Page 260 - Industrial Power Engineering and Applications Handbook

Installation and maintenance of electric motors 10/239
Bearing insulation

Driven equipment
Driven Equipment

earing insulation

Insulation at the foundation
Figure 10.8 Alternative arrangements to eliminate shaft currents

Loose fit of the bearing housing, causing both inner Fault in the switching device. Sometimes, the
and outer races of the bearing to rotate inside the housing switching device may not be functioning properly.
Corrosion or the presence of foreign matter in the To give an instance, one 150 h.p. squirrel cage
bearing. motor was selected for a centrifugal air compressor.
The motor starting torque was adequately chosen
Any of the above reasons may result in noise and an to start the compressor through an autotransformer
increase in temperature and must be corrected. Critical starter with a 40% tapping. The motor started but
installations such as a refinery, a petrochemical plant, a locked up somewhere in the middle of the full
chemical plant or a petroleum pipeline may require special speed and did not-accelerate further. The supply
precautions and control to avert any excessive heating of voltage and the transformer tapping were in order.
the bearings, which may become fire hazards. For these A thorough check revealed that the voltage at the
installations, bearing temperature detectors with a relay motor terminals was much less than 40%. A long
and alarm facility may also be installed in the control cable drop from the A/T to the motor was suspected
circuit of the switching device to give warning or trip the but when the voltage was measured at the A/T
motor if the temperature of the bearing exceeds the pre- outgoing terminals, it was almost the same as at
set safe value. the motor terminals, thus eliminating the possibility
of a longer cable drop. In fact the A/T was faulty
or not properly connected so that it was producing
10.5 General problems in electric only 25% voltage instead of 40% in the secondary
motors and their remedy circuit thus seriously affecting the torque and the
motor’s starting performance.
4 Motor not taking up load. This may be due to incorrect
Only the most general types of problems are discussed stator connections. In general, motors of 3 h.p. and
here: above, except HT motors, are wound for delta
connections and all the six terminals are located in
Bearings make a churning noise or overheat: the terminal box to facilitate YlA starting. These
Check condition and level of grease as well as any terminals are connected in delta through metallic
skin effects or watermarks on the races, balls or links (Figure 10.9). If the motor is inadvertently
rollers. If there are watermarks, the bearings should connected in star, each motor phase will receive
be replaced. Otherwise wash and regrease them, as only 118 times the rated voltage. This will reduce
explained earlier. the torque to one third of the rated torque (Section
For creaking and harsh noises, check for misalign- 4.2) which may not be adequate to pick up the load.
ment and belt tension. It may even damage the windings if the motor remains
Motor not picking up speed: energized for a while due to excessive load current
Check all phases for supply continuity. (Z’R losses).
Check the voltage. While connecting a delta-wound motor through a Yl
Check the starter connections and contacts in all A starter, the metallic shorting links should be
three phases. Also check proper contacts and brush removed. Otherwise the starter will have a dead short-
pressure in slip-ring motors. If these are satisfactory circuit at the motor terminal box and may burn the
and the motor still does not pick up, check the starter, damage the motor terminal box and even
suitability of the motor for the type of load and line cables.
switching method. Motor takes longer to pick up. When conditions 3