-phase motor and a phase splitter, the wiring will be in place, the motor will be at the desired location, and the expenses will be cut down. There is also less maintenance on three-phase motors; this one fact will often influence the use of a phase splitter.

Q3. How is regulation obtained in a motor?

A3: The motor at start is similar to a transformer with a shorted secondary. The current in the rotor and stator will be high. As the motor approaches its rated speed, the rotor induces a voltage into the stator in opposition to the line voltage; this is called counter emf. The line current is then reduced in proportion to the speed.

Q4. Does an ac motor (other than a synchronous motor) run at synchronous speed?

A4: No. It must slip below synchronous speed so that an effective voltage will be produced.

Q5. What is a synchronous motor?

A5: A synchronous motor is almost exactly the same as an alternator. The field must be excited by direct current. The motor runs at the same speed or at a fixed multiple of the speed of the alternator supplying the current for its operation. Should it slip, the motor will pull out and stop since it must run pole for with pole the alternator.

Q6. What is a synchronous capacitor?

A6: It is a synchronous motor running without mechanical load on the line, with its field overexcited for power-factor correction.

Q7. If the field on a dc motor were opened, what would happen?

A7: The motor would try to run away with itself, or, in other words, the motor would reach a very high speed and might destroy itself.

Q8. What is a universal motor?

A8: This is a motor built like a series dc motor. However, the stator and armature are both laminated, designed for high speeds, and may be used on either ac or dc, although the speed and power will be greater on dc.

Q9. How can the direction of rotation of a universal motor be changed?

A9: By reversing either the field leads or the armature leads, but not both.

Q10. How can the direction of rotation of a dc motor be changed?

A10: By reversing either the field leads or the armature leads, but not both.

Related Post: 

1. 13 Most Important Question and Answers on AC Motor - Part-1
2. 18 Most Important Question and Answers on DC Motors - Part-1.

Read More

Why is it preferable to generate alternating current rather than direct current?

Alternating current can be changed in voltage by means of transformers. This is necessary because to transmit power over any distance it must be at high voltage. Too much power is lost when transmitting at low voltage. Direct current cannot be changed in voltage without first changing it to alternating current and then raising the voltage; the operation must then be reversed at the receiving end.

Read More

On an alternator, does the dc part (the poles) or the ac part (the conductors) rotate?

It is immaterial which part rotates. However, the dc field is usually made the rotating part, and the stator is usually the ac part of the device. This is because the dc field excitation can be of relatively low voltage, and it is easier to insulate a rotating part for low voltage than for high voltage. The ac output is usually a much higher voltage, and it is much more practical to insulate the stator for the high voltage. Also, with this arrangement there are

no brushes required on the output side.

Read More

If transformers with different electrical characteristics are connected in parallel, what will happen?

They won’t distribute the load equally; one transformer will tend to assume more of the load than the other. This leads to overheating and, in severe cases, the destruction of the transformer(s).

Read More

What is an induction regulator?

This device is similar to a booster transformer. It has a primary and a secondary winding, which are wound on separate cores. The primary can be moved in either direction; this is usually done by an electric motor. In turning, the primary bucks or boosts the line voltage, as required. The amount of bucking or boosting is anticipated by the current being drawn by the line.

Read More

What is the relationship between the current and voltage in the high side of a transformer and the current and voltage in the low side of a transformer? Draw a diagram showing this relationship.

With respect to the turns ratio, the current in one side of a transformer is inversely proportional to the current in the other side, whereas the voltage across one side of a transformer is directly proportional to the voltage across the other side. These are illustrated in Figure-1.

Read More

12 Most Common Interview Questions and Answers on Transformer - Part 2

Q. 1: What is induction?

A. 1: The process by which one conductor produces, or induces, a voltage in another conductor, even though there is no mechanical coupling between the two conductors.

Q. 2: What is inductance?

A. 2: The property of a coil in a circuit to oppose any change of existing current flow.

Q. 3: What is self-inductance?

A. 3: The inducing of an emf within the circuit itself, caused by any change of current within that circuit. This induced emf is always in a direction opposite to the applied emf, thus causing opposition to any change in current within the circuit itself.

