Inverter frequency hertz issue

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PNjunction
PNjunction Solar Expert Posts: 762 ✭✭✭
in Solar Beginners Corner #1
I noticed that most of my small msw type inverters run well below 60hz, typically around 40-50hz when measured with a Kill-A-Watt meter. Granted this isn't laboratory equipment, and my cfl lighting seems to have no problem with it.

When I measure the frequency of my pure-sign-wave inverter output, it seems to be rock-solid at 60hz. I prefer to run most of my stuff from this, and save the non-critical things like lighting for the msw inverters.

I don't usually see anyone checking/testing the frequency that their inverter runs at. Has anyone had any problems related to this?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: Inverter frequency hertz issue

    A K-A-W will not read frequency accurately on a MSW inverter. The reason being that the output waveform is a series of Voltage steps to and from a lower-than-sine-wave peak Voltage which can confuse it. Since not all MSW inverters have the same step profile the meter will not necessarily read the same on any two for Voltage, current, Watts, or frequency.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #3
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    Re: Inverter frequency hertz issue

    As Cariboocoot suggested, it's vertually 100% that it's not the inverter Hz being out of wack, rather that the meter cannot properly read the mess of sharp pulses and spikes coming out of the MSW inverter.
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #4
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    Re: Inverter frequency hertz issue

    You guys are absolutely right. I had forgotten that my Fluke 87V multimeter is an rms and measures frequency. Guess what - 60hz on the msw type in a jumpstarter, and 61 hz in the other mobile msw type.

    While I like to blame my tools, I still count this as operator-error. :) Thanks for the heads up!
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #5
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    Re: Inverter frequency hertz issue

    Well this thread made me curious so I plugged my Kill-A-Watt meter into the cheap Duracell 400W MSW inverter that I keep in my truck and I hooked up my old Tektronix 2236 scope to it. Surprisingly it was dead on. There must be some variation among KAW meters. It could also be that there is variation in the duty cycle of different inverters "modified sine wave" - (in this case a square wave) which could effect the KAW readings. Mine looks to be right about 50%.

    Attachment not found.
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  • john p
    john p Solar Expert Posts: 814 ✭✭✭
    #6
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    Re: Inverter frequency hertz issue

    Cariboocoot."""Since not all MSW inverters have the same step profile the meter will not necessarily read the same on any two for Voltage, current, Watts, or frequency"""" Not completely true. They are accurate to the nearest ounce on a Wed andSun, and accurate to the nearest inch every other day ending in "Y"
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #7
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    Re: Inverter frequency hertz issue
    mtdoc wrote: »
    Well this thread made me curious so I plugged my Kill-A-Watt meter into the cheap Duracell 400W MSW inverter that I keep in my truck and I hooked up my old Tektronix 2236 scope to it.

    Attachment not found.

    WOW! that scope waveform is definitely not MSW, rather it's as square wave as "cheap" inverters come! :D
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
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    Re: Inverter frequency hertz issue

    It isn't the K-A-W meters that vary (within the brand) it's the inverters. As Wayne pointed out that one is totally square wave. Curiously that means more accurate frequency for the meter as there is only one 'pulse' peak for it to count. When there's a bunch of steps up to the Voltage peak the meters get confused. Any sort of bad power with high harmonic distortion will produce the same result.
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #9
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    Re: Inverter frequency hertz issue
    Cariboocoot wrote: »
    It isn't the K-A-W meters that vary (within the brand) it's the inverters. As Wayne pointed out that one is totally square wave. Curiously that means more accurate frequency for the meter as there is only one 'pulse' peak for it to count. When there's a bunch of steps up to the Voltage peak the meters get confused. Any sort of bad power with high harmonic distortion will produce the same result.

    Good point. I wish I had some other MSW inverters around to put on the scope and test. My assumption has always been that most of the cheap "MSW" inverters just put out square waves. I suppose the better MSWs put out a stepped waveform that better approximates a sine wave. Ironic to think that KAWs might be more accurate with the el cheapo MSW inverters.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,440 admin
    #10
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    Re: Inverter frequency hertz issue

    Usually, from what I have seen the MSW wave form is simply a "shoulder" at 0 volts for a few milliseconds. It probably also allows them to raise the peak MSW voltage a bit too (true sine wave voltage is higher than SW or MSW peak voltage--and it can cause "low voltage" issues with some electronic power supplies). Overall, a slightly better approximation of a sine wave.
    Square Wave
[FONT=fixedsys]--    ----
  |  |   |
  |  |   |
  ----    ----[/FONT]

Modified Square wave with shoulder at 0 volts
[FONT=fixedsys]----      ----
|  |_    _|  |_
     |  |      |
     ----      ----[/FONT]

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #11
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    Re: Inverter frequency hertz issue

    Yeah, I didn't know there were any actual square wave inverters still on the go, and definitely not any being promoted as MSW.
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #12
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    Re: Inverter frequency hertz issue

    Pretty funny huh. Here's the tech spec sheet from my inverter. Under AC waveform it says "Modified Sine Wave"

    Must be difficult to modify a sine wave to get a square wave.. True Square wave would be more accurate!

