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afergie

Member Since 15 Jun 2012
Offline Last Active Jun 16 2012 03:42 AM
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Topics I've Started

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.
Details:
Supply: 240VAC, 50 Hz, Single phase
Motor : Conventional Run & Start windings
LRA 90~100 amps
Operating current: 18.9 amps
Speed: 2900 rpm

Circuit:
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)

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

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

Action:
Replaced C1

Result:
Identical behaviour

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

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

Questions:
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