Operation
Operating Voltage and Frequency
Variations of applied stator voltage and frequency
from the rated nameplate values will result variation of
machine performance. Torque, efficiency, power factor,
heating and stator current will change. Also, noise and
vibration levels may change. The torque varies as the
square of the voltage; therefore, a 10 percent decrease in
voltage will decrease the torque by 19 percent. For best
operating performance, nameplate voltage and frequency
should be maintained.
The machine will operate successfully, under
running conditions and at rated load. with variations in
voltage or frequency up to the limits indicated below:
1. Plus or minus 10 percent of rated voltage, at
rated frequency.
2. Plus or minus 5 percent of rated frequency, at
rated voltage.
3. A combination of variation in voltage and
frequency of 10 percent (sum of the absolute quantities)
of the rated values, provided that the variation in
frequency does not exceed plus or minus 5 percent of its
rated value.
Performance of the machine within these voltage
and frequency variations will not be in accordance with
the values established for operation at rated nameplate
voltage and frequency.
Line-to-line Voltage Balance
Polyphase machines are sensitive to unbalance in
the applied line voltages. If unbalances exist in the
applied line voltage, unbalance in phase currents will
result. The resulting unbalance in currents will, in
general, be significant. For example, the locked-rotor
current will be unbalanced by the same percentage as the
voltage, but at operating speed the percentage unbalance
of the current will be 6 to 10 times the percent unbalance
of the voltage. Percent Voltage Unbalance is defined as
follows:
Percent
Maximum Voltage Derivation
Average Voltage
GE Energy Motors GEEP-427-I
GEEP-427-I Pegasus, Horizontal Induction Motor
Voltage
Unbalance
Where Average Voltage is arithmetic average of
the three line voltages and Maximum Voltage Derivation
is the greatest line voltage deviation from the average.
Unbalanced line voltages result the production of
negative sequence currents in the machine that produce
fields which rotate in a direction counter to the normal
field. This results in an increase in current, losses and
heating with reduction in torque, efficiency and power
factor. Accordingly, line voltages should be as closely
balanced as can be determined on a voltmeter.
If line voltage unbalance exists, the machine
may be damaged and should be derated in accordance
with Figure 20-2 of NEMA Standard MG-20.55, in order
to reduce the possibility of such damage. Derating
factors, for several values of line voltage unbalance, are
given below.
Percent Voltage Unbalance
Operating Factor
In addition, the selection and setting of the
machine overload-protective device must consider the
derating factor and the increase in current, resulting from
line voltage unbalance. This is a difficult procedure
which must be done by a person familiar with setting
protective devices to adequately protect the machine. It is
recommend that the nearest General Electric office be
contacted if assistance is required.
Copyright 2009, The General Electric Company, USA 15
TEWAC, Sleeve Bearing
=
1
0.99
x 100
2
3
4
5
0.95
0.89
0.82
0.75