Question Set 1

1) Which is the most efficient way of controlling a DC shunt motor below rated speed?

a. Armature resistance control

b. Armature voltage control

c. Field resistance control

d. Diverter resistance control in field circuit

2) Which of the following is an interface between Electric Power Supply and the motor?

a. Load

b. Sensing Unit

c. Power Modulator

d. Mechanical Gear

3) Fixed voltage Fixed Frequency AC can be converted to Variable Voltage Variable Frequency AC by:

a. AC voltage regulator

b. Cycloconverter

c. Fully controlled converter

d. Half controlled converter

4) Which of the following is called the inertial torque?

where $J$ is moment of inertia in $kg\cdot m^2$, B is the coefficient of viscous friction in $N\cdot m$/(rad/s) and $\omega_m$ is the rotor speed in rad/sec.

a. $J\cdot \dfrac{d\omega_m}{dt}$

b. $J\cdot \dfrac{d^2\omega_m}{dt^2}$

c. $B\cdot \omega_m$

d. $B\cdot \omega_m^2$

5) A motor drives two loads. One has rotational motion. It is coupled to the motor through a reduction gear with a=0.2 and efficiency of 95% . The load has moment of inertia of 5 $kg\cdot m^2$ and load torque of 20 $N\cdot m$. The other load has translational motion and has a weight of 500 kg which has to be lifted at a constant speed of 1 $\frac{m}{s}$. The coupling between the translational load and the motor has an efficiency of 90%. The motor inertia can be taken as 0.5 $kg\cdot m^2$ and the motor runs at a speed of 960 rpm. The equivalent inertia referred to the motor shaft is:

a. 0.54 $kg\cdot m^2$

b. 0.64 $kg\cdot m^2$

c. 0.74 $kg\cdot m^2$

d. 0.84 $kg\cdot m^2$

6) Given, $T_O=48.68$ N.m and $F_2=500$N The power developed by the motor in as is:

a. 3873 Watt

b. 4873 Watt

c. 5873 Watt

d. 6873 Watt

7) Which of the following Load Speed-Torque characteristics represents a hyperbola?

a. Fan type of load

b. Traction load

c. Low speed hoist load

d. Constant power load

8) Which of the following types of friction is independent of speed?

a. Coulomb friction

b. Static friction

c. Viscous friction

d. Windage friction

9) Which of the following load offers a constant load torque?

a. Traction load

b. Low speed hoist

c. Fan type of load

d. High speed hoist

10) A motor-load combination has the following speed torque characteristics:

$T = 100 - 0.1 \omega_m$ ($N\cdot m$)

$T_L = 0.05\cdot \omega_m$ ($N\cdot m$)

where T = motor torque in $N\cdot m$, $T_L$ = load torque in $N\cdot m$ and $\omega_m$ = speed of the motor-load combination in rpm. The steady state speed of the drive is:

a. 333.33 rpm

b. 455.55 rpm

c. 666.66 rpm

d. 721.66 rpm

11) A drive has the following motor and load speed-torque equation

Motor: $T=1+2\cdot \omega_m$

Load: $T_L=3\cdot \sqrt{\omega_m}$

The steady state equilibrium speeds are:

a. $1$ & $1/4$

b. $1$ & $1/2$

c. $2$ & $1/4$

d. $2$ & $1/2$

12) The drive in Q11 will operate in the steady state at a speed of:

a. $2$

b. $3/2$

c. $1$

d. $1/4$

13) What is the most appropriate way to smoothen the motor torque for a pulsating type load torque?

a. By connecting an inductance in series with the motor.

b. By connecting a flywheel with the motor-load combination.

c. By controlling the triggering angle of the converter feeding the motor.

d. By introducing a gear with appropriate gear ratio.

14) The motor torque speed characteristic is given by the equation $\omega_m= 100 - T$ where $\omega_m$ is in rad/sec and T is in $N\cdot m$. The initial motor torque is $10~N\cdot m$. A step load torque of $50~N\cdot m$ is applied at $t = 0$. If the total inertia of the motor load combination is $5~kg\cdot m^2$, the value of the motor torque after 5 sec is:

a. 50.0 $N\cdot m$

b. 45.2 $N\cdot m$

c. 35.2 $N\cdot m$

d. 25.2 $N\cdot m$

15) The speed of the motor in rad/sec in Q14 at t=5 sec is

a. 100 rad/s

b. 64.7 rad/s

c. 356.9 rad/s

d. 37.7 rad/s