Application Guide

63-7062 20
V
o
l
t
s
Phase L1
400
300
200
100
0
-100
-200
-300
-400
Time ( ms )
0
2018
1614
12108
6
4
2
10
7.5
5
2.5
0
-2.5
-5
-7.5
-10
A
m
p
s
7.5 kW Motor No Load
Voltage
Voltage
Current
Current
Fig. 21. Motor voltage and current under no load conditions.
Note the shape of the current waveform in Fig. 21 with respect to the voltage. It is the non sinusoidal format of the current
waveform that causes the harmonics on the network and can ultimately cause voltage waveform distortion.
V
o
l
t
s
Phase L1
400
300
200
100
0
-100
-200
-300
-400
Time ( ms )
0
2018
1614
12108
6
4
2
40
30
20
10
0
-10
-20
-30
-40
A
m
p
s
7.5 Motor Full Load
Voltage
Voltage
Current
Current
Fig. 22. Motor voltage and current under full load conditions.
Note that the peak current in Fig. 22 has risen from 7.5 amps to almost 40 amps. Motor full load current is 13.5 amps.
Motor and VFD Tests
In the past, the VFD was seriously criticized for causing motor problems- in particular causing the motor to overheat to the
extent where motor winding insulation was damaged, resulting in
motor burnout
. As a result, it was often recommended by
suppliers/manufacturers/consultants to down rate a VFD driven motors by 10 percent.
Many advancements have been made in the control design of the modern VFD to an extent now that for centrifugal fan and
pump type loads, no derating is required. The following sections contain results of a sequence of tests indicating the heating
effects of running a standard 7.5 kW squirrel cage motor connected to the commercial power supply and to a general purpose
VFD. The loading was provided by a calibrated dynamometer. Thermal imaging equipment was used to identify the
hot spots
on the outside of the motor body. Type K thermocouples were secured to the cleaned surface of the motor body at these points
using epoxy resin and then thermal insulation applied.