Application: VSD failure
Introduction
• A variable speed drive on a critical system, the
fault indicator LED is on
• According to the VSD (variable speed drive), the
error code is F4; this is: the drive has detected an
“under voltage” condition and shuts down
How do you identify if the failure is in the drive,
in the motor or in a distorted supply voltage?
A typical industrial electrical distribution system
with non-linear loads
Diagnosing VSD failure
Using a DMM
• Check the input line voltage (figure 1)
– A typical multimeter will display either average
or rms voltage values
– Depending on the type of distortion, the rms or
peak value may not reveal if a problem exists
• Check the VSD dc bus voltage
– The dc bus voltage is directly proportional to the
peak of the input line voltage
– Any distortion or error in peak amplitude of the
line voltage can cause an over- or undervoltage error
• The drive dc bus voltage in figure 2
shows around 20 % under the nominal
value of 160 V
Do you replace the drive
controller and/or motor?
Figure 1. Measuring the line voltage at
the input to a Pulse Width Modulated
drive, appears to be normal
Figure 2. Checking the dc bus voltage
level is less than nominal revealing a
potential problem
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