Question.
What are routine maintenance carried out on an a.c. motor and
Explain the meaning of single phasing in a.c. machinery
State the dangers associated with single phasing
& enumerate the protective devices normally fitted to counteract such
dangers.
Answer.
The maintenance requires that for cage rotor
induction motor are very simple. Keep insulation resistance high, contact
resistance low, lubricate correctly, maintain even air gap, ensure both the
interior and exterior are always clean and dry.
All running a.c motor is
to be felt by hand to check for overheating. Check current drawn is within
limits. Check that there is no vibration and is running smooth. Regularly clean
the external surface of the motor. Periodically grease all running motor
bearing with correct quantity and type of grease recommended by manufactures.
Check brush gap and slip ring. The shaft of
stationary motor should be periodically rotated quarter turn to minimize
vibration damage to the bearing. Periodically Check insulation, tightness of
terminals, connection and foundation bolts, bearing
removed as per PMS.
Check for damage renew if necessary, stator and
other motor control gear regularly inspected to check and maintaine
following items.
Enclosure – check for accumulation of dirt and
rust, any corroded parts must be cleaned and painted, examiner startor fixing bolt and earth connection
Contactor and relay – check for any sign of
overheating and loose connections, remove dust and grease from insulating
components. Ensure magnet armature of contactor moves freely.
Contacts – examine for excessive pitting and
roughness due to burning. a thin smear of electrical
contact lubrication helps to prolong life of all contact. Check contact spring
pressure. Check power and control fuse contact.
Connections - examine all power and control
connection for tightness
OCR - check for proper size (relate to motor full
load current) periodically overhaul motor.
Following safety precaution should be observed.
Take permission from responsible officer for doing
specific work.
Make sure to switch off supply from both ends.
Place caution danger notice near the work place and near switching terminals.
Badly contaminated motor requires total immersion
in solvent. Care is taken because it is toxic, ample ventilation should be
provided.
Check bearing cover, if damaged or crack should be
replaced.
Stator winding look for damaged insulation. Examine
stator core for signs of rubbing with rotor. Check I.R. rotor inspected for
sign of damage or overheating in cage winding and laminated core. Clean all
ventilating ducts. Internal fans must be in good condition to provide adequate
venting.
b) [Single Phasing is where one of the 3-phase's
supplying the Motor becomes disconnected.
The Motor will continue to run if this happens and
can result in Motor Burnout.
The effect of Single Phasing is to increase the
Current in the two remaining Lines and cause the Motor to become very noisy due
to uneven Torque produced.]
1) The meaning of single phasing is where one of
the three phases is lost in an AC motor.
2) The effect of single phasing is an imbalance in
the other two phases, which can lead to rough operation of the motor, or if the
motor is stopped and you try starting the motor, it will make a humming noise
and will not start.
Protection against single
phasing.
Thermal overload
Single phase preventer
3 bimetal thermal strips one in each phase, the
ends of which projects between 2 parallel guides one fixed and the other
movable and spring controlled. All 3 strips can respond in unision
to balanced 3phase overload but one or two of the strip will act independently
in event of single phasing. This action is accentuated by open circuited phase
and cooling of and thereby deflecting in opposite direction.