KEY TECHNOLOGY
Restarting Motors that are Already Spinning Using
Variable Frequency Drive Technology
our facility has experienced a brown
Y
out. Your motors are beginning to slow
down. Typically, you would need to wait
until all motors have stopped completely before restarting – resulting in longer
down-time and increased power usage on
start-up. But with software solutions built
into some variable speed drives (VFD’s), restarting an already spinning motor can be
fast and efficient.
In a normal induction motor starting operation with a drive, the output of the drive
is synchronized with the induction motor
flux which is derived from the applied stator voltage and resulting current flow. The
output frequency starts from an initial value
of zero, motor speed is also zero and ramps
up synchronized together.
However, starting on an already spinning
motor either in the same or reverse direction is a challenge. In the case of across the
line starting, it is usually required that the
system come to rest before starting. In the
case of adjustable speed drives, the drive
may have difficulty since it normally starts
at zero speed and the motor is turning at a
different speed. High currents result and
the drive may shut down.
Toshiba MTX Nema 3R MV ASD
Drive manufacturers recognizing the
need to start into rotating loads developed
a software solution that usually does the following steps on starting after power loss:
1.
The drive output voltage is set to
some low voltage such as 10%.
2.
The drive starts outputting varying frequency at the same fixed low voltage
from zero to full speed.
3.
The motor rotor is at some rotational speed different from the drive frequency. When the drive puts low voltage
to the motor at the wrong frequency, the
current will go to a specific level. As the
drive is scanning the frequency band forward and reverse, there will occur at some
point where the rotor speed will match the
44 November 2009 PROCESSWest
Toshiba T300MVi ASD
stator scanning frequency, at which time
the current in the stator drops dramatically and the drive software recognizes the
instantaneous current change as the rotational speed of the rotor.
4.
The drive software once it has determined the correct rotational frequency
raises the voltage to normal and the drive is
now in control of the motor.
The disadvantages of the above method
are many. The scanning takes time and the
motor may lose significant speed. If you
scan too quickly, the software may not detect the motor speed. In the case of low
inertia systems, the motor speed may be
dynamic and the software cannot react
quickly enough. This method works generally well for high inertia loads such as a
fan using a voltage source drive. However
in case of high inertia loads and with use
of current source inverter technology, the
motor may self excite with the drive motor filter capacitors and not have the motor
field collapse; to avoid such self excitation,
an output contactor may be needed to temporarily disconnect from the drive.
With voltage source drives, it is commonly known that induction motors will
no longer have a terminal voltage on the
motor shortly after the loss of power (2-4
seconds) even if the motor is still rotating.
Not commonly known, the induction motor has magnetic hysteresis in the iron of the
rotor that makes it a miniature three phase
generator. Any induction motor, even if
power has been off for a long time will generate a low three phase voltage if rotated
by external means. This voltage is used to
detect rotation in electronic devices called
backspin detectors that are commonly used
in the oilfield for delaying start on submersible pump motors turning backwards due
to a falling column of oil.
Some drive manufacturers utilize this
three phase residual voltage in the motor
exactly the same as an encoder feedback to
determine motor speed and direction before starting. The drive has additional custom hardware internally that would make
quadrature pulses similar to a commercial
encoder A and B output directly from the
sine wave voltage being detected. The advantage of this method allows the drive instant information of the motor speed and
direction which allows the drive to take immediate control and start with no delay.
When the direct detection method described above is enabled, the drive keypad
display shows the frequency the motor is rotating as soon as power is turned on. When
a start command is enabled, the drive starts
exactly at the frequency of the rotating
motor, ramps voltage to normal and then
accelerates or decelerates the motor to the
commanded speed and direction.
Toshiba Dura Bull MV Motor
Induction motors that have too much
power back for the drive to return the load
to normal speed will torque limit at an adjustable setting until the dynamic conditions return to reasonable levels and then
motor will resume normal commanded
speed.
If you have questions regarding this control system, or
any other motor and/or drive questions, please contact:
Angelo Cultrera, Power Products Manager,
Tundra Boiler & Instrumentation Ltd.
Authorized Distributor for Toshiba Motors & Drives
Ph: 403.861.1225, E-mail: angeloc@tundrasolutions.ca
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