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Synchronous Generator MCQs – Alternator Questions

Synchronous Generator MCQs – Alternator Questions

Synchronous generator or alternator is AC electrical machine used to convert mechanical power to ac electric power. In a synchronous generator, the DC current is applied to the rotor winding. The dc current produced a rotor magnetic field. The rotor of the generator is turned by prime mover thus producing rotating magnetic field within the machine. The rotating magnetic field in turn induced a three-phase set of voltage in the stator windings of the generator.

Synchronous Generator is also known as

Synchronous Generator is also known as:

  1. Alternator
  2. DC Generator
  3. Rosenberg generator
  4. None of these

Correct answer: 1. Alternator

Explanation: The term alternator is used for the generator since it produces AC power. It is also known as a synchronous generator because it must be driven at synchronous speed in order to produce AC power of the desired frequency.

The field winding of alternator is excited with

The field winding of alternator is excited with:

  1. AC
  2. DC
  3. Both of these
  4. None of these

Correct answer: 2. DC

Explanation: The term field winding is referred to the winding which produces main magnetic field in any machine. In synchronous generator, field windings are located on rotor. The term rotor winding and field windings are interchangeably used. In alternator a DC current should be applied to the field circuit of rotor in order to produce rotating magnetic field.

The magnetic pole on rotor of alternator are of

The magnetic pole on rotor of alternator are of:

  1. Salient type construction
  2. Non-salient type construction
  3. Both of these
  4. None of these

Correct answer: 3. Both of these

Explanation: The magnetic poles on the rotor can be of either salient or non-salient construction.

Salient construction: The term salient implies “protruding” or “sticking out”. Such a salient pole is a magnetic pole that sticks out from the surface of the rotor. Salient construction is used for rotors with four or more poles.

Non-salient construction: Nonsalient pole is a magnetic pole constructed flush with the surface of the rotor. Such rotor construction is normally used for two- and four-pole rotors.

The armature winding of an alternator is generally

The armature winding of an alternator is generally

  1. Star connected
  2. Delta connected
  3. Both of these
  4. None of these

Correct answer: 1. Star connected

Explanation: In an alternator armature winding refers to the stator winding wherein the main voltage is induced. Alternator is generally connected in star configuration primarily because of economical operation and availability of neutral point.

Economical operation: The phase voltage is 0.57 times of the line voltage resulting in less voltage stress and lower insulation costs. Considering the insulation, breakdown strength and conductor material requirement the overall operation becomes more feasible in terms of economy.

Neutral availability: The neutral point can be grounded in order to provide path of circulating currents. The availability of natural point is one of most important factors for an alternator. This neutral connection provides elimination/reduction of harmonics, and a path for zero sequence currents.

The output frequency of synchronous generator depends on

The output frequency of synchronous generator/alternator depends on

  1. Rotational speed
  2. Pole number
  3. Both of these
  4. None of these

Correct answer: 3. Both of these

Explanation: The output frequency of alternator can be expressed mathematically by the formula:

f = P x N / 120

The output frequency of alternator is dependent on both pole number as well as rotational speed of alternator.

Most cost-effective method supplying dc power in smaller synchronous machines is via

Most cost-effective method supplying dc power in smaller synchronous machines is via:

  1. Slip rings and brushes
  2. DC power source mounted on shaft
  3. Auto-transformer
  4. Any of above

Correct answer: 1. Slip rings and brushes

Explanation: Slip rings are the metal rings that completely encircle shaft of a machine however are insulated from it.

One end of dc rotor winding is tied to each of the two slip rings on the shaft of the synchronous machine.

The brush is made of carbon compound similar to graphite. A stationary brush rides on each slip ring.

The brush conducts electricity freely but has very low friction so that it doesn’t wear down the slip ring. The application of slip ring and brush demands more maintenance in machine. However for small synchronous machines, it presents most cost-effective method for applying dc field current.

If number of poles of alternator is increased, the generated EMF

If number of poles of alternator is increased, the generated EMF:

  1. Increases
  2. Decreases
  3. Remains constant
  4. None of these

Correct answer: 1. Increases

Explanation:

EMF generated per phase E = 2.22 fZΦ

f = NP/120

In armature winding of an alternator, the coil span falls short of full-pitch by 60° (electrical). The pitch factor will be

In armature winding of an alternator, the coil span falls short of full-pitch by 60° (electrical). The pitch factor will be:

  1. 0.25
  2. 0.5
  3. 0.866
  4. None of these

Correct answer: 3. 0.866

Explanation: Pitch factor (Kp) = cos (α/2) = cos(60°/2) = 0.866

The rating of an alternator is expressed in

The rating of an alternator is expressed in:

  1. HP
  2. kW
  3. kVA
  4. kVAR

Correct answer: 3. kVA

Explanation: kVA is kilo volt-amperes, wherein a volt-ampere is the unit for the apparent power in any circuit. The apparent power is defined as the product of root mean square voltage and root mean square current. Volt-amperes are usually used for analyzing alternating current (AC) circuits. Since alternator is an AC machine, the rating of an alternator is expressed in kVA.

An alternator operates on principle of

An alternator operates on principle of

  1. Self-induction
  2. Mutual induction
  3. Self or mutual induction
  4. Electromagnetic induction

Correct answer: 4. Electromagnetic induction

Explanation: An alternator operates on the principle of electromagnetic induction.

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