Current draw from a Fridge compressor circuit

[NorthernGuy]

Please forgive any newbie errors in this post, my electronics days are long ago! I just wanted to see if my assumptions are correct regarding a fridge current draw.

The fridge operates from 240v AC, 50Hz. The brushless compressor motor 'run' winding has a cold resistance of 18ohms and is wired in series with a 4uF run capacitor. At cold startup, a second 'start' compressor winding (23 ohms cold) is briefly energized in parallel with the run winding, via a PTC (thermistor) which has 13 ohms cold resistance. The PTC is designed to quickly heat up with current draw, until when hot it is a few K ohms, so effectively ceases powering the start winding. I've no idea what the reactance is of the two windings. The compressor is nominally about 125w from tech info.

Looking at the 4uF Capacitor, the reactance is 1/(2 Pi f C) = 796 ohms. So if the compressor motor side was, at a worst case scenario, completely shorted out, the current draw would be limited by the capacitor at from this formula 0.30amps or 72 watts. Any lesser compressor failure would I assume give more impedance so less current/power draw.

Are these assumptions correct? I'm trying to get my head around the (rare) fire risks associated with fridges/freezers. Are my sums correct, that a run capacitor in series with the motor, which not all machines have, greatly limits the power that can be drawn, or am I missing something?

 

[Diver300]

Plug fuses are not designed to protect against a motor stalling. It would be difficult to find a fuse that would always withstand a motor starting and would protect against a stall, as the only difference is time.

 

[Nigel Goodwin]

You raise some valid points. And we are mixing a lot of different technologies here too. Fridges from an era before electronic controls have a pretty ancient, motor with PTC/Relay startup system. CRT TVs were, correct me if wrong, mostly transformer driven, well the ones I knew were. But transformers have all but vanished it seems now, in favour of switch mode supplies, well known for heavy start-up transients.

My Ctek car battery charger probably can only deliver 75w output at full pelt, but is fused at 13A. Presumably a 3A fuse would lead to in-warranty fails, so financially ruinous to the manufacturer. I had the 12v 2A adaptor to an external hard drive fail once at switch-on. This didn't touch the 13A fuse in the socket adaptor, but it instantly took out the 32A ring main MCB (not a residual current device). The response of metal wire fuse clearly being very slow compared to even a much a higher rated MCB.

I suppose a fuse blows by melting metal, and as alarming as a transient may be on your peak hold clamp meter, you have to factor in its duration, less visible, to work out if the heating affect will melt metal. My guess is that it rarely does; melting metal isn't fast.

I do think the demise of the 5A fuse as a manufacturer fitted item was a bad turn for the consumer. A corded power drill (remember those?) drawing several hundred watts must now be fused at 3kW, or in reality far more as the BS1362 standard says a 1.9x overload (say 6kW) should blow it 'within 30 mins'. If something hasn't caught fire within than power/time window, it likely never will. These plug fuses seem little more than short-circuit protection in safety terms.

The reason for my original post was with safety in mind, fixing up an old fridge and pausing to ponder about that device in almost every house, buzzing away 24/7, using very little power but with that little risk we all overlook. All safety seems to hang on the bimetal overload, the only thing that should disconnect upon a high stall current if the plug fuse won't. And it must repeatedly disconnect every time it cools to return on again, perhaps unnoticed for days in a garage etc. Yes it is my risk to fit a 5A, but with nothing more than beer in this one, I think I'll see if I am as lucky as Diver300 has been.

You seem confused about the role of the fuse in the plug?, essentially it's there to blow if the mains lead is shorted out, most equipment is fused internally as soon as the mains lead enters the equipment. However, as far as I'm aware, fridges don't have internal fuses.

Bear in mind, the UK is one of the few (if not the only?) country to have fused plugs.