Weird Washing Machine Problem - Page 1
Forum section: electronics / projects

author: wilheldp
author_rank: Regular Contributor
timestamp: October 24, 2022, 06:51:28 pm
content: I am trying to diagnose a bizarre issue I'm seeing with modern washing machines, and I have reached the end of my electrical expertise without a satisfying answer. I was hoping I could harness the intelligence of this forum to come up with an explanation.The issue began as premature corrosion of the metal fittings on washing machine hoses (as in, they were corroding and fracturing after only a year or so). I figured out that the corrosion was being accelerated by a high DC voltage between the metal fitting attached to the water valve and ground. With some further tracing, I have found that a temperature sensor embedded in the plastic valve body is somehow leaking DC into the water supply, and it us using the water as a path to get to the metal hose fitting.This phenomenon only occurs on washing machines that use a BLDC motor for the main agitation motor. These are mostly newer models because older models use AC motors. The power cord plugs directly into a big DC power supply which essentially just rectifying and smoothing the 120 Vac into 170 Vdc. The temperature sensor in question only operates on 5 Vdc. If you measure the voltage across the two conductors routed to the sensor, you measure 5 Vdc, but if you measure from each wire to ground, you get 170 and 165 Vdc. So, my first question is, how does that work? How do you "float" the DC ground so that you just get a 5 Vdc difference, but the ground reference is still at 170 Vdc?My second question is, how on earth is the voltage getting from the temperature sensor into the water supply? I am fairly certain that the sensor is just a thermistor, and it is embedded in a rigid plastic housing. There are no exposed wires or metal parts sticking out of the sensor housing. I have measured as much as 110 Vdc between a hose fitting attached to the valve and ground, so most, if not all, of that 170 Vdc is getting directly injected into the water supply.We have gotten washer hoses with degraded fittings that are only rubber between the two metal fittings AND hoses that have a stainless steel protective braiding on the outside of the hose. There is no continuity between the two metal fittings on the rubber hoses, but there is a direct short between the metal fittings on the stainless steel braided hoses. How can any current be flowing through the fitting on the rubber hoses if there is not a direct path to ground?Nothing about this is making any sense at this point. I hope you guys can shed some light on it before I go completely insane. TIA.

author: moffy
author_rank: Super Contributor
timestamp: October 24, 2022, 09:29:34 pm
content: It sounds like they have a non isolated power supply, no isolation transformer, and are depending on insulation to provide isolation. Transformers are expensive, especially mains, so if they can eliminate that then they reduce their costs. It's a bit weird though, as all the metal work should be grounded, and if you had significant leakage current, as you suspect, you would tend to trip an RCD breaker, unless that circuit doesn't have an RCD breaker. If the leakage is coming from that temperature sensor then it is faulty. Check your metal work grounds, and if the metal work fittings are connected to insulated fittings you could ground them as well.P.S. A temperature sensor also suggets a heater element, which is immersed in the water, a possible leakage source.

author: wilheldp
author_rank: Regular Contributor
timestamp: October 24, 2022, 10:04:52 pm
content: Quote from: moffy on October 24, 2022, 09:29:34 pmYeah, I sectioned a valve today and isolated the temperature sensor. There is a 0.5mm hole in the plastic sensor housing, partially exposing the thermistor. That provides a plausible explanation for how the electricity is getting into the water supply. But this is a re-occuring issue in several different makes and models of washer. I guess it's possible that they are all sourcing these thermistors from the same place, and they have a systemic manufacturing defect.Quote from: moffy on October 24, 2022, 09:29:34 pmThere's no heating element. There are hot and cold water supplies to the valve. They are routed to separate solenoids, then both are routed to a third solenoid. The supplies are mixed at the third solenoid to get "warm" water, and the thermistor is in the mixed warm water stream to detect how hot it is.

author: moffy
author_rank: Super Contributor
timestamp: October 24, 2022, 10:28:55 pm
content: Quote from: wilheldp on October 24, 2022, 10:04:52 pmSo, you must be a washing machine repair man/technician?P.S. I only ask because if so, then you are the expert.P.P.S. What about grounding of the metal work? Let me guess no stainless steel tubs/rotors (I don't know the proper term)?

author: Bud
author_rank: Super Contributor
timestamp: October 24, 2022, 10:49:02 pm
content: Can't trust any appliances made in China. Also, one company may own multiple brands which is common in electrical appliance industry. They will share same part numbers.

author: wilheldp
author_rank: Regular Contributor
timestamp: October 24, 2022, 11:17:48 pm
content: Quote from: moffy on October 24, 2022, 10:28:55 pmForensic Engineer. I usually investigate fires, but the mechanicals brought me in on this one because it was an electrical issue.Everything seems to be grounded well.

author: moffy
author_rank: Super Contributor
timestamp: October 24, 2022, 11:25:15 pm
content: It looks like you have the problem and the cause diagnosed. Have you been able to measure the level of leakage current? There should be corrosion present on the thermistor unless the corroded connectors have become sacrificial. I must say this is an interesting if not somewhat baffling problem.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 24, 2022, 11:38:00 pm
content: If you can identify the kind of sensor and get a replacement, epoxy it to the outside of the plastic fitting.

author: wilheldp
author_rank: Regular Contributor
timestamp: October 24, 2022, 11:48:50 pm
content: Quote from: moffy on October 24, 2022, 11:25:15 pmYeah, the hole in the sensor housing explained some things. But I'm still baffled how and why there is 170 Vdc present at a sensor designed to operate on 5 Vdc.I have a couple of power supply boards coming for additional testing. I may try to extract one of them from the potting compound to try to make a circuit diagram. But it's the hard potting compound, so I don't like my chances. I also plan to try to get an accurate current measurement. I know a few of us have gotten shocked while testing this stuff.Quote from: NiHaoMike on October 24, 2022, 11:38:00 pmI wish we could identify the sensor. There are no markings on this thing at all.

