# Choosing a braking resistor

#### daxliniere

Joined Aug 7, 2021
52
Hey gang,
I have 2 projects I'm working on at the moment that have DC motors. One is a motorised axis on a milling machine using a DC servo motor (not RC servo) that has a huge planetary gearbox with maximum output of 60 rpm. The other is a treadmill motor that I'm installing into a metal turning lathe to replace a 1/4HP AC induction motor.

My question is around how to choose the appropriate resistance and current capacity of the braking resistors for each of these. On both, I'm planning to add a DPDT relay with connections for motor speed controller (input), DC motor (output) and dummy load (braking resistor).
I know that I can simply short the motor terminals, but I was concerned about the long-term effects (especially since the treadmill motor has a whopping flywheel on it). From this, I know that the lower the resistance, the faster the stop, but what is too low?

For power handling, though the treadmill motor is over 2000W, I suspect I won't need a braking resistor that can handle anywhere near that, since it's only going to be for a second or two.

The mill's servo motor already stops pretty quickly (that's what a bazillion:1 planetary gearbox will do for you!), but I would love to have it stop on a dime.

I'm still using the treadmill's motor controller, but it seems to take a fairly long time to come to a stop. This doesn't have to stop instantly, but it would be helpful to reduce the spin-down time as much as possible

Any help with selection would be greatly appreciated.

All the best,
Dax Liniere.

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#### MrSalts

Joined Apr 2, 2020
2,767
Stopping on a dime with a whopping flywheel on it will be a challenge without almost short circuiting the leads. A big MOSFET with low on-resistance is a good option.

#### MrChips

Joined Oct 2, 2009
28,155
To stop on a dime you need a negative resistance.
This may sound like an impossibility but it is doable with electronics.

#### daxliniere

Joined Aug 7, 2021
52
Thanks @MrSalts and @MrChips (still waiting for MrPepper and MrVinegar.. )
As I mentioned, the motor with the flywheel doesn't need to stop instantly, but it would be nice to reduce the deceleration-to-stop time since it'll be on a metal turning lathe.

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#### MrSalts

Joined Apr 2, 2020
2,767
Well, it's input is 1300 watts at 100v, so it's drawing 13 amps.
As soon as you switch it off, it will be generating about the same power, same voltage and amperage.

Assume you place a 10 ohm resistor across the leads, the 100v potential across the leads will be a braking force of 1000w in the first instant. 100v/10ohms = 10 amps. 10amps * 100v = 1000w. This may be good enough for you and you can likely get by with a resistor rated at much less than 1000 W because the wattage will drop immediately as the motor slows. As the motor slows, the voltage will drop linearly with rpm but power will drop much faster.

A 1000w x 10 ohm resistor will cost about $70. An alternative that may be cheaper and brake faster would be two 500W incandescent bulbs wired in parallel. The bulbs are about$10 each and sockets the same so, \$40 plus shipping.
https://www.1000bulbs.com/product/1...vqucXaZfwL5rmr-eOsbI8ycJhvE8KaLUaAoSYEALw_wcB

https://www.1000bulbs.com/product/2...MDScQgLY6DKUgxyz8S649EKkqE5exRwkaAo5zEALw_wcB

The good news about the bulbs is, they will provide less resistance with less voltage applied to them. So, as the lathe slows and outage drops, the lamp's resistance drops and the motor stops faster than a fixed resistor would.

The bulbs should stop the motor within about 2x the time it takes to start the motor to full speed. Likely less.

A simple switch controls as follows.

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#### daxliniere

Joined Aug 7, 2021
52
@MrSalts, the lamps are a GREAT idea. I think it might be even better to use something compact like a halogen tube. The question is 1x 1000W 240v or 2x 500W 110v in parallel?

Thanks again,
Dax.

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Joined Jul 18, 2013
26,707
As I mentioned, the motor with the flywheel doesn't need to stop instantly, but it would be nice to reduce the deceleration-to-stop time since it'll be on a metal turning lathe.
In most machine tool applications where a TM motor is utilized, it is a general practice to remove the flywheel.
The TM is quite a different application to a machine tool spindle.

#### MrSalts

Joined Apr 2, 2020
2,767
@MrSalts, the lamps are a GREAT idea. I think it might be even better to use something compact like a halogen tube. The question is 1x 1000W 240v or 2x 500W 110v in parallel?

Thanks again,
Dax.
The resistance of a 240v filament in a 1000 watt bulb is higher than the resistance of a 1000w 120v bulb's filament. Don't use that.
Use the two 500w bulbs wired in parallel. Also, make sure to keep them mounted behind something that won't get dusty and you may possibly touch. There will be real power on those that can shock you or cause a fire.

If you can get that flywheel off without bending the shaft, @MaxHeadRoom has an excellent point.

Joined Jul 18, 2013
26,707
Many of these are threaded on, so you need to know the original motor direction as some are left hand thread. !

#### Tonyr1084

Joined Sep 24, 2015
7,251
Every TM I've dismantled has had a CW motor (Clock Wise). I've always seen the motor mounted on the left side of the machine with the spindle to the left. That's so it can connect to the tread belt. I can't say for sure that all TM's are CW motors, but I think it's a pretty good bet.

Joined Jul 18, 2013
26,707
can't say for sure that all TM's are CW motors, but I think it's a pretty good bet.
Search the web, there is a slew of instances where TM LH thread has been the issue.

#### daxliniere

Joined Aug 7, 2021
52
In most machine tool applications where a TM motor is utilized, it is a general practice to remove the flywheel.
The TM is quite a different application to a machine tool spindle.
Thanks @MaxHeadRoom, I figured that the extra inertia would be a good thing, helping to maintain a constant speed during cuts, but I can easily remove the drive pulley and machine off the flywheel part of it.

Are you sure this wouldn't be a useful thing to have? I'm not opposed to making the modification but it is pretty much a one-way operation.

#### Tonyr1084

Joined Sep 24, 2015
7,251
The flywheel also doubles as a fan to cool the motor.

#### daxliniere

Joined Aug 7, 2021
52
Okay great, @MrSalts, I'll order a pair of the 110v 500W halogens. And yes, I'll be sure to keep them well insulated in every regard. Thanks again for the lamp suggestion.
I'll keep everyone posted as to my progress.

#### daxliniere

Joined Aug 7, 2021
52
The flywheel also doubles as a fan to cool the motor.
Good point, but not in my particular case, as mine has a fan on the opposite end. Good shout, though.

#### MrSalts

Joined Apr 2, 2020
2,767
Okay great, @MrSalts, I'll order a pair of the 110v 500W halogens. And yes, I'll be sure to keep them well insulated in every regard. Thanks again for the lamp suggestion.
I'll keep everyone posted as to my progress.
They each make some heat but only for a few seconds with decreasing power. As long as you aren't starting/stopping several times per minute, I doubt you'll have to worry about heat buildup in an enclosure.

#### daxliniere

Joined Aug 7, 2021
52
Ha! It turns out someone has already found the solution for this...

Joined Jul 18, 2013
26,707
The flywheel also doubles as a fan to cool the motor.
But the extra load also creates heat, the motor shown appears to have the fan on the back end.

Joined Jul 18, 2013
26,707
Ha! It turns out someone has already found the solution for this...
The down side is, that braking resistors also create more heat in the motor.