Re: [Emc-users] Brake resistor question

[Kirk W. <kwa...@wa...>]

On 04/17/2014 06:56 AM, Viesturs Lācis wrote:
> Hello!
>
> I have three chinese servo drives in the waterjet machine and they are
> constantly faulting with "overvoltage" error. Since I have not yet added
> any braking resistors, it seems obvious that I should do so.
> The question is: how do I determine appropriate resistance and rated power
> values for the resistor?
> Motor parameters are here:
> http://www.cutting.lv/fileadmin/user_upload/tg-motor.pdf
>
> Currently max current in motors is set to 1,4A, which makes me think that
> actual motor power is 500W.
> Experienced guy suggested braking resistor with 100-150 ohm resistance. But
> how do I determine correct rated power? Or is it that I just take the
> biggest power available as it cannot be too high?
>
> Viesturs

Some VFD manuals have sections covering braking resistors. Some resistor 
modules have sensors or protection devices of some type.

The Ohms and voltage values will set the current going through the 
resistor.

V = I * R
or
V / R = I,

let's say 240V / 100 Ohms = 2.4 Amps.

Watts = V * I or 240 * 2.4 Amps = 576 Watts.

If you go higher in Ohms, you will get less braking. The VFD's braking 
circuit will likely have a maximum current rating as well as the motor.

One of my shop-made braking modules (four gold colored resistors) is 
bolted to the back of my VFD here:
http://www.wallacecompany.com/cnc_lathe/HNC/00024-1a.jpg


-- 
Kirk Wallace
http://www.wallacecompany.com/machine_shop/
http://www.wallacecompany.com/E45/