I’m piecing together my first PV system and I hit a snag with respect to sizing my inverter (high frequency, 24 VDC to 120VAC). Based on my research, the inverter needs to handle a surge in power from an inductive motor that surges, or inrushes, for a fraction of a second during startup. I understand that this is why the inverters will be listed as a certain wattage with a higher rating for surges, usually double the rated capacity. I have a central vacuum cleaner that I’d like to include in my system. The motor is an 11 amp, 120 volt. I tried getting the inrush current from the manufacturer but to no avail. I also don't know what NEMA rating it is (motors are assigned a letter rating which tells you about the watts per hp, or something along those lines, sorry, not an expert). I also read that the best way to know for sure is to use an ammeter that can record/retain the maximum current. Since this vacuum motor plugs into an outlet, I also need an AC circuit splitter in order for the clamp-on meter to read the live wire. Before I invest in both the meter and splitter, I have two questions. 1. Is there any way to calculate the inrush current? 2. If not, any recommendations as far as the meter and splitter? Thanks in advance for any help.
Sizing Inverter to Account for Inrush Current
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gnubie
Solar Wizard
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Any chance of a good picture of the specs plate / sticker on the motor? There might be some notation on there that gives the locked current or mutiplier factor. An induction motor can take 4 and more times the run current during initial start up with a big peak at power on ramping down as the speed comes up. If the motor takes a while to come up to speed its not just the initial surge that needs to be covered.
An 11Amp motor under blower duty load is going to need 40amps startup current for about 10 full seconds...
you MIGHT be able to swing that with a 3000W HF inverter... but I doubt it.
A decent 2000 watt LF inverter should handle it ok... might need 2500
My manual for Samlex inverter, a quality brand. States under "Inverter Sizing Factor" listing various devices, for small industrial motors such as pumps and blower fans, to size at 3 times Running Active Power Rating (Watts) of the appliance. That seems really high to me. But here it is from the book.
MichaelK
Solar Wizard
Nope! Depending on the motor, 3X can be rather low. As smoothjoey recommends, I also use a meter with "inrush current" capability. I started out with an expensive Fluke meter like he pictured, but I've since purchased a much cheaper alternative called UniT.
UNI-T UT216C 600A Digital Clamp Meters NCV LCD Test ACDC Auto Range Multimeters 747045037461 | eBay
Specifications Range UT216C. AC current (A) 600A ±(2.5%+5). DC current (A) 600A ±(2.5%+5). Jaw opening 30mm. Analog bar graph. VFC, low pass filter. Display count 6000. Auto range √.
www.ebay.com
Please note that UniT makes many different meters, and the price goes up with the features. In-rush is one of the more advanced features, but you can still get one for about 60$.
Now that I have an inrush meter, I've gone around testing everything to see what the initial draw is. My submerged well-pump had the greatest inrush, at 3.8X. An AC box fan in the window was lowest, at 1.1X. Without testing it, I'd guess that the initial load on a central vacuum is not going to be that high.
One thing I consistently tell everyone is to plan for 2X of your needs. If you think you'll run 2000W of loads, get at least a 4000W inverter. 3X is even better. It's really hard on electronics when you try running them at the ragged upper edge of what they are designed for.
Thanks so far for all the feedback. The comments are steering me more towards a low frequency inverter. 2000W units seem to have a 6000w peak for up to 20 seconds, so I think that puts me in the right ballpark. I wanted to make certain that I don't buy one undersized. So, I was searching for clamp meters and found several including the one noted above that read inrush current. So, no problem there. However, finding a higher amperage AC line splitter is another story. like I mentioned, this motor is a plug in unit and I am not cutting and separating the line just to get the inrush current. I figured the AC splitter was the way to go but the only ones I can find have a maximum of 15A. From what I've read, that is what they are designed for and no more (give or take a few amps). Does anyone happen to know of a high amperage line splitter? if not, then there is no point in buying the clamp meter.
MichaelK
Solar Wizard
You stated above that the unit you want to test is plugged in, so is it plugged into a standard plug, or is it on a special 20+Amp circuit? Does the plug have the special 20A right-angle blades, or standard 15A parallel blades? What's the rating of the breaker that controls that circuit? Is it 15A, or 20A? You might be worrying for nothing.
