MCQs on Electrical Machines

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 = (N_nL - N_fL) X 100/N_fl.

02․ An electric motor with constant output power will have a torque speed characteristics in the form of a

In case of motor E_aI_s = 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

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

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

Mechanical power developed by the motor is given by P_m = E_bI_a = VI_a - I_a²R_a. Mechanical power deeloped by the motor will be maximum. If dP_m/dI_a is zero. i.e. V - 2I_a = 0 - I_a. R_a = V/2. ⇒ Back emf E_b = V - V/2 = V/2.

06․ The direction of rotation of a DC motor can be determent by

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

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

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

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

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