1. The PWM control of DC motor varies _______
- Linearly with speed
- Inversely with speed
- Parabolically with speed
- Exponentially with speed
2. The speed of DC shunt motor can be increased above its rated speed by
- Increasing the supply voltage within its rated value
- Increasing the flux per pole
- Decreasing the flux per pole
- Connecting additional resistance in series of the armature
3. A separately excited dc motor has an armature resistance Ra = 0.05 Ω. The field excitation is kept constant. At an armature voltage of 100 V, the motor produces a torque of 500 Nm at zero speed. Neglecting all mechanical losses, the no-load speed of the motor (in radian/s) for an armature voltage of 150 V is
- 600 RPM
- 1200 RPM
- 300 RPM
- 900 RPM
4. A 200 V d.c. shunt motor with armature resistance of 0.2 Ω and carrying a current of 50 A is running at 960 r.p.m. If the flux is reduced by 10% at constant torque and with negligible iron and friction losses, the speed will be near.
- 1280 r.p.m.
- 1170 r.p.m.
- 1100 r.p.m.
- 1060 r.p.m.
5. A 250 V dc shunt motor has an armature resistance of 0.2 Ω and a field resistance of 100 Ω. On no-load at rated voltage, it draws an armature current of 5 A and runs at 1200 RPM. When the motor is operating on load is it draws a total line current of 50 A at rated voltage, with a 5 % reduction in the air-gap flux due to armature reaction. The voltage drop across the brushes is 1 V. The speed of the motor, in rpm, under this loaded condition, is closest to:
6. A dc series motor with resistance between terminals of 1 Ω, runs at 800 RPM from a 200 V supply taking 15 A. If the speed is to be reduced to 475 rpm for the same supply voltage and current the additional series resistance to be inserted would be approximately
- 2.5 Ω
- 3 Ω
- 4.5 Ω
- 5 Ω
7. The DC Motor, which can provide zero speed regulation at full load without any controller is
8. Consider the following statements related to the speed control of a D.C. motor
(a) Speed may be controlled by changing the pole flux.
(b) Speed may be changed by changing voltage across the armature
9. A 240 V shunt motor with the armature resistance of 0.1 Ω runs at 850 r.p.m. for an armature current of 70 A. If its speed is to be reduced to 650 r.p.m., the resistance to be placed in series for an armature current of 50 A is nearly
- 0.82 Ω
- 1.13 Ω
- 1.24 Ω
- 1.34 Ω
10. A 250 V dc shunt machine has armature circuit resistance of 0.6 Ω and field circuit resistance of 125 Ω. The machine is connected to 250 V supply mains. The motor is operated as a generator and then as a motor separately. The line current of the machine in both cases is 50 A. The ratio of the speed as a generator to the speed as a motor is _______
11. A shunt DC motor’s Armature resistance is 1 ohm and field coil resistance is 100 ohms. The back emf constant is 9 volts for 100 RPM. The DC supply voltage is 100 V and it is running at a speed of 1000 RPM. What is DC supply current?
12. A separately excited 300 V DC shunt motor under no-load runs at 900 rpm drawing an armature current of 2 A. The armature resistance is 0.5 Ω and leakage inductance is 0.01 H. When loaded, the armature current is 15 A. Then the speed in rpm is
- 840 RPM
- 880 RPM
- 850 RPM
- 830 RPM
13. A 120 V DC shunt motor takes 2 A at no load. It takes 7 A on full load while running at 1200 rpm. The armature resistance is 0.8 Ω and the shunt field resistance is 240 Ω. The no-load speed, in rpm is
- 2141 RPM
- 1241 RPM
- 1125 RPM
- 1535 RPM
14. A dc series motor of resistance 1 Ω across terminals runs at 1000 rpm at 250 V taking a current of 20 A. When an additional resistance of 6 Ω is inserted in series and taking the same current, the new speed would be
- 142.8 rpm
- 166.7 rpm
- 478.3 rpm
- 956.6 rpm
15. A 230-V DC shunt motor has an armature resistance of 0.25 Ω and runs at 1100 rpm, taking an armature current 40 A. It is desired to reduce the speed to 750 rpm. If the armature current remains the same, find the additional resistance to be connected in series with the armature circuit.
- 1 Ω
- 1.5 Ω
- 1.25 Ω
- 1.75 Ω