NorthernGuy
New Member
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?
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?