smps for dc motor  - Page 1
Forum section: electronics / projects

author: daniel444
author_rank: Regular Contributor
timestamp: July 28, 2019, 04:23:33 am
content: i have acquired a 1kw 180v treadmill motor that im going to use a for a disc grinder project.im planning on using mean-well LRS-350-15 as the power supplyi realize that its only 350w at 15v , that will give me the low speed i want with some torque tooi dont need the full power of the motor for the type of grinder im building and ive already tested it on some 100w smps an it is almost powerful enoughmy question is do i have to put anything in between the motor and the meanwell smps to protect it ?thanks

author: beanflying
author_rank: Super Contributor
timestamp: July 28, 2019, 04:32:27 am
content: The Meanwell is on the better end of the Cheap and Cheerful Chinese market and most likely has either an internal fuse (minimum but lift the lid and check or add one if not!) or short circuit protection. The problem you will face with any power supply (Linear or SMPS) and a DC motor is the inrush current on startup. If there is any sort of resistance to overcome initially the current will be way higher than a running current.

author: daniel444
author_rank: Regular Contributor
timestamp: July 28, 2019, 04:39:33 am
content: thanks , ive already tested the motor on a cheap 100w laptop supply and its almost powerful enough , so im thinking the meanwell should do the job

author: Siwastaja
author_rank: Super Contributor
timestamp: July 28, 2019, 07:27:44 am
content: A quick approximation:180V 1kW nominal = 5.5A nominalStall current at 180V likely 10x that, so 55A. (A ballpark number.)Stall current at 15V = 15V/180V * 55A = 4.6A.Meanwell supply: 350W/15V = 23A.So it's more than enough.Of course, the RPM will be very low. Also, just 8% of nominal voltage is so low that it won't necessarily produce high enough torque, even at low RPM.An imaginary, ideal motor with zero equivalent DC resistance would give you full torque at zero RPM by just anything >0 voltage.Imotor = (Vsupply - Vbemf)/Rmotor,where Vbemf is the voltage the motor generates by rotating, linearly depending on the RPM. For example, a motor rated to runat 5000 rpm at 100V with no load, has approximately 0.02 V/rpm. At zero RPM, the current is simply Vsupply / Rmotor.And motor manufacturers want to minimize the R, because it's only causing losses.So now you see that an efficient motor, with almost zero Rmotor, absolutely requires being driven from an actualcurrent source, not a voltage source, because driven from a voltage source, you get infinite current. The torque produced cannot be infinite, though; the iron saturates, and the efficiency plummets. Overcurrent protection is not needed - it just trips all the time - active current limiting is. (And with active limiting, you never get overcurrent.) It's exactly like driving an LED or charging a battery, voltage is secondary, current is controlled.Inrush is the wrong term. Of course there's inrush, because every motor sometimes starts at 0 RPM, but the same thing happens whenever you have a lot of mechanical load. Inrush only applies when the motor spins up on the lab bench, with no load. This model is too simplistic for real-world use. (It's like saying that LEDs need current limiting because of "inrush". No, current limiting is neededall the time.)Your case is an exception to the rule. You have such a ridiculously low voltage (8% nominal), that you are not going to have issues with too much current with any practical, less than perfect motor. Try it, and if you have enough torque for your task, you are fine.You can measure the worst case stall current, but I'm 100% positive it's going to be less than 23A (your supply rating), and it's very likely lower (or not too much higher) than the rated running current of the motor (around 5.5A we calculated as the first thing).But, running at low RPMs prevent the motor cooling (based on an internal fan, for example) from working properly. You may need to measure the winding and brush temperatures in extended high-torque use.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 28, 2019, 07:30:48 am
content: Quote from: daniel444 on July 28, 2019, 04:39:33 amDid you measure that the 100W supply voltage sags? It's highly likely that the voltage doesnotsag - if that's the case, a higher current rated power supply won't change anything. You may need to up the voltage instead of / in addition to power rating.But if you go too far, you may need to get into proper current limiting. Driven from a voltage source, your torque will fluctuate a lot depending on RPM, and you may have too high current at very low RPM, producing more torque than you need at the lowest speeds, and causing excessive motor heating.

author: beanflying
author_rank: Super Contributor
timestamp: July 28, 2019, 07:49:03 am
content: Quote from: Siwastaja on July 28, 2019, 07:27:44 amPlease explain why inrush is the wrong term? In the OP's intended grinding application peak power causing the SMPS to shutdown or go into overload is most likely at startup and the inrush current is exactly the correct term to discuss. Useage load is a different matter and that is set by how hard you make the motor when grinding which is an operator governed issue.The OP provided nearly no information on usage and yet you have waxed lyrical with no idea of the motors efficiency and if the 1000W is BS inflated PMPO style or working power or anything other than marketing wank. Your attempted 'specific maths' is therefore not much better than useless.

