MCQs on Electrical Machines

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01․ The no load and full load speed of DC mototr are 1400 rpm and 1200 rpm respectively. What is the percentage speed regulation of the motor?

% speed regulation = (NnL - NfL) X 100/Nfl.

02․ An electric motor with constant output power will have a torque speed characteristics in the form of a
Circle about the origin.
Rectangular hyperbola.
Straight line through the origin.
Straight line parallel to the speed axis.

In case of motor EaIs = Tφ for the constant output power, Tφ = Constant. So that torque-speed characteristic is the form of straight line parallel to the speed axis.

03․ The output power of any electrical motor is taken from the
Coupling mounted on the shaft.
Motor frame.

The output power of any electrical motor is alway from the coupling mounted on the shaft because the motor converts electrical power to mechanical power.

04․ The output indicate on the name plate of any motor is always the
Power drawn in KW.
Power drawn in KVA.
Output power at the shaft.
Gross power.

In case of electric motor (AC or DC), the mechanical power is available at the shaft in horse power.

05․ When the mechanical power is developed by a DC motor maximum
Back emf is equal.
Back emf is equal to half of applied voltage.
back emf is equal to applied voltage.
None of above.

Mechanical power developed by the motor is given by Pm = EbIa = VIa - Ia2Ra. Mechanical power deeloped by the motor will be maximum. If dPm/dIa is zero. i.e. V - 2Ia = 0 - Ia. Ra = V/2. ⇒ Back emf Eb = V - V/2 = V/2.

06․ The direction of rotation of a DC motor can be determent by
Amperes law.
Flemings left hand rule.
Flemings right hand rule.
Lenzs law.

If a current carrying conductor is placed in a magnetic field, mechanical force is experienced on the conductor, hence the conductor moves in the direction of force is determined by Flemings left hand rule.

07․ In case of a conductively compensated DC series motor, the compensating winding is provided
As a separate unit.
In parallel with the armature winding.
In parallel with the field winding.
In series with the armature winding.

In DC series motor, compensating winding is provided in series with the armature windings which reduce sparking in commutation and prevent field owing to armature reaction.

08․ The DC compound motors are generally
Differentical compound.
Cummulative compound.
Level Compound.
None of these.

The DC commulative compound motor, starting torque is high upto 400% depending the degree of compounding and speed regulation is varying depending upon of compounding. So it is used where high starting torque, fairly constant speed are required. But in case of differently compound motor, the differential arrangement of the windings is a disadvantage in very heavy overload as field may be weakend too much and the motor may tend to race, taking a very large current.

09․ In a DC compound motor the field regulator is provided to
Control the flux.
Demagnetize the field partially.
Limit the armature current.
None of above.

In a DC compound motor, speed control is involving the variation of flux by means of filed regulator.

10․ The armature shaft must be able to with stand
Bending moment due to weight of the armature.
Any unbalanced magnetic pull on the armature core.
Twisting stains due to transmission of torque.
All of the above.

The armature shaft is able to with stand (i) Any unbalanced magnetic pull on the armature core. (ii) Bending moment due to weight of the armature and commutator. (iii) Twisting stains due to transmission of torque