Re: [CAD_CAM_EDM_DRO] Re: Sizing of braking resistors for a VFD

Bob McKnight wrote:

>To: CAD_CAM_EDM_DRO@yahoogroups.com
>From: "washcomp" <jeff@...>
>Date sent: Wed, 10 Mar 2004 23:43:18 -0000
>Subject: [CAD_CAM_EDM_DRO] Re: Sizing of braking
>resistors for a VFD
>Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com
>
>
>I don't understand 'braking resistors' on a 3 phase induction motor.
>Can someone please clarify???
>
>

When VFDs are decelerating an induction motor, energy is extracted
from the motor and sent back to the DC supply. This causes a rise
in voltage that can eventually destroy the power transistors in the drive.
So, some means must be provided to dissipate this energy. The largest ones
use synchronous inverters to send the power back to the mains. Otherwise,
resistors are connected momentarily to the DC bus to bleed off the
excess energy.

Jon