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2019, 09:10:49 am
content: But the original poster doesn't want to run the motor at full power, otherwise I would have suggested the old fashioned approach of a buck/auto-transformer and rectifier.I suggest they actually measure the motor current, when the motor is driving the expected mechanical load, to get an idea of the actual power consumption, before selecting an appropriate power supply. Note it's very important to measure the current, whilst the motor is loaded, as the no load current could be an order of magnitude less.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 28, 2019, 09:19:54 am
content: Quote from: beanflying on July 28, 2019, 07:49:03 amI already explained, but I try to rephrase and elaborate (I'm trying to help here):Wikipedia defines inrush current as:"Inrush current, input surge current, or switch-on surge is the maximal instantaneous input current drawn by an electrical device when first turned on"But the need to limit motor current has nothing to do with the initial state alone. The exact same situation happens whenever there is an increase in mechanical load, and RPM drops near zero. Inrush limiting doesn't do anything in this case, yet the user expects either of the following:1) Automatic safety shutdown, or more often:2) Limited current (and hence torque) to a safe level, whatever it is depending on the case. Current limiting for limiting winding and brush heating (and prevent core saturation, because the extra current over saturation limit won't produce any more torque, but still takes more power in). Torque limiting for mechanical reasons.3) Combination of the two. Torque limiting for some time, automatic cutoff if torque integral exceeds some value.Any standard "in-rush" limiting circuit, such as a soft start voltage ramp, naturally fails miserably, because it's only acting on startup, not during operation. A proper always-active limiter, OTOH, limits the current in all cases,includingbut not limited to startup.The logic is simple:1) Inrush limiter only handles one case2) Generic limiter handles all the cases,3) Almost always (especiallyin a grinder), many cases exist in addition to startup.So while it is correct to say that motors have inrush current, it's not very meaningful because adding an inrush limiter won't usually solve the problem.You couldthinkthat simple motor applications like a fan would only need inrush control. Yet, about 15-10 years ago, all BLDC fans started implementing an in-operation current limiting mode with hickup recovery. Because fans get gummed up with dust, and the torque and current increases over time.Why such "nitpicking" on terminology? Because searching for "inrush limiting" gives a lot of results of circuits and concepts that don't solve the problem! (If it exists, at all. If it doesn't, inrush limiting isn't needed either.)Phenomena where initial energy delivery is needed to get a system to a steady state, is called inrush. An example is charging the output capacitor in voltage supply. But this is not the case with battery charging, LED drive, or most motor drives, where current limiting is needed as a part of normal operation, not only as a special startup case. Term "inrush" is not properly used in any of these cases. These loads are all equivalent to a voltage source and a very small series resistor, typically insufficient to limit current properly.QuoteNo, it likely isn't. Depends on actual grinding process, but it's highly likely that the grinder is often turned on without much mechanical load. The peak current is the same, but decays quickly as the RPM ramps up.During grinding, I can see that the user pressed the grinder down hard, which could create a much longer-lasting surge than during the startup condition. Same current, but more energy -> more problematic if not handled. It may stall for say, for a second before the user reacts - this is already way longer than no-load rampup (inertia charging).But let's not derail this more. If you need more information on this, please open another thread to get more help. As said, I strongly suspect at that such low voltages, no current limiting is actually needed at all (neither inrush, nor during operation). And I don't often say this! Full stall current at such low voltage is unlikely to be high enough to cause damage to the motor, and the supply is likely big enough to handle full stall current, for indefinite time. This is easy to verify by measuring the current, and looking for excessive heating in the motor during extended stall.QuoteWhat the heck is your problem? I'm trying to advocate math and understanding, even with initial napkin approximations, instead of cargo cult engineering. By all means, substitute for more accurate numbers once you got them. Math and understanding is important in engineering. I think you are on a wrong forum.

author: beanflying
author_rank: Super Contributor
timestamp: July 28, 2019, 09:21:09 am
content: Quote from: Zero999 on July 28, 2019, 09:10:49 amWhat does full load at 180V have to do with 15V 'full load' based on a likely BS power claim it is still BS. Any figures will be rubbery at best to just wrong. Taking a 'best guess' and keeping an eye on temperature is about as good a plan as any to setting a safe load point at 10% of the rated voltage.Best '[guess' would see the motor needing to be derated to 1-200W providing the brushes will handle the higher current.

author: beanflying
author_rank: Super Contributor
timestamp: July 28, 2019, 09:23:06 am
content: Quote from: Siwastaja on July 28, 2019, 09:19:54 amMy PROBLEM is you are using NON FACTS as facts! There is no figures to make ambit claims on, you are not doing Engineering anything but trying to add numbers where they don't exist.

