Repairing an LG WM3997 washer/dryer - Page 1
Forum section: electronics / repair

author: ciradrak
author_rank: Contributor
timestamp: January 15, 2025, 06:09:03 pm
content: Over a few days my washing machine spun slower & slower, until the drum would not rotate. After disassembling the stepper motor everything appeared to be in good condition. Eventually I measured the incoming voltage lines using a multimeter. Running the machine's test sequence for the motor showed no voltage using either AC or DC test options. The wiring all appeared to be fine & ohmed out correctly back to the main circuit board. Further testing showed that no measure able voltage was being produced by the inverter on the main circuit board. The board was in cased by rigid plastic & what appears to be clear silicone. I cut away the plastic housing, which revealed that the board was discolored inside the silicone. I don’t see the white ash which comes from silicone decomposing from heat, so the discoloration is probably from something else on the board, flux possibly? At this point I stripped off the silicone from the backside of the board to electrically reach the components. The silicone was sticky & softer in the discolored areas.Honestly, I’m not sure what I should be doing to test the board or figure out what is wrong. None of the capacitors seem obviously exploded. Any ideas?

author: BILLPOD
author_rank: Frequent Contributor
timestamp: January 15, 2025, 07:04:06 pm
content: Good Morning Ciradrak, Your description makes it sound as if the drum is seized up. Can you turn the drum by hand? I've never heard of stepper motors in washing machines, somaybe you are talking about the main motor that operates the drum. I've had a washer that the transmission, (which is powered by the motor), seized up. Your problem may not be electrical, but mechanical.

author: ciradrak
author_rank: Contributor
timestamp: January 15, 2025, 07:19:26 pm
content: Yes, the drum & motor spin freely. I took the motor all apart & everything is clean & in good order. The windings have resistance values which match the specification too.I think it is a stepper motor? How could we identify one?

author: coromonadalix
author_rank: Super Contributor
timestamp: January 15, 2025, 07:26:18 pm
content: a stepped motor will rotate in degrees and you feel them, a brushless one turn freely

author: ciradrak
author_rank: Contributor
timestamp: January 15, 2025, 07:53:10 pm
content: The motor is controlled by three wires & turns freely. So a brushless one.

author: fzabkar
author_rank: Super Contributor
timestamp: January 16, 2025, 10:02:13 pm
content: Just for reference ...https://research.encompass.com/ZEN/sm/WM3997HWA.pdf(service manual)QuoteCan you measure the voltages across the two 680uF 250V bulk capacitors before and after the motor is started?

author: tom66
author_rank: Super Contributor
timestamp: January 16, 2025, 11:03:58 pm
content: If the motor was turning slower than expected, it sounds like the machine may be unable to determine the motor's position. Brushless motors require the controller to know the motor's position accurately to time the sequence of phases correctly. On most non-drone applications, motor position detection is done by an external transducer like a hall effect sensor or optical encoder. (Drones and some RC applications detect motor position by back EMF, but this can't work at very low speeds.)

