1. Member
Join Date
Feb 2007
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40

## DC bus voltage

I was wondering what DC bus voltage means? I have a distribution center that has VFDs controlling 480v 3 phase motors and there is a display that you can watch that shows DC bus voltage. The incoming voltage is set at nameplate 480 but when you go to the display for bus voltage it says 600 and some volts. Can someone explain what this is for me. Oh there is no electric breaks or anything eles just the motor.
Thanks Tim

2. Moderator
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The input side of the VFD, the 480V, is a rectifier that feeds a DC bus. The DC bus is then inverted to become the output. It is not uncommon to see DC voltage greater than 600V and even into the 700V range. You need to be able to see this voltage for troubleshooting purposes.

3. Member
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Feb 2007
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40
Are you saying that 480v line in turns into 600 plus volts dc and then turns back into 480 load volts to the motor?

4. gar
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Ann Arbor, Michigan
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091013-2021 EST

Timboe:

Consider a 480 V sine wave. What is the peak value? What is the capacitor voltage in a capacitor input power supply relative to the input AC voltage?

The DC bus as said above supplies the inverter that generates the AC output to the motor. If there is energy fed back to the DC bus from the motor in dynamic braking, then the DC bus voltage will rise. CNC machines that do a lot of accelerating and decelerating have shunt resistors that get switched across the DC bus when the voltage rises too high. Other more expensive means can regenerate this energy back into the AC source.

Also what is the peak voltage to your 480 motor when supplied with 480 V AC?

.

5. Originally Posted by Timboe
Are you saying that 480v line in turns into 600 plus volts dc and then turns back into 480 load volts to the motor?
Yes it does. If you don't have a manual for the drive I suggest you obtain one online. There is usually an explanation in the manual that explains how it operates.

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