Choosing a braking resistor - Page 1
Forum section: electronics / projects

author: daxliniere
author_rank: Regular Contributor
timestamp: November 13, 2022, 01:10:37 am
content: 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 possibleAny help with selection would be greatly appreciated.All the best,Dax Liniere.

author: james_s
author_rank: Super Contributor
timestamp: November 13, 2022, 02:14:02 am
content: I'd go by the max voltage of the motor and the max current that can be handled by the relay and whatever else is between the motor and the resistor. For the power rating as long as it can handle the peak, I would expect the duty cycle to be fairly low. Chances are something of just a few ohms will work fine, I had an electric lawnmower that used a coil of about 20 feet of #18 wire as a braking resistor.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 13, 2022, 09:46:09 am
content: Quite simple - when the motor spins at rated RPM, it produces rated voltage. By R = U/I, you can choose the resistor to produce the rated current, and hence rated power. This also means rated torque!The only issue is - when the speed ramps down, generated voltage ramps down linearly, too. If you still want to keep the same torque - same current - linearly decreasing power - you would need to ramp the resistor value down, too! With fixed resistor size, current (torque) reduces too, and power reduces quadratically. Net effect is a lot of braking initially, but braking getting weaker when slowing down.Constant current circuit (of basically any type) fixes the issue, but sometimes simpler solution is another resistor in parallel which can be switched in with a relay at low speeds, for more braking.And finally, you can exceed rated current by maybe 2x, even 3x for short times - tens of seconds, maybe even minutes, it's not like the motor instantly blows up. Dead-short is usually no-no, but no different from applying full voltage from non-current limited supply, which many types of motors are fine with, even though highly sub-optimal.

author: H.O
author_rank: Frequent Contributor
timestamp: November 13, 2022, 09:50:35 am
content: Are you saying that you're going to switch the motor connections between the output of the speed controller and the resistor? If so, is the speed controller rated for having the motor disconnnected under load? Usually that's a very bad idea.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 13, 2022, 09:55:28 am
content: Yeah, the right way to do it is to use a motor controller which can do braking, and maybe going for another controller would be a good idea. Then the controller takes care of the constant-current operation, given current setpoint, and just boost the generated voltage into the DC bus. Then, if the controller does not have built-in braking resistor, one needs to limit the DC bus voltage by some kind of circuit which triggers on overvoltage - a comparator driving power transistor + resistor is fine, for example. If the input supply is battery, braking will be regenerative and such resistor arrangement is not needed (except for special cases such as battery full or disconnected).

author: daxliniere
author_rank: Regular Contributor
timestamp: November 13, 2022, 09:48:43 pm
content: Thanks everyone, your responses are greatly appreciated.I'm not sure about either of the two motor drivers' ability to handle disconnection from load, but this will all be controlled by Arduino, so when the machine/axis is commanded to stop, the controller will set speed to zero and do the motor-to-braking-resistor changeover.The treadmill motor for the metal turning lathe is being powered by the original treadmill control board (for now). The high gear ratio servo motor driving the mill axis driving is being powered bythis 0-100v PWM unit.In both cases, the motor controllers were cheap/free, so if they die, it's not the end of the world.I've had a lot of trouble finding DC motor speed controllers that operate at more than 60V. Any suggestions? My original idea was to use a 4000W SCR-based AC motor controller to feed a 50A bridge rectifier...

author: daxliniere
author_rank: Regular Contributor
timestamp: November 13, 2022, 11:50:42 pm
content: Ha! It turns out someone has already found the solution for this...

author: Siwastaja
author_rank: Super Contributor
timestamp: November 14, 2022, 11:23:46 am
content: If the controller goes into zero duty cycle when commanded with zero speed, I see absolutely no harm being done by just connecting the resistor in parallel by a relay, no need to disconnect the controller from the load. You may want to dimension resistor power handling so that if the controller accidentally tries to apply full input voltage, no harm is done. And/or add a thermal fuse in series with the load resistor.Others please chime in if I miss something.

author: Terry Bites
author_rank: Super Contributor
timestamp: November 14, 2022, 03:33:34 pm
content: https://inverterdrive.com/brake-resistor-calculator.aspxBut be warned- hard braking can cause mechanical damage to the rotor and bearings.The rotor will have an energy of K = ½Iω^2 in addtion to stored magnetic energy. I^2L . Its worth looking at the numbers and deciding on a ramp down time that wont shear the rotor off the shaft.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 14, 2022, 03:36:54 pm
content: Quote from: Terry Bites on November 14, 2022, 03:33:34 pmThis is a brake resistor selector for an inverter. Totally different to what OP is asking about. Such braking resistor is connected to the DC bus voltage of the inverter, and inverter does voltage conversion from the motor.

author: coppercone2
author_rank: Super Contributor
timestamp: November 15, 2022, 01:20:24 am
content: What is the best resistor for this? Ceramic composition or wire wound?