Switching power supply broken after connection with refrigeration compressor - Page 1
Forum section: electronics / projects

author: federico
author_rank: Contributor
timestamp: September 13, 2022, 06:49:56 pm
content: Hi Guys, an info for a problem that I'm not sure there is ..I connected a DC 12v 60A, 700W switchng power supply to a small refrigerator compressor that consumes only DC 12V - 150W.I was testing it, but after 12 hours of operation, the power supply died.I realized that every time the compressor switches off, it gives a good blow, and I wondered that it would give a discharge back to the power supply .... being a motor that is started with capacitors for starting, I'm afraid that the immediate stop causes the capacitors to discharge in return towards the power supply.Do you think it is possible? or simply broke the power supply .. (it was almost new).I was thinking of putting a non-return diode or a voltage arrester like this:https://www.amazon.com/LRSO1-Lightning-Arrester-Protection-Uptell/dp/B07C9FD1X1Do you think it makes sense?Thanks to all in advance.

author: moffy
author_rank: Super Contributor
timestamp: September 13, 2022, 09:15:05 pm
content: Quote from: federico on September 13, 2022, 06:49:56 pmI would have thought that at 12V DC, it would be a simple DC motor with brushes, no capacitor start. That still wouldn't solve the problem of where the current/energy stored in inductances goes when it switches off. A problem also is the start current which could be quite large, e.g. 5x the max load current or greater if starting under load, this might be an issue.

author: Benta
author_rank: Super Contributor
timestamp: September 13, 2022, 10:12:01 pm
content: Energy returning from the "induction" in a brushed DC motor is a superstition that keeps cropping up. It doesn't happen, and if some spikes occur, it's because of brush switching/sparking, which should be damped with noise caps anyway.Much more to the point: is your power supply current limited at all? Starting a DC motor under load (which a compressor certainly is) will theoretically result in infinite starting current. Staring a DC motor without æoad gives the same result, but for a much shorter time, perhaps letting the PSU output caps deal with it.This is alleviated by wire, brush and winding resistances, but it's no trivial matter.

author: james_s
author_rank: Super Contributor
timestamp: September 13, 2022, 10:28:57 pm
content: Quote from: moffy on September 13, 2022, 09:15:05 pmHermetic refrigeration compressors never use brushed motors, the DC ones have always had brushless DC motors with an external driver.

author: moffy
author_rank: Super Contributor
timestamp: September 13, 2022, 10:30:33 pm
content: Quote from: Benta on September 13, 2022, 10:12:01 pmThere are two major inductances in a DC motor:1. Field2. RotorIf you interrupt the current through these inductors, like any inductor, it will produce voltage spikes, so it is better to ramp up/down the voltage across DC motors. Brush sparking is due to the brushes not being at the 0V switching point. This point is offset by load currents and compensating magnetic poles are provided in large motors to compensate for this shift. Unbalanced brushes can lead to a "flash over" which in a 1500V/1000A motor running at 2000 RPM is terrifying.P.S. Brush sparking can also be due to mechanical reasons, uneven commutator, brushes not properly bedded etc.

author: moffy
author_rank: Super Contributor
timestamp: September 13, 2022, 10:32:37 pm
content: Quote from: james_s on September 13, 2022, 10:28:57 pmThank you for that, it clarifies the situation.

author: Benta
author_rank: Super Contributor
timestamp: September 13, 2022, 10:45:28 pm
content: Quote from: moffy on September 13, 2022, 10:30:33 pmYes, well, let's stay with 12 V, 150 W here, shall we? If the question was about domestic AC mains, I wouldn't throw in a nuclear plant for comparison just to make a point.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 14, 2022, 01:30:29 am
content: What is the model of the power supply? Is it from a known reputable brand? It's possible the power supply failed simply due to a defect, through no fault of the compressor.These DC refrigeration compressors actually have much lower starting current compared to AC compressors. Back EMF shouldn't be an issue, as this should already be mitigated in the driver circuit for the BLDC motor (think about it - if there was enough back EMF to kill a power supply, certainly it would also damaged the motor driver itself too!).

