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  • SunEagle
    replied
    Originally posted by inetdog View Post
    i believe that the amperage numbers from the KAW reflect magnitude only, so amps times volts == VA.
    The watt figure is obtained from an actual wattage measurement (integrating instantaneous volts times amps), and the PF is derived from the ratio of the two. But based on that the PF for the fan should have been .97.
    So something in the way the KAW gets the numbers is not quite right, at least at the 2% level. Now the amp value was only given to two significant digits, so the "real" value could be off by as much as +/-3%.

    I agree that to see any effects of capacitive versus inductive reactance cancellation you will need to find a good inductive load.
    Also, though, the CFL and LED will have significant distortion PF included in their measurement.
    I should have recorded the VA measurements as well to see how they compared to the other metrics. I will go back and check tomorrow but my voltage is a little high today so the next time all of my measurements may be different. I was surprised with the fan PF being 0.99. It did bounce a little between 0.98 and 1.0 but settled at 0.99.

    The KAW is pretty neat but I am still learning about it. The listed accuracies run between 0.2 to 2% depending on which function it is measuring.

    I am currently getting a profile of my small frig so I really know what it will take to keep it running on a battery system. It is drawing about 81 watts now but the compressor is not running so I want to get a 24 hour run on it. I am also checking on some 12v Pure Sine wave inverters since my cobra is a Modified Sine wave and will not really run the frig.

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  • inetdog
    replied
    Originally posted by SunEagle View Post
    I'm not sure how the kill a watt makes these measurements unless it takes into account the PF and then calculates the amperage. You would expect a higher va due to a lower PF.
    i believe that the amperage numbers from the KAW reflect magnitude only, so amps times volts == VA.
    The watt figure is obtained from an actual wattage measurement (integrating instantaneous volts times amps), and the PF is derived from the ratio of the two. But based on that the PF for the fan should have been .97.
    So something in the way the KAW gets the numbers is not quite right, at least at the 2% level. Now the amp value was only given to two significant digits, so the "real" value could be off by as much as +/-3%.

    I agree that to see any effects of capacitive versus inductive reactance cancellation you will need to find a good inductive load.
    Also, though, the CFL and LED will have significant distortion PF included in their measurement.

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  • SunEagle
    replied
    Originally posted by Naptown View Post
    Look at the amps and watts on the CFL vs the LED
    Amps were higher on the cfl but wattage was lower than the LED
    I'm not sure how the kill a watt makes these measurements unless it takes into account the PF and then calculates the amperage. You would expect a higher va due to a lower PF.

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  • Naptown
    replied
    Look at the amps and watts on the CFL vs the LED
    Amps were higher on the cfl but wattage was lower than the LED

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  • SunEagle
    replied
    kill a watt test

    I did my own testing with a kill a watt P3 P4460 meter which I just purchased.

    I used a 13 watt CLF, a desk top fan and a 20 watt LED lamp. All values were given by the kill a watt meter.

    CLF - .59 PF, 0.18 A, 13.4 W, 122.4 V
    Fan - .99 PF, 0.27 A, 34.1 W, 122.4 V
    LED - .90 PF, 0.16 A, 18.6 W, 122.4 V
    All three added together - 0.61 A, 66.1 W

    All three measured together - .94 PF, 0.56 A, 65.1 W, 122.4 V

    There was a very small difference between the added and measured values for all three items so it seem even though the CLF and LED lamps may have some type of electronic power supply it is not affecting the amperage vectors that much. Even the PF seems to be in line considering the CLF has the lowest but is also the smallest W load.

    I need to find me a small 120v pump or inductive load to see how that affects the measurements. Interesting experiment.
    Last edited by SunEagle; 07-13-2013, 11:40 AM. Reason: corrected statement

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  • bcroe
    replied
    Power Factor

    I don't think anyone mentioned it yet, but PF for sine waves is just the simplest case. For those you
    could add the real currents & reactive currents, and from the totals calculate a combined PF. But with
    electronic stuff (switchers, or even rectifiers) the current waveform could be about anything. In theory
    you could have one load with some current spikes & gaps, another with reversed pattern so the spikes
    & gaps cancel each other, and get a PF of 1 (don't expect it to happen!). And caps aren't going to help
    as much for non sine current waveforms.

    The good news is, some equipment now uses switchers to get a much better power factor. Bruce Roe

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  • inetdog
    replied
    Originally posted by Robert1234 View Post
    Previous reading was 183.9 Watts
    (28.6v @ 6.43amp)

    Like I say, precision is impressive. Question remains if it is accurate.

    Used my other amp clamp as a back up. It's lowest scale is 200a. It read 6.7 as compared to the 6.83. I don't even know what brand that clamp is.

