Page 361 - Improving Machinery Reliability

Extending Motor Life in the Process Plant Environment 327

net voltage imbalance in the phase with the highest current (2rNET2). A 3.5% net volt-
age imbalance will cause a 25% increase in temperature.
Assume the voltages to a motor are 456 volts to phase A, 440 volts to phase B,
and 432 volts to phase C. The average voltage is 456 volts. The voltage imbalance is
as follows:

472 - 456
Voltage imbalance = x 100 = 3.5%
456
Temperature increase = 2 x 3S2 = 25%

Causes of voltage imbalance include unequal phase system loading, unequal tap
settings, poor connections to the power supply, open delta transformer systems,
capacitor bank (power factor correction) malfunctions, single-phasing (loss of a
phase), etc. Voltage imbalances over 1% require derating the motor per NEMA stan-
dard MG1-14.35. The motor in the above example should be derated to operate with-
in its specified temperature rise limits.
As mentioned earlier, another power problem affecting motor insulation and
motor life is voltage surges caused by the high switching frequencies of modern
PWM inverters (adjustable-speed drives or ASDs) utilizing IGBTs. While rapid
IGBT switching eliminates audible noise and the “cogging” problems of earlier AC
drives, the wave forms generated include transient spikes of 1600 volts or higher in a
nominal 460 V system. This is more than enough to overcome the 600-volt “with-
stand capability” of the winding insulation.
Repeated exposure to these high voltage spikes punctures and breaks down the
winding insulation, leading to early motor failure. Such problems can be minimized
by special motor winding methods, special winding insulation or both.
Other damaging power surges can be caused by lightning strikes, capacitor dis-
charges, and utility problems, as well as locked-rotor conditions resulting from
motor system malfunction.

Over-Current Insurance

Over-current protection should be provided in the motor feeder circuit and good
engineering practice emphasizes using an overload relay in each phase of the motor to
give protection from voltage imbalance conditions. Controllers and overload relays
must be sized in accordance with the National Electrical Code (NEC) and are nomi-
nally designed to accommodate 600% of the full-load current. The recently intro-
duced NEMA Design E, high-efficiency motor may require a larger controller since
the design parameters allow a locked rotor current up to 900% of full-load current.