Service manual

With a phase loss, the current waveform of the remaining
phases would take on the appearance shown in
Illustration 6.17.
0.0 ms 5 ms/Div.
130BX144.10
Input B
Illustration 6.17 Input Current Waveform with Phase Loss.
Always verify the condition of the input voltage waveform
before forming a conclusion. The current waveform follows
the voltage waveform. If the voltage waveform is incorrect,
proceed to investigate the reason for the AC supply
problem. If the voltage waveform on all three phases is
correct, but the current waveform is not, the input rectifier
circuit in the frequency converter is suspect. Perform the
static soft charge and rectifier tests and also the dynamic
diode module test.
6.4.6
Output Imbalance of Motor Supply
Voltage Test
Check the balance of the output voltage and current to
measure the electrical functioning between the frequency
converter and the motor. In testing the phase-to-phase
output, both voltage and current are monitored. Conduct
static tests on the inverter section of the frequency
converter before this procedure.
If the voltage is balanced but the current is not, it
indicates the motor is drawing an uneven load. This could
be the result of a defective motor, a poor connection in
the wiring between the frequency converter and the
motor, or a defective motor overload.
If the output current is unbalanced as well as the voltage,
the frequency converter is not gating the output properly.
It could be the result of a defective power card or an
improper connection of the output circuitry.
NOTE
Use an analog voltmeter for monitoring output voltage.
Digital voltmeters are sensitive to waveform and switching
frequencies and commonly return erroneous readings.
The initial test can be made with the motor connected and
running its load. If suspect readings are recorded,
disconnect the motor cables to isolate the problem further.
1. Using a voltmeter, measure AC output voltage at
frequency converter motor terminals 96 (U), 97
(V), and 98 (W). Measure phase-to-phase checking
U to V, then U to W, and then V to W.
All three readings must be within 8 V AC of each other.
The actual value of the voltage depends on the speed at
which the frequency converter is running. The volts/hertz
ratio is relatively linear (except in VT mode) so at 60 Hz the
voltage should be approximately equal to the mains
voltage applied. At 30 Hz, it is about half of that, and so
on, for any other speed selected. The exact voltage
reading is less important than balance between phases.
2. Next, monitor three output phases at the motor
terminals 96 (U), 97 (V), and 98 (W) with the
clamp on the ammeter. An analog device is
preferred. To achieve an accurate reading, run the
frequency converter above 40 Hz as this is
normally the frequency limitation of such meters.
The output current must be balanced from phase to phase
and no phase must be more than 2-3% different from
another. If these tests are successful, the frequency
converter is operating normally.
3. If a greater imbalance exists than described
above, disconnect the motor cables and repeat
the voltage balance test.
Since the current follows the voltage, it is necessary to
differentiate between a load problem and a frequency
converter problem. Should a voltage imbalance in the
output be detected with the motor disconnected, it is
necessary to test the gate drive circuits for proper firing. If
output voltage is unbalanced, measure from ±UDC to the
output phases U, V and W respectively, to find out which
IGBT is not firing correctly. Then replace the power card to
determine if there is a gate firing problem. This applies for
frequency converters with separate IGBT modules. For
frequency converters with IGBTs on the power card, the
solution may already be found.
If the voltage was balanced, but the current imbalanced
when the motor was connected, the load is suspect. There
could be a faulty connection between the frequency
converter and motor or a defect in the motor itself. Look
for bad connections at any junctions of the output wires
including connections made to contactors and overloads.
Also, check for burned or open contacts in such devices.
6.4.7
Input Terminal Signal Tests
The presence of signals on either the digital or analog
input terminals of the frequency converter can be verified
on the frequency converter display. Digital or analog input
status can be selected or read in parameters 16-60 to
16-64.
Test Procedures
VLT
®
HVAC Basic Drive Service Manual
MG90X202 - VLT
®
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