Factor Correction On Induction Motors
Very often power factor
correction capacitors are applied to an induction (asynchronous)
motor circuit to reduce the inductive or lagging current associated
with the magnetizing current of the induction motor. In many
applications (normally when the motor is large), a single
power factor correction capacitor or filter will be applied
to the load side terminals of the motor controller as illustrated
in Figure 1 below. For this application, the capacitor is
energized when the motor controller is closed (motor running).
The benefit to this type of application is as follows:
The reactive power requirements of the
motor are only supplied when the motor is running. This
effectively provides automatic control of power factor.
Total equipment cost are reduced as
the motor controller performs the capacitor switching
- The voltage profile to the motor is improved.
A major drawback to
this type of capacitor application, however, is improper
sizing of the capacitor can lead to motor failure. To
large of a capacitor leads to self-excitation of the
motor, which can result in motor insulation failure.
occurs when the capacitive reactive current from the
capacitor is greater than the magnetizing current of
the induction motor. When this occurs, excessive voltages
can result on the terminals of the motor. This excessive
voltage can cause insulation degradation and ultimately
result in motor insulation failure. Figure 2 below illustrates
a simplified circuit diagram for the disconnected capacitor
- Typical Power Factor Correction
Capacitor Application On Induction Motor
To understand the phenomenon of self-excitation and why high
voltage can occur, it must first be understood that when the
motor is connected to the source with the motor controller
closed, a rotating magnetic field is set up between the stator
winding and the rotor winding of the induction motor (in the
air-gap of the motor). This rotating magnetic field can be
thought of as stored energy. When the motor is switched off,
the stored energy still present in the air-gap of the motor
begins to collapse and produce a current in the rotor winding.
This rotor current induces a voltage on the stator winding
and terminals of the motor which are disconnected (the motor
becomes a generator). Because the motor has just been disconnected,
it is still spinning due to its rotating inertial speed which
will decrease in time. The decaying speed produces a subsequent
voltage (and current flow through the capacitor) at a decaying
frequency (starting at a value near 60 hertz). When the frequency
of the motor terminal voltage equals the resonant frequency
of the motor and capacitor reactance combination, high voltage
may be produced. This high voltage can lead to insulation
failure on the motor.
2 - Simplified
Circuit Diagram for Motor Controller and Power Factor Correction
Capacitors (Diagram Shown For Motor Controller in Open Position)
To create self-excitation,
the capacitive reactance of the capacitor must be less than
that of the motor reactance (this occurs when to large of
a capacitor is chosen). This combination of reactance will
result in a resonant frequency below 60 hertz (for the circuit
in the above diagram). Therefore, as the motor slows in speed,
the frequency of the motor terminal voltage will decrease
from a value of near 60 hertz toward zero. When the motor's
terminal voltage frequency passes through the resonant frequency
setup between the capacitor reactance and the motor reactance,
the terminal voltage will become very high, only limited by
the properties of the iron. Depending on the inertia of the
motor, this resonance (or high voltage) may be present for
a considerable period of time.
On the other hand, if the
capacitive reactance is greater than the motor magnetizing
reactance (this occurs for a properly sized capacitor), the
resonant frequency is greater than the motor speed (greater
than 60 hertz). Under this condition, when the motor is disconnected,
the frequency of the decaying terminal voltage will never
correspond with the resonant frequency of the motor and capacitor
reactance combination. Therefore, a high voltage condition
will not occur.
Figure 3 below helps to illustrate
how to large of a capacitor can result in an over voltage
condition on the motor. The figure shows a plot of the capacitor
and motor magnetizing voltage verses current waveforms. As
can be seen, the motor magnetizing curve is sloped over, which
is a characteristic of iron. The capacitor characteristic
is a strait line. Two capacitor characteristics have been
drawn on the plot, one which represents a properly sized capacitor,
and one which represents an improperly sized capacitor (to
large of a capacitor). The curve labeled "A" is
sized properly because its capacitive current is less than
that of the magnetizing current at nominal voltage. The curve
labeled "B" is sized improperly because its capacitive
current is greater than the magnetizing current at 1 per-unit
voltage. When disconnected, the "B" curve in figure
3 shows a valid operating point at 140% voltage. This voltage
may occur as the motor slows in speed and passes through its
Figure 3 - Typical
Motor Saturation Curve and Capacitor Characteristic Curves
at a given frequency
When applying capacitors on terminals of motors, it is important
that the capacitor be sized correctly. To large of a capacitor
can cause self-excitation of the motor and lead to motor insulation
NEPSI recommends one of the
following techniques be used when applying capacitors or harmonic
filers directly on the terminals of an induction motor.:
Request a recommended kvar rating from
the motor manufacturer.
Size the capacitor at 80% of the no-load
current rating (magnetizing current) of the motor. In
no case should the rating be greater than 90%.
Utilize recommended capacitor sizing
tables induction motors. Motor tables, however, do not
guarantee a properly sized capacitor and may not account
for newer more efficient motor designs. The values published
in these tables, have been found in many cases to be acceptable.
If tables are utilized, NEPSI recommends the motor terminal
voltage be checked on commissioning of the motor.
Measure the no-load motor current and
size the capacitor at 80% of the no-load current rating
of the motor.
Power Systems, Inc.
66 Carey Road
Queensbury, New York 12804
Copyright © 1999
- 2009 Northeast Power Systems, Inc.