Cause for DC bus voltage to drop when the motor is loaded?

Product Line:
All Variable Frequency Drives.

Environment:
All models, All serial numbers.

Cause:
As the speed of the motor increases, and/or as the load on the motor increases, the DC bus voltage is dropping and the motor current is increasing, while the main input voltage stays the same or drops very slightly.

Resolution:
There are multiple possible causes for this situation, but one that should be ruled out first is "flat topping" of the incoming power. The DC bus capacitor bank in the drive is charged to the peak to peak voltage of the incoming AC mains power.  To determine what the DC bus voltage should be, multiply the incoming AC voltage as measured by an RMS meter by the square root of 2.  Approximately 1.414 X the input RMS voltage.  As an example, if you read 480V AC incoming voltage RMS, the peak to peak AC voltage would be 678.8.   The DC bus would be charged to 480 X the square root of 2, or 678.8VDC

When you measure the incoming voltage with a voltmeter, you are measuring the RMS voltage.  So the DC voltage on the DC bus will be 1.414 X the RMS reading of your incoming AC voltage.  If your incoming power is overloaded slightly, your peak to peak incoming voltage will dip slightly.   This will have a very small change on your RMS meter reading, but a more significant change on the DC bus voltage.   As the DC bus voltage drops, the motor current will increase to maintain the same power at the motor.  If you were to look at your incoming power to the drive with an oscilloscope, you would see the top of the waveform would be flattened slightly.  This is an indication that your incoming power is being overloaded.  Steps should be taken to increase the amount of power available on your incoming power to the drive.