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...

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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...

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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...

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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)...

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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...

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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. ...

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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...

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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...

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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...

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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...

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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...

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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...

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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...

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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...

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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...

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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....

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