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Old 20th Apr 2017, 10:53 am   #1
PsychMan
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Default Fridge compressor capacitors

I have a working late 50s Frigidaire fridge. I'm going about rewiring it as the insulation is dropping to bits. It has been fitted with an electronic starter circuit to engage the start windings briefly, and up until now has been run on a timer plug as the thermostat has failed. I'm adding an arduino board with a temperature sensor, and a hefty solid state relay to engage the compressor depending on temperature. I've tested this as a lash up and it works very well, so I'm going about installing it all properly.

I notice the fridge doesn't seem to have a run capacitor, but the compressor does work without this. When reading about compressor wiring online I see a lot of references to run capacitors, I'm assuming these aren't needed for all compressors? I just want to make sure I'm not missing something, as I cannot locate a run capacitor on this model.
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Old 20th Apr 2017, 12:00 pm   #2
Herald1360
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Default Re: Fridge compressor capacitors

There's various types of single phase induction motors..... they all need some sort of rotating field, either temporary or permanent, or a manual turn to get them started. Once they're going the simple oscillating field from the single phase will keep them going as long as the load doesn't stall them.


Capacitor Start where a capacitor feeds a second winding briefly before being disconnected by a centrifugal or time delay switch.


Capacitor Run where a capacitor feeds a second winding continuously.


Shaded Pole where a short circuit round part of the magnetic circuit generates a phase shift in part of the field.


Yours sounds like it might have been a capacitor start type where the capacitor and centrifugal switch arrangement has been replaced by the electronic starter you describe. Is there a largish (few uF ) ac rated capacitor in there?
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Old 20th Apr 2017, 12:05 pm   #3
Boater Sam
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Default Re: Fridge compressor capacitors

Many relay or centrifugal switch start induction motors do not use a capacitor. With the start relay the start winding is switched in by the stall current in the run winding, as the speed comes up, that current drops and the relay opens, disconnecting the start winding. Beware, often the relay drops out under gravity, don't use it upside down!
Centrifugal switch works on the speed of the motor operating the switch, closed when speed is low.
If there is a capacitor on such a motor it is often in the Start winding to increase the start up torque.
Capacitor run motors generally do not use a switched starting system. The capacitor is in the start winding constantly
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Old 20th Apr 2017, 12:26 pm   #4
PsychMan
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Default Re: Fridge compressor capacitors

Quote:
Originally Posted by Herald1360 View Post
Yours sounds like it might have been a capacitor start type where the capacitor and centrifugal switch arrangement has been replaced by the electronic starter you describe. Is there a largish (few uF ) ac rated capacitor in there?
I think you're right, There is a largish capacitor in there, I cant recall the specs though and it would be a pain to get to it now and check. Ive done my best to replicate the wiring as I found out, with the main addition being the solid state relay that now switches the whole lot on or off.

There are 3 terminals on the starter, one goes to the start winding, the other 2 are Live & Neutral. The other 2 terminals on the compressor are connected to Live and Neutral. The compressor is a little loud generally, due to age Im assuming, but not that bad once its running. The start windings engage for around a second or so which I can hear by the difference in noise levels.

I did wonder if I would be better off with a new starter, but this one works and I've no idea what spec the compressor is as I cant see any identifying markings on it
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Old 20th Apr 2017, 12:53 pm   #5
merlinmaxwell
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Default Re: Fridge compressor capacitors

The way I look at non capacitor start motors is they are inductive start, the main winding has a lot of inductance (shifting the phase, just like a capacitor but the other way round) and the start winding has much less (inductance) causing the motor to spin.
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Old 20th Apr 2017, 4:58 pm   #6
hannahs radios
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Default Re: Fridge compressor capacitors

Make sure you add an MOV across your SSR otherwise the back EMF from the motor might kill it!
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Old 21st Apr 2017, 11:34 am   #7
PsychMan
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Default Re: Fridge compressor capacitors

I think this will be ok. Its a hefty one, 25 amp rated. It has an integrated snubber network (resistor and capacitor) and the datasheet stated a high current capacity for inductive loads and good tolerance of high voltage surges.
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Old 21st Apr 2017, 7:10 pm   #8
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Default Re: Fridge compressor capacitors

All fridge and freezer compressors I have ever seen are what is called "split phase start", and have no capacitor for run or start, frequently operated by a current sensing relay, or these days sometimes via a thermal starter
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