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Member Since 15 Jun 2012
Offline Last Active Jun 16 2012 03:42 AM

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Possible Failure Of Soft Start Of Domestic Air Conditioner Motor

15 June 2012 - 02:46 PM

I'm trying to determine if the soft start module OR the compressor unit is at fault.
Supply: 240VAC, 50 Hz, Single phase
Motor : Conventional Run & Start windings
LRA 90~100 amps
Operating current: 18.9 amps
Speed: 2900 rpm

Power supplied from a "power Relay" (my phraseology) activated by the inside unit control circuit board, which supplies active to the common © motor terminal
It also simultaneously supplies an input terminal on the soft start module
There is a connection between a 2nd terminal on module and the Run terminal of the motor
There is a connoction from a 3rd terminal on module to neutral
The remaining 4th terminal on module connects to a pair of parallel capacitors (start capacitors) nomonated as C3 & C4 - 60 & 98 mfd
The other side of thess parallel capacitors connects to Start terminal of motor
There is a 3rd capacitor - C1 50 mfd - also connectect between Start terminal and Neutral
All three capacitors are start capacitors (unless run capacitors now look like start caps)

Compressor motor starts but cuts out on internal overload after 5 ~ 10 seconds.

Capacitor check revealed C1 450v 50 mfd to be quite dead

Replaced C1

Identical behaviour

1. Motor overload due to seizing compressor? (impossible to check mechanically)
2. Failure of soft start module?

Cannot source a replacement module except to import from OEM (China) at considerable cost

1a. Is there some way I can disconnect module and DOL start to check compressor seizure or internal motor failure (At present motor windings seem OK: Run = 0.9 ohms, Start = 1.9 ohms Total across R & S Terminals = 2.6 ohms)?
1b. Should I modify capacitor circuitry to do this?
1c. If so, what modification?

2. Is it possible to check and diagnose the soft start module behaviour?

I am limited to 10 amps current measurement

I'd appreciate any knowledgeable person's advice