5.1.5 Motor/Load Problems
Problems with the motor, motor wiring or mechanical load on the motor can develop in a number
of ways. The motor or motor wiring can develop a phase-to-phase or phase-to-ground short re-
sulting in an alarm indication. Checks must be made to determine whether the problem is in the
motor wiring or the motor itself.
A motor with unbalanced, or non-symmetrical, impedances on all three phases can result in un-
even or rough operation, or unbalanced output currents. Measurements should be made with a
clamp-on style ammeter to determine whether the current is balanced on the three output phases.
See Output Phase Imbalance Test procedure.
An incorrect mechanical load will usually be indicated by a torque limit alarm or warning. Discon-
necting the motor from the load, if possible, can determine if this is the case.
Quite often, the indications of motor problems are similar to those of a defect in the adjustable
frequency drive itself. To determine whether the problem is internal or external to the adjustable
frequency drive, disconnect the motor from the adjustable frequency drive output terminals. Per-
form the output phase imbalance test procedure (6.3.10) on all three phases with an analog
voltmeter. If the three voltage measurements are balanced, the adjustable frequency drive is
functioning correctly. The problem therefore is external to the adjustable frequency drive.
If the voltage measurements are not balanced, the adjustable frequency drive is malfunctioning.
This typically means that one or more output IGBT is not switching on and off correctly. This can
be a result of a defective IGBT or gate signal from the gate drive card. Perform the IGBT gate
signal test (6.3.11).
5.2 Internal Adjustable Frequency Drive Problems
The vast majority of problems related to failed adjustable frequency drive power components can
be identified by performing a visual inspection and the static tests as described in the test section.
There are, however, a number of possible problems that must be diagnosed in a different manner.
The following discusses many of the most common of these problems.
5.2.1 Overtemperature Faults
In the event that an overtemperature indication is displayed, determine whether this condition
actually exists within the adjustable frequency drive or whether the thermal sensor is defective.
Of course, this can easily be detected by feeling the outside of the unit, if the overtemperature
condition is still present. If not, the temperature sensor must be checked. This can be done with
the use of an ohmmeter in accordance with the thermal sensor test procedure.
High Power Service Manual for VLT
®
FC Series
Adjustable Frequency Drives
5 Adjustable Frequency Drive and Motor Ap-
plications
MG.90.L1.22 - VLT
®
is a registered Danfoss trademark
5-5
5
Summary of Contents for VLT HVAC FC 100
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