Free Electrical Machines MCQs with Answers

• A. 200 Hz
• B. 50 Hz
• C. 2 Hz
• D. 0.2 Hz

Explanation: Rotor emf frequency is f2 = sf1, so f2 = 0.04 × 50 = 2 Hz. Rotor frequency is much lower than the 50 Hz supply frequency during normal…

• A. less than 1%
• B. 1.5%
• C. 2%
• D. 4%

Explanation: At no load, the rotor runs close to synchronous speed, so only a small slip is needed to produce the torque required for losses.

• A. high speed steel
• B. stainless steel
• C. carbon steel
• D. cast iron

Explanation: The shaft must withstand torsional stress, so carbon steel is commonly used because it is strong and tough.

• A. silicon steel
• B. cast iron
• C. aluminium
• D. bronze

Explanation: Cast iron is commonly used for induction-motor frames because it provides mechanical strength, rigidity and good protection.

• A. only if the load current is zero
• B. only if the load current is not zero
• C. irrespective of the value of load current
• D. none of the above

Explanation: A series generator needs load current to flow through its series field winding before that winding can build useful flux and voltage.

• A. Lenz's law
• B. Ohm's law
• C. Faraday's law of electromagnetic induction
• D. none of the above

Explanation: A generator produces emf when conductors cut magnetic flux, as described by Faraday's law of electromagnetic induction.

• A. will be less than 250 V
• B. will always be 250 V
• C. may be greater or less than 250 V
• D. none of the above

Explanation: The full-load voltage depends on the compounding: a cumulatively compounded generator may maintain or raise voltage, while a…

• A. 150 V
• B. less than 150 V
• C. greater than 150 V
• D. none of the above

Explanation: Increasing load current strengthens the series-field flux, so the generated emf of a series generator generally rises with load current…

• A. only if the resistance of the field circuit is less than critical value
• B. only if the resistance of the field circuit is greater than critical value
• C. irrespective of the value of the resistance in the field circuit
• D. none

Explanation: Self-excitation requires the shunt-field resistance to be below the critical resistance at the operating speed, allowing residual…

• A. 100% regulation
• B. infinite regulation
• C. 50% regulation
• D. 1% regulation

Explanation: Voltage regulation measures the change from no-load to full-load voltage, so a smaller percentage is preferred for a generator.

• A. same kW rating
• B. the same operation r.p.m.
• C. the same drooping voltage characteristics
• D. same percentage regulation

Explanation: Drooping voltage characteristics make the load division between parallel generators stable: a generator that takes more current develops a…

• A. to increase the series flux
• B. to increase the generated e.m.f.
• C. to reduce the combined effect of ar-mature reaction of both the machines
• D. so that the two identical machines will pass approximately equal cur¬rents to the load

Explanation: The equalizer bar connects the armature circuits so that the series-field effects remain balanced, allowing identical generators to share…

• A. Shunt generators
• B. Series generators
• C. Compound generators
• D. None of the above

Explanation: Shunt generators have a naturally drooping external characteristic, which gives stable load sharing when units operate in parallel.

• A. Self-excited generator
• B. Separately excited generator
• C. Level compounded generator
• D. All of the above

Explanation: A level-compounded generator is designed so that the rise in generated emf offsets internal voltage drops as load increases, keeping…

• A. reverse the field connections
• B. increase the field resistance
• C. increase the speed of primemover
• D. check armature insulation resistance

Explanation: A self-excited generator needs residual magnetism of the correct polarity to begin voltage build-up; reversing the field connections…

• A. their rising voltage characteristics
• B. unequal number of turns in their series field windings
• C. unequal speed regulation of their primemovers
• D. unequal series field resistances

Explanation: Flat and over-compounded generators have voltage characteristics that do not share load stably, particularly because the over-compounded…

• A. to increase the speed and hence generated e.m.f.
• B. to increase the series flux
• C. so that two similar machines will pass approximately equal currents to the load
• D. to reduce the combined effect of ar-mature reaction of both machines

Explanation: In parallel series generators, the equalizer bar provides a common connection for the armature currents and keeps the series-field effects…

• A. rising voltage characteristics
• B. identical voltage characteristics
• C. drooping voltage characteristics
• D. linear voltage characteristics

Explanation: The drooping voltage characteristic of a shunt generator gives negative feedback in parallel operation: the unit taking extra current…

• A. 40 percent
• B. 25 percent
• C. 10 percent
• D. 5 percent

Explanation: The demagnetizing component of armature reaction usually reduces the main air-gap flux by only a small amount, commonly approximated as…

• A. field
• B. brushes
• C. armature
• D. load

Explanation: Critical resistance is the maximum resistance of the field circuit at which a self-excited generator can build up voltage.