1. When a DC motor generates maximum power, the ratio of applied voltage to back emf is
2. A DC series motor running at the speed of 800 rpm draws 18 A from the supply. If the load is changed such that the current drawn by the motor is 55 A, calculate the speed of the motor (in rad/sec on the new load. Armature and field winding resistances are 0.2 and 0.4 Ω, respectively. Assume a supply voltage of 230 V and also that the flux produced is proportional to the current.
- 25 rad/s
- 20 rad/s
- 15 rad/s
- 10 rad/s
3. A separately excited dc motor runs at 1500rpm under no load with 200V applied to the armature. The field voltage is maintained at its rated value. The speed of the motor, when it delivers a torque of 5 Nm is 1400 rpm. The rotational and armature reaction is neglected. The armature resistance of the motor is
- 2 Ω
- 3.4 Ω
- 4.4 Ω
- 7.7 Ω
4. A dc shunt motor is connected to the source through a 3-point starter. Suddenly if the starter handle is moved fastly from off to on position, then the ______
- Motor will draw a large current
- Motor will not start
- Motor will burn
- All of the mentioned
5. A DC motor is used in a conveyor belt. The belt has been wrapped around the rollers, and one of the rollers is connected to the motor. The series-connected motor rotates at 1200 rpm with the torque of 12 N-m and the field current of 40 A. Find output voltage, assuming RA = Rf = 0.
- 37.69 V
- 25.48 V
- 45.24 V
- 40.44 V
6. With an armature voltage of 100 V and rated field winding voltage, the speed of a separately excited DC motor driving a fan is 1000 rpm, and its armature current is 10 A. The armature resistance is 1 Ω. The load torque of the fan load is proportional to the square of the rotor. Neglecting rotational losses, the value of the armature voltage (in Volt) which will reduce the rotor speed to 500 rpm is
- 50 V
- 47.5 V
- 35.5 V
- 30 V
7. For a 100 hp 250 V, compound dc motor with compensating winding has a field current of 5 A to produce a voltage of 250 V at 1200 rpm. What will be the shunt field current of this machine at no load?
- 5 A
- 5.6 A
- 4 A
- 0 A
8. A 250-V DC shunt motor has a shunt field resistance of 150 Ω and an armature resistance of 0.3 Ω. For a given load. The motor runs at 1400 rpm and draws 20 A current. If the resistance of 75 Ω is added in series with the field, find the new armature current. Assume the load torque remains constant.
- 32.32 A
- 18.84 A
- 16.60 A
- 27.42 A
9. A DC shunt motor when connected to 250 V DC supply runs at 1500 rpm. When the same motor is connected to a 300 V DC supply, then what will be the speed of the motor neglecting losses and saturation?
- 1200 rpm
- 1500 rpm
- 1800 rpm
- 750 rpm
10. For a dc shunt motor of 5 kW, running at 1000 RPM the induced torque will be _______
- 47.76 N
- 57.76 N
- 35.76 N
- 37.76 N
11. It is advised not to run dc series motor with no load Why?
- Because zero torque at no load will make speed infinite
- Because zero torque as no-load will not let machine start
- Because infinite torque will be produced
- None of the mentioned
12. The flux and the internally generated voltage of a dc machine is a ________ function of its magneto-motive force.
13. The basic difference between 4 point starter when compared to 3 point starter is:
- The holding coil is removed from the shunt field circuit
- The overcurrent release is connected in the supply line
- Field circuit is completed through starting resistance
- The starting resistance is gradually cut out till when the armature reaches the running position
14. Voltage equation of a dc motor is
- V = Eb + IaRa
- V = Eb – IaRa
- V = EbIa – Ra
- V = EbIa + Ra
15. The back EMF of the DC motor is _______ of the applied voltage.
16. For speed control of a DC motor, which technique provides speed above base speed?
- Field control method
- Supply current control method
- Supply voltage control method
- Armature resistance control method
17. If the field of a DC shunt motor is opened while running, what will happen to the speed?
- Speed of the motor will remain constant
- The motor will get locked
- Speed of the motor will reduce
- Speed of the motor will become dangerously high
18. High-speed protection of DC shunt motor is not provided by:
- 3 point starter
- 2 point starter
- 4 point starter
- 5 point starter
19. The connection of NVC in DC 3 point starter, is connected across DC supply in series with:
- field winding
- overload relay
- starting resistor
20. Three-point starter can be used for