Trane TR150 - Output Imbalance of Motor Supply Voltage Test

Illustration 6.15 Normal AC Input Voltage Waveform
The waveform shown in Illustration 6.16 shows the input
current waveform for the same phase as shown in
Illustration 6.15 while the frequency converter is running at
40% load. The 2 positive and 2 negative jumps are typical
of any 6-diode bridge. It is the same for frequency
converters with diode modules.
Illustration 6.16 AC Input Current Waveform with Diode Bridge
With a phase loss, the current waveform of the remaining
phases would take on the appearance shown in
Illustration 6.17.
Illustration 6.17 Input Current Waveform with Phase Loss
Always verify the condition of the input voltage waveform
before drawing 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 3-phases is correct,
but the current waveform is not, the input
rectier
circuit
in the frequency converter is suspect. Perform the static
soft-charge and
rectier
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 that 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 and the voltage are unbalanced, it
indicates that the frequency converter is not working
properly. It could be the result of a defective power card or
an improper connection of the output circuitry.
NOTICE
Use an analog voltmeter for monitoring output voltage.
Digital voltmeters are sensitive to waveform and
switching frequencies and commonly return erroneous
readings.
Perform the initial test with the motor connected and
running its load.
If suspect readings are recorded, then:
•
Stop the motor and wait until the motor has
stopped rotating.
•
Set the frequency converter to coast.
•
Disconnect the motor cables to isolate the
problem further.
Then:
1. Using a voltmeter, measure AC output voltage at
frequency converter motor terminals U, V, and W.
Measure phase-to-phase checking U to V, then U
to W, and then V to W.
All 3 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 V/Hz ratio is relatively linear (except
in VT mode). For example, if the rated motor
frequency is 60 Hz, the voltage should be approx-
imately equal to the applied mains voltage. At
30 Hz, it is about half of the applied mains
voltage for any other speed selected. The exact
voltage reading is less important than balance
between phases.
2. Stop the frequency converter and disconnect
mains.
Test Procedures
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