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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
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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Yours sounds like it might have been a capacitor start type where the capacitor and centrifugal switch arrangement has been replaced by the electronic starter you describe.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
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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
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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
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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
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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
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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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Old 10th May 2017, 2:54 pm   #9
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Default Re: Fridge compressor capacitors

trsomian , I think I got this fridge from you

I was trying to find the message in my inbox, but must have cleared them out.

Ive got the fridge running well with my new thermostat setup. One thing I wonder about now is overload protection on the run windings, I.E whether it needs any. I've no experience with fridges before this and it seems a lot of information online relates to newer fridges.

When the thermostat activates the relay, the run winding is connected directly to live, the common terminal to neutral, and the start winding has one connection to the starter box, with the starters other 2 terminals connected to live and neutral. It is conservatively fused, but I wonder if that alone is enough

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Old 10th May 2017, 9:15 pm   #10
trsomian
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Default Re: Fridge compressor capacitors

I thought it might be my late mother's fridge that was the subject of this discussion. Personally, I would want more protection than just a fuse, because fuses, are frankly, crude devices, and getting the correct rating and type of fuse would be a surprisingly complex task. The original start relay fitted has an overload device, which assuming it is still working I would leave in circuit
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Old 11th May 2017, 9:05 am   #11
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Default Re: Fridge compressor capacitors

Thanks a lot for your reply, that makes a lot of sense to me now. I did remove it as I believed it was purely an unused starter , and initially the fridge wouldn't start - that may have purely been a wiring issue though looking back. I should still have it so I could have a go putting it back.

There are generic overload devices available fairly cheaply, if I couldn't use the original I might buy a few (starting with lowest power rating) and see which lowest HP one allows the fridge to start and run properly. Looking at how they're wiried, it seems they are connected between the common terminal and neutral, so I can see how its difficult replacing that with anything else as they need to be tolerant of current draw at startup, and protect against overload under running conditions.

With regards fuses, I was thinking of measuring current to the start windings and run windings then individually fusing them with glass fuses on inline fuse holders, better than nothing but not great.

Im sure I can either get the original back or find a generic device that would do the job. The compressor on this is still very good, and once running with the thermostat the compressor doesn't run very much at all and maintains a nice temperature inside
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Old 17th May 2017, 8:00 pm   #12
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Default Re: Fridge compressor capacitors

The original overload is cutting out after running a while, and I've measured the compressor is only pulling 0.9 amps when running so I suspect it may be past it.

Ive read the specs off the compressor ( R 1/2 25 Volts 230 cycles 50 Amps 9.4). Either im misreading the corroded compressor plate, or 9.4amps is some form of starting peak.

When I did measure the starting peak, the DMM saw 4.5 amps, a quick flash to 14, then running consistently at 0.98 amps. Im not sure of the 14amp reading, as the meter is only specced to read 10, and with a 5 amp fuse on the whole appliance which has not blown to date, I cant see 14 being accurate. In addition the 5amp fuse is also protecting the arduino PSU, and I think the arduino itself with relay control and temp sensor must pull a few hundred ma.

Im looking into some combined start/overload devices like the Supco RCO 820, which will also tidy up the wiring. Their products are often ranged from say 1/12 HP to 1/6, So in terms of overload Im not 100% I'd be happy with a generic product. What I might do is also add a hall effect current sensor to the arduino and add my own overload control in the software as its fun and I like over-engineering
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Old 7th Jun 2017, 7:17 pm   #13
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Default Re: Fridge compressor capacitors

Well after much waiting I've received the following product:

http://www.supco.com/web/supco_live/...20.htmlhttp://

Had to be ordered from the USA, but I'm hoping to fit in the next few days and will report back the results. Despite the web listing the packaging clearly shows its for 1/12 - 1/8 compressors (not 1/12 -1/5 HP). It has an overload function, but providing it works I will also add a current sensor to the arduino unit, that way when I poll the temperature I can also poll the current reading while running and find a suitable current threshold to set where the unit cuts the relay power, triggers a small piezo buzzer, and halts the main loop code.

If its successful I'll look to see what I can do about the door seal. Its not too bad but doesnt seal too great.
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Old 15th Jun 2017, 9:01 am   #14
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Default Re: Fridge compressor capacitors

Well, The fridge works great with the new Supco unit installed, seems to start a little smoother too.

When adjusting the compressor mounting bolts (one had worked loose), I noticed on top of the compressor is a round disc, feels almost like hardboard material.

Its fixed to the compressor by a fixing in the middle, and can be rotated as its a bit loose. Does anyone know what this is?? Modern compressors don't seem to have them.

When I got the fridge there was a bit of old foam wedged here between this disc and the top of the housing, I presume to quieten it down a bit, as its evident a lot of the noise comes from this disc rattling.

I considered gluing it down or even removing it, but need to know what it might be before I consider that.

OK I found a picture of another compressor that has a similar disc on top, any ideas? Picture attached
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Old 15th Jun 2017, 2:03 pm   #15
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Default Re: Fridge compressor capacitors

I think the idea was for condensation to drip onto it, and be evaporated by the hot compressor.
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Old 15th Jun 2017, 2:20 pm   #16
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Default Re: Fridge compressor capacitors

Interesting Bill. Now I look at that picture, there do seem to be water marks on the disc, and I cant think why else such a material would be used unless it was intended to be partially absorbent.

I imagine its better than water drops sitting longer on the compressor casings and potentially rusting it. Or, in fact dripping down into the wiring terminals.

If I cant find any other advice, I might use a few drops of heat resistant epoxy to hold it in place and stop it flapping.
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Old 15th Jun 2017, 2:38 pm   #17
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Default Re: Fridge compressor capacitors

Most modern fridges have a drip tray mounted so that they get a bit of bottom heat from the compressor. It is to get rid of the drips from the slime hole at the back.
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