MSW inverter Hz reading

Socratic MonologueSocratic Monologue Solar Expert Posts: 30 ✭✭
I have a small off-grid system that uses a 1500w MSW inverter. It has run lights, small power tools and a garage door opener perfectly. I recently got an electric lawn mower to run off the system -- the mower runs great from the inverter power; I can't tell the difference when running it on utility power.

Here's my question: I hooked a KillAWatt meter to see how many watts the mower draws (450-700, depending on load) and whether I need a heavier gauge extension cord (definitely, I do). When I checked the Hz reading from the inverter, it read 20 Hz under no load, and fluctuated up to about 55 Hz under varying loads. A little internet reading suggests that this is caused by the inability of a KillAWatt meter to read modified square waves accurately, but (A) I would enjoy knowing more about why this is (i.e. what is it about the waveform and the way the meter reads it that yields low/fluctuating Hz measurements), and (B) I should make sure I'm not really damaging my new mower.

The meter reads 59.9 Hz under all loads when connected to utility power, so I think the meter works fine.

Comments

  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: MSW inverter Hz reading

    Your killer watt meter is only reasonably accurate reading a sine wave A MSW inverter has a modified square wave and cant be read accurately with a simple device thats designed to read a sine wave.. you will not get an accurate AC voltage reading of a MSW inverter for same reasons., the only way to measure the current the mower draws is to measure the DC side of the inverter.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: MSW inverter Hz reading

    Modified Square Wave inverters are wrongly called Modified Sine Wave by merchants for sales purposes. It's DEFINITELY a square wave, and a very "dirty" one, that's slightly modified. There's nothing at all that originates as, or resembles a sine wave. That said, their output waveform, as mentioned, is anything but clean and smooth. There are sharp peeks, spikes, more or less flat stretches, and short periods where there is no output at all between spikes and pulses. So in reality, it's a pure wonder that anything designed to operate on sine wave will operate at all in MSW. It's like one car with round wheels rolling nicely along the street, while another car has 6 or 8 flat sides and sharp lumps on it's wheels. Quite the difference in the ride would be an understatement. And that's what sensitive electrical items have to deal with when operated on MSW.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MSW inverter Hz reading

    I'll try to explain the why of the K-A-W's inaccurate frequency reading. (Engineers should look away now; it won't be pretty.)

    On sine wave the meter is counting pulses. Every peak of the wave is a pulse, and it is above a certain Voltage. The K-A-W counts these up over time (very small amount of time) and averages them out to so-many pulses per second: the frequency. As in 60 Hz is 60 pulses per second.

    MSW - Multiple Square Wave, Multiple Stepped Wave, definitely not sinusoidal. Instead of nice clean parabolic sweeps of up and down Voltage you have what looks like a staircase. Sometimes a pretty darn unmanageable one, as they are not all the same. Worst case: one step up and then down. That's pure square wave. Better units will have more, smaller steps up in Voltage and back down again. The amount of Voltage per step will differ from one inverter to another as will the duration at each step. This confuses the pulse counter of the K-A-W. It's looking for peaks over 'X' Volts and instead getting peaks that may not even meet 'X' Volts. As a result, two or three of the inverter's peaks may get added together as one, so you get a reading of 30 Hz or whatever instead of 60 Hz. Basically it just cant make sense out of the jagged, inconsistent rise and fall of the Voltage. Put an induction load on that and the wave form distorts more, as there is feedback from the induction unit to the inverter which can smooth the pattern and increase the Voltage and cause all sorts of other 'blips' depending on what is being powered. Motors fed of transformers, btw, are notorious for pushing residual magnetism backwards through the circuit with often not-so-hilarious results. Let's just say circuit protection is a really good idea.

    The short form; the K-A-W meter is too unsophisticated to make sense out of the electrical chaos that is the output of an MSW type inverter.
  • Socratic MonologueSocratic Monologue Solar Expert Posts: 30 ✭✭
    Re: MSW inverter Hz reading

    Thanks -- that's the sort of explanation I needed to make sense of the readings.

    So, how much damage does this sort of power do to an AC motor that will get run a couple dozen hours a year at most?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MSW inverter Hz reading
    Thanks -- that's the sort of explanation I needed to make sense of the readings.

    So, how much damage does this sort of power do to an AC motor that will get run a couple dozen hours a year at most?

    In that application probably not that much. Your lawnmower will start with no load on it, unlike a refrigerator or water pump, so there's not as much resistance to spinning up to speed. That reduces the initial current need, which is largest and is where the heat build-up is worst. While running it will vary according to how hard the grass is to cut; more load on the motor, more current drawn. The trouble is the MSW is going to "look like low Voltage" to the motor so it will probably draw around 20% more than it normally would no matter what.

    Definitely get the biggest gauge extension cord you can manage, as every bit of V-drop is bad. It'd be silly to do anything more complex like add in a Variac and remote meter so you can adjust the Voltage to the mower up and down as needed to keep it within operating range.

    Take a look at the Voltage output of your inverter too, especially at the end of the extension cord. With a cheap meter it will probably read 90 Volts. With a good RMS meter it will look like 120 as it should. Again; difference in the wave form affecting the instruments.

    For more fun get an oscilloscope on it and you can see the waveform. That can be scary on utility power sometimes too. :p
  • Socratic MonologueSocratic Monologue Solar Expert Posts: 30 ✭✭
    Re: MSW inverter Hz reading

    Thanks for all that. I need to run about 200' of cord, so I ordered up some 12awg (10awg failed the diminishing returns test of cost vs. voltage drop) - was running this morning on 16awg, which I won't do again. The voltage readings from the inverter actually made sense -- ~120v no load, down to about 112 under load, which is in line with the expected drop in the cord I was using.

