Free Electrical Machines MCQs with Answers

• A. diesel engine
• B. hydraulic turbine
• C. steam turbine
• D. gas turbine

Explanation: Hydraulic turbines, especially those used with large water heads, generally operate at lower speeds than steam, gas and diesel prime…

• A. To excite the poles of main exciter
• B. To provide requisite starting torque to main exciter
• C. To provide requisite starting torque to generator
• D. None of the above

Explanation: A pilot exciter is a small auxiliary generator whose output supplies the field winding of the main exciter.

• A. Induction motor
• B. Under excited synchronous motor
• C. Over excited synchronous motor
• D. D.C. generator

Explanation: A synchronous condenser is a synchronous motor running without mechanical load and operated with overexcitation.

• A. water cooled
• B. natural air cooled
• C. forced air cooled
• D. hydrogen cooled

Explanation: Large turbo-generators are generally hydrogen-cooled because hydrogen removes heat effectively while producing less windage loss than air.

• A. Supercharged engine
• B. Diesel engine
• C. Petrol engine
• D. All of the above engines will equal power

Explanation: For equal cylinder size and speed, a petrol engine generally develops less power because its mean effective pressure is lower than that of…

• A. kW
• B. kVAR
• C. kWh
• D. kVA

Explanation: A transformer is rated in kVA because its heating is determined by voltage and current, while its losses do not directly depend on the…

• A. Electrical power developed in armature - copper losses
• B. Mechanical power input - iron and friction losses
• C. Electrical power developed in armature - iron and copper losses
• D. Mechanical power input - iron and friction losses - copper losses

Explanation: Generator output is obtained from mechanical input after rotational losses such as friction and iron losses, followed by subtraction of…

• A. It will rotate at the same speed as that with its field winding closed
• B. It will rotate at less speed as that with its field winding closed
• C. It will rotate at dangerously high speed
• D. None of these

Explanation: With the shunt field open, only weak residual magnetism remains, so the motor can accelerate to an excessively high and unsafe speed.

• A. when under loaded
• B. while over-excited
• C. only at synchronous speed
• D. below or above synchronous speed

Explanation: Synchronous torque is produced when the rotor is locked magnetically with the rotating stator field, which occurs at synchronous speed.

• A. the rotor excitation only
• B. the supply voltage only
• C. the rotor excitation and supply volt-age both
• D. the rotor excitation, supply voltage and maximum value of coupling angle (90°)
• E. none of the above

Explanation: For a cylindrical synchronous motor, maximum power is proportional to EV sin 90° divided by synchronous reactance, where E depends on…

• A. directly proportional to the square of the applied voltage
• B. directly proportional to applied voltage
• C. inversely proportional to applied voltage
• D. none of the above

Explanation: The breakdown or pull-out torque corresponds to the maximum power angle and is proportional to EV/Xs.

• A. static condenser
• B. condenser
• C. synchronous condenser
• D. none of the above

Explanation: A synchronous motor running without mechanical load and over-excited draws leading current, supplying reactive power to the system.

• A. the stator current will be zero
• B. the stator current will be very small
• C. the stator current will be very high
• D. the back e.m.f. will be more than the supply voltage
• E. none of the aboveElectromechanical Devices

Explanation: At no load with negligible losses, the motor needs no real power, so its ideal stator current is zero.

• A. rotor and stator teeth
• B. rotor and the stator poles of opposite polarity
• C. rotor and the stator poles of the same polarity
• D. none of the above

Explanation: The load angle is the angular displacement between the rotor magnetic axis and the stator resultant field.

• A. locked-rotor torque
• B. synchronous torque
• C. pull up torque
• D. reluctance torque

Explanation: Locked-rotor torque is the maximum torque available when the rotor is stationary, considering the possible rotor angular positions at…

• A. lower stability limit
• B. high stability limit
• C. good speed regulation
• D. good voltage regulation
• E. none of the above

Explanation: A low short-circuit ratio means a relatively high synchronous reactance, reducing the maximum power-transfer angle and hence the stability…

• A. pull-up torque
• B. pull-in torque
• C. pull-out torque
• D. none of the above

Explanation: Pull-in torque is the maximum constant load torque at which a synchronous motor can accelerate and lock into synchronism under the…

• A. a series motor
• B. an induction motor
• C. an alternator
• D. a rotary converter

Explanation: A synchronous motor has the same basic construction as an alternator: a three-phase stator armature and a DC-excited rotor field system.

• A. no-load and greatly over-excited fields
• B. no-load and under-excited fields
• C. normal load with minimum excitation
• D. normal load with zero excitation

Explanation: For power-factor correction, a synchronous motor is run lightly loaded or at no load with over-excitation, causing it to draw leading…

• A. Eddy current losses in the conductors
• B. Iron losses in the stator
• C. Copper losses in the slot portion of the conductors
• D. Windage losses
• E. None of the above

Explanation: Windage loss is caused by air friction from the rotating rotor and is not dissipated by the stator core surface.