DC motors and their properties during overload conditions. - Page 1
Forum section: electronics / projects

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 03, 2023, 09:09:28 pm
content: I have a 24Vdc power supply and a few 12Vdc motors.And of course, a 12Vdc motor is not going to suddenly explode on 13Vdc, but 24Vdc does seem a bit much.So I wonder, what sort of margins do the voltage ratings of these motors have.Small motors can often be bought in the voltage you need (6V, 12V 24V are common) but the bigger 12Vdc motors are common as windscreen wiper motors (and lead acid batteries already go over 14V anyway).Another nice source for motors is from second hand (bigger) accu drills. Here in the EU these get recycled in the home improvement store, and you can steal them from the collection bins (Yeah, a bit naughty). Usually the batteries are dead (so put those back in the recycle bin) but the rest of the drill is often in good condition.And also, as these motors are going to be speed controlled with PWM, would there be any downsides of PWM'ming these motors form 24Vdc with a 50% duty cycle?Edit: As PWM frequency get mentioned a lot in the replies: I do use a low PWM frequency, a few hundred Hz. the humming this causes is less ten the noise from the gearing, and with low frequency PWM I had much better control at very low rpm. I think the torque ripple gets its over the friction, while it still stalls at a small gentle push from a higher frequency PWM.I do have a bit of experience:I once bought a 3.6V small handheld drill (the size that fits in your fist) and PWM'med it from a 14Vdc power supply. It was a bit "temperamental" and I had a limited range because I did not PWM it over 25% duty cycle, but apart from that it did all seem to work. However, after about 10 years of operation, the motor quit on me, and after taking it apart it turned out the brushes were still fine, but there was no copper anymore on the commutator, so it was game over for that motor.The motor operated twice a day (roller curtains up, and down again) and each cycle ran for about 12 seconds, so that is 10*365*2*12 = 87600 = 24.3hours.I always tried to treat the motor nicely, and starting the motor slowly by the PID algorithm helped with that, During early tests the brushes sparked significantly, but I got that under control quickly. I also had guards on the PID algorithm, and when the motor was blocked or did not start within two seconds of ramping up the PWM, the motor was turned off again with a fault condition.25 hours does not seem much of a service life for a DC motor, but maybe the drill was too cheap (it was a "Skill", not great, but there are much crappier motors in that form factor) and I would have expected more of that motor.The commutator was however just a plastic bushing with about half a mm thick (thin) copper strips. Quite crappy.Or, to go back to the main topic.What are the downsides of using a 24Vdc power supply for a 12V motor, and PWM'ming it at 50% duty cycle.What is it that determines the limit of a "safe" motor voltage?Some known factors:* Insulation breakdown is unlikely to be below a few hundred volts.* Servo motors are often rated for twice or thrice the torque (= current) for intermittent duty (but not over voltage?)* Over current and overheating is a problem, but can be managed.

author: Zero999
author_rank: Super Contributor
timestamp: August 03, 2023, 09:14:52 pm
content: The main issue is over-speed which can damage the bearings and brushes. Reduce the speed to the rated RPM and it'll be fine.

author: Benta
author_rank: Super Contributor
timestamp: August 03, 2023, 09:27:07 pm
content: PMDC motors of the kind you mention (automotive/industrial, 20...200 W) are extremely robust and hard to damage. I've used a lot of them in RC speedboats (with water cooling) and they just go.Heat and overspeeding are the main issues.Operating at 50% duty cycle at 24 V is completely unproblematic.BUT:If you think you can do it at 20 or 40 kHz, forget it. The iron losses will be immense = no torque.My own experience is to keep the PWM modulation frequency below 3 kHz, and if you really want fine regulation, 300...500 Hz is more the range.But play around, the risk of damaging the motors is small.

author: Circlotron
author_rank: Super Contributor
timestamp: August 03, 2023, 10:06:55 pm
content: If you wanted to PWM it at high frequencies you could always put a series powdered iron toroid and shunt cap between the catch diode and the motor. Essentially a buck regulator. That would keep high voltage pulses out of the motor and keep iron losses under control too.

author: Benta
author_rank: Super Contributor
timestamp: August 03, 2023, 10:29:04 pm
content: I forgot something, @circlotron's post reminded me:It's good practice on these motors to place noise suppression caps directly on the motor. This is to suppress radiated noise from the brushes (you'd be surprised how much it can be!)47...100 nF between the connectors, 10...22 nF from each connector to motor case.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 04, 2023, 05:43:11 am
content: With a PWM controller, voltage itself is not a problem:Overspeeding is bad for brushes, and double the voltage gives double the maximum speed. If you control the speed though, it's then of no issue.Too much current causes heating (in brushes and windings), and in case of non-current-controlled stupid drive scheme, double the voltage doubles the current (I = (Vsupply - Vbemf)/Rtotal). But you really should have current sense and active current limiting, then it does not matter. (More voltage causes quicker current rise rate, but these motors have so much inductance it's still slow enough even for cheaper types of current sense amplifiers / comparators / ADCs / whatever you'd use to control current).You seemingly get a better torque at low PWM frequency but that's not thanks to the low frequency itself, it's the different transition point from DCM to CCM and therefore different average current. At low fixed duty, you are just pushing more current into the motor when you lower the frequency. Once you add current sense and regulation, this difference goes away, and you should use high enough frequency to get rid of torque ripple, I don't think it's common that vibration actually helps. If you can accept the audible noise, somewhere around 5kHz is pretty much the sweet spot, but anything 1-20kHz does it.