    AC Volt amps are still ~190. AC Watts are 134.
    Just as a VFD for a motor may be able to send excess power fed back by the motor during braking to the AC supply (or it may be forced to waste it as heat in an internal resistor), an inverter may be able to store internally the reactive power component of the output demanded of it. But it is not guaranteed. Some or all of the reactive power may be wasted as heat inside the inverter rather than being efficiently reused for the next part of the voltage cycle.

    You are probably not looking at 100% efficiency converting 1834 watts to 190VA, nor are you looking just as a very low efficiency at converting 184 watts to 134 watts, but rather something in between.
    (Although the latter is quite possible, since you do not know what the inverter drain is for zero (or very low) output. It is not unheard of for a 1000W inverter to have a no-load power drain of 50 watts.)

    The efficiency of the inverter at any particular power output usually depends, to first approximation, on two terms. The first a constant no-load drain and the second a loss roughly proportional to the output power.

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  • Robert1234
    replied
    Clamp on meter has dual scale, 400A and 40A. I was at the 40A level. Will look for the other meter as it is also dual scale. I have another in-line DC amp meter as well, but I'm not going to disconnect wires at this point. Had a wide open day.

    Only got 108 ah thru the CC today - another rainy, overcast day in the southeastern US. VA calcs say system should be at 3% below target cutoff levels at 8 am. Watts calc say batts will only be half of target discharge. Setting the plant beds for constant flooded instead of drain-fill just in case the pumps go off.

    ---------------------------------

    Just took another DC side reading for grins & giggles with pumps and lights on.

    6.83 amp @ 26.9 v = 183.7 Watts

    Previous reading was 183.9 Watts
    (28.6v @ 6.43amp)

    Like I say, precision is impressive. Question remains if it is accurate.

    Used my other amp clamp as a back up. It's lowest scale is 200a. It read 6.7 as compared to the 6.83. I don't even know what brand that clamp is.

    AC Volt amps are still ~190. AC Watts are 134.
    Last edited by Robert1234; 07-12-2013, 06:07 PM. Reason: Another reading

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  • inetdog
    replied
    Originally posted by Sunking View Post
    Fluke is good but I suspect your clamp-on is a 600 Amp Version? They are not accurate at low current readings. Best accuracy if you want to spend a few coins is a SHUNT, then use th eFluke volt meter to read the voltage drop and convert to amps.

    Ohm's Lab makes really good ones and there are dozens of other less expensive ones out there. Just make sure not to buy one any larger than you anticipate needing. Best accuracy is at or near full scale.
    For very temporary use, just for testing, you could also increase the precision and the accuracy of a clamp-on ammeter by running several (N) turns of the DC wire through the clamp. This will make the meter reading N times the actual current.

    Now I do not expect you to be doing this with your 1/0 or other large actual DC wiring, but just with temporary wiring that will support the inverter drain when powering just the loads you are testing. If you are going to modify the wiring though, just putting in the shunt temporarily will give you better accuracy.

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  • Sunking
    replied
    Originally posted by Robert1234 View Post
    Have to agree on the numbers looking weird.

    To directly answer your query, DC Power is measured by using a multimeter at the inverter input lugs (volts) and using a clamp meter on the 3-0 positive cable at the inverter. Precision of the measurement was pretty good (low standard deviation) but I cannot account for the accuracy other than saying this is pretty decent test equipment (Fluke). I have multiple volt meters and they all read the same. If there is a significant error, it is probably in the amps measurement. I do have another amp clamp that I can get another reading with (if I can dig it up). I also have a 150 amp shunt system but elected not to install it on these lines figuring I'd just clamp them whenever I wanted. Unless I was going to set up some method to integrate the shunt reading, I really couldn't see much of a purpose for it other than having a fancy digital display to look at.
    Fluke is good but I suspect your clamp-on is a 600 Amp Version? They are not accurate at low current readings. Best accuracy if you want to spend a few coins is a SHUNT, then use th eFluke volt meter to read the voltage drop and convert to amps.

    Ohm's Lab makes really good ones and there are dozens of other less expensive ones out there. Just make sure not to buy one any larger than you anticipate needing. Best accuracy is at or near full scale.

    Leave a comment:


  • Robert1234
    replied
    Have to agree on the numbers looking weird.

    To directly answer your query, DC Power is measured by using a multimeter at the inverter input lugs (volts) and using a clamp meter on the 3-0 positive cable at the inverter. Precision of the measurement was pretty good (low standard deviation) but I cannot account for the accuracy other than saying this is pretty decent test equipment (Fluke). I have multiple volt meters and they all read the same. If there is a significant error, it is probably in the amps measurement. I do have another amp clamp that I can get another reading with (if I can dig it up). I also have a 150 amp shunt system but elected not to install it on these lines figuring I'd just clamp them whenever I wanted. Unless I was going to set up some method to integrate the shunt reading, I really couldn't see much of a purpose for it other than having a fancy digital display to look at.