    It does seem to work fine to charge my iPad or Laptop when on the road which is how I've used it.

    Interesting this spec sheet says under license by Battery-biz.inc.

    On the bottom of mine it says "Manufactured by Xantrex" and "Designed in Canada/Manufactured in China" :roll:
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
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    Re: Inverter frequency hertz issue
    mtdoc wrote: »
    On the bottom of mine it says "Manufactured by Xantrex" and "Designed in Canada/Manufactured in China" :roll:

    Oh sure: blame Canada! :p
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #14
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    Re: Inverter frequency hertz issue
    Cariboocoot wrote: »
    Oh sure: blame Canada! :p

    Hahahahaha You tell him Coot! Hahahahaha
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  • mtdoc
    mtdoc Solar Expert Posts: 600 ✭✭
    #15
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    Re: Inverter frequency hertz issue

    Ha. I love Canada. I listen to the CBC every day since it comes in better than the US stations. Victoria is the closest big city to me. Heck, on a clear day I can see Canada from my office!

    And I can say from first hand experience, those Canucks know how to design a damn fine true square wave inverter!:p
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  • PNjunction
    PNjunction Solar Expert Posts: 762 ✭✭✭
    #16
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    Re: Inverter frequency hertz issue

    The KAW model I have is a P3 P4400, but is a few years old. I ran the test again just to be sure under load / no load conditions - on the KAW it oscillates 40-55 hz for my msw's. I even had a backup Extech 470 multimeter (rms - freq capable) and it showed the true 60 - 61hz condition for those same inverters.

    Makes me wonder how accurate the PowerFactor option on the 4400 is when using an msw inverter - I'll bet not very good.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,440 admin
    #17
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    Re: Inverter frequency hertz issue

    The Kill-a-Watt is not that accurate... With sine wave and inductive/resistive loads, the PF is probably OK. On MSW, probably not very.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #18
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    Re: Inverter frequency hertz issue
    mtdoc wrote: »
    Pretty funny huh. Here's the tech spec sheet from my inverter. Under AC waveform it says "Modified Sine Wave" ...
    Must be difficult to modify a sine wave to get a square wave.. True Square wave would be more accurate!
    That casts a whole new light on the nomenclature all right. Most two step waveform MSW inverters call themselves Modified Sine Wave, and I prefer to call them Modified Square Wave since they are closer in nature to square than sine. But you could not call this one a Modified Square Wave since the square wave is really unmodified!
    What I do call this is a lot of marketing chutzpah. :-)
    You could also call this one Pure Square Wave, depending on whether the intended audience values truth or purity more.
    SMA SB 3000, old BP panels.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #19
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    Re: Inverter frequency hertz issue
    inetdog wrote: »
    That casts a whole new light on the nomenclature all right. Most two step waveform MSW inverters call themselves Modified Sine Wave, and I prefer to call them Modified Square Wave since they are closer in nature to square than sine. But you could not call this one a Modified Square Wave since the square wave is really unmodified!
    What I do call this is a lot of marketing chutzpah. :-)
    You could also call this one Pure Square Wave, depending on whether the intended audience values truth or purity more.

    technically they are all square wave, but those with enough smaller square wave steps in voltage smooth out with an average. if that average is under 5% total harmonic distortion then it qualifies as a sine wave even though it may have hundreds of square wave steps for each cycle.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #20
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    Re: Inverter frequency hertz issue
    waynefromnscanada wrote: »

    WOW! that scope waveform is definitely not MSW, rather it's as square wave as "cheap" inverters come! :D
    No, it isn't. A square wave would only have two "flat" voltage regions, not three.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #21
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    Re: Inverter frequency hertz issue
    ggunn wrote: »
    No, it isn't. A square wave would only have two "flat" voltage regions, not three.

    Hahaha You caught me! I was looking for a step in the voltage between the zero line and the full voltage line and missed the extended zero time. :D
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