Compressor trips thermal switch and wont start

by Ryan
(Louisiana)

I have a cheap 15A direct drive HF compressor that won't start and trips the thermal switch but not the breaker. It will start from empty every time but wont start when the cut-in kicks on.


The compressor is on a dedicated 20A circuit I ran for this and I have checked the check valve which seems to be good. The unloader valve releases air when the compressor stops. Oil been recently changed. The confusing part is after getting it to cycle normally once, I can get it to start normally for up to about 30-45 minutes afterwards but if it sits for an hour or so, or more, it will go back to tripping the thermal switch.

It's in my garage on the opposite side of my breaker panel in the house (so I can't move it closer unless I want a compressor in my bedroom). I did test it on another circuit by bringing it into my kitchen which is about 15ft closer to the panel than the garage, and it kicked on fine. I'm in Louisiana so while it's chilly here, I wouldn't think it's cold enough to keep it from starting, especially since it starts up in the kitchen.



Comments for Compressor trips thermal switch and wont start

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Oct 20, 2016
@ 2nd Capacitor by: Bilbal44
by: Doug in s.d.ca

That would be the run capacitor.

With a bad run cap, your motor draws too much current and protect itself by stopping.

A bad connection to the cap can also cause this, as can a bad winding in the motor.

But, usually, it's the cap.


Sep 20, 2016
2nd Capacitor
by: Bilbal44

I have the HF 90221 pancake model that starts well but refuses to run for more than 20 seconds.

I assume the Start Capacitor is the 370 vac one, what is the function of the 120 vac one?
It still looks good when checked with an ohm meter.

Thanks for any information,
Bilbal44

Jan 02, 2016
won't start
by: Doug in s.d.ca

18V is way too much drop.

What size wire did you use and how long is the run?

Jan 02, 2016
Compressor trips thermal switch and wont start
by: Ryan

I've checked the cap with a digital meter. It started off with low ohms then gradually increased to OL so I don't think it's a bad cap but maybe it's failing like you said. I also disconnected the line from the check valve to the compressor head while air was in the tank. The compressor started fine and actually kicked on quicker than normal (no startup sluggishness).\

I can't measure startup current since I don't have an amp meter but I did measure voltage drop when the compressor kicks on. Voltage at the outlet is 118V but drops to 100V whenever the compressor struggles to start. I'm thinking maybe it's trying to pull too much current but I don't know why it would do that.

Jan 02, 2016
HF compressor trips thermal switch
by: Bill

I think your compressor has a weak or failing start capacitor on the motor, Ryan.

See the page on checking those, and when you do, if it isn't that, add a comment here please.

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