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ci \ ampere meter would show almost a constant
f ai bi starting current. The operator, possibly also judging
the speed of the ball mill, may have paused in the
star position before changing it over to delta but
by then the motor had failed. Thus, it was a wrong
application of the switching device for the type of
load.
7 Motor trips and/or fuses blow during start
0 Check selection of fuses for the type of load and
(a) Terminal box (b) A connected winding switching. Perhaps they are under-rated for the
starting inrush current and its duration. For
selection of fuse ratings see Section 12.10.4.
Figure 10.9 Terminals inside a terminal box
0 Supply voltage may be low, causing the motor to
take longer to start.
Starter relay selection or relay setting may not be
and 4 above have been checked but this problem matching the starting requirements. The relay
still arises, then motor selection may not be thermal characteristics must match motor
appropriate for the type of load or the method of characteristics. In heavy drives with prolonged
switching. For example, a ball mill at a thermal start times the relay may not operate if it is not
power station required a 75 kW motor employing a properly selected. For heavy loads, with large
Y/A manual switching (an old installation). The moments of inertia and requiring a longer starting
method of switching was overlooked at the time of time, a timer can be introduced into the relay
motor selection. The motor burnt during the start circuit, to bypass this until the motor accelerates
while the starter was still in the star position. The to a reasonable speed. Then the relay circuit is
stator was found to be completely charred and the reactivated. The most common practice is to
rotor damaged to the extent that its short-circuiting provide a CT-operated thermal overload relay
rings also melted due to excessive starting heat and (Figure 13.53; see also Section 12.4.1). The CTs
some molten metal was scattered over the stator have a low saturation point. In the event of a high
overhangs causing extensive damage. The reasons starting current, they become saturated at two to
for such a failure can be as follows: three times the full load current and bypass the
The starter was manually operated and thus the excessive starting kick through the relay and thus
changeover time, from star to delta, largely avoid a false trip during a prolonged start.
depended upon the skill of the operator, who may 8 Motor vibrates. A motor already tested at the manu-
have been unaware of the implications of a longer facturer’s works for vibration limits, as shown in
starting period in the star position. Moreover, on Table 1 1.3, may still appear to have more vibrations
load, the starting heat in a Y/A starting is much at site during operation. This may be due to the
more than on a DOL starting as discussed in Section following reasons:
2.7.3. The foundation bed or the structure on which the
motor is mounted is not rigid, is tilted or is uneven.
Tighten foundation bolts and check for proper
Note A similar situation may be found in an automatic
YlA starter when the setting of the timer, to changeover alignment of the motor and the drive. Make the
from star to delta, is at a higher value to allow a longer foundation or structure as rigid as possible.
starting time to pick up speed. Then also the motor would 0 Single phasing, in which case the motor may make
meet the same fate if the starting time exceeded the thermal a humming noise and cause vibrations due to
withstand time of the motor. uneven flux distribution.
Bearing-end play may also cause such vibrations.
The fuse rating, if used before the starter, may It is possible that the driven equipment to which
have been inadvertently high, inconsistent with the the motor is coupled has a higher vibration level
thermal withstand capacity of the motor. Thus the than the motor, resulting in quantum imbalance
fuse did not blow which would have protected the and more vibrations than when the motor was
motor from such severe and persistent overloading. tested. All attempts must be made, to bring the
The over-current relay selection or its setting may vibration level of the drive and the driven system
not have been appropriate, otherwise the starter within the limits as prescribed in Table 1 1.3.
would have tripped under such a faulty condition. 9 The motor makes rumbling noise and the stator
Selection and over-current setting of the relay current fluctuates. The rotor circuit may be broken
should have been consistent with the motor’s and should be repaired. If the motor also overheats,
thermal capacity. there may be an inter-turn fault or a short-circuit
A ball mill requires a high starting torque (Section between the phases. Detect these and rectify if
2.5 and Table 2.3) to accelerate its heavy masses. possible, otherwise rewind the motor.
In the star position, the motor torque diminishes 10 If heavy rain causes a flash flood at the site of
to one third the starting torque as on DOL. The installation, the motor may be submerged in water
mill would take a long time to accelerate and the for some time. A reasonably good motor, with good