    Thanks once again for the discussion.

    P.S. As this is a hobby, I data log all the daily charge / discharge readings. Have done so with resistive loads prior to this for months on end and have good records of coulombic efficiency for charge / discharge and can make extremely accurate predictions for when the inverter will go off line when the drain has high power factors on the AC side. The readings as of late with the new loads APPEAR to suggest the system is draining at the volt-amp rate rather than the watt rate. I'll know better tomorrow am. We've had about two weeks of really overcast skies and rain here. Unless we get some really good sun today (to the tune of 120 amp-hr into the batteries minimum), the inverter will choke at 7 am tomorrow due to low volts when the lights kick in at the aquaponic units (assumin the drain is indeed following the volt-amp relationship). If it's following AC watts, my batteries should only be 40% of the way to shut down and all will be well with the world.

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  • Sunking
    replied
    Originally posted by Robert1234 View Post
    For pumps, lights, pumps & lights respectively:

    AC Watts - 34.8, 102.0, 134.0
    DC Watts (at inverter) - 35.3, 158.5, 183.9
    % Efficiency - 99%, 64%, 73%
    Robert something is just not right with the numbers which makes me suspect test equipment error. How are you measuring DC input power?

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  • inetdog
    replied
    Originally posted by Robert1234 View Post
    Good thing I don't do this for $$ then.

    Forgive my ignorance, but quite honestly I'm "sweating the small stuff" simply because I'm curious. If understand what you are saying correctly, it is that the inverter capacitors (behind where I measure) that should correct for the reactive power I measured. Somewhat curious as to how that works as the required capacitive value moves around even in my simple examples.

    Referring back to the data I posted, I did exactly as you suggest - measured DC Watts and compared to AC Watts.

    For pumps, lights, pumps & lights respectively:

    AC Watts - 34.8, 102.0, 134.0
    DC Watts (at inverter) - 35.3, 158.5, 183.9
    % Efficiency - 99%, 64%, 73%

    This is a Magnum 4024 inverter. Published efficiency is 87% min I believe. Measured AC/DC efficiency is well below that in 2 of the 3 examples.
    Basically, the inverter output (and the intermediate stage of the internal DC to DC converter that feeds the DC to AC convertor in some designs) can store a certain amount of power. If the output stage is appropriately designed, the ebb and flow of the unreal part of the apparent power (reactive power) will simply charge and discharge this capacitance as needed. The extent to which this is used will not require a change in the original size of this storage, as it will be factored into the minimum PF rating of the inverter.
    In a generator, the same sort of exchange of energy into and out of the generator will cause a varying mechanical load placed on the engine by the generator, and the flywheel effect of the motor/generator system will be able to level this out reasonably well, minimizing the extra fuel needed. But the full current is actually flowing through the generator windings, and although the extra power consumption may not be an issue, the heating of the generator windings will be. For that reason, generators too can be specified in terms of both the maximum real power (capacity of the prime mover engine) and maximum VA at a PF above a specified limit (usually somewhere between .6 and .8, so check the actual generator specs.)

    I am not sure why your inverter is so much more efficient in supplying a very inductive motor load than a resistive load (possibly some of the efficiency factors that Dereck mentioned), but that makes it a particularly good inverter for that use. And there might be a problem with your AC power measurements.

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  • Robert1234
    replied
    Good thing I don't do this for $$ then.

    Forgive my ignorance, but quite honestly I'm "sweating the small stuff" simply because I'm curious. If understand what you are saying correctly, it is that the inverter capacitors (behind where I measure) that should correct for the reactive power I measured. Somewhat curious as to how that works as the required capacitive value moves around even in my simple examples.

    Referring back to the data I posted, I did exactly as you suggest - measured DC Watts and compared to AC Watts.

    For pumps, lights, pumps & lights respectively:

    AC Watts - 34.8, 102.0, 134.0
    DC Watts (at inverter) - 35.3, 158.5, 183.9
    % Efficiency - 99%, 64%, 73%

    This is a Magnum 4024 inverter. Published efficiency is 87% min I believe. Measured AC/DC efficiency is well below that in 2 of the 3 examples.

    Leave a comment:


  • Sunking
    replied
    Originally posted by inetdog View Post
    Could you elaborate a bit on why a higher VA (lower power factor) load will require more fuel energy from a generator but will not require more battery energy from an inverter?

    I would expect the two cases to be fairly similar.
    Output capacitors on the inverter capture what is reflected and put back into the circuit. Generators cannot do that as it is mechanical and inverter is electronic.

    Every work with PWM motor speed controllers in a DC series wound motor? Why is it you can have a 600 amp controller pushing 600 amps into the motor, while only taking 100 amps from the battery? I will give you a clue. Back EMF.

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