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Possible Failure Of Soft Start Of Domestic Air Conditioner Motor


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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

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Measure the voltage acoss the motor terminals. If you have full voltage and the compressor does not tuen, then you have a mechanical lock up.
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  • 1 month later...
Soft starters on capacitor start single phase motors is a tricky business. Are you sure this was properly thought out? The reason is, when the start capacitor is in the circuit, the capacitor charging current looks like a shrt circuit to the SCRs in the soft starter, and can cause them to mis-fire. At the same time, the high harmonics created by the soft starter heats up the capacitors, causing them to swell and fail. It's usually a race to see which one happens first. Does this Chinese mfr regulary design a soft starter into their unit? If not, did you ask for it? If so, they may not have understood the risks.
"He's not dead, he's just pinin' for the fjords!"
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Hi Jeff


Never had any problems starting cap start motors or cap start cap run motors. The start capacitor is in series with the start winding so the capacitive current is limited by the start winding.

Non capacitive start motors are usually much less effective with a soft starter than the cap start ones.

If you can reconfigure the motor conntections so that you soft start the run winding only, that gives the best results.

We have started a large number of capacitor start motors without issues, I am interested in you experiences.


Best regards,


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50 mic for the start cap does not sound high enough.


I think, do as Marke says...only maybe check current at startup instead...should be easy with clipon - if it does not drop before ol operates then starting torque is crap or motor is overloaded from seizedcompressor . Probably a reason why start cap is dead..try 130ish and see what happens. ps your two runs in par should be rated at 400ish Volts and the start should be about 230 ish..If not then your start cap is probably incorrect.

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