Application Guide

63-706221

Test 1. Motor Run with Varying Loads on Commercial Power Supply

Fig. 23 indicates performance of a standard squirrel cage motor driven directly by a commercial power supply at 100%, 66%

and 33% load. On 100% load, motor body temperature rises quickly to 64°C, while ambient temperature remains stable around

20°C. When the load is reduced to 66%, body temperature quickly falls to 45°C, as expected under reduced load conditions.

Reducing load to 33% produced a further reduction in motor body temperature, to 40°C. However, the test was curtailed due to

lack of time. It is evident from the slope of the graph at this load, that the motor temperature would have continued to fall

several degrees before stabilizing at an estimated 38°C.

7.5 kw 4 pole motor

100%

66%

33%

°C

Motor

Ambient

Fig. 23. Motor run at 100%, 66%, and 33% load on commercial power supply.

Test 2. Motor Driven by VFD with Varying Loads.

Fig. 24 indicates the results of a similar test to test 1. However, the motor was driven by a VFD running at 50Hz, rather than a

commercial power supply. Early in the test, the VFD tripped. The diagnosed cause was VFD thermal overload. Caused by

setting the VFD to a level too low for the load. This overload setting was adjusted and the test restarted. At 100% load, the

motor body temperature reached 65°C when the load was reduced (see Fig. 24). However, had time allowed, the temperature

would have probably reached 70°C. As the load was reduced to 66 percent, the motor body temperature fell to 46°C. Reducing

the load to 33 percent caused the motor body temperature to fall to 38°C. During the test, ambient temperature was recorded

and was similar to that of test 1.

A comparison between the two tests indicates that when the motor under test was run on a VFD, under maximum load

conditions, at rated speed i.e. 50 Hz, the motor body temperature was increased by 5°C over that reached when run on the

commercial power supply. This small increase in temperature is still within the operating range of the machine and clearly

indicates that there is no need to derate a motor driven by a VFD, when the motor is run at full rated speed.

7.5 kw 4 pole

°C

Inv Trip

100%

66%

33%

Ambient

Motor

Fig. 24. Motor run at 100%, 66%, and 33% load on VFD.