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2019, 09:28:06 am
content: Quote from: beanflying on July 28, 2019, 09:21:09 amWhere did I stay full load at 180V? It should have been obvious I meant full load at whatever voltage, the original poster is intending to power the motor with.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 28, 2019, 09:31:09 am
content: The very first words were, "a quick approximation".Why do you think the motor ratings are bullshit? If they are, the math is garbage in, garbage out, indeed.Yes, the most practical answer is, just try it, it's likely to work just fine with no engineering whatsoever. I said that, too.But I'm trying to be educative. Don't belittle others, people are not stupid. They are capable of putting their own numbers in! Not limited to this grinding application. The concepts and formulae are important here, and they are correct. If not, please show a non-fact, instead of attacking and derailing.QuoteI said that, and he's referring to me.It was used tocalculatea meaningful parameter, in this case, stall current at 15V. It's an approximation (neglecting non-linearities such as core hysteresis loss, friction...), and assumes that the specification is not bogus, of course.This is because motors arevery oftenrated by simply giving a full load voltage, rpm, power and/or current ratings. You often need to derive the relevant parameters from the parameters you have!

author: beanflying
author_rank: Super Contributor
timestamp: July 28, 2019, 09:34:41 am
content: Quote from: Zero999 on July 28, 2019, 09:28:06 amWhat would 'measuring' a current that is going to smoke the brushes achieve? Points of failure on the motor exist as does the PS. Running a motor at circa 10% nominal voltage without the data, best guess is in this case all you can do without creating a motor power curve at that voltage.I have done this a lot with R/C motors over the years looking at BEP and performance but this is OTT for most without the gear to do it.My main point here is you can't simply look at the PS but the motor under running load is likely the weak point with brush failure or overheating.

author: daniel444
author_rank: Regular Contributor
timestamp: July 28, 2019, 02:06:55 pm
content: thanks for all the responsesnow im thinking i might go for 24v 14A version of the same line of power supply to get a bit more torque , it has a cast iron flywheel so there is energy stored in that tooim spinning these diamond discs i bought off ebay , i stuck them to the flywheel with magnets for testing and was able to grind the tips of a screwdriver flat again , i dont want it to spin to fast , its for fine wet grinding and sharpeningi have another power supply that can run it at full speed but it would throw water everywhere and you couldn't get as close to it. so i want to get a dedicated lightweight power supply like the mean-welli got the motor from a used treadmill , took out and cleaned the brushes , they are only 30% used

author: Benta
author_rank: Super Contributor
timestamp: July 28, 2019, 05:12:13 pm
content: Make it simple:just measure the resistance of the motor (I assume it's a PMDC type), moving the armature until you have the lowest resistance. This will tell you the maximum current the motor is ever going to draw, at startup or under load.Design accordingly.

author: Zero999
author_rank: Super Contributor
timestamp: July 28, 2019, 10:13:04 pm
content: Quote from: Benta on July 28, 2019, 05:12:13 pmUnfortunately that will end up in a grossly oversized the power supply.Quote from: beanflying on July 28, 2019, 09:34:41 amWhat the heck are you on about? I said nothing about measuring a current which is going to smoke the brushes!It's simple. Power the motor with the mechanical load connected, measure the current and size the switched mode power supply accordingly. Choose a power supply with plain old constant current limiting, not one which will shut down or fold-back and the inrush current will be a non-issue.

author: Benta
author_rank: Super Contributor
timestamp: July 28, 2019, 11:44:56 pm
content: Quote from: Zero999 on July 28, 2019, 10:13:04 pmYou completely missed the "Design accordingly" part. This was a hint to estimate the appropriate current limiting. I never suggested going to the max. but just to get a picture of the motor characteristics.

author: beanflying
author_rank: Super Contributor
timestamp: July 29, 2019, 01:18:32 am
content: Quote from: Zero999 on July 28, 2019, 10:13:04 pmThe OP stated initiallyhe was going to use a Meanwell Power Supply and you ran off to reinvent the world with a current limited one he doesn't have. Operating current on some now additional information from the OP will most likely see currents lowish when running or he will be ripping the diamonds off the discs rather than grinding. I have had the issue with Chinese Power Supplies simply detecting a short circuit and not starting on larger motors if this is the case then additional current limiting or PWM would be a possible fix.Meanwhile KISS this is a home use case not one that needs to be Engineered to death with imagined OTT solutions.

author: daniel444
author_rank: Regular Contributor
timestamp: July 31, 2019, 02:58:48 am
content: updateso i bought a HWS150A-24/A , by TDK-Lambda24v, 6.5ait goes up to 28vi got a good price for it on ebay, i think its new old stockthe motor resistance measures from 6 ohms to 30 ohmsthese diamond discs are good for hand use as wellthanks for all the tips