author: Simon
author_rank: Unknown
timestamp: August 04, 2023, 07:15:00 am
content: I have an application where I will run a 24V motor at 48V, but it is used only briefly so will be fine.One thing to bear in mind is RFI, if you PWM the motor you just create a broadband radio transmitter and hammer the commutator even more. The motor will have quite a bit of capacitance so each PWM cycle will create one hell of a current pulse even with the motor spinning, this then leads to ground bounce issues and it's all a mess. As suggested make a "poor mans buck converter". This then means that you can have capacitance across the motor to dull the noise which with straight PWM would just add to the current spikes and make even more RFI.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 04, 2023, 07:33:59 am
content: Quote from: Simon on August 04, 2023, 07:15:00 amBrushed motor is a broadband radio transmitter already; PWMing it does not make it any worse.QuoteComplete nonsense. Motors have very little capacitance. The equivalent circuit is inductor and resistor in series with ideal voltage source/sink (this voltage being linearly proportional to rotational velocity). Therefore, PWMing a motor does not cause any sort of current spikes; instead, a nice triangular current waveform. This is how motor controllers work; brushless or brushed, doesn't matter.I would be careful against adding too much capacitance across the motor terminals for the reason mentioned. Placing the controller as close as the motor as possible helps because then the motor wires do not radiate that much, and you get the filtration at the DC link (input to the controller).Proper circuit layout, i.e., the freewheeling switch (diode or synchronous MOSFET) close to the switching MOSFET, is important.QuoteDon't overcomplicate it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2023, 09:38:07 am
content: Video related,They don't happen to say what voltage it was at, but the welder at the end will be on the order of 30V, and apparently they were doing a metric shitload of amperes, so there's that.Yeah, note that PWM directly into a motor is a standard method, saving the effort of an output filter -- when a short connection and direct grounding (of the chassis/frame preferably) is possible, or where shielded cabling or conduit is available (as in many industrial drives, not that they're usually very well grounded, leaving quite a bit of noise to the environment!). The winding inductance (namely the leakage part i.e. what field isn't coupled into the rotating EMF) does the job of PWM filter, at some expense to wire/core losses, give or take PWM frequency and motor design.Tim

author: Simon
author_rank: Unknown
timestamp: August 04, 2023, 10:29:08 am
content: Quote from: Siwastaja on August 04, 2023, 07:33:59 amYea, I tried to not over complicate it, it was only a 400mA motor, instead it got really complicated and we kept blowing the MOSFET driver up..... Brushed motors usually have a ceramic capacitor across them to suppress the noise, that won't bode well with a PWM drive. Take it off and rfi get's worse.OK we tried to drive the thing with non ideal wiring but, if it was that bad that it destroyed several drivers in a row until we stopped doing that, then I think that it was far from ideal by any other standard.Putting an inductor in series will help with inrush currents through any capacitance, the motor already had a back emf diode and that in itself is as much of a buck converter as you need. I would do this especially if you want to run a lower voltage motor from a PWM source.

author: Simon
author_rank: Unknown
timestamp: August 04, 2023, 10:39:42 am
content: Quote from: T3sl4co1l on August 04, 2023, 09:38:07 amI'm not sure of the point here, other than the same as every photonic induction video, most performance fans will take off even at their rated voltage, that one does look powerful. A fan like that can typically be 3-400W, brushless versions can be 850W for a 305mm fan, I've seen 4 produce enough pressure to lift the steel shroud they were mounted to, bearing in mind that only flow would go up, not the pressure.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 04, 2023, 10:48:22 am
content: Thank you all for your reactions. Thanks to you I've come to the conclusion that using a 24V power supply is no issue at all, as long as it's done properly.As I've written before, the motor died after 10 years and now has to be replaced, but I'm not in a hurry, and I'll take the opportunity to do some serious measurements about the behavior of Brushed DC motors.While photoninduction is entertaining, I'd rather get more then 10 years of use out of my next motorBack then I only had a Rigol DS1052E and it's FFT capabilities are worse then a joke.I've been longing for a more modern Siglent (It's FFT is quite good, better then newer Rigols) and I guess I can do half decent EMC measurements with it in combination with a near field probe and a wide band amplifier. I still have to look into that topic.I will also do more measurements on the inrush currents during PWM cyles and various circuit topologies.

author: Simon
author_rank: Unknown
timestamp: August 04, 2023, 11:03:29 am
content: Remember that brushed motors do not have long lives, they are usually rated at between 1000 and 5000 hours with some lasting 10'000 hours. The life is usually down to the brushes.One neat trick I discovered is PWM your motors and vary the frequency, I was using a half bridge driver at the time. Measure the temperature of the driver as you vary the frequency, there will be a sweet spot where you are going fast enough to keep the motor inductance in continuous conduction, the drivers temperature will drop as you increase from a very low frequency to one high enough for continuous conduction, if you go higher the temperature goes back up as the switching losses now take over.

author: amyk
author_rank: Super Contributor
timestamp: August 06, 2023, 09:33:57 pm
content: As others have mentioned (and PhotonicInduction has demonstrated), the brushes and commutator are the biggest limiting factor, and to a lesser extent overheating. Brief overloads are well-tolerated, to the extent that in some applications e.g. automotive starter motors, it is normal.