Application Note

Application Note
Optimizing electric-motor
efficiency – Part II
F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y
In Part I of this short application note series
we discussed inspection points on motors
to make during an energy audit or other
efficiency program. Part I covers basic
motor efficiency strategy and describes
tests and cost savings for voltage and
current unbalance and for power factor.
Part II covers inspection points that are
best incorporated into regular, long-term
preventive maintenance:
• Off-design voltage
• Bad grounding and connections
• Insulation resistance
This application note also explains how to
conduct inrush current testing, either dur-
ing new high-efficiency motor installation or
when troubleshooting new installs.
Off-design voltage
A three-phase induction motor is designed to
operate within ± 10 percent of the rated voltage
on its nameplate. Operating motors significantly
above or below their rated voltages (“off-design
voltage”
1
) affects their efficiency and other operat-
ing parameters.
Under-voltage can cause damage to motors if it
is excessive. A fixed mechanical load on a motor
requires a certain amount of power to do the
work. Since the amount of power the motor must
draw is roughly the product of the voltage and
the current, if the voltage decreases, the current
(amps) must increase to do the same amount of
work. If the current drawn exceeds the full-load
value specified on the motor’s nameplate, over-
heating and damage likely will occur over time.
Under-voltage might cause production stop-
pages because the effects of low voltage are
compounded during starting and acceleration.
That is, under-voltage can affect the ability of a
motor to overcome the inertia of its load at startup
and slow its acceleration to full running speed.
Typically the running speed stabilizes just slightly
below normal but within ± 10 percent of its
nameplate value. However, so-called breakdown
torque is reduced, and the motor is less capable
of driving through a brief torque overload without
stalling.
Over-voltage, in full-load situations and when
it’s within the +10 percent range of the nameplate
value, actually enhances the efficiency of a motor.
However, at reduced loading, efficiency improves
with lower voltages as long as they are within
-10 percent of the nameplate value. Since under-
voltage increases current, it might seem that
over-voltage will decrease current. That is not the
case. In fact, at significant over-voltage, a motor
draws more current in attempting to compensate
for the effect of the over-voltage on the windings.
The result is overheating.