Thread: Dynamic Brake Resistor

I have run a Powtran 2.2 kW 240/240 VFD 0-50 Hz on my CNC mill for a couple of years without a dynamic brake resistor.
Currently re-building the controls to include an emergency stop.

Checked the VFD rating plate, and it has the suffix "Z" denoting rated for brake resistor.
But, the manual does not state just what the resistor rating should be.

Could someone please confirm that the dynamic brake resistor should be 70 ohm, 300 watts.

I have a 200 x 55 x 30 mm extruded aluminium housed resistor of this rating on my shelf, and I think (?) it came with the VFD.
But I am not sure.

Keep well,
John.

Originally Posted by electrosteam

I have run a Powtran 2.2 kW 240/240 VFD 0-50 Hz on my CNC mill for a couple of years without a dynamic brake resistor.
Currently re-building the controls to include an emergency stop.

Checked the VFD rating plate, and it has the suffix "Z" denoting rated for brake resistor.
But, the manual does not state just what the resistor rating should be.

Could someone please confirm that the dynamic brake resistor should be 70 ohm, 300 watts.

I have a 200 x 55 x 30 mm extruded aluminium housed resistor of this rating on my shelf, and I think (?) it came with the VFD.
But I am not sure.

Keep well,
John.

It sounds right and You won't hurt the VFD by trying it.

And, for others who might not be lucky enough to have a large/expensive resistor on the shelf,

try some halogen light bulbs, or an old radiator element mounted in a box !

Originally Posted by nigelpearson

And, for others who might not be lucky enough to have a large/expensive resistor on the shelf,

try some halogen light bulbs, or an old radiator element mounted in a box !

When I tried an old radiator element and then 2x150W incandescents, they generated a short/bright pulse of yellow light from coils and the motor went THUMP and was stopped dead in its tracks.
Changing the decoration time didn't seem to make a lot of difference - possible something to do with element resistance changing with temp?
OK for some emergencies but I wouldn't usually subject any motor or machine to that sort of regular treatment - plus there is always the chance of bits like chucks coming of machines.

I fitted a VFD and a proper brake resistor to a old 12" radial arm saw at the mens shed. Even though it was about 70 years old it had very good bearings and after being switched off it would coast silently for many minutes representing a significant hazard especially for older operators with limited vision. When I added the brake I experiment with braking times starting and 10s and went down from there. It could be easily stopped in 2s but the motor made a loud screeching noise plus there was slo the possibility of loosening the blade at those sorts of decelerations. Eventually we settled on 4s - no motor noise and blade never loosened over the 4 years the shed had the saw.

I added a VFD with the view of adding a braking resistor to my WW bandsaw because it too ran on fairly quietly for ~90s after being switched off. However I found a foot brake to be more flexible as I change blades (6mm to 32mm) fairly regularly and the thought of having a single deceleration time for all blades did not appeal. With blades >13mm I can stop the blade in ~3s but with thiner blades I like to take it a bit slower.

Originally Posted by BobL

When I tried an old radiator element and then 2x150W incandescents, they generated a short/bright pulse of yellow light from coils and the motor went THUMP and was stopped dead in its tracks.
Changing the decoration time didn't seem to make a lot of difference - possible something to do with element resistance changing with temp?

Yes. Filament devices have serious PTC (positive temperature co-efficient):

xSspL.jpg

I think I used several small bulbs in parallel, to match the specified resistance, and so that they barely glow.


I haven't ever seen a thumping stop. When too-few globes were used, they flashed on, but the motor didn't slow down much. (no probs with radiator element)

Thanks BobL and others,
The question arose because the resistor had loop lugs on the connections, the Powtran VFD accepts ferrules or bare cores only..
A few minutes of thought said it was probably a purchased item, simply thrown in the box when the VFD shipped.
I agree to just suck and see, 70 0hms at 320 Vdc or so will take about motor full load current, so should stop in a couple of seconds.

It looks like the resistor will be mounted externally on the power/AC/VFD box for the mill, steppers etc in another box.

Keep well,
John.

Originally Posted by electrosteam

Thanks BobL and others,
The question arose because the resistor had loop lugs on the connections, the Powtran VFD accepts ferrules or bare cores only..
A few minutes of thought said it was probably a purchased item, simply thrown in the box when the VFD shipped.
I agree to just suck and see, 70 0hms at 320 Vdc or so will take about motor full load current, so should stop in a couple of seconds.

It looks like the resistor will be mounted externally on the power/AC/VFD box for the mill, steppers etc in another box.

If by "loop lugs" you mean "ring crimps" then I generally use these on all my VFD and motor conn
Screen Shot 2021-10-28 at 3.35.19 pm.png

The last thing I want to happen is for one of the phases to disconnect while the motor is under load.