18 Most Important Questions and Answers on DC Motors - Part-1.

A brushed DC electric motor generating torque from
DC power supply by using an internal mechanical commutation. 

Q. 1. How may the direction of rotation of a d.c. motor be reversed?

Ans. 1. By reversing either the field current or current through the armature.  Usually, reversal of current through the armature is adopted.

Q. 2. What will happen if both currents are reversed?

Ans. 2. The motor will run in the original direction.

Q. 3. What will happen if the field of a d.c. shunt motor is opened?

Ans. 3. The motor will achieve dangerously high speed and may destroy itself.

Q. 4. What happens if the direction of current at the terminals of a series motor is reversed?

Ans. 4. It does not reverse the direction of rotation of motor because current flows through the armature in the same direction as through the field.

Q. 5. Explain what happens when a d.c. motor is connected across an a.c. supply?

Ans. 5.

1. Since on a.c. supply, reactance will come into the picture, the a.c. supply will be offered impedance (not resistance) by the armature winding.  Consequently, with a.c. supply, current will be much less.  The motor will run but it would not carry the same load as it would on d.c. supply.
2. There would be more sparking at the brushes.
3. Though motor armature is laminated as a rule, the field poles are not. Consequently, eddy currents will cause the motor to heat up and eventually burn on a.c. supply.

Q. 6. What will happen if a shunt motor is directly connected to the supply line?

Ans. 6. Small motors up to 1 kW rating may be line-started without any adverse results being produced. High rating motors must be started through a suitable starter in order to avoid the huge starting current which will-

     (i)  Damage the motor itself and 

     (ii) Badly affect the voltage regulation of the supply line.

Q. 7. What is the function of interpoles and how are interpole windings connected?

Ans. 7. Interpoles are small poles placed in between the main poles.  Their function is to assist commutation by producing the auxiliary or commutating flux.  Consequently, brush sparking is practically eliminated. Interpole windings are connected in series with the armature windings.

Q. 8. In rewinding the armature of a d.c. motor, progressive connections are changed to retrogressive ones.  Will it affect the operation in any way?

Ans. 8. Yes.  Now, the armature will rotate in the opposite direction.

Q. 9. A d.c. motor fails to start when switched on.  What could be the possible reasons and remedies?

Ans. 9. Any one of the following reasons could be responsible :

1. Open-circuit in controller–should be checked for open starting resistance or open switch or open fuse.
2. Low terminal voltage–should be adjusted to name-plate value.
3. Overload–should be reduced if possible otherwise larger motor should be installed.
4. Excessive friction–bearing lubrication should be checked.

Q. 10. A d.c. motor is found to stop running after a short period of time.  What do you think could be the reasons ?  How would you remedy each?

Ans. 10. Possible causes are as under:

1. Motor not getting enough power–check voltage at motor terminals as well as fuses, clups and overload relay.
2. Weak or no field–in the case of adjustable-speed motors, check if rheostat is correctly set.  Also, check field winding for any ‘open’.  Additionally, look for any loose winding or broken connection.
3. Motor torque insufficient for driving the given load–check line voltage with nameplate voltage.  If necessary, use larger motor to match the load.

Q. 11. What are the likely causes if a d.c. motor is found to run too slow under load?  And the remedy?

Ans. 11. 

1. Supply line voltage too low–remove any excessive resistance in supply line, connection or controller.
2.  Brushes ahead of neutral–set them on neutral.
3. Overload–reduce it to allowable value or use larger motor.

Q. 12. Why does a d.c. motor sometime run too fast when under load? Give different possible causes and their remedies.

Ans. 12. Different possible causes are as under:

1. Weak field–remove any extra resistance in shunt field circuit.  Also, check for ‘grounds’.
2. Line voltage too high–reduce it to name-plate value.
3. Brushes back of neutral–set them on neutral.

Q. 13. Under what conditions is sparking produced at the brushes of a d.c. motor?  How would you remedy it?

Ans. 13.

1. Commutator in bad condition–clean and reset brushes.
2. Commutator either eccentric or rough–grind and true the commutator.  Also, undercut mica.
3. Excessive vibration–balance armature.  Make sure that brushes ride freely in holders.
4. Brush-holding spring broken or sluggish–replace spring and adjust pressure to recommended value.
5. Motor overloaded–reduce load or install motor of proper rating.
6. Short-circuit in armature circuit–remove any metallic particles between commutator segments and check for short between adjacent commutator risers.  Locate and repair internal armature short, if any.

Q. 14. Sometimes a hissing noise (or brush chatter) is heard to emanate from the commutator end of a running d.c. motor.  What could it be due to and how could it be removed?

Ans. 14. Any one of the following causes could produce brush chatter:

1. Excessive clearance of brush holders–adjust properly
2. Incorrect angle of brushes–adjust to correct value
3. Unsuitable brushes–replace them
4. High mica–undercut it
5. Wrong brush spring pressure–adjust to correct value.

Q. 15. What are the possible causes of excessive sparking at brushes in a d.c. motor?

Ans. 15.

1. Poor brush fit on commutator–sand-in the brushes and polish commutator.
2. Brushes binding in the brush holders–clean holders and brushes and remove any irregularities on surfaces of brush holders or rough spots on brushes.
3. Excessive or insufficient pressure on brushes–adjust pressure.
4. Brushes off neutral–set them on neutral.

Q. 16. Why does a d.c. motor sometime spark on light load?

Ans. Due to the presence of paint spray, chemical, oil or grease etc. on commutator.

Q. 17. When is the armature of a d.c. motor likely to get over-heated?

Ans. 17.

1. When motor is over-loaded.
2. When it is installed at a place having restricted ventilation.
3. When armature winding is shorted.

Q. 18. What causes are responsible for over-heating of commutator in a d.c. motor?

Ans. It could be due either to the brushes being off neutral or being under excessive spring pressure. Accordingly, brushes should be adjusted properly and the spring pressure should be reduced but not to the point where sparking is introduced.


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