Variable Frequency Drives: Motor Regeneration, Overhauling Loads,
and Starting Into a Rotating Motor
Copyright © 2004 Francis J. Martino
Motor Regeneration and Overhauling Loads
Upon power failure, the electric field of a motor winding will collapse. As the field
collapses, the motor becomes a generator and is capable of supplying an initial outrush
current that is equal to the motor starting inrush current. In addition, if the inertia of
the load is greater than the motor rotor inertia, then the load will be driving the
motor. The load is referred to as an overhauling load and the motor is then
referred to as being overhauled. An overhauling load will cause generator action
within the motor which will cause the motor to send current into the drive.
An example of an overhauling load is a punch or stamping press that has energy storage
in a flywheel. The flywheel will lower in speed as it delivers its stored energy during
the stamping portion of the duty cycle. The motor will then be required to accelerate the
flywheel to its maximum speed.
The inertia of the flywheel as it is decreasing in speed will then overhaul the motor at the
moment the motor attempts to accelerate the flywheel.
A blower or fan that is decelerating after a power failure will also present an overhauling
load if an attempt is made to restart the motor while the motor is still spinning.
Another typical example of an overhauling load is a machine that has cutting blades
that make repeated cuts of the process material. The cutting blades will also have a
Current flowing out of the motor due to overhauling is called regenerative current.
Regenerative current will pass into a variable frequency drive, increase the charge
on the DC bus capacitors and cause the DC bus voltage to rise. The DC bus voltage may be
observed to rise incrementally. After a number of cycles, the drive will initiate a high DC
bus voltage trip.
There are three types of drives that may be used to prevent a regenerative trip:
1. An eddy current drive that consists of a three phase motor winding, a magnetic clutch
that utilizes eddy current interaction, and a low powered controller that supplies DC
excitation to the clutch. An eddy current drive will not trip off with an overhauling
load. To prevent the clutch from being driven by the motor, an eddy current brake must be
added to the drive.
2. A regenerative drive that will accept the entire regenerative current of a motor and
transmit that current from the DC bus to the incoming AC power line of the facility system.
Regenerative drives are available for use with both AC and DC motors.
3. The third is a typical PWM variable frequency drive that will accept approximately
a ten per cent regenerative load without tripping. The higher the duty cycle is then
the less will be the regeneration that can be accomodated.
4. A typical PWM variable frequency drive with an active dynamic braking system. The dynamic
braking system will monitor the DC bus volts and, at a predetermined level, will pass the
regenerative current into the dynamic braking resistors where the current will be dissipated
as heat. Consult the manufacturer's specifications for duty cycle and ohmic values
of the braking resistors.
Starting Into a Rotating Motor
When starting or restarting into a rotating motor, the drive must be capable of dealing
with the regenerative current passing from the motor.
If dynamic braking resistors or a regenerative drive are not used then the drive must not
only sense the speed of the motor but must also be programmed to have a time delay long
enough to allow the motor to slow to a speed at which it will not present a regenerative
load that is greater than the nominal ten percent regenerative capability of the drive.
Ride Through on Power Failure
If incoming power fails momentarily, the drive microprocessor will remain active if it
derives power from the DC bus. However, if the microprocessor continues to fire the output
devices so as to drive a heavily loaded motor, the DC bus capacitors will quickly discharge
and arrive at the low voltage trip point.
If bypass contactors are used with the drive, a momentary power failure of sixty milliseconds
or longer will cause the contactors to open. The drive will then shut down. It may then be
repowered with the auto-restart feature.
A quick restart into a spinning motor is seldom needed. It is generally accepted
practice to allow the motor to come to rest and then utilize the auto-restart feature of
the drive to restart the motor.
Typical applications that may require a restart into a spinning motor are machinery
that drive sensitive processes or a blower that supplies positive pressure to a hospital
To maintain power on the bypass contactors and on the drive microprocessor, a double
conversion uninterruptible power supply (UPS) must be used. A double conversion unit
maintains a continuous low current to the load. When the power fails, the UPS will apply
full power instantaneously.