refrigerator damage from ST head???

[cschuerm]

Anyone have any feedback regarding the possibility that power from an ST head could be damaging refrigerator compressors?
I've had three refrigerators die now and each time, it was after a couple of days running on my Lister genny with an ST head.  Voltages and freqs were quite good as measured by my DMM (around 125VAC and 59-61hz depending on load).  Seems like too much to be just a coincidence....
thanks
Chris

[trigzy]

Hi,
      Was the voltage measured at an outlet?   Or measured at the outlet legs on the generator?   Maybe there is a lot of loss between the generator and actual loads causing low voltage?   Also, some fridges that I have seen use a solid state device to supply power to the compressor, models with a relay are much more reliable.

Steve

[mike90045]

125VAC is on the high end, but should be less internal heating.   Should not have mattered.

[Doug]

Shouldn't be an issue are these new fridges?


Does a regular electirc motor start, sound and opperate normal?

[cschuerm]

Voltage was measured at the house breaker box so should closely represent outlet voltage.  Other loads seemed generally fine although two of my older UPS' wouldn't sync to the ST's power.  Fan blower in my furnace started nicely with no light dimming or other signs of problems.  When the fridges cycled, they did sound a little "different", but I never saw voltage drop below 110VAC at any time.  Just wondering if some aspect of the ST's output (which we all know isn't the greatest) is particularly hard on compressors.

chris

[PaulMoffenger]

cschuerm,
Three refrigerators? Plug in another one and monitor it carefully!
Seriously, did you lose all of these while running on the generator? Did they only run on the generator once? For a long time? Were they all new, or old?
More specifics could help identify your problem more quickly, and accurately.

Paul

[SHIPCHIEF]

Let's be fair:
Were these cheapo used refrigerators that you got for free or near free? I know that's what I always buy. Who knows what damage they might have already had, or how may operating hours are on them by the we get them?
If it was a new fridge, it would be under warrenty. 

[M61hops]

There is nothing about ST generated electricity that should harm refrigerators, except maybe the frequency being way off.  Do you know exactly what failed in the fridges?  (start winding, run winding, overload switch or start relay?)  Low voltage is hard on compressors but they usually are pretty hard to kill dead.  Most of the time that compressors won't run in refrigerators it's because of an electrical problem that is not too hard to fix.  It's rare that a compressor fails permanently but it does happen, and for no apparent reason .

[mobile_bob]

some of the st heads have horrible harmonic distortion, at least by modern standards
iirc the line is supposed to be better than 5% THD or no worse

seems like i have heard of some st heads being somewhere around 15+% THD

also some of the modern refrigerators are built to a price point, meaning the motors are no bigger
than they have to be to get by,, higher harmonic distortion results in more heating of the motors internals.

also if these are new units with electronic stuff, the distortion might be kicking their ass too, or

start caps can have issues with high distortion as well? maybe you lose a start cap and toast the start winding, or
lose a run cap and have other issues?

bottom line, maybe check the heads THD to see if it is really excessive for some reason?

excess heat can make the overtemp safty trip out to save the compressor motor, but this is not meant
to be a continous operation safety feature, so maybe it trips a few dozen times over a few days and when the
compressor cuts off you think it is normal shutoff and don't notice it is short cycling?  if it cuts off and then cuts back
on too soon it can cause a start against high head pressure which will overload the motors start winding.

or maybe i am all wet?  wouldn't be the first time!

reaching at straws here, seems like if you got anywhere from about 110-128volts and 58-62hz or so, steady and reliably
especially at startup, maybe you gotta look at THD as maybe causing issues?  perhaps checking the voltage droop
on startup to make sure you don't have a huge spike that the gen can't service and the voltage drops too far making
the unit unable to come off the start windings as soon as it should?

bob g

[jzeeff]

Any waveform that isn't a perfectly shaped sine-wave has harmonics in it that cause a little bit of heating.    But some people run motors on highly distorted modified sine-waves (aka square-wave with some time at 0V).  On the other hand, I run almost everything from a pure sine-wave inverter and even with this  I'm not convinced that some of my load devices won't ruin the waveform for other devices.

I've also used a ferroresonant transformer to clean up power for a refrigerator.

It is pretty easy to use a PC and a current probe to look at voltage and current waveforms.

(typed before bob's response)

[WGB]

Hey jzjeeff and Bob, can you go into the PC and current probe to see sine wave form some more?
Most PC stuff that I looked at was as expensive as a small electronic scope.
Thanks

[mobile_bob]

they make some relatively inexpensive scopes for the pc one of which is interestingly called "pcscope"

but for around 50bucks plus shipping one can get a full function dual channel oscope off of ebay that is used
of course

i picked up a nice 10mhz scope for 65bucks from a local guy off of craiglist that does a nice job of what i need to see.

also for the pc scope thing, they make them to plug into your soundcard, so use a desktop and not your laptop
cause if you get a highvoltage spike it can take out the sound card which is replacable on a desktop and on the mother
board of a laptop, don't know how big a threat that is,, but i would be wary of killing a high end laptop to just do some
odd testing.

there are also usb scopes that are fully isolated and can work with a pc and are safe for laptops i understand
but they can be well over a hundred for a single channel to several hundred for a nicely featured 4 channel unit.

i am no expert on scopes, so maybe you should wait to see what others might add to the question

bob g

[jzeeff]

That's accurate.   To use a PC sound card, you need the voltage to be in the +/-1V range and complete isolation.  You can also spend another $4 and use a cheap USB sound card to not risk your PC's internal one (I don't bother). http://www.dealextreme.com/details.dx/sku.5831

For current, I use a Fieldpiece ACH 1 mv/amp clamp-on probe.  This is completely isolated from what you are measuring.

For voltage, a 120V->6V transformer and then a couple of resistors (say 900 ohms and 100 ohms) to act as a voltage divider to reduce to .6V.  This is also isolated.

Then you just need $10 for Zelscope.  www.zelscope.com.

[cschuerm]

More data eh?
Okay, all 3 units were purchased new.  Ages ranged from 5 to 15 years.  They were all reasonably good quality brand name units.  The failures occurred at entirely different times but always after running for several days on Roid power.  I am normally careful to only switch my transfer switch when the generator is up and running but on one occasion, I did run out of fuel which caused a "soft" power down.  I'm primarily thinking about the THD issue.  During our last major power outage (when I lost the last fridge), I did have a data-logging Fluke DMM connected and it showed that I didn't have any significant voltage or frequency excursions (beyond what one would normally expect from such a generator anyway).  I did not perform a post-mortum on the refrigerators or attempt to have them repaired.  (wife thought it was a great excuse to get new stuff).  One unit started getting very noisy right before it quit working and the other two failed while I wasn't around.   Guess that's about all the factual data I have.  Either the poor power quality did contribute to their demise or it was just coincidence that they finally failed when they did.

Chris

[compig]

Probably something to do with the waveform coming off the ST and the fact that fridges have frequent start/stop cycles. Any rubbish in the waveform will cause heat and resonance in the motor windings which would lead to premature failure.