author: langwadt
author_rank: Super Contributor
timestamp: October 24, 2022, 11:55:09 pm
content: Quote from: wilheldp on October 24, 2022, 11:48:50 pm5V supply is not isolated and phase and neutral is swapped?

author: WattsThat
author_rank: Frequent Contributor
timestamp: October 25, 2022, 12:36:13 am
content: FWIW for those outside the US: It is highly unlikely the washer is on an RCD breaker, you don’t see them in general use here in the USA.Our National Electrical Code (NEC) specifies that any 120V outlet within 6 feet of a water source must have a ground fault interrupting outlet (GFCI). These provide the same function except they’re distributed at the point of use, not at the point of supply. But, having a GFCI is not something you can count on in older homes, many have never been updated. So, the washer may be powered by a source that allows leakage current from line to neutral or line to ground without any protective trip level.Even worse, there are many older homes in the US that lack a dedicated ground. It was very common in those installations that you’d find the line and neutral swapped somewhere within the structure.

author: james_s
author_rank: Super Contributor
timestamp: October 25, 2022, 12:47:45 am
content: Quote from: wilheldp on October 24, 2022, 11:48:50 pmI don't find this particularly surprising. A lot of appliances use a transformerless power supply which is non-isolated. Essentially a buck converter, or in the case of the motor controller in my washing machine just a string of dropper resistors. It's not uncommon to have low voltage electronics floating around 170V, this was commonly referred to as a "hot chassis" design in TV sets and such. In these devices there should be no exposed conductive surfaces that are connected to anything that can be electrically live. It's possible the designer assumed that the sensor used would have better isolation between the housing and the terminals.Have you checked to see if the 170V can deliver a significant load? A light bulb or power resistor is a reasonable test load here, to rule out capacitive leakage.

author: wilheldp
author_rank: Regular Contributor
timestamp: October 25, 2022, 01:13:44 am
content: Quote from: james_s on October 25, 2022, 12:47:45 amThanks for the explanation. I've been looking into non-isolated power supplies since it was mentioned a few times so far. Looks like that is the culprit. I really need to find out more information on that sensor to see if the hole in the housing is intentional or not. It looks like a bad/off-center molding, but it may be done on purpose to increase response time to temperature changes. If that is the case, the sensor manufacturer might be appalled to know that the sensor is being powered by a non-isolated power supply.I know that the 170V power supply in the washer has a lot of load capacity since it has to run the main agitation motor. How much of that can leak through the sensor and water supply remains to be seen. I'm hoping to be able to test that later this week when the exemplar power supply boards show up.

author: james_s
author_rank: Super Contributor
timestamp: October 25, 2022, 01:45:24 am
content: Well there could well be more than one 170V supply, I would expect the motor drive to be a separate board connected to the incoming line but I don't know for sure, it's possible they have integrated it all on yours. If the sensor is indeed connected directly to a circuit powered by a non-isolated power supply then I would expect it to be safely protected where nobody can reach it, unless the sensor itself is defective and has an insulation fault but I wouldn't want to rely on that.I don't know how widespread it is, but I have been appalled at some of the engineering choices I've seen at least in older (as in early 2000's) appliances. I get the feeling that the guys that were designing appliances for many years were out of their element when it came to the modern digital electronics that started showing up. The big string of power resistors in the motor driver of my Maytag Neptune being a good example, I was shocked they took such a crude approach at providing power to the logic circuitry.

author: wilheldp
author_rank: Regular Contributor
timestamp: October 25, 2022, 04:49:31 pm
content: Quote from: james_s on October 25, 2022, 01:45:24 amI don't think so. The connector routed to the thermistor is on the same power supply board where the power cord for the washing machine connects. I can see the rectifier, choke, filter caps, and smoothing caps protruding from the potting compound. But I don't see another voltage regulator for the 5V supply. I think it might be surface mount and/or embedded in the potting compound.

author: 2N2222A
author_rank: Regular Contributor
timestamp: October 28, 2022, 04:02:07 pm
content: Positive versus negative depends on what you're using for ground or common when you measure the Voltage on the sensor, and also what they're using for common. Chassis positive or positive common things do exist.When you measure with the negative probe on the negitive side of a bridge rectifier and the test probe on the AC input, both AC terminals will measure as positive.If the temperature sensor is connected to the DC negitave, then the AC side (hot and neutral), which includes ground and the water pipes, are now positive when measured with the temperature sensor as the negative reference point. The temperature sensor is negative so no corrosion occurs. The water pipe is now positive and it corrodes.I assumed you are referring to the water pipes and valves on the building on the supply side of the hoses that connect to the washing machine. A non grounded washing machine chassis would explain this. If the washing machine is grounded, then the corrosion should remain within the washing machine, unless all the plumbing going from the temperature sensor and out to the hoses is either plastic or non grounded.

author: amyk
author_rank: Super Contributor
timestamp: October 30, 2022, 01:20:57 am
content: Quote from: wilheldp on October 24, 2022, 06:51:28 pmThe hose ends are metal, but are the inlet pipes where they attach to the machine also metal? If not, I don't see how current could flow through the fitting to cause corrosion, and the voltage in the water may be a red herring (while the leakage current may be finding a path to ground through the plumbing, corrosion only occurs when the galvanic cell that's formed is in the path of the current.)If you can post some pictures of what's happening, that might help us identify more what's going on.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 31, 2022, 07:24:27 pm
content: Chuck in a Miele and relax.