Chapter 8 - Troubleshooting & Maintenance
8-3
8.3 Troubleshooting
Condition Checking and Fixing Procedures
The motor does not start. Check if red light on keypad is not flashing (no VFD fault).
Check if display shows FWD when start command is applied.
• If not, check parameter I/O-28 for FWD and REV run bits, only one of
them should be 1.
1. If not, check start contact wiring and closure and check the PNP-NPN
dip switch position.
2. If yes, check parameter SET-15 for Run Prevention setting.
• If yes, check if speed reference is greater than 0.5Hz
1. If not, check speed reference selection in SET-10, analog signal wiring
and level.
2. If yes, check the motor wiring, motor disconnect or contactor. All
disconnecting devices should be closed and motor should have
connection to VFD output terminals. Check VFD output voltage
corresponding to the output frequency. If VFD is running at some
frequency and there is no output voltage, call Cerus tech support.
The motor rotates in
opposite direction.
If VFD is running and shows FWD but motor rotates in reverse direction, stop
and power down VFD, wait at least 10 minutes, and then swap any two motor
wires to change the motor rotation.
The VFD stays at high
speed when speed
reference decreases.
The electrical noise in analog signal can cause VFD not to follow a speed
reference, especially for 0-10VDC signal with non-shielded cable. Increase I/O-
1 filtering time up to 500ms. If this does not help, install shielded cable.
The Motor current is
higher than FLA rating.
Check mechanical system for conditions that can overload the motor (dirty
filter, closed valve or brake, etc.) In a new installation, check motor windings
wiring. If motor windings wiring is correct, switch FG2-60 to Sensorless mode
and run FG2-61 Auto Tuning.
The motor runs at very
low speed regardless of
speed reference signal.
Increase FG1-71 Stall level if FG1-70 is set to Yes.
Decrease FG2-68 Torque Boost level.
The motor speed is not
stable (oscillates).
Check motor winding configuration wiring. If motor speed oscillates because of
speed reference oscillation, check analog signal for noise. Check mechanical
system for conditions that can create load level oscillation (vibration,
unbalanced load, etc).
If distance to the motor is more than 1500 feet, install output filter.
8.4 Checking VFD Power Components
The VFD input and output power components are semiconductors and can be checked by multimeter with
diode check mode. For the direct polarity test, when multimeter positive (red) lead is connected to Diode
anode (triangle) and negative (black) lead to cathode (bar), the reading should be for open diode state
0.3~0.5 V. For reverse polarity test when multimeter leads are swapped, the reading should be more than
2.5V ("OL" on some meters) for closed diode state. If diode is shorted, the reading in direct and reverse test
will be close to 0V. If diode is open (exploded), the reading in direct and reverse test will be more than
2.5V ("OL" on some meters). The IGBT transistors cannot be checked with multimeter but diodes across
IGBTs can.
8.4.1 Diode Bridge and IGBT Module Test for 7.5~40HP VFDs
Before checking the power components, disconnect AC input power and wait for about 10 minutes.
Check that there is no DC voltage on P1+ and N- terminals.
Disconnect power wires from R, S, and T terminals and motor leads from U, V, and W terminals.
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