Sizing of Braking Resistor

[kimseunghee]

I'd like to check braking resistor sizing of VFD. What philosophy is used in sizing braking resistor, i.e. how to size braking resistor.

 

[burnt2x]

Please contact the manufacturer. They'll be more than willing to assist you. Or Google "Braking resistor, VFD".

 

[DickDV]

While I agree with the above advice to Google for information first, I have just one little word of advice on this subject. Resistors are rated in continuous wattage. Power resistors can generally be hit with 10 times that wattage for 10 seconds if they are cold to start with.

So, if your only purpose is one stop per hour or e-stop service only, you can figure your braking kw and then buy a resistor rated one-tenth that wattage.

Saves a little money.

 

[blacksea]

Calculation of Shunt resistor value maybe performed as was sighed out above from energy point of view. I hope good reference would be AN 5448 from Galil(inside Motors and Amplifiers session) -

http://www.galilmc.com/literature/technotes.html

 

[LionelHutz]

There are 2 things to look at.

First is the rotational kinetic energy that you want to dissipate. The resistor has to be sized large enough power wise not to melt down when this energy is dumped into it.

Second is the current capability of the VFD. The manufacturer will either give a maximum current or minimum resistor size in the specification. This is used to find the ohms of the resistor.

 

[controlsdude]

http://www.postglover.com/

go here, if you buy from a vfd manufacturer, they usually resell from these guys. Used to be called IPC.

good info on site

 

[ozmosis]

If this is a specific enquiry then check that the actual VFD has the capability to 'dump' the regenerative energy into a resistor. VFD's need a 'brake chopper' (a 7th transistor) that is switched on when the internal DC voltage reaches a point before it would trip the VFD on overvoltage. Not all VFD's have this brake chopper included and if not, then you need to connect one externally. The current rating of this transistor will also determine the rating of the resistor.
As already pointed out, the VFD manual will typically inform you of the resistor ratings if the brake chopper is integral.

 

[cdevine]

Continuing on the theme of inertia, if you try to slow the motor down too quick it can lead to VFD trip due to overvoltage on the DC link.

I agree, contacting the VFD manufacturer is the way to go but they do need some specific info on the load that you are trying to pull up. It is not a trivial calculation you need velocity info of the load and masses.

Chris Devine

http://www.icande.com.au

http://www.redbusbar.com