PSW v. MSW and refridgerators

[eagle19952]

I have read a variety of postings that have pretty much said that refridges would not survive (if I read correctly) if powered by a MSW inverter....That PSW's were the only ones that would work.
I have owned 2 inverters, one a Trace 2512 and the other a Xantrex Freedom Marine 3000 with 2 echo chargers, both MSW inverters.

and I still use the same refidgerator that has been in the coach since 1998......What am I missing.....I have lost zero electrical anything.

PS. All electric coach, I have no propane..except my outdoor grill.

[stevet903]

I have 2 Heart EMS 2800 MSW inverters in my 89, and the original Amana refrigerator is still going strong (knock on wood)....  Your mileage may vary...

[eagle19952]

I had to look, mine is a GE...

[stevet903]

Mine is very old fashioned - none of those fancy digital controls or anything - turn the dial inside the fridge to set the temperature...  Probably why it's worked for so long.

[Geoff]

Link:

http://www.mrsharkey.com/busbarn/electrics/chapt6.htm

[eagle19952]

Link:

http://www.mrsharkey.com/busbarn/electrics/chapt6.htm

MSW inverters aren't a good choice for running induction (capacitor-start) motors, and some cordless tool battery charger systems malfunction and burn up when fed MSW.

BUT they seem to work...and i often wonder why.
and I am aware that they are refered to as True SW but i think of them as Pure SW...

[TomC]

The only true sine wave on an alternating voltage is created by a mechanical generator.  Modified sine wave inverters have relatively big squared steps that duplicate the sine wave shape.  The only difference in a modified and a pure sine wave inverter is the size of the squared steps going up and down duplicated the sine wave. On a pure sine wave, these steps are much smaller closer duplicating the true sine wave of a generator.

I have a 2512 Trace MSW inverter that is almost 20 years old.  There are two things that it powers on my bus that the performance is different-the microwave-which hums when on inverter and the cooking times are longer, and the stove hood fan motor runs slower (its' speed varies also according to the load on the inverter-more load on the inverter, the faster it runs-sort of interesting) [I don't run my A/C's on the inverter]. My electronics and LCD TV run just fine on the MSW inverter-including charging the lap top.

Now with more and more electronic gizmos available, on my truck conversion, I am going to use either an Outback, or Magnum 2800 watt pure sine wave inverter/charger.  Having a built in 130amp smart battery charger AND the pure sine wave inverter is worth the money (near $2,000.00).  Good Luck, TomC

[junkman42]

Just a quick comment!  All MSW inverters are not created equal.  The better quality inverters use a greater number of steps so the voltage is more paltable to certain devices.  So called pure sine wave inverters are MSW with a wave shaping network on the output!  Perhaps a Pi filter or you choose the word.  The reason You might see a Microwave run quieter with a greater load because the heavy load causes the inverter output to be a little smoother because the load on the internal transformer of the inverter starts smoothing out the leading edge of the square wave, anyone remember eli the iceman?  If Your small wall wart charger is real light it probably not like MSW if it is heavy it has a transformer and will probable work just fine.  I use a freedom 3000 and it runs several led LCD TVs, computer  and anything I would plug in at home!  It is very possible to produce without a rotating alternator a very precise sine wave IE radio transmitters, signal generators etc!  Okay turn up up the heat.  John L

[Sean]

I have read a variety of postings that have pretty much said that refridges would not survive (if I read correctly) if powered by a MSW inverter....That PSW's were the only ones that would work.
...

I don't think anyone has said that.  This is a highly complex subject, and thus it is impossible to describe the entire domain in a single sentence, however I generally advise against using MSW inverters to power induction motors such as those found in household refrigerators, roof air conditioners, and other common appliances.

To put this in perspective, note that your fridge is likely designed to run on 110-125 VAC.  If you connect it to a 95 VAC source, will it work? Sure.  Is that good for it?  No way.  I think most people grok that notion right away -- if you lower the input voltage enough, you will actually start to notice the fridge struggle: the motor will get hotter than normal, it will cycle on longer, and you will have trouble keeping your food cold.  Run it long enough on low enough voltage, and eventually you will burn the motor out.

Now it is likely that if you ran that fridge on, say, 107 VAC, you would hardly notice a problem.  And the fridge might run for decades that way.  But you would still be heating the motor up just a bit more than normal, and the motor will run for longer each cycle.  You would ultimately pay for this loss of efficiency on your electric bill.