author: ciradrak
author_rank: Contributor
timestamp: January 18, 2025, 07:06:43 am
content: I reassembled the machine with test leads to one of the big 680µF 250V capacitors capacitors. Powering the machine on took the voltage across the capacitor to 168.8V Starting the test sequence caused the motor to shrug & wobble back & forth a bit before going still. At the height of the wobble the voltage across the capacitor dropped to 162V and then went back up. Retesting gave the same result, as did hooking the leads to the other capacitor.The way the motor shrugs when trying to start, it does seem to be failing to figure out the motor’s position. I do not see anything in the motor that looks like a hall sensor & one does not appear in the wiring diagram for the machine. The display used to give a read out of rpm for the drum & was capable of spinning at speeds as low as one or two revolutions per second. The spec says “0-1300 rpm”. Could it still be using back EMF to know the position of the drum?I read about brushless DC motors & how simple controllers use six mosfets to control them. I don’t see anything that looks like an array of six mosfets. This board is more sophisticated & uses something else, sine wave generators?I also created a survey of the electrolytic capacitors on the board using a component tester. I’m not sure if it tells us anything useful as some of the in circuit capacitances are wildly different from the individual components. Some groups appear to be connected in parallel and give similarly far out readings. With sealant still on the topside of the board it is difficult to tell where some traces connect. Some show up as diodes & one registers as ’unknown’. Of the ones that give reasonable readings several are borderline on a basic ESR chart, but one CE181 is way out. So that could be a problem? That capacitor is also in an area of the board which shows the most discoloration under the silicone.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: January 18, 2025, 02:19:42 pm
content: Maybe try to identify some of the ICs, find the voltage supply pin via data sheet lookup, and then measure their supply voltage? Confirming stable supply voltages is always a good starting point.If you can't identify hall encoders, rotary encoders, etc. then I would also guess that it is using back EMF. It might be worth investigating the integrity of the wires/connectors between the PCB and the motor. Washing machines definitely see a lot of vibration.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 19, 2025, 03:52:18 am
content: 115 x 1.414 =162.Your 680uF caps are OK.The electrical circuit does not show any standalone sensors that control the motor so I assume it must be on the Control PCB.https://electronics.stackexchange.com/questions/365796/how-do-single-phase-bldc-motors-start-in-proper-directionDoes the machine throw up an error in LOAD TEST MODE?

author: ciradrak
author_rank: Contributor
timestamp: January 20, 2025, 03:14:15 am
content: I'm working on understanding & carrying out the other instructions.Putting the machine into LOAD TEST MODE shows three rotating codes on screen.15:47U:11d:04I don't know what that means, but it does not seem to be an error.

author: ciradrak
author_rank: Contributor
timestamp: January 20, 2025, 04:10:49 am
content: Here are the coils & magnets inside the motor from when it was disassembled. I count 12 magnets & 36 coils.

author: Original
author_rank: Contributor
timestamp: January 20, 2025, 04:46:20 am
content: if the drum does rotate a few degrees (not full turns) on its own and then stop, then your problem is here:

author: ciradrak
author_rank: Contributor
timestamp: January 20, 2025, 04:55:40 am
content: I got really excited for a few minutes there, that maybe the problem was about to be solved, but as you can see in the photo of the windings, this machine does not have a rotor position sensor switch. :'(

author: Original
author_rank: Contributor
timestamp: January 20, 2025, 05:06:15 am
content: the sensor is on the back take off the motor assembly!

author: ciradrak
author_rank: Contributor
timestamp: January 20, 2025, 05:52:24 am
content: Eating humble pie & hoping for a miracle I took the machine apart again.I see the red-yellow-blue wire harness that runs the motor, and a green wire with a yellow strip which seems to be a ground connection, but that's all there is here. Having learned about brushless DC motors, I can now see how each winding skips to each third coil, & all of them tie together in a star configuration using a little block a bit to the left of the wire harness in the image. (Will this image be rotated sideways? Not quite sure why portrait images are rotated into landscape.)

author: Original
author_rank: Contributor
timestamp: January 20, 2025, 06:07:48 am
content: Unbelievable!!!! I am quoting from the service manual which says there should be.... Nice Job LG !!!

author: Original
author_rank: Contributor
timestamp: January 20, 2025, 06:10:26 am
content: I would send you the service manual, but I'm no longer sure how much of it applies

author: ciradrak
author_rank: Contributor
timestamp: January 20, 2025, 06:41:29 am
content: Mine is remarkably similar, except missing that crucial bit in the diagram...I'm sad but it would require something of a miracle to get this fixed now. This machine is scheduled to be hauled away tomorrow. I thought maybe I could do something about that, but I'm lost as to what I could do now.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 21, 2025, 07:32:53 am
content: The machine maybe gone by now but it would be interesting to see a pic of the rear of the PCB to see how bad the burnt areas were.What components on the other side were generating all the heat?Measuring the DC voltages across that group of Electrolytic Caps in the "Burned" area could have been informative as they are probably all the DC supplies for the PCB components, relays and the motor.

author: ciradrak
author_rank: Contributor
timestamp: January 21, 2025, 05:13:30 pm
content: Sadly it is gone now. :c I'm having a little bit of a mourning moment. I'm planning on trying to fix something else in a couple days.