This set of DC Machines online quiz focuses on “Starting of DC Motors – 2”.

1. In three-point starter, as a starting handle is rotated __________

a) The resistance is added into armature circuit

b) The resistance is removed from field circuit

c) The resistance is added into field circuit

d) Resistance is neither added nor removed

View Answer

Explanation: The starting resistance is arranged in steps between conducting raised studs. As the starting handle is rotated about its fulcrum, it moves from one stud to the next, one resistance step is cut out, and it gets added to the field circuit.

2. How much torque is ensured by resistance adding arrangement in 3-point starter?

a) Low and non-zero

b) Infinite

c) High finite

d) Zero

View Answer

Explanation: As a starting handle is rotated one resistance step is added into field circuit. There is a short time wait at each stud for the motor to build up speed. This arrangement ensures a high average starting torque.

3. The resistance of NVC is _______

a) Small

b) Large

c) Infinite

d) Zero

View Answer

Explanation: The resistance of no volt coil (NVC) is small. NVC resistance forms a part of field resistance when resistor rotating handle of three-point starter is moved from min. to max. position. Starting resistance is also added to the field circuit.

4. NVC will release the handle electromagnetically when ________

a) In the case of failure of field current

b) If the resistance is very high

c) At the end of each rotation

d) At the start of each rotation

View Answer

Explanation: In case of failure of field current (due to accidental or otherwise open circuiting) NVC coil releases the handle (held electromagnetically), which goes back to the OFF position under the spring action.

5. Over-load coil performs the function when __________

a) In the case of failure of field current

b) If the resistance is very high

c) If armature current increases beyond certain value

d) At the start of each rotation

View Answer

Explanation: The contact of this relay at armature current above a certain value (over load/ short circuit) closes the NVC ends, again bringing the handle to OFF position. NVC and OL release are protections incorporated in 3-point starter.

6. In 4-point starters the resistance is added in series with NVC because __________________

a) To increase field current

b) To increase armature current

c) To limit the NVC current

d) To limit armature current

View Answer

Explanation: To overcome the problem caused when the field current is low, NVC is connected across the two lines, one line connected to F terminal through the starter and other directly to the second line from another L terminal of the starter. To limit the NVC current a protective resistance R is connected in series with it.

7. What will be the γ value for starter taking 4 steps, where ratio of resistance at maximum allowable current to armature resistance is equal to 1.8?

a) 1.2164

b) 1.8

c) 2.2468

d) 0.8220

View Answer

Explanation: γ is defined as the ratio of upper current limit to the lower current limit in starters of DC machine. γ

^{n-1}= ratio of resistance at maximum allowable current to the armature resistance. Substituting values for n=4, we get γ=1.2164.

8. Maximum allowable current for a 240-V DC shunt motor is equal to 65 A. Minimum allowable current for same is equal to 40 A. What will be the value of γ?

a) 0.6153

b) 1.265

c) 1.625

d) 2.652

View Answer

Explanation: γ is defined as the ratio of upper limit current value to the lower limit current value. So, γ will be the ratio of 65/40. From calculations, we get γ= 1.625. Note that γ is unitless quantity.

9. For a certain machine having γ = 1.8, we are using starter with 5 steps. What will be the resistance at step 3 if step 2 resistance is equal to 2 Ω?

a) 2.111

b) 1.111

c) 3.6

d) 10.8

View Answer

Explanation: γ is defined so as to calculate step resistance from the given maximum limit of current and minimum limit of the same. This γ = r

_{n}/r

_{n-1}. So, for calculating resistance at step 3 we’ll substitute the corresponding values in the equation, which will give step 3 resistance as 1.1111Ω.

10. Which of the following is the correct formula for calculating step resistance?

a) γ = r_{n}/r_{n-1}

b) γ = r_{n}/r_{n+1}

c) γ = r_{n}*r_{n-1}

d) γ = r_{n}*r_{n+1}

View Answer

Explanation: For given values of armature currents upper and lower, corresponding equivalent resistances are calculated. So, by the induction formula we calculate the value of γ and the step resistance as well, where n denotes the number of steps.

11. Which of the following starter can sufficiently start the DC series motor?

a) 3-point starter

b) 4-point starter

c) 2-point starter

d) Cannot be determined

View Answer

Explanation: 2-point starter is enough to start the DC series motor. Since armature and field winding are in series already high armature current will not flow. No requirement of 3 or 4-point starter in DC series motor.

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