refrigerator damage from ST head???

[Jim Mc]

This is what I love about these forums - the ability to go on and on about what each of assume to be true...

11hz do you mean periodic oscillation of frequency at that 11Hz?  With massive flywheels the oscillation should not be that big like 2 to 5hz spread around 60Hz even at that 11Hz. 

Just guessing you don't run anything electrical from your Listeroid?  If you did, you'd probably not think of the flywheels as massive.  They are in fact barely marginal for AC generator service...

Shouldn't be a problem, even that oscillation at 11Hz is not high frequency enough for a induction motor. 

You're missing my point.  (not your fault - I did a poor job explaining it...)  Here's the thing - As the Listeroid output cause the 60 Hz output to vary up and down by a few Hz (at an 11 Hz rate) induction motors will be caused to deliver a varying torque output.  In other words, they are constantly trying to speed up and then slow down their load at this 11 Hz rate.  So, right after TDC on the power stoke, the induction motor sees increased voltage and frequency and tries to speed up.  Then it coasts a bit, or is actually less than fully loaded during the remainder of the 4-stroke cycle.   Then it gets hit with another interval where it has to accelerate the load. 

So, the load on the induction motor (the compressor in this case) is continually being sped up and slowed down, at 11 Hz. 

Wouldn't you agree?  (If so, I can attempt to explain why this could be the cause of increased heating in the motor)

Having designed switchers myself, I understand high frequency currents, and their effect on capacitors.  But I don't see how that's the issue here, with an inductive load which, as I'm sure you know, has higher impedance at higher frequencies - the opposite of a capacitor.

[Wizard]

I'm not talking about high freq fuzz like I see with SMPS on AC input and radiating out.  I'm talking about "NOISY" distorted, lumpy sinewave from ST with bit of spikes on it.

What kind of flywheel did you use in that case?  To this better, was that SOM flywheels (these weighs in excess of 200lb each IIRC, or wasn't that 250lb each?) on lister with 100lb or so flywheel on the ST?  Like the original SOM system was set up?

And I do understand now what you mean by the freq variations around 11Hz cycle.

Cheers, Wizard

[Jim Mc]

I'm not talking about high freq fuzz like I see with SMPS on AC input and radiating out.  I'm talking about "NOISY" distorted, lumpy sinewave from ST with bit of spikes on it.

I know.  But still doubt that's the cause of the problem.  A noisy waveform as you describe is caused by high frequency (>60 Hz) content.  And, as I mentioned, you may see it in the voltage waveform, but I have my doubts about whether it shows up much in the current waveform of an inductive motor.

What kind of flywheel did you use in that case? 

  Good Q - I'm ASSUMING the OP has a Listeroid, which AFAIK tend to have too-light flywheels for use as AC generator prime movers.

And I do understand now what you mean by the freq variations around 11Hz cycle.

Good.  Allow me to take the next step and show why I think this is the root cause of the toasted compressors...

Let me make a few simplifying assumptions...

Imagine the AC load (the refrigerator motor in this case) draws 4A rms from an ideal 60 Hz source.

Now, imagine that the internals of the motor present resistive losses of, say, 5 ohms.  In that case, it's easy to see the resistive losses would account for 80 Watts (twinkle twinkle little star, power equals I squared R, right?)

Now, let's assume the Listeroid freq and voltage vary so wildly that the motor only does work for 1/2 of the time with the other half of the time spent decelerating, or slowing down, as the engine coasts down in speed after the power stroke.  If it's required to do the same total average work (as I suppose it would in this case), you'll probably agree it needs to work twice as hard in the first part of the cycle, and then loafs along for the 2nd half, right?

Now, let's go on to assume that when the motor is doing twice the work, it draws twice the current.  (It's a stretch, but not too far from reality).  So our motor draws twice the current for half the time.  And let's go on a bit more, and say that during the 2nd half of the cycle, where the motor is coasting, it draws zero current.The average current would still be our original 4A rms, right?  It would draw 8A for a bit, then 0A for the next bit, and the average works out to 4A.  So no big deal, eh?

But wait.  What about our resistive losses?  We now have I^2R losses of 320W for 1/2 of the time and 0W for the other half.  And the average losses are now 160W.  Yep - same average value, but the losses have doubled.

Is the reality exactly like this, where the motor losses double?  No.  But I'd argue they do increase.  And increased losses mean more heat.  And that leads to toasted compressors...