SCR test part I
1.
Remove the power card mounting plate.
2.
Unplug the MK1802 connector from the inrush
board.
3.
The plug has three pairs of wires, one for each
SCR module. Measure the resistance of each pair.
Red is the SCR gate and black is the SCR cathode.
A proper reading is be 5–50 Ω. A significantly higher
reading or an open circuit indicates a failed SCR or a faulty
connection.
SCR test part II
1.
Check the connections of the gate cables to the
SCR/diode modules.
2.
If the connections are good, replace the failed
SCR/diode module.
If the SCR checks are successful and there is still no DC bus
voltage, replace the inrush board.
6.4.9 Output Imbalance of Motor Voltage
and Current
Checking the balance of the adjustable frequency drive
output voltage and current is a way to measure the
electrical functioning between the adjustable frequency
drive 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 adjustable frequency
drive before performing this procedure.
If the voltage is balanced but the current is not the motor
could be drawing an uneven load. This could be the result
of a defective motor, a poor connection in the wiring
between the adjustable frequency drive and the motor, or,
if applicable, a defective motor overload.
If the output current is unbalanced as well as the voltage,
the adjustable frequency drive is not gating the output
properly. This could be the result of a defective power
card, gate drive, connections between the gate drive card
and IGBTs, or the output circuitry of the adjustable
frequency drive being improperly connected.
NOTICE!
Use a PWM-compatible digital or 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.
Monitor three output phases at adjustable
frequency drive 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 adjustable frequency drive above
40 Hz, which is normally the frequency limitation
of such meters.
A balanced output current from phase to phase is correct.
A variation of more than 2–3% is not correct. If the test is
successful, the adjustable frequency drive is operating
normally.
2.
Using a voltmeter, measure AC output voltage at
adjustable frequency drive 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.
A variation of more than 8 V AC among the three readings
is not correct. The actual value of the voltage depends on
the speed at which the adjustable frequency drive is
running. The volts/hertz ratio is relatively linear (except in
VT mode) so at 50 Hz/60 Hz the voltage is approximately
equal to the AC line voltage applied. At 25 Hz/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.
If a greater imbalance exists, disconnect the motor leads
and repeat the voltage balance test.
Since the current follows the voltage, it is necessary to
differentiate between a load problem and an adjustable
frequency drive problem. If a voltage imbalance in the
output occurs with the motor disconnected, test the gate
drive circuit for proper firing. Proceed to
chapter 6.4.10 IGBT Switching Test
If the voltage was balanced but the current imbalanced
when the motor was connected, then the load is suspect.
There could be a faulty connection between the adjustable
frequency drive 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.
Test Procedures
Service Manual
82
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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