This set of DC Machines Multiple Choice Questions & Answers (MCQs) focuses on “Starting of DC Motors – 1”.
a) Back emf of these motors is zero initially
b) These motors are not self-starting
c) These motors have high starting torque
d) To restrict armature current as there is no back emf at starting
Explanation: At the time of starting (n=0), the induced emf of a motor is zero such that current drawn by armature, from rated voltage supply would be Ia= V/Ra. Since armature resistance is very low, armature current drawn is very high and will damage the machine.
2. For which motors DOL starter can be used?
a) Up to 5 H.P
b) Up to 10 H.P
c) Up to 15 H.P
d) Up to 20 H.P
Explanation: DOL starters are limited to the small rating motors where distribution system (mains supply) can withstand high starting currents without excessive voltage dips. For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points, increases the life of starter.
3. A three-point starter is used for _________
a) Shunt motors
b) Shunt as well as compound motors
c) Shunt, compound and series motors
d) Not for DC motors
Explanation: 3-point starters are used only for shunt and compound motors, they are not used for series motors. Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed.
4. The starting resistance of a DC shunt motor is generally ______
b) Around 0.5 kΩ
c) Around 5 kΩ
d) Infinitely large
Explanation: Starting resistance of a DC shunt motor and DC compound motor is low. Well, that’s the reason why we use starters in a DC motors, in order to limit the armature current flowing through the armature and to protect machine circuitry.
a) Over load relay is connected in series and no volt relay in parallel with the load
b) Over load relay is connected in parallel and no volt relay in series with the load
c) Over load relay and no volt relay are both connected in series with the load
d) Over load relay and no volt relay are both connected in parallel with the load
Explanation: In simple manual shunt motor starter like 3-point starter over load relay coil is kept in series with DC mains while no volt coil is kept in parallel with DC mains. Both the coils are equally important in a motor starter circuit.
6. What will happen if DC motor is used without starter?
a) Heavy sparking at brushes
b) It’ll start smoothly
c) Will not start at all
d) Depends on load
Explanation: It would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. Sudden development of large torque will cause mechanical shock to the shaft, reducing its life. Such heavy current cannot be generally permitted to be drawn from the source of supply.
7. Motor will start quickly when used without starter.
Explanation: The only thing in favour of direct starting must be mentioned here. Since the torque of the motor with direct start is much higher, the motor starts much more quickly. As a consequence, the Joule input per start is much less than that with resistance start.
8. In shunt and compound motor starting the shunt field should be made on with full starting resistance in ____________
a) Series with field
b) Parallel with field
c) Series with armature
d) Parallel with armature
Explanation: In shunt and compound motors starting the shunt field should be switched on with full starting resistance in armature circuit. A short time delay in this position allows the field current to build up to the steady value of the inductive field transients.
Explanation: I1=55 A, I2 =35 A
So, γ= Ratio of upper limit to the lower limit = 55/35 = 1.57, R1= 200/55= 4 Ω
Now, γn-1 = 4/0.4 = 10. By substituting γ value, we get n = 6.
10. γ is given as 1.585. Resistance at maximum allowable current is given equal to 4 Ω, what is the 5th step resistance?
Explanation: γ is given as 1.585. So γ-1 is equal to 0.631. R1 is provided and it is equal to 4 Ω.
r1= (1-0.631) * 4 = 1.476 Ω
r2= 1.476*0.631= 0.931 Ω, similarly calculating till r5= 0.235 Ω.
11. Four-point starter is used when ______________
a) Motor field current is varied in narrow range
b) Motor speed is varied in small range
c) Motor field current is varied over wide range
d) Can be used anywhere
Explanation: Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed while four-point starter is used when motor field current can vary over a wide range and so does the motor speed.
Sanfoundry Global Education & Learning Series – DC Machines.
To practice all areas of DC Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.