If you get this fault, the motor will coast to a stop. The fault code will be displayed on the digital operator, and the fault relay will be activated.

This Fault Code is seen when the inverter main circuit senses DC Bus Voltage at or below the appropriate trip point.
For a 240VAC inverter (low voltage) application, the default trip point is 190 VDC.
For a 480 VAC inverter (high voltage) application, the default trip point is 380 VDC.

You can measure the voltage of the DC Bus at terminals +1 to -.

Common causes of this fault and actions to take include:
Low Input Voltage to the Inverter at terminals R/L1, S/L2, and T/L3. Check the input circuit voltage with a voltmeter on the AC scale at the terminals. If it is low, find the cause, the inverter is not the problem. Set the voltmeter to the appropriate range based on the inverter model.

Failed Input Diode of the rectifier circuit. With no power on the inverter, set the Multimeter to Volts DC and on the highest scale, check the DC Bus Voltage <10VDC by measure positive lead to +1 terminal and negative lead to the - terminal. Then change your multimeter to the diode test check. With the positive lead on the R/L1, and the negative lead on the +1 terminal, you should measure a low reading (about .5vdc). Repeat this check with the positive lead on S/L2 to +1, and T/L3 to +1. All should be a low reading. Switch your leads with the negative lead at R/L1 and the positive lead at +1. The reading should now be high (overlimit). Repeat for S/L2, and T/L3 to +1. Again the readings should be high. Now measure between the - terminal and R/L1, S/L2, and T/L3. With the positive lead on the - terminal, and the negative lead at R/L1, S/L2, and T/L3 the readings should be low (about .5vdc). Reversing the leads with the negative lead on the - terminal, and the positive lead at R/L1, S/L2, and T/L3 the readings should be high (OL). If you read a low and a low as you read across the diode, the diode is shorted and is the cause of your fault.

Acceleration time constant for your model of inverter has been changed and is too short. The inverter tries to ramp up to full load too quickly and is dragging down the DC Bus voltage. Check the Inverter constant for the accel constant and make sure it is set properly. Contact Milnor Customer Support for more guidance as needed.

CSH 01-14-09