Q. 4: What is mutual inductance?

A. 4: The linkage of flux between two coils or conductors, caused by the current flowing within one or both of the coils or conductors.

Q. 5: Name several methods by which an emf can be generated.

A. 5: By conductors being cut by-

• A magnetic field (as in generators) 
• Chemical reactions (as in batteries)
• Heat (as in thermocouples)
• Crystal vibration (as in piezoelectricity) and 
• Friction (as in static electricity).

Q. 6: What is a transformer?

A. 6: A device that transforms electrical energy from one or more circuits to one or more other circuits at the same frequency but usually at a different voltage and current. It consists of a core of soft-iron laminations surrounded by coils of copper-insulated wire.

Q. 7: Draw a diagram of two coils, such as the coils of a transformer winding, and indicate the self-inductance and the mutual inductance.

A. 7: Self-inductance is produced within the primary coil, and mutual inductance exists between the two transformer coils, as shown bellow this Figure:

Figure: Self-inductance and mutual inductance in the coils of a Transformer.

Q. 8: What always surrounds a conductor when a current flows through it?

A. 8: A magnetic field.

Q. 9: There are two basic types of transformers. What are they?

A. 9: The isolation type, in which the two windings are physically isolated and electrically insulated from each other, and the autotransformer type, in which there is only one coil with a tap or taps taken off it to secure other voltages (the primary is part of the secondary and the secondary is part of the primary).

Q. 10: What is an oil-immersed transformer?

A. 10: The core and coils are immersed in a high-grade mineral oil, which has high dielectric qualities.

Q. 11: What is Transformer Oil?

A. 11: Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil-filled transformers, some types of high-voltage capacitors, fluorescent lamp ballasts, and some types of high-voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Transformer oil is most often based on mineral oil, but alternative formulations with better engineering and/or environmental properties are growing in popularity.

Q. 12:  Why is oil used in a transformer?

A.12: To increase the dielectric strength of the insulation, to keep down the possibility of arcing between coils, and to dissipate heat to the outer case so that the transformer can carry heavier loads without excessive overheating.

Courtesy: 

1. Book of Paul Rosenberg
2. Wikipedia
3. Internet.

Download this Article as PDF:

Read More: 10 Most Common Interview Questions and Answers on Transformers - Part-1         

Read More

What is optical fiber cable? What are the advantages of optical fiber cable?

Optical fiber cable:

An optical fiber cable is a cable containing one or more optical fibers that are used to carry light. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed. Different types of cable are used for different applications, for example long distance telecommunication, or providing a high-speed data connection between different parts of a building.

Figure: Optical Fiber Cable.

Advantages of optical fiber cable:

• High Bandwidth Over Long Distances - Fiber optics have a large capacity to carry high speed signals over longer distances without repeaters than other types of cables. The information carrying capacity increases with frequency. This, however, doesn't mean that optical fiber has infinite bandwidth, but it's certainly greater than coaxial cables. 
• Less signal degradation - The loss of signal in optical fiber is less than in copper wire. 
• Data Security - Optical fibers are difficult to tap. As they do not radiate electromagnetic energy, emissions cannot be intercepted. As physically tapping the fiber takes great skill to do undetected, fiber is the most secure medium available for carrying sensitive data.  
• Safety - Since the fiber is a dielectric, it does not present a spark hazard. 
• Ease Of Installation - Increasing transmission capacity of wire cables generally makes them thicker and more rigid. Such thick cables can be difficult to install in existing buildings where they must go through walls and cable ducts. Fiber cables are easier to install since they are smaller and more flexible. They can also run along the same routes as electric cables without picking up excessive noise. 
• Light, flexible and cheap - Optical fiber cable can be used in corrosive environments and is light, flexible and cheap. 
• Non-flammable - Because no electricity is passed through optical fibers, there is no fire hazard.

Read More: What are the various kind of cables used for transmission?

Read More

What are the various kind of cables used for transmission?

