# Choosing a braking resistor

#### daxliniere

Joined Aug 7, 2021
52
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.
The lathe will definitely be start/stopped multiple times per minute during short operations, but I can put them in free air inside the 150kg cast iron base of the lathe to keep them out of the way.

#### Tonyr1084

Joined Sep 24, 2015
7,247
How does a treadmill stop the tread nearly instantaneously when you hit the emergency stop? I'm thinking they must be using that choke to stop the motor too. I don't know this for a fact, I'm just speculating, but wouldn't a choke do the same job as the resistor? My thinking is that when a choke is substituted for the resistor and you hit the brake the choke, which resists changes starts out as a high impedance device acting like a high value resistor. But as the magnetic field increases the impedance drops. When the choke is magnetically saturated the impedance drops to near zero. Is this the wrong thinking? To me it looks like high resistance at the brake command but quickly drops to low resistance (near zero when saturated). I could be wrong.

And no - this isn't an attempt to hijack the thread. Throwing different ideas out there is the intent of this post. To spur different thinking. Sometimes we all get locked into a particular vane of thinking and can miss an otherwise alternative solution to the problem. Treadmills stop in short order. How? Of all the TM's I've disassembled I've never seen any high wattage resistors. So to me the resistor approach seems like not the best solution.

#### daxliniere

Joined Aug 7, 2021
52
Hey Tony, yes, I think that's a valid question. Honestly, I think the only time I used a treadmill in my life was a couple times as a teenager.
Once I have the motor installed, I'll test it out, but logic tells me that it will probably be needed since others have talked about this issue.
Plus, it's not just the treadmill motor that I want a brake for, but also the servo motor I mentioned in the first post.

All the best,
Dax.

#### MisterBill2

Joined Jan 23, 2018
14,191
On my electric lawnmowers there has been a switch that short circuits the motor whenever the drive lever is released. THis greatly reduces motor life and so I always remove that function. The result is that I also need to avoid sticking my hands into the spinning blade, which is a simple safety rule. Removing the flywheel and replacing it with a fan is the way to go. And all of the metal lathes that I have used have a separate mechanical brake to halt rotation when the drive is switched off. On some, the brake operation released the detent of the drive switch, to remove motor power.

#### MisterBill2

Joined Jan 23, 2018
14,191
On consumer appliances with the operation of an "emergency stop" it is quite acceptable to greatly reduce the life of the product. The primary intent is to reduce product liability, all other considerations are a distant second priority. So short-circuiting the motor and causing damage that reduces product life is not generally a consideration on consumer goods.

Joined Jul 18, 2013
26,629
How does a treadmill stop the tread nearly instantaneously when you hit the emergency stop? I'm thinking they must be using that choke to stop the motor too.
On the popular MC2100 TM boards that are used, a large reverse EMF diode is used directly across the motor. this cannot be used in a bi-directional controlled motor controller , however, for obvious reasons, unless different switching arrangement is used for Bi-directional control.
@daxliniere , do you need braking on the servo motor with a 60rpm output GB?
I know planetary GB's can be back-driven, but I would have thought the inertia would be sufficient.
.

#### sagor

Joined Mar 10, 2019
796
Ha! It turns out someone has already found the solution for this...
Poor example of electronics in the video. Those metal "100W" resistors are 100W only if mounted on a heatsink. Without a heatsink, at best they are 10W to 20W. Measuring the resistance to determine if they are "still good" is wrong. If the resistance has changed, even a bit, the resistor may be partially damaged and next use may cause total failure.
Now I would agree that 8 of those, on a small heatsink, would provide a good brake for that motor. Braking time would be inversely proportional to the lower resistance, so 8 of those would brake quickly. Based on his video, I would say even 8 x 50W resistors would still suffice, if mounted on a heatsink. But, one has to experiment with your specific motor setup to be sure.
Most resistors have a wattage rating curve over time. Short periods of overcurrent usually work out ok. There are graphs (from some vendors) for those metal clad resistors, showing current/wattage over time, usually in the milliseconds to a few seconds. Same holds true for wire sizes and even things like fuses.
Also, be very careful if overloading one of those metal clad resistors. I overloaded one (50W), once, and it blew out both ends like a pistol.

#### Tonyr1084

Joined Sep 24, 2015
7,247
For some reason I like to be the guy who points out overlooked issues. This time I'm pointing out that you can stop the motor at a given timeframe by whatever means you're using. The point is that if this is going to be a lathe of any kind, the material on the drive head also needs to be considered for weight. Imagine a steel shaft who's diameter is 6" and maybe 10" long. That's a lot of spinning mass. If the motor braking system is expected to stop the motor in (just making up numbers for a moment) 1.5 seconds and the motor itself has a spinning mass of (again, made up numbers) 10 pounds - stopping that motor all by itself may occur within the parameters. But if there's something with a spinning mass of 20 pounds (and I don't know how to calculate these numbers) it could take 8 times longer. The reason why I'm GUESSING at this number is because I figure twice the mass will take four times longer (inverse square law). So something twice the weight of the motor should have (maybe) 8 times longer to stop. In other words, in this absurd example, it could take 12 seconds to stop.

Get a watch and pay attention to 12 seconds as they go by. It's by no means a short period of time.

OK, I forgot if we're talking about a gear box or not. I think someone said something about that. That may help, but it's still going to need to be add into the equation. Spinning mass at any RPM has inertia. And if you stop the motor too fast you might break the gear box if the spun mass is great enough.

