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 resulting 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 uneven or rough
operation, or unbalanced output currents. Measure with a
clamp-on style ammeter to determine whether the current
is balanced on the three output phases.
A torque limit alarm or warning usually indicates incorrect
mechanical load. Disconnect the motor from the load if
possible to determine whether 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.
Perform the output imbalance of supply voltage test
procedure 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 type of
malfunction typically means that one or more output IGBTs
are not switching on and off correctly. A defective IGBT or
gate signal from the gate drive card can cause this.
Perform the IGBT gate signal test.
5.2 Internal Adjustable Frequency Drive
Problems
Most 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. However, there are 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
When an overtemperature indication is displayed,
determine whether this condition actually exists within the
adjustable frequency drive or whether the thermal sensor
is defective. This can easily be detected by touching the
outside of the unit to see if the overtemperature condition
is still present. If not, check the temperature sensor with
an ohmmeter.
5.2.2 Current Sensor Faults
An overcurrent alarm that cannot be reset, even with the
motor cables disconnected, sometimes indicates current
sensor failure.
The adjustable frequency drive experiences frequent false
ground fault fault trips due to the DC offset failure mode
of the sensors.
An explanation of the internal makeup of a Hall effect type
current sensor helps to explain these faults. Included inside
the device is an op-amp to amplify the signal to usable
levels in the receiving circuitry. The output at zero input
level (zero current flow being measured) is zero volts,
exactly halfway between the plus and minus power supply
voltages. A tolerance of +/-15 mV is acceptable. In a three-
phase system that is operating correctly, the sum of the
three output currents is always zero.
When the sensor becomes defective, the output voltage
level varies by more than the 15 mV. The defective current
sensor in that phase indicates current flow when there is
none. This results in the sum of the three output currents
being a value other than zero, which is an indication of
leakage current flowing. If the deviation from zero (current
amplitude) approaches a specific level, the adjustable
frequency drive assumes a ground fault and issues an
alarm.
To determine whether a current sensor is defective,
disconnect the motor from the adjustable frequency drive,
and then observe the current in the adjustable frequency
drive display. With the motor disconnected, the current
should be zero. An adjustable frequency drive with a
defective current sensor indicates some current flow.
Because the current sensors for the higher horsepower
adjustable frequency drives have less resolution, an
indication of a fraction of one amp is tolerable. However,
that value should be considerably less than one amp. If the
display shows more than one amp of current, a current
sensor is defective.
Adjustable Frequency Drive ...
Service Manual
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Danfoss A/S © Rev. 2014-02-10 All rights reserved.
MG94A222
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Summary of Contents for VLT FC 103
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