author: jonpaul
author_rank: Super Contributor
timestamp: September 14, 2022, 12:02:24 pm
content: PSU/UPS not designed for ANY motor or inductive loads:10..30x startup current surges, which generate kilovolts and jules of inductive transients on stopping.After wa few compressor cycles it was bound to happenFinally 99% of cheap Chinese PSU and UPS are far underrated and have zero overcurrent nor transient protections.Use a decent USA or UK brand like APC or Triplite, rated at least 2 kVa for a 150 VA frig.Jon

author: Vincent
author_rank: Regular Contributor
timestamp: September 14, 2022, 01:55:55 pm
content: I'll also say the motor's starting current is the likely cause of the PSU's failure. Only the DC resistance of windings, plus that of the wires, limits the current until the motor spins and provides EMF to gradually null out most of the applied voltage. In that application, adding some sort of soft-start circuitry wouldn't be a bad idea at all.As for the possible poweroff voltage spike, I'm not as sure. Freewheeling diode?

author: federico
author_rank: Contributor
timestamp: September 14, 2022, 02:06:15 pm
content: Thanks everyone for the answers, I have read and tried to understand everything. yes, the motor has no brushes, it is a BLDC with driver attached.I attach link from alibaba, I bought a complete kit of radiators to work as an air conditioner, the link is for the engine only:https://www.alibaba.com/product-detail/12V-24V-DC-mini-deep-freezer_1659744559.html?spm=a2700.details.0.0.2e0341cdSQBE8PQuoteit is a poor Chinese power supply, paid less than 30.00 Dollars (But it is oversized, 700w to support 150w of compressor), in fact now I would like to buy a Meanwell.QuoteI'm in Italy, I don't know these brands,Meanwellisn't it good?QuoteThe engine is managed by an external thermostat that starts it at 1000RMP, to go up to 3500 RMP at the most. I don't know if it acts as a softstarter, but in theory it starts at idle speed. but with an oversized PSU it shouldn't be able to handle it?QuoteAre you going to insert a diode to prevent it from returning to the power supply?Could the fact that I have not connected the ground of the power supply (because it is absent from the warehouse where I tested it) have helped to break the power supply?So you do not recommend any diode / voltage arrester, you say that with a good power supply I can solve it?Before spending a lot of money on an expensive power supply and finding it broken after a few hours, I thought I'd ask for advice before proceeding.Thank you all

author: Benta
author_rank: Super Contributor
timestamp: September 14, 2022, 04:28:07 pm
content: Quote from: jonpaul on September 14, 2022, 12:02:24 pmLet's stay realistic. The inductances are negligible compared to mechanical inertia and back EMF.

author: Vincent
author_rank: Regular Contributor
timestamp: September 14, 2022, 06:13:46 pm
content: Quote from: federico on September 14, 2022, 02:06:15 pmWhat I mean by soft-start is a circuit that limits the current to a specific value by itself. That thermostat may provide that effect if it starts the motor at a reduced speed. But maybe it's not low enough to limit the inrush current to a safe value. Because of the compressor the current needed to get the motor going could be too high for the power supply. This is something you'll probably need to figure out before choosing another power supply. At least that's what I would do, then find how to reduce that inrush current as much as possible, then choose a power supply capable of delivering that current + a certain % like 10 or 20, just to be sure.Quote from: federico on September 14, 2022, 02:06:15 pmActually it'd be this:where R & L are the DC motor. As previously mentioned the inductance of the DC motor is probably not high enough to generate damaging voltage spikes, but if it were the diode would clamp that voltage spike to the forward voltage of the diode.

author: Benta
author_rank: Super Contributor
timestamp: September 14, 2022, 07:11:33 pm
content: Quote from: Vincent on September 14, 2022, 06:13:46 pmR and L are NOT the DC motor, they are theparasiticsof a DC motor. An ideal DC motor is a back EMF voltage source proportional to the input voltage. On top of that, motor and load inertia come into play.

author: Vincent
author_rank: Regular Contributor
timestamp: September 14, 2022, 07:38:09 pm
content: Quote from: Benta on September 14, 2022, 07:11:33 pmRight. Though I was simply making a connection between the diagram and the real-life component, not claiming a motor is simply a series RL circuit. For the purpose of freewheeling, the back-EMF nature of the motor isn't exactly relevant is it? An ideal motor wouldn't even need it in the first place.

