Design of Electrical Machines Questions and Answers – Motor Starters, Calculation of Resistance Steps & Design of Field Regulators

This set of Design of Electrical Machines test focuses on “Motor Starters, Calculation of Resistance Steps & Design of Field Regulators”.

1. What is the function of the motor starter with respect to current?
a) to slow the low current flow
b) to prevent the low current flow
c) to allow the large current flow
d) to prevent the large current flow

Explanation: There are varied type of starters which vary among themselves according to the function. The main function is to prevent the excessive current at the starting.

2. What is the work of the starter with respect to the mechanical stress?
a) to allow large mechanical stress
b) to restrict large mechanical stress
c) to allow small mechanical stress
d) to restrict small mechanical stress

Explanation: The main function of the starter is to prevent the excessive current at the starting. The other work of the starter is to restrict the large mechanical stress from acting on the machines.

3. What is the relation of the current with the starting torque in the starter concept?
a) the starter should restrict current to prevent low starting torque
b) the starter should restrict current to produce high starting torque
c) the starter should send current to prevent low starting torque
d) the starter should send current to produce high starting torque

Explanation: The main function of the starter is to prevent excessive current flow. At the same time it should allow current to produce good or high starting torque.

4. When does the starter take up liquid rheostat?
a) when the resistance can be varied heavily
b) when the resistance can be varied gradually
c) when the resistance cannot be varied
d) when the resistance should not be varied

Explanation: The starter actually picks up either liquid rheostat or metallic resistance during its operation. The starter takes up liquid rheostat during the situation where the resistance is gradually varied.

5. The starter take up metallic resistance when the resistance should not be varied in steps.
a) true
b) false

Explanation: The starter actually picks up either liquid rheostat or metallic resistance during its operation. The starter takes up metallic resistance when the resistance is to be varied in steps.
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6. What happens when the starter takes up metallic resistance?
a) voltage fluctuates from high to low
b) voltage fluctuates between fixed upper and lower limits
c) current fluctuates from high to low
d) current fluctuates between fixed upper and lower limits

Explanation: The starter actually picks up either liquid rheostat or metallic resistance during its operation. The starter takes up metallic resistance when the resistance is to be varied in steps. When taken with resistance steps, the current fluctuated between upper and lower limits.

7. What is the product of the ratio of the current and the useful flux per pole?
a) product of ratio of current and useful per pole = (useful flux per pole due to upper limit current * upper limit current) / (useful flux per pole due to lower limit current * lower limit current)
b) product of ratio of current and useful per pole = (useful flux per pole due to upper limit current + upper limit current) / (useful flux per pole due to lower limit current + lower limit current)
c) product of ratio of current and useful per pole = (useful flux per pole due to upper limit current – upper limit current) / (useful flux per pole due to lower limit current – lower limit current)
d) product of ratio of current and useful per pole = (useful flux per pole due to upper limit current * lower limit current) / (useful flux per pole due to lower limit current * upper limit current)

Explanation: First the product of useful flux per pole due to upper limit current and lower limit current and then the product of useful flux per pole due to lower limit current and upper limit current. On substitution the product of ratio of current and useful per pole is obtained.

8. How many machines are considered in the calculation of the resistance steps?
a) 2
b) 3
c) 4
d) 5

Explanation: There are 3 machines considered in the calculation of the resistance steps. They are starters for dc shunt motors, starters for dc series motors, starters for three phase slip ring induction motor.

9. What is the concept of notching operation?
a) process of decreasing the voltage
b) process of increasing the efficiency
c) process of cutting out the resistance
d) process of adding on the resistance

Explanation: The process of notching operation occurs In the dc shunt motors. The concept of notching operation means the cutting out the resistance.

10. What happens in the dc shunt motor when the notching process occurs?
a) flux remains constant
b) speed remains constant
c) voltage remains constant
d) current remains constant

Explanation: Notching operation means cutting out the resistance. During the process of notching speed remains constant.

11. What is the formula of the ratio of the lower limit to upper limit of current with respect to the resistance?
a) lower limit of current / upper limit of current = motor resistance / resistance to limit the starting current
b) lower limit of current / upper limit of current = (motor resistance / resistance to limit the starting current)1/number of resistance
c) lower limit of current / upper limit of current = (motor resistance * resistance to limit the starting current)1/number of resistance
d) lower limit of current / upper limit of current = motor resistance * resistance to limit the starting current

Explanation: The motor resistance, resistance to limit the starting current and number of resistance is calculated. On substitution the ratio of lower limit to upper limit of current is obtained.

12. What is the relation of the ratio of rotor current limits and the ratio of lower limit to upper limit current?
a) ratio of rotor current limits = ratio of lower limit to upper limit current
b) ratio of rotor current limits > ratio of lower limit to upper limit current
c) ratio of rotor current limits < ratio of lower limit to upper limit current
d) no relation between ratio of rotor current limits and ratio of lower limit to upper limit current

Explanation: The ratio of rotor current limits and the ratio of lower limit to upper limit current are first calculated. The ratio of rotor current limits is approximately equal to the ratio of lower limit to upper limit current.

13. How many machines are considered for the design of field regulators for dc machines?
a) 2
b) 3
c) 4
d) 5

Explanation: Two machines are considered for the design of field regulators for dc machines. They are the shunt generators and shunt motor.

14. What is the first step in the design of the field regulators for dc machines?
a) calculation of the resistance of section
b) calculation of the total field circuit resistance
c) calculation of the field circuit resistance
d) resistance to be inserted

Explanation: 2 machines are considered in the design of field regulators for dc machines. They are the shunt generators and shunt motor. The first step is the calculation of the field circuit resistance.

15. What is the second step in the design of the field regulators for dc machines?
a) calculation of the resistance of section
b) calculation of the total field circuit resistance
c) calculation of the field circuit resistance
d) resistance to be inserted

Explanation: 2 machines are considered in the design of field regulators for dc machines. They are the shunt generators and shunt motor. The second step is the calculation of the resistance to be inserted.

16. What is the last step involved in the design of field regulators for shunt generators?
a) calculation of the resistance of section
b) calculation of the total field circuit resistance
c) calculation of resistance of section
d) resistance to be inserted

Explanation: 2 machines are considered in the design of field regulators for dc machines. They are the shunt generators and shunt motor. The last step is the calculation of resistance of section.

17. What is the first step in the design of the field regulators for dc motor?
a) number of sections
b) shunt field circuit resistance
c) shunt field resistance
d) resistance of step

Explanation: 2 machines are considered in the design of field regulators for dc machines. They are the shunt generators and shunt motor. The first step is the calculation of number of sections.

18. What is the plot of the magnetization curve?
a) field current in the y axis vs voltage in x axis
b) field current in the x axis vs voltage in y axis
c) armature current in the y axis vs voltage in x axis
d) armature current in the x axis vs voltage in y axis

Explanation: The magnetization curve is the curve which is used to obtain the various values required in the design of field regulators. The curve is the plot of field current in x axis and voltage in y axis.

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