Page 360 - Improving Machinery Reliability

326 Improving Machinery Reliability

Motor System Tuneup

Proper tuning of the motor system is required to avoid operation at natural (criti-
cal) frequencies or speeds that will create resonance and large damaging oscillations
that can literally cause components to “shake themselves to death.” The complete
system must be connected and operating to check for these critical frequencies since
they are determined by total mass as well as speed. “Bounce” from unresisted belt
pulls, impact loads from hammer mills and vibrations from reciprocating compres-
sors can all cause damaging resonance in the motor system.
With systems operating at a single speed, the critical frequency should be kept
well to the high side of the operating speed. With multiple-speed systems, there are
multiple critical frequencies (or harmonics) that are encountered as the speed
changes. Where electronic adjustable speed drives are used to control the system,
they may have to be programmed to avoid operation at these speeds, once they are
determined.

Pumping and Piping
Vertical pump motor mountings commonly experience resonance when one end of
the motor is unrestrained. The pump head itself is occasionally rather unsupported,
leading to reed frequency vibration. Changing the mass of the vertical motor by
adding weights, additional bracing, or spring washers under the hold-down bolts
may eliminate vibrations, but proper system design is preferable.
External piping can also transmit vibrations to the pump drive system. Surges in
the fluid system can cause unrestrained piping to “hammer” or shake uncontrollably
and affect the motor and pump drive. Thermal expansion of long runs of pipe can
also affect the alignment of the pump drive and put stresses on the motor bearings.

Power Points

All of the manufacturer’s ratings for motor output, temperature rise, and expected
life assume that the characteristics of the incoming power fall within certain varia-
tions of voltage and frequency (Hz). The total limit for these variations is *lo%.
Operating the motor outside this limit affects both motor life and motor efficiency.
Operating a motor at reduced voltage causes the motor to draw increased current
to satisfy the torque demand. The motor may be unable to develop adequate starting
torque and higher current means higher 12R losses.
Operating a motor at higher voltage increases core losses and insulation-damaging
heat buildup. In general though, low voltage creates greater problems than high volt-
age and has the same damaging effect on winding insulation as overloading.
Voltage imbalance among the three phases is also damaging to motor life. Tem-
perature rise in the windings increases at a rate equal to two times the square of the