Application Guide

63-7062 22
Test 3. Motor Run on VFD with Fan Load Simulation.
The purpose of this test is to prove that the motor will not suffer harmful effects of overheating, when driving a fan or pump type
centrifugal load at varying speeds. The dynamometer was configured to produce similar characteristics to those of a centrifugal
fan. The motor was then run up to 50 Hz an the load adjusted to 100%. After a short time, it can be seen from Fig. 25, that the
motor body temperature rose to a similar level as that reached in test 1 and 2 in the same time frame. The speed of the motor
was then reduced in steps from 50 Hz to 40, 30 and 20 Hz. In each case the temperature of the motor body fell as the speed of
the motor reduced. No auxiliary cooling was provided for the motor i.e. the only cooling air flow was provided by the shaft-
mounted fan on the motor. Clearly, there are no harmful heating effects caused by driving a motor with a VFD, on centrifugal
type loads and there is no need to derate the motor.
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7.5 kw 4 pole
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H
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Ambient
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Fig. 25. Fan load with motor running at various frequencies.
Test 4 Motor Run on VFD with Constant Torque Load.
This test was by far the most arduous for the motor and VFD since the dynamometer was configured to hold the torque stable
across a speed range from 50 Hz down to 15 Hz. This was not a test to prove the capabilities of the VFD to produce full load
torque at reduced speeds, rather it was to show the heating effects under these operating conditions. Most modern VFDs can
cause the motor to produce full load torque down to 1.5 Hz or less.
Time constraints limited the amount of test time at the higher speeds, thus not allowing the motor body temperature to stabilize
at each step in the speed range, however a full load 50 Hz test had already been undertaken in test 2. It can be seen from Fig.
26 that under constant torque conditions, the motor body temperature will rise with each step reduction in speed.
30 Hz is probably the minimum speed at which a standard motor can be run continuously under full load torque conditions,
however for short periods of time, much lower speeds at full load torque can be tolerated without adverse effects. If regular
operation at speeds lower than 30 Hz and at full load torque are anticipated, then an auxiliary fan should be installed, in order
to produce the desired cooling for the motor. Brooke Hansen and other motor manufacturers can provide standard motors with
auxiliary cooling fan fitted as standard.