Need advice re testing a motor (problem with airconditioner) - Page 1
Forum section: electronics / repair

author: Kevin H
author_rank: Contributor
timestamp: September 28, 2023, 07:18:29 am
content: I have a problem with a Daikin ducted reverse cycle air conditioning system. Every time it is turned on and any mode is engaged, one of two 5A fuses blow. This basically cuts all power to the indoor unit and control panels.Through a process of elimination I believe the problem lies with the evaporator fan motor.I’ve replaced the motor run capacitor, but that didn’t help. It doesn’t make any difference whether the mode is heating, cooling or fan only.I have removed the fan and motor from the evaporator unit in the attic I’m hoping someone can tell me how to test the motor.The fan can be spun freely by hand so it’s not a bearing.It’s a Fasco motor, model# 8555NVA-A17, 240V, 50Hz, 315W and it has two speeds. Other than and earth and two wires to the motor run capacitor (4uF), it has five other wires – brown, blue, purple/mauve, grey and white.

author: Gyro
author_rank: Super Contributor
timestamp: September 28, 2023, 09:15:00 am
content: Have you checked for resistance between the motor winding connections and mains earth? It could be a winding insulation failure. Ideally you would use a proper high voltage insulation tester but a DMM might show something.

author: SeanB
author_rank: Super Contributor
timestamp: September 28, 2023, 09:39:10 am
content: It is Diaken, time to replace the motor.

author: Kevin H
author_rank: Contributor
timestamp: September 29, 2023, 06:26:47 am
content: Thanks SeanB.It is a Daikin system. I would just change the motor, but they're over $500 plus freight@ 7kg, so I want to make sure the problem is the motor before I do that.Thanks Gyro.A DMM isn't showing any shorts between the windings and earth.I found the wiring diagram for the indoor unit and the wires have the following labels;Blue=NBrown=HGrey=MViolet=LWhite=LLI should also mention that the motor has a built in overload protection with auto reset.I have attached schematics for the indoor and outdoor units.The whole system gets its power from a 3 phase input to the outdoor unit, which in turn supplies 240v to the indoor unit via connector X1M. The fuses that blow (one or the other) are F1U and F2U marked in red on the schematic for the outdoor unit. The fan motor is on the right hand side of the schematic for the indoor unit.If the fan motor is disconnected and the blown fuse is replaced, the system powers up and the control panels work. Once the fan motor is connected if the system it turned on in any mode, the fuse blows immediately. However, you can very briefly hear the fan motor try to start. I tried turning the system on while spinning the fan by hand, in case it was just starting current that was the problem, but the fuse still blows.Does anyone have any other ideas how to troubleshoot this? (to confirm whether it is the fan motor that is the problem).EDIT: I've added an image of the outdoor schematic so people don't have to download the PDF version!

author: Gyro
author_rank: Super Contributor
timestamp: September 29, 2023, 08:42:25 am
content: Any condition that causes more than one of the motor tap relays (RY F1-F3) to energise at the same time would probably cause the fuse to blow (maybe not imediately) - and result in the motor briefly 'trying to start'.

author: Uup
author_rank: Regular Contributor
timestamp: September 29, 2023, 08:44:46 am
content: Based on your diagnosis so far, it is very likely that the indoor fan motor is faulty (shorted turns in the motor winding).You can test the motor independently to confirm. Since it has a run capacitor, which is 4uF, it is probably less than 400W and a PSC type of motor, which is simple to connect power to for testing.The motor connections are as follows:N - (Blue) : Motor common (Neutral)H - (Brown) : High SpeedM - (Grey) : Intermediate-high SpeedL - (Violet) : Intermediate-low SpeedLL - (White) : Low SpeedCapacitor wires are probably orange and on an independent cable.For safety, connect the wires to a terminal strip and be sure to connect earth ground too. The non-connected wires will have a voltage on them when power is applied, so if you are not using a terminal strip you should use screw connectors or insulate the exposed wires too. Also, use something with over-current protection, such as a extension power board with integrated circuit breaker. Otherwise, you will likely trip a power circuit breaker if the motor has a significant short.If you don't have anything at hand for over-current protection then you could use something like a portable electric heater or other similar simple high power appliance. A heater with a high/medium/low setting would be convenient as the current limiting amount would be controllable. A dead short will not take out a circuit breaker either. Make a test lead with a power plug and two sockets and wire the sockets in series. One socket will be for the current limiting device and the other socket for a plug/lead to the fan motor under test. An appliance that is rated at around 1500-2000W would be fine. If the motor is faulty then the appliance used for current limiting will power up/work and the voltage to the motor will be low. If the motor is good then the reverse will be true.The power connections to the motor are straight forward. Neutral (blue) from the test lead to Neutral (blue) on the motor. Active (Brown) on the test lead toone of the speed wires only. I would start by testing on the lowest speed first. If the motor seems to work fine then test the next speed up, then progress to high speed. If the motor is faulty it may still appear to run normally, but will be drawing excessive current and will eventually overheat.Also consider that the motor will draw more current than rated when it is outside of the unit (no static pressure). You could face the blowers downwards against a floor, if it seemed to work normally and you wanted to test for a longer period of time.Lastly, seriously consider your safety before testing. That is, both electrical and mechanical safety. Eg. if the motor does spin-up, the fans will be blowing air which could cause the assembly to turn or flip, taking out your test wiring and causing everything to crash and tumble... depending on how you set everything up. You obviously have ability, since you removed the fan assembly and got to this point. However, since you're asking how to test the motor, it is reasonable to consider that you may not be aware of other potential risks to your safety.