Cables, which are used for transmitting power, can be categorized in three forms:

• Low-tension cables, which can transmit voltage upto 1000 volts.
• High-tension cables can transmit voltage upto 23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.

Read More: What is optical fiber cable? What are the advantages of optical fiber cable?

Read More

How do synchronous motors differ from alternators?

Figure: Diagram of a simple alternator with a rotating magnetic core (rotor) and stationary wire (stator) 

They may be just like alternators; however, if they are, they won’t be self-starting and will have to be started by some means until they approach synchronous speed, at which time they can be connected to the line and pull into speed. Most synchronous motors have a squirrel cage winding in addition to the dc field. They start as a squirrel cage motor, and when they are about up to the speed of the alternator, the dc field is energized. The poles then lock in position with the revolving field of the armature, and the rotor revolves in synchronization with the supply circuit.

Read More

What are the advantages and disadvantages of dc motors compared with ac motors?

Advantages and disadvantages of DC motors compared with AC motors:

Advantages of DC Motor:

• Speed control over a wide range both above and below the rated speed: Speed control of DC motors is much easier, making them more versatile for use where a wide range of speeds is required. 
• They may be used for dynamic braking; that is, a motor on an electric train will act as a motor when required, but when going downhill, it can be used as a generator, thereby putting current back into the line. 
• Free from harmonics, reactive power consumption and many factors which make DC motors more advantageous compared to AC induction motors.
• In generating, it requires power, so it acts as a brake. 
• Quick starting, stopping, reversing and acceleration.

Disadvantages of DC Motor:

• High initial cost.
• DC motors, however, require more maintenance than most AC motors. 
• Cannot operate in explosive and hazardous conditions due to sparking occur at brush (risk in commutation failure).
• In order to use a DC motor, special provisions must be made.

Applications:

Some of the applications of DC motors are Steel mills, paper mills, cranes and elevators, electric trains and much more etc.

Read More

What is the difference between a dc motor and a dc generator?

Fundamentally, there is none. A dc motor will generate electricity if driven by some prime mover, and a dc generator will act as a motor if connected across a dc source.

Read More:  If a dc motor is connected across an ac source, what will be the result? Why?

Read More

If a dc motor is connected across an ac source, what will be the result? Why?

Direct-current flow is obstructed only by resistance, whereas alternating current is obstructed by both resistance and inductive reactance. Therefore, when a dc motor is connected across an ac source, the current on ac will be much less than that of dc. The motor would run, however, but it would not carry the same load as it would on dc. There would be more sparking at the brushes. The armature is made up of laminations, but the field is not. The eddy currents in the field would therefore cause the motor to heat up and eventually burn out on ac; this would not happen on dc.

Read More

What are the conditions for parallel operation of alternators?

Figure: Parallel operation of shunt generators.

To synchronize AC generators, several important factors must be checked:

• Equal terminal voltages- The terminal voltage of the incoming machine must be approximately equal to bus-bar voltage. This is obtained by adjustment of the incoming generator’s field strength.

• Equal frequency- The frequency of the incoming machine must be equal to those bus-bars. This is obtained by adjustment of the incoming generator’s prime-mover speed.

• Phase voltages in proper phase relation- The phase of the incoming machine voltage must be the same as that of the bus-bar voltage relative to the load i.e. the phase voltages of the incoming machine and the bus-bar should be in phase opposition. This implies that there will be no circulating current between the windings of the alternators already in operation (the bus-bars) and the incoming machine.

Note: The above figure shows the shunt generator No.1 is connected to the bus-bars and delivering load. The shunt Generator No.2 is connected in parallel to the Generator No.1. When the load on the generator No.1 increases beyond its rated capacity, immediately the second shunt generator operate and wish the first generator to come across the raised load demand.

Read More

13 Most Important Questions and Answers on AC Motor - Part-1

Figure: Asynchronous AC Motor (Induction)

Q1. What are the three basic types of ac motors?

A1. Series, synchronous, induction.

Q2. Series motors are generally used to operate what type of equipment?

A2. To power small appliances.

Q3. Why are series motors sometimes called universal motors?