    Once I explain to my wife why we needed $120 worth of extension cords, I'll ask about buying an ocilloscope.;)
  • bluewickedburnerbluewickedburner Solar Expert Posts: 78 ✭✭✭✭
    Re: MSW inverter Hz reading
    Modified Square Wave inverters are wrongly called Modified Sine Wave by merchants for sales purposes. It's DEFINITELY a square wave, and a very "dirty" one, that's slightly modified. There's nothing at all that originates as, or resembles a sine wave. That said, their output waveform, as mentioned, is anything but clean and smooth. There are sharp peeks, spikes, more or less flat stretches, and short periods where there is no output at all between spikes and pulses. So in reality, it's a pure wonder that anything designed to operate on sine wave will operate at all in MSW. It's like one car with round wheels rolling nicely along the street, while another car has 6 or 8 flat sides and sharp lumps on it's wheels. Quite the difference in the ride would be an understatement. And that's what sensitive electrical items have to deal with when operated on MSW.

    I beg to differ. While cheap or yesteryear MSW inverters aren't the best thing for sensitive electronics, most consumer products do not fall into the sensitive category. Computers and related systems (except laser printers), most LCD TVs and so on work just fine with MSW inverters of good quality and recent manufacture. Again, I'm talking about most commonly purchased electronics most people buy.

    If you look at the wave form for MSW inverters of high quality and recent manufacture, they look nothing like the description offered. Their "steps" are very small and very short in duration so for most practical purposes, it doesn't matter.

    Also, since we're getting into this, a tire is not perfectly round, it is in fact a series of very small flats, so small the tire seems round, offering a smooth ride. If you take an octagon shaped tire and spin it fast enough, the ride is smooth. The point being the comparison is like apple and oranges.

    You can get an O scope to check wave forms and sine wave inverter to run something. Even then something else will usually fail and be completely unrelated to the electrical system.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MSW inverter Hz reading

    Wayne's description is, for practical purposes, accurate.
    So is Bluewickedburner's.

    No, I'm not joking. All depends on the inverter and the equipment. Some of it, on either side of the plug, is junk. Other things not so much so.

    Take "wall warts". Please! You can bet they're a candidate for "don't run on MSW" because the internal circuitry is pretty cheap. Most likely no more than a transformer, one rectifier, and a small filter cap. Oh yeah; that will produce some clean DC! :roll: On the other hand your desktop PC turns AC into DC, oscillates it rapidly, runs it through a transformer, reconverts it to DC, monitors the DC output Voltage to regulate the oscillation so the V+ and V- can be kept withing a hair's breadth of specs at ll time. And we're talking redhead here! :p

    To carry that analogy to the groaning point ...

    If we want to split hairs (ARGH!) the "pure sine wave" inverters are many, many precise little square steps up and down the sinusoidal path with some very good filtering to keep the distortion under 3% as per utility spec.

    Some equipment is better than others. We can only deal with generalized statements, which inevitably are paradoxical in their nature to be wrong and right at the same time.

    Everybody understand that? :roll:
  • bluewickedburnerbluewickedburner Solar Expert Posts: 78 ✭✭✭✭
    Re: MSW inverter Hz reading
    Wayne's description is, for practical purposes, accurate.
    So is Bluewickedburner's.

    No, I'm not joking. All depends on the inverter and the equipment. Some of it, on either side of the plug, is junk. Other things not so much so.

    ....

    Everybody understand that? :roll:

    I got it, but then there is something wrong with me, or so I've told myself.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: MSW inverter Hz reading

    CARIBOOCOOT Take "wall warts". Please! You can bet they're a candidate for "don't run on MSW" because the internal circuitry is pretty cheap. Most likely no more than a transformer, one rectifier, and a small filter cap. Oh yeah; that will produce some clean DC!you must be finding some supercheap "wallwarts" plug packs.?? All the ones I open have 1 very small transformer 1 surfacemount IC.1 transistor 5 large caps 4x,1 caps6 diodes
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MSW inverter Hz reading

    John;

    You probably don't get dumped on with the Chinese $1 store specials that flood the market here. I often question whether it is even safe to plug in to the wall!
    I'm sure the inside of the power supplies for things like my Sharp calculator are of the quality you describe. But you should see some of the trash that's marketed here. :cry: And no it doesn't have to be approved; counterfeit goods are a serious problem on these shores.
  • bobdogbobdog Solar Expert Posts: 192 ✭✭
    Re: MSW inverter Hz reading

    EDIT: Never mind I found my answer though a search...duh
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: MSW inverter Hz reading
    bobdog wrote: »
    EDIT: Never mind I found my answer though a search...duh

    I've run both desk tops and lap tops on MSW with no problems, as I have TV's. This is NOT to say that all puters or TV's will run on MSW, just that the ones I happen to have/have had, had no obvious problems with MSW.
  • john pjohn p Solar Expert Posts: 814 ✭✭✭
    Re: MSW inverter Hz reading

    Cariboocoot Here in Aus it illegal to import or sell any electrical device not approved, shipments get checked at import point by customs. And rightly so.

    Im rather supprised that a country like Canada would have such lax import rules. And im rather supprised the electrical authorities allow them to be sold.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MSW inverter Hz reading
    john p wrote: »
    Cariboocoot Here in Aus it illegal to import or sell any electrical device not approved, shipments get checked at import point by customs. And rightly so.

    Technically it is here too. Yet there are stores full of such knock-offs; faked brands, faked certifications, unlicensed goods, et cetera. It is a very big problem here. :grr
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