author: james_s
author_rank: Super Contributor
timestamp: September 14, 2022, 08:05:28 pm
content: Once again I think it's important to clarify that this is NOT a DC motor, it is a BLDC motor with its own electronic driver box. This is a hermetic compressor, the entire motor and compressor assembly is permanently sealed inside a welded steel dome and the entire assembly is continually doused in a mixture of oil and refrigerant. If you made something like that with a brushed DC motor the metering device would likely get clogged with carbon particles.

author: Benta
author_rank: Super Contributor
timestamp: September 14, 2022, 08:31:53 pm
content: @james_s, point taken and I agree.

author: Zero999
author_rank: Super Contributor
timestamp: September 14, 2022, 08:44:42 pm
content: Quote from: james_s on September 14, 2022, 08:05:28 pmIt's a standard fridge motor, wound for lower voltages, with an inverter attached to it.I can't see any mechanism for it to destroy the power supply. I doubt the inverter will do any braking, when it stops.

author: janoc
author_rank: Super Contributor
timestamp: September 14, 2022, 08:48:02 pm
content: Quote from: federico on September 14, 2022, 02:06:15 pmIf you expect a sub $30 AliExpress supply to actually be able to reliably handle 700W, you have to be kidding. Be glad that it didn't burn your house down. That thing would have probably struggled with 200W already and every corner has been cut.To have a decent quality supply you need to likely add one zero to that price.Quote from: federico on September 14, 2022, 02:06:15 pmMeanwell is good - butnot for your application. There is quite a difference between an 2kW UPS which also comes with all sorts of protections and a "naked" industrial supply like the boxes from Meanwell that are intended for powering "well behaved" loads.

author: Vincent
author_rank: Regular Contributor
timestamp: September 14, 2022, 09:06:45 pm
content: Quote from: james_s on September 14, 2022, 08:05:28 pmI'm not arguing about that at all. Maybe this is just me who goofed up on the wording, but I never intended to claim anything regarding the nature of the motor in OP's refrigerator compressor, or motors in general. I used "DC motor" as a generic term, not particularly focusing on its construction or other characteristics. My point was just that if that motor creates voltage spikes when disconnected, connecting a freewheeling diode to it would be a good idea, that's all! But that tangent on diode freewheeling is probably pointless anyway.The main problem is "make SMPS + fridge compressor work", so let's just focus on that, shall we?

author: MathWizard
author_rank: Super Contributor
timestamp: September 14, 2022, 09:26:36 pm
content: So if you had a computer ATX 700 W 12V PSU, typically a forward converter type SMPS, and is meant more for capacitive loads like logic circuits switching on/off......what gets changed to make a SMPS to power a typical motor ? I haven't gotten deep enough into the AC analysis of all this stuff yet, but I'll get there.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 15, 2022, 02:04:20 am
content: Quote from: federico on September 14, 2022, 02:06:15 pmJust buy a surplus server PSU.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 15, 2022, 04:44:55 am
content: Quote from: federico on September 14, 2022, 02:06:15 pm700W power supply for $30... enough said. That thing could just as well have failed while powering nothing at all. I've seen cheap power supplies defective right out of the box, and others that fail prematurely. Never mind the fact that such cheap power supplies often fail to deliver anywhere close to rated power, or promptly overheat and let the smoke out. Also such cheap power supplies could even be dangerous as the electrical isolation between primary and secondary may or may not be sufficient. Never cheap out on power supplies.I highly doubt the cause was overloading of the power supply. Any half decent quality power supply should be able to handle an overload (or even a short circuit) without damage to the power supply.Meanwell is a good choice for decent power supplies that won't break the bank.Quote from: federico on September 14, 2022, 02:06:15 pmThose are UPS brands. This person likely misunderstood and thought you're trying to run a regular AC powered fridge.

author: james_s
author_rank: Super Contributor
timestamp: September 15, 2022, 06:31:38 am
content: I have a bunch of different server PSUs, I never paid much more than the cost of shipping for one and I've yet to have one fail. I use them to power chargers for my RC airplane batteries, I've loaded them up to the point where the overcurrent shutdown kicked in.