A3. They operate on either ac or dc.

Q4. What determines the number of field poles required to establish a rotating magnetic field in a multiphase motor stator?

A4. The number of phases in the applied voltage.

Q5. What is the angular displacement between field poles in a two-phase motor stator?

A5. 90º .

Q6. What is the major difference between a two-phase and a three-phase stator?

A6. Number and location of field poles.

Q7. What requirement is the synchronous motor specifically designed to meet?

A7. Constant speed required by some loads.

Q8. Why is the ac induction motor used more often than other types?

A8. They are simple and inexpensive to make.

Q9. The speed of the rotor is always somewhat less than the speed of the rotating field. What is the difference called?

A9. Slip.

Q10. What determines the amount of slip in an induction motor?

A10. Load.

Q11. What type of ac motor is most widely used?

A11. Single-phase induction motor.

Q12. How do split-phase induction motors become self-starting?

A12. By using combinations of inductance and capacitance to apply out-of phase currents in starting windings.

Q13. Why are shaded-pole motors used to drive only very small devices?

A13. They have very weak starting torques.

Download this Article as PDF:

Related Post: 

1. 18 Most Important Questions and Answers on DC Motors - Part-1.
2. 10 Interview Questions and Answers on Motor - Part-2

Read More

21 Common Interview Questions & Answers on AC Generators - Part-2

Figure: Three Phase Generator Output

Q1. Magnetic induction occurs when there is relative motion between what two elements?

A1. A conductor and a magnetic field.

Q2. What is the part of an alternator in which the output voltage is generated?

A2. Armature.

Q3. What are the two basic types of alternators?

A3. Rotating armature and rotating field.

Q4. What is the main advantage of the rotating field alternator?

A4. Output voltage is taken directly from the armature (not through brushes or slip rings).

Q5. Most large alternators have a small dc generator built into them. What is its purpose?

A5. To provide dc current for the rotating field.

Q6. How are alternators usually rated?

A6. Kilovolt-amperes (volt amperes).

Q7. What type of prime mover requires a specially designed high-speed alternator?

A7. Steam turbine.

Q8. Salient-pole rotors may be used in alternators driven by what types of prime movers?

A8. Internal combustion engines, water force and electric motors.

Q9. What does the term single phase indicate?

A9. One voltage (one output).

Q10. In single-phase alternators, in order for the voltages induced in all the armature windings to add together for a single output, how must the windings be connected?

A10. In series.

Q11. What determines the phase relationship between the voltages in a two-phase ac generator?

A11. Placement of armature coils.

Q12. How many voltage outputs are available from a two-phase three-wire alternator?

A12. Three.

Q13. What is the relationship of the voltage at C in figure 3-7 to the voltages at A and B?

A13. C is 1.414 times greater than A or B.

Q14. In a three-phase alternator, what is the phase relationship between the individual output voltages?

A14. Each phase is displaced 120º from the other two.

Q15. What are the two methods of connecting the outputs from a three-phase alternator to the load?

A15. Wye and Delta.

Q16. Ships’ generators produce 450-volt, three-phase, ac power; however, most equipment uses 117volt, single-phase power What transformers and connections are used to convert 450-volt, three-phase power to 117-volt, single-phase power?

A16. Three single-phase, delta-delta, step-down transformers.

Q17. What two factors determine the frequency of the output voltage of an alternator?

A17. Speed of rotation and number of poles.

Q18. What is the frequency of the output voltage of an alternator with four poles that is rotated at 3600 rpm?

A18. 120 Hz.

Q19. The variation in output voltage as the load changes is referred to as what? How is it expressed?

A19. Voltage regulation. As a percentage.

Q20. How is output voltage controlled in practical alternators?

A20. By varying the voltage applied to the field windings.

Q21. What generator characteristics must be considered when alternators are synchronized for parallel operation?

A21. Output voltage, frequency, and phase relationships.

Download this Article as PDF:

Related Post: 

1. 16 Common Interview Questions and Answers in Generators - Part-1
2. 37 Interview Questions and answers on DC Generators - Part-3

Read More