Question Set 2

16) In which of the following control methods, the no-load speed of a shunt DC motor can be increased beyond rated value?

a. Armature resistance control

b. Armature voltage control

c. Field resistance control

d. Controlling resistance in parallel with armature

17) What is the ideal no-load speed for a series DC motor?

a. $V/(K\phi)$

b. $V/(K\phi)^2$

c. Zero

d. In finite

18) A 200 V , 11 A, 1500 rpm DC shunt motor has armature and field resistance of 0.5 $\Omega$ and 200 $\Omega$ respectively. The load torque can be assumed to be constant at rated value. What is the motor speed if a resistance of 5 $\Omega$ is inserted in the armature circuit?

a. 1115 rpm

b. 1222 rpm

c. 1328 rpm

d. 1404 rpm

19) A full controlled converter is feeding an R-L load with R = 10 $\Omega$ , L=31.83 mH. If the input frequency is 50 Hz, the impedance angle ($\theta$) of the load is

a. $30^o$

b. $45^o$

c. $60^o$

d. $75^o$

20) For Q19, if the input voltage is 230 V / 50 Hz and $\alpha=30^o$. What is the average output voltage assuming continuous conduction?

a. 149V

b. 169V

c. 179V

d. 189V

21) A 230 V , 50 $H_Z$ single phase supply feeds a fully controlled FWR. The FWR is used to power the armature of a separately excited DC motor. The DC motor ratings are 200 V , 10 A (armature current), 1000 rpm, $R_a$ = 1 $\Omega$. The no-load speed of the motor supplied from the FWR is

a. 179 rad/s

b. 169 rad/s

c. 159 rad/s

d. 129 rad/s

22) Discontinuous current operation for a SP-FC-FWR fed separately excited DC motor occurs for speeds

a. Higher than the critical speed

b. Lower than the critical speed

c. Higher than 50% of the critical speed

d. Lower than 50% of the critical speed

23) Extinction angle($\beta$) for a SP-FC-FWR can be calculated by

a. A closed form solution of the relevant equation

b. An iterative solution of the relevant equation

c. Computation of no load speed of the motor

d. Computation of converter output voltage

24) During coasting period in a SP-FC-FWR, the armature voltage is

a. The same as input ac voltage

b. The same as the drop across the armature inductance

c. The same as the armature back-EMF

d. The same as the drop across the armature resistance

25) In which of the following methods, the supply voltage to the armature of a separately excited DC motor is reversed?

a. Forward motoring

b. Forward regenerative braking

c. Forward dynamic braking

d. Forward plugging

26) For a semi-controlled converter fed DC motor, what is the region for $\pi \lt \omega t \lt \beta$ called?

a. Duty interval

b. Coasting interval

c. Freewheeling interval

d. Zero-current interval

27) A 200 V , 900 rpm, 100 A separately excited DC motor has an armature resistance of 0.05 $\Omega$. It is fed by SP-FC-FWR supplied by an ac voltage of 230 V, 50 $H_Z$. Assuming continuous conduction, calculate the firing angle for rated motor torque and 900 rpm speed.

a. $75^o$

b. $65^o$


d. $15^o$

28) For Q27, calculate the motor speed for rated torque and $\alpha=60^o$.

a. 861.2 rpm

b. 746.5 rpm

c. 505.9 rpm

d. 454.7 rpm