5 HP, and increase as the horsepower goes down.
Since 'I' represents current (amps) and 'R' stands for the amount of
resistance (Ohms), we can multiply the two numbers to get the IR
drop of any motor. Typical values of IR drop for RPM III motors
range between approximately 5-10% of rated voltage (25-50 V on
500 V motors or 12-24 V on 240 V motors) at rated horsepower.
IR drop compensation is an adjustment feature of most Reliance
Electric V
HS DC drives that largely compensates for the armature
IR drop of the DC motor while under load. As load changes on a
DC motor, the armature current, "I" also changes in the same
direction, i.e., if the load increases, the armature current increas-
es. With a voltage regulated drive, this causes the speed to
change as the load changes because armature terminal volts (Vt)
equals the sum of the IR drop plus the motor CEMF (Vt = CEMF +
IR). If the IR drop increases, then the CEMF must decrease
because the terminal volts are being held or regulated constant by
the control. It is desired to maintain the motor CEMF as nearly
constant as possible as the load changes. The IR drop
compensation adjustment accomplishes this as well as can be
reasonably expected.
One word of caution: IR drop compensation must never be
used with a speed regulator that uses a tachometer for
feedback. Drive instability will most likely result. The IR
compensation adjustment is to be used with voltage regu-
lated drives ONLY.
Instability is an unwanted variation in speed, usually both up and
down, at some frequency; 0that causes the drive to sound as
though it is 'chattering".
Warning: There must always be at least one other person
present when any drive adjustments are undertaken. This
is for safety reasons and is only good, common sense.
To adjust IR compensation, consult the instruction manual for the
drive under consideration. In general, the procedure is as follows:
1. Turn OFF the POWER ON/OFF circuit breaker or open the main
AC disconnect.
2. Locate the IR compensation adjustment in the drive cabinet. It
will usually be set from the factory to the fully counterclock
wise, or zero position.
3. Remove the load from the driven machine. The driven machine
itself should remain coupled to the motor or reducer. The term
"remove the load means to remove the product from the
conveyor, or to remove the board from a printer-slotter
machine, or remove the pellets from a plastics extruder, and so
forth. In short, we want to remove the VARIABLE load from
the motor.
4. Restore power and, after a suitable warm-up period, set the
speed potentiometer to run the motor at the speed at which
regulation is most critical: usually the machine linespeed. It
will be necessary to use a hand tachometer, oscilloscope,
strobe or some other precise RPM measuring device to
determine this speed, in RPM. Note this speed carefully and
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Publication D-7728 March 2000
© 2000 Rockwell International Corporation All Rights Reserved Printed in USA
write it down. DO NOT move the drive speed setting potentiome-
ter once this critical speed has been reached and noted.
5. Stop the drive. Remove power and add the load on the
machine.
6. Restart the drive, allowing adequate warm-up time.
7. Adjust the IR compensation potentiometer within the drive
cabinet until the motor runs at the same speed determined and
written down earlier. Do not set for higher than this speed
because it can result in instability. If instability does exist
under these operating conditions, reduce the IR compensation
potentiometer setting slightly until instabililty disappears.
IR compensation has now been properly adjusted and the best
speed regulation using the equipment you have as a voltage
regulator will be attained.
ANY VARIATION CALLED FOR IN AN INSTRUCTION MANUAL
TAKES PRECEDENCE OVER THE PROCEDURE GIVEN ABOVE.
NOTE: This material is not intended to provide operational instructions. Appropriate Reliance
Electric Drives instruction manuals precautions should be studied prior to installation, operation, or
maintenance of equipment.