Sorry to be the guy who finds problems with the solutions instead of finding solutions to the problem. But sometimes you just need to identify all factors that are critical in engineering a machine. And I'm not an engineer.

#### daxliniere

Joined Aug 7, 2021
52
@MisterBill2, @MaxHeadRoom, @sagor and @Tonyr1084 - thank you all for your input, those last comments especially were all gold.

Tony, I've been thinking about this. I don't need it to stop instantly, just faster than a 'free-fall' spindown which would get boring really fast. Considering everything that's been brought up by everyone, I think that I will try it out first and if I'm unhappy with the time it takes to come to a complete stop, I think I will design a physical braking system, perhaps a thick leather strap around a large wheel with a solenoid to engage it when the motor is stopped. (Better still, when the motor is engaged, the solenoid could be engaged to release the brake. That would make it failsafe.)

By the way, I machined off the flywheel and now I'm noticing that on the lowest speed settings, I can hear the motor speed is self adjusting when spinning in free air. I'm hoping that adding some load to the shaft will fix that issue. Time will tell!

Joined Jul 18, 2013
26,629
By the way, I machined off the flywheel and now I'm noticing that on the lowest speed settings, I can hear the motor speed is self adjusting when spinning in free air.
What is meant by "self adjusting" ?

#### MrSalts

Joined Apr 2, 2020
2,767
@MisterBill2, @MaxHeadRoom, @sagor and @Tonyr1084 - thank you all for your input, those last comments especially were all gold.

Tony, I've been thinking about this. I don't need it to stop instantly, just faster than a 'free-fall' spindown which would get boring really fast. Considering everything that's been brought up by everyone, I think that I will try it out first and if I'm unhappy with the time it takes to come to a complete stop, I think I will design a physical braking system, perhaps a thick leather strap around a large wheel with a solenoid to engage it when the motor is stopped. (Better still, when the motor is engaged, the solenoid could be engaged to release the brake. That would make it failsafe.)

By the way, I machined off the flywheel and now I'm noticing that on the lowest speed settings, I can hear the motor speed is self adjusting when spinning in free air. I'm hoping that adding some load to the shaft will fix that issue. Time will tell!
That can be a bearing issue, a binding issue or some looseness in the overall system causing a drone pitch change.

#### daxliniere

Joined Aug 7, 2021
52
What is meant by "self adjusting" ?
The motor speed is 'drifting' aka changing on its own by small amounts. It's sporadic and jerky. Only happens when running at lower speeds and started after I removed and machined the flywheel (done extremely carefully, no chance that I damaged the motor or bent the shaft), which was obviously 'evening out' the rotation. (Would you call that inertial damping??)

That can be a bearing issue, a binding issue or some looseness in the overall system causing a drone pitch change.
I don't think that is the cause in this situation because if I grab the motor shaft (with thick gloves) and slow it down, after releasing, the motor over-speeds, then settles back down.
I guess the MC2100 (rev C) controller I have has some smarts in it as it's monitoring the motor output without a tach sensor being connected. (monitoring of back-EMF?)

My guess is that the PID in the controller was programmed with that particular motor and flywheel combination in mind. It would be great if there was some way I could reduce the Derivative parameter, butyou probably need a special programmer and proprietary software to make such adjustments.

But like I said, once it's installed it'll have some load on it and it's possible this problem will go away.

Thanks again, guys!

Joined Jul 18, 2013
26,629
So what are you using to generate the 20Hz PWM control signal?

#### daxliniere

Joined Aug 7, 2021
52
So what are you using to generate the 20Hz PWM control signal?
At the start, just the TM console. Once I've verified everything works as expected, I'll use this code as the basis for an Arduino controller which will also feature RPM sensor + (maybe) PID, LCD display + rotary encoder, forward/reverse (with delay when changing direction).

What were you thinking about?

Joined Jul 18, 2013
26,629
There are some schematics based on a 555 out there, I put one together with a 8 pin Picmicro for more precise PWM.

#### daxliniere

Joined Aug 7, 2021
52
Ahh cool, I don't think I realised you had done a TM repurpose project, too.

#### MrAl

Joined Jun 17, 2014
9,757
@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?
What does your gut say? Both are readily available.

Thanks again,
Dax.
Hi,

I would stay away from bulbs they can fail unexpectedly.
A better choice would be a "load cone". These are devices that are made with a ceramic 'cone' with coiled nichrome wire wound around the perimeter. The resistance is a lot more constant too. They can be installed in ceramic ceiling light sockets and that makes it easy to wire them up to a circuit. They are not too expensive but obviously more expensive than light bulbs (ha ha).
They come in sizes at least 600 watt and 1000 watt probably more sizes too. 120v and 220v types.
Also called 'heater cones' or 'cone heaters'.
They are used to test some high power equipment such as high power converters.
Not sure how much they cost these days. Could be $20 to$30 each but they last a very long time.

Examples:
Light Bulb Depot® Product Listings

So now you can add "MR AL" to the list

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Joined Jul 18, 2013
26,629
You already have a built in method with the reverse diode, in any case, if using the termination of the PWM input command to stop, there is slight coasting built in with the MC2100.

#### shortbus

Joined Sep 30, 2009
9,804
Instant braking may be a bad thing, if your lathe chuck is the screw on type. You may see it fly off onto the floor, or worse into your lap.

#### Tonyr1084

Joined Sep 24, 2015
7,247
Instant braking may be a bad thing, if your lathe chuck is the screw on type. You may see it fly off onto the floor, or worse into your lap.
Wish I had thought of that.