author: floobydust
author_rank: Super Contributor
timestamp: September 15, 2022, 08:18:42 pm
content: DiodeGoneWild did test one of those super cheap chinese USD $26 12V 50A 600W SMPS - which of course died after 1hr, rectifier above 100°CHis autopsy of what was too cheap and failed is very good.

author: james_s
author_rank: Super Contributor
timestamp: September 15, 2022, 08:34:09 pm
content: Quote from: Zero999 on September 14, 2022, 08:44:42 pmI'm not certain of that, I have one that that shows on the drawing a 3 phase delta winding whereas typical fridge compressors use a start and run winding. It's from a portable cooler that had failed due to loss of charge so I might just cut open the compressor sometime to take a look. That said, they do make 3 phase hermetic compressors, not sure if anyone makes one as small as that in a domestic fridge though.

author: james_s
author_rank: Super Contributor
timestamp: September 15, 2022, 08:35:41 pm
content: Quote from: floobydust on September 15, 2022, 08:18:42 pmI see no reason to consider buying one of those when you can get a top notch server PSU for about the same price. The surplus market is flooded with them from decommissioned servers, sometimes you even find brand new ones.

author: TheMG
author_rank: Frequent Contributor
timestamp: September 15, 2022, 11:30:52 pm
content: Quote from: james_s on September 15, 2022, 08:34:09 pmThey're actually brushless DC (BLDC) motors with a permanent magnet rotor, specifically made for DC powered applications.The Danfoss website has some good information on DC compressors (they also do mention that they use a permanent magnet motor):https://www.danfoss.com/en-us/products/dcs/compressors/compressors-for-refrigeration/direct-current-compressors/#tab-overviewUnfortunately, startup current does not seem to be specified in any of the datasheets. I do have a fridge out at the cabin that has one of these compressors. Rated 55W while running, 12 or 24V DC supply. I have it running off a 150W Meanwell 48V-to-24V DC-DC power supply (runs off my solar system which has a 48V battery bank). No problem whatsoever. The manufacturer of the fridge does sell separately a 24V 5A (120W) power brick for those that want to run in on AC power, so it can be assumed the startup current is probably no more than 2x the running current.Next time I'm out there I'll bring my clamp meter and measure how high the startup current is. I'm guessing it's not going to be much as these compressors do have a soft startup, which is one of the many benefits of BLDC motors in general, they can be run at any speed from zero to rated speed, it's not all-or-nothing like a standard compressor with an induction motor.

author: coppercone2
author_rank: Super Contributor
timestamp: September 16, 2022, 02:27:37 am
content: I got a freakin crazy ideas, put a oscilloscope on it to measure if it is in fact doing something bad with voltage or currentI swear benta and whoever is being tossed into all the motor threads, its like the family guy chicken man fight and its getting into other shows

author: coppercone2
author_rank: Super Contributor
timestamp: September 16, 2022, 04:22:26 am
content: Because I have a hunch that what happened is that someone put a cheap preventative maintenance replaced server power supply on eBay, and that it broke because it was going to break anyway. Like some data center crap.30A 12V sounds like a server tower supply. I had a small form factor 60A 12V server supply once, I think that maybe they are repurposing them as power supplies or something. There must be tons of them in existence. Maybe they are taking old server supplies apart and making power supplies out of them, it would be one way to make a cheap 30A supply. I have a hunch its dodgy power transistors. I think a name brand used server supply would be better then what you have, but its still likely got a TON Of hours on it.I would imagine they build DC-compressors with decent parameters, because they are designed for portable power applications. I think there would be too many people complaining about them killing solar converters and stuff if they were that bad a load. Like trailer/RV people might seriously depend on those refrigerators working. I have a feeling if you plug it into a quality power supply its going to be just fine.... but you really should check with an oscilloscope.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 16, 2022, 04:43:23 am
content: Quote from: coppercone2 on September 16, 2022, 04:22:26 amSounds more like one of those cheap 3D printer PSUs.

author: coppercone2
author_rank: Super Contributor
timestamp: September 16, 2022, 05:00:22 am
content: ok so that would mean that its meant to run a purely resistive load? that would explain it. I assume 90% of the energy is meant for the warmed bed and extruder? I feel like the current consumption of the positioning motors might be really low, proportionally.