Q.1. Explain what happens when a d.c. motor is connected across an a.c. supply?
Ans. 1.Since on a.c. supply, reactance will inherit
the image , the a.c. supply are going to be offered impedance (not resistance)
by the armature winding. Consequently, with a.c. supply, current are going to
be much less. The motor will run but it might not carry an equivalent load
because 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
sector poles aren't. Consequently, eddy currents will cause the motor to heat
up and eventually burn on a.c. supply.
Q.2. what is going to happen if a shunt motor is
directly connected to the availability line?
Ans. Small motors up to 1 KW rating could also be
line-started with none adverse results being produced. High rating motors must
be started through an appropriate starter so as to avoid the large starting
current which can
(i) damage the motor itself and
(ii) badly affect the voltage regulation of the
availability line.
Q.3. How may the direction of rotation of a d.c.
motor be reversed?
Ans. By reversing either the sector current or
current through the armature. Usually, reversal of current through the armature
is adopted.
Q.4. what is going to happen if both currents are
reversed?
Ans. The motor will run within the original
direction.
Q.5. A d.c. motor fails to start out when switched
on. What might be the possible reasons and remedies?
Ans. Anybody of the subsequent reasons might 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 call
pole value.
3. Overload should be reduced if possible otherwise
larger motor should be installed.
4. Excessive friction bearing lubrication should be
checked.
Q.6. A d.c. motor is found to prevent running after
a brief period of your time. What does one think might be the reasons? How
would you remedy each?
Ans. Possible causes are as under:
1. Motor not getting enough power check voltage at
motor terminals also as fuses, clips and overload relay.
2. Weak or no field within the case of adjustable
speed motors, check if rheostat is correctly set. Also, check field coil for
any ‘open’. Additionally, search for any loose winding or broken connection.
3. Motor torque insufficient for driving the given
load check line voltage with name plate voltage. If necessary, use larger motor
to match the load.
Q.7. What are the likely causes if a d.c. motor is
found to run too slow under load? and therefore the remedy?
Ans. 1. supply route voltage too low remove any
excessive resistance in supply route , connections or controller.
2. Brushes before neutral set them on neutral.
3. Overload reduce it to allowable value or use
larger motor.
Q.8. Why does a d.c. motor sometime run too fast
when under load? Give different possible causes and their remedies.
Ans. 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.9. Under what conditions is sparking produced at
the brushes of a d.c. motor? How would you remedy it?
Ans. 1. Commutator in bed condition-clean and reset
brushes.
2. Commutator either eccentric or rough-grind and
true the commutator. Also, undercut mica.
3. Excessive vibration-balance armature. confirm
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 brief between
adjacent commutator risers. Locate and repair internal armature short if any.
Q.10. What are the possible causes of excessive
sparking at brushes during a d.c. motor?
Ans. 1. Poor brush fit on commutator-sand within the
brushes and polish commutator.
2. Brushes binding within the brush holders-clean
holders and brushes and take away 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.
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