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 29, 2023, 03:17:03 pm
content: Yeah, strap it down good before powering, you don't want things moving unexpectedly with exposed 240 around. Also try to work outside in case of smoke.Stupid question: Would shorted turns in this case show up with ring testing the winding? That could be done without playing with 240, just don't know that I've seen it done with lower frequency iron core type stuff.

author: blauerscharik
author_rank: Regular Contributor
timestamp: September 29, 2023, 05:09:09 pm
content: Quote from: Gyro on September 29, 2023, 08:42:25 amYou are looking at the outdoor fan. He is having issues with the indoor fan.

author: blauerscharik
author_rank: Regular Contributor
timestamp: September 29, 2023, 05:11:48 pm
content: QuoteWhat more troubleshooting do you need?What's the model of the whole unit?

author: Kevin H
author_rank: Contributor
timestamp: September 30, 2023, 02:46:43 am
content: Quote from: blauerscharik on September 29, 2023, 05:11:48 pm

author: Uup
author_rank: Regular Contributor
timestamp: September 30, 2023, 05:32:34 am
content: Quote from: Kevin H on September 30, 2023, 02:46:43 amDC resistance is not always helpful as an indicator with a partially shorted winding on a PSC type, more useful with three phase motors. Having said that, the resistance is higher than what I would expect for that motor, so I think there are shorted turns between the intermediate and high speeds.As for the discrepancy between the selectable number of speeds on the wall control and the available speeds on the motor, it is by design. You are right in that the controller automatically chooses the appropriate speed and the user just has the option of high and low speeds. This is due to the ability of the air conditioner to cater for different installation static pressures. This is normally configured in the set-up options by the installer, as each installation has different characteristics which can be catered for, to a certain degree. Depending on the configuration of the installation settings, high speed on the wall control may actually run intermediate low on the motor rather than the motor's actual high speed.Most of the more recent commercial and larger home units use BLDC motor(s), which have a greater range of adjustment for static pressure and which are also more energy efficient, albeit more expensive too.

author: Uup
author_rank: Regular Contributor
timestamp: September 30, 2023, 05:56:49 am
content: Quote from: BrokenYugo on September 29, 2023, 03:17:03 pmDepends on the type of ring tester and even then it would be more comparable to signature analysis than to a straight good/bad indication. If it's one of those old types for testing EHT transformers for CRTs, with a good/bad indicator, then doubtful.

author: Kevin H
author_rank: Contributor
timestamp: September 30, 2023, 07:32:16 am
content: I have now tested the motor with a 5A fuse in the active line . The motor runs fine at the two lowest speeds. At intermediate-high speed there is the briefest sound of the motor starting (less than a second) then the fuse blows. At high speed the fuse blows instantly. So I'm going to order a new motor.Thanks to everyone for your suggestions/advice!

author: blauerscharik
author_rank: Regular Contributor
timestamp: September 30, 2023, 07:55:48 am
content: Quote from: Kevin H on September 30, 2023, 07:32:16 amI think you have found the issue earlier already (disconnecting the load/fan from the board).But you basically have confirmed it now outside of the system.Good job!

author: Kevin H
author_rank: Contributor
timestamp: October 08, 2023, 06:40:39 am
content: Success!!Replaced the motor and the system is now working perfectly.Thanks to all for the advice!