Electronics > Repair

Need advice re testing a motor (problem with airconditioner)

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Kevin H:
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.

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.

It is Diaken, time to replace the motor.

Kevin H:
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;

I 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!

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'.


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