OK, while running a motor on MSW is not exactly the same as running it on low input voltage, the effects are similar.  Just as in the low-voltage example, there are differing levels of inefficiency -- a low-quality inverter that produces only a few steps per cycle will create more trouble than a higher-quality MSW inverter that produces more steps.

The issue is that induction motors will actually resist or fight the sudden voltage change from one step to the next.  This resistance to change actually consumes power, which is immediately transformed into heat in the motor windings.  Note that this happens with any MSW inverter; with higher-quality models, less heat is produced.

Will your refrigerator run under these conditions?  Sure.  Is it good for the fridge? No way.  But, again, depending on the quality of the inverter output AND the quality of the motor construction in the fridge, it is possible the unit will run for decades.  Quite possibly the fridge will die a natural death from slow loss of refrigerant, control system failure, compressor bearing failure, or any number of other factors long before the extra heating of the motor windings will do it in.

All of that said, one thing is unequivocally true:  Your refrigerator will use more energy from your battery bank on any MSW inverter than it will on a "true sine" inverter.  This is an incontrovertible fact and can easily be measured with the right tools.

If you only use your inverter to power the fridge while driving from pole to pole, wherein the power is coming from the engine alternator, perhaps this is not an issue.  But if you ever use it to run the fridge on battery power while parked, then you will experience lower time-between-recharge on your system.  Moreover, using power stored in batteries is enormously expensive, anywhere from $0.25 to $0.50 per kWh stored and retrieved  (see my write-up on this issue here:http://ourodyssey.blogspot.com/2011/07/about-those-batteries.html), so over the long term, the cost difference between an MSW inverter and a true sine model of the same rating can easily be overshadowed by the extra battery expenditure.

This is the reason that I recommend to people who are designing their system or shopping for components to plan for a true sine model if they wish to operate air conditioners or household refrigerators from an inverter.  However, when I see existing installations with quality MSW inverters already in place, I generally do not recommend changing them out unless the user is experiencing a problem -- the cost of the extra energy or even the shortened life of the household appliance will not likely be greater than the cost to replace a perfectly good MSW inverter with a true sine model.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[eagle19952]

But if you ever use it to run the fridge on battery power while parked, then you will experience lower time-between-recharge on your system.

I think I get this...but what % of improvement could one expect in battery available duration....

[jjrbus]

I have read a variety of postings that have pretty much said that refridges would not survive (if I read correctly) if powered by a MSW inverter....That PSW's were the only ones that would work.
I have owned 2 inverters, one a Trace 2512 and the other a Xantrex Freedom Marine 3000 with 2 echo chargers, both MSW inverters.

and I still use the same refidgerator that has been in the coach since 1998......What am I missing.....I have lost zero electrical anything.

PS. All electric coach, I have no propane..except my outdoor grill.

Thanks for speaking up.

  Some of these gentleman are great sources of information and I appreciate their input and have learned much from them over the years.

But I and many others used MSW for many years with no problems.  OK my fan hummed and the clock on the microwave was not accurate. the hum was not irritating, but I could tell when it was on inverter.  I'm retired so I don't care what time it is.

  I did notice that some had more money in their electrical systems than I had in my conversion!

   Its a motor coach not a NASA project!

   You do realize your refrigerator, will self destruct in the next 10 days           JIm

[Lin]

I would prefer a pure sine inverter, but that's not what we have now.  We use an RV frig, so that is not an issue.  The microwave does not really sound happy on the inverter, but it does okay for a small warming job.  Our Trace 2500 watt MSW inverter will not run an AC unit (which I do not need it to do), nor will it run my portable compressor (which would be actually be nice).  The frig compressors may be more forgiving.

I think that Sean's advice is right on.  It always seems to be, but that does not mean I always follow what he says.  He generally gives the facts and quality recommendations.  If I were full timing, my attitude could be different, but as a vacation user, I am open to less than first class compromises.  However, I sure like to know the details and implications of the compromises I am making.

[jjrbus]

That rings a bell. Have you tried running your AC on your 2500?  There have been some posts on some AC's not starting on 2000W generators and it is not the 2000W, it is an issue with the AC's. Probably applies to the compressors as well?    JIm

  Something to do with a hard start capacitor