One thing to consider is that the amperage rating of wire is based on continuous service. Engineers have demonstrated that domestic wiring can transmit thousands of amps if the current is flowing for just a fraction of a second. It takes time for the resistance to generate enough heat to be damaging. If you use a standard 15A splitter, and just run current through it just long enough to get an inrush reading, you can shut the unit off before in it gets damaging hot.
If you are really freaked out about it, then just make your own splitter using 20A plug/sockets and a short length of 12gauge wire. You can get the parts at Home Depot or Lowes for a few bucks.
If you haven't selected an inverter yet, I suggest seriously considering this Schneider.
ressupply.com
I have one for my workshop, and have found it to be a hardy, no fuss unit. It can produce both split-phase 120/240V AC, and has a built-in charger circuit. You simply wire a generator 240VAC output to it's AC2 input, and it can charge your batteries directly.
One thing to consider is that the amperage rating of wire is based on continuous service. Engineers have demonstrated that domestic wiring can transmit thousands of amps if the current is flowing for just a fraction of a second. It takes time for the resistance to generate enough heat to be damaging. If you use a standard 15A splitter, and just run current through it just long enough to get an inrush reading, you can shut the unit off before in it gets damaging hot.
If you are really freaked out about it, then just make your own splitter using 20A plug/sockets and a short length of 12gauge wire. You can get the parts at Home Depot or Lowes for a few bucks.
If you haven't selected an inverter yet, I suggest seriously considering this Schneider.
Schneider Electric Conext SW4048-120/240 Inverter/Charger - RES Supply
Free Shipping! Schneider Electric Conext SW4048-120/240 Inverter/Charger, 4000W, 120/240VAC, 50/60Hz, 30A Transfer, 48VDC, 45A Charger, RNW8654048
ressupply.com
Hedges
I See Electromagnetic Fields!
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Not sure about central vacuums, but portable ones are brush type. Which kind of motor does yours have?
So long as the inverter doesn't crap out, a brush type motor should start no mater what current is available and no matter how low voltage drops due to current limit of inverter.
I could get a scope shot of our 7A upright vacuum and 15A Milwaukee grinder starting current if you'd like.
The outlet's breaker is 20 amp but the vacuum's plug is the standard straight blade for 15 amp so I guess I am worried about nothing. But I like the idea of making my own splitter. Why didn't I think of that?
I don't know what kind of motor it is but I have another inquiry into the manufacturer so I'll see what happens. I will follow up. Thanks.
I don't know what kind of motor it is but I have another inquiry into the manufacturer so I'll see what happens. I will follow up. Thanks.
Well, I was actually able to get in touch with an engineer that works for the central vacuum company that made my unit (VacuFlo) and here is his response:
"Yes, the in-rush current at start up for the M-560 motor is approximately 145 amps. As you know, this only happens at initial start up and only reaches that level for milliseconds. The maximum current draw on this motor is 14.5 amps, but this only happens with no tubing, hose or accessories attached so you shouldn't see readings this high. The average current draw for this motor at 120 volts is between 12.0 and 12.5 amps."
That equates to 17,400 W at 120 V. My planned system won't have a surge capacity anywhere near that. So, unfortunately, it made my decision on the vacuum rather easy (unless anyone has any further advice).
I do appreciate all the feedback. I found a short extension cord that has been sitting in my garage forever, so I cut open the outer casing and was able to separate the wires. I intend to purchase an ammeter with the inrush current capability and do some testing on the vacuum (out of curiosity) and some other potential devices to tie into my system. I will follow-up on this thread with my results.
"Yes, the in-rush current at start up for the M-560 motor is approximately 145 amps. As you know, this only happens at initial start up and only reaches that level for milliseconds. The maximum current draw on this motor is 14.5 amps, but this only happens with no tubing, hose or accessories attached so you shouldn't see readings this high. The average current draw for this motor at 120 volts is between 12.0 and 12.5 amps."
That equates to 17,400 W at 120 V. My planned system won't have a surge capacity anywhere near that. So, unfortunately, it made my decision on the vacuum rather easy (unless anyone has any further advice).
I do appreciate all the feedback. I found a short extension cord that has been sitting in my garage forever, so I cut open the outer casing and was able to separate the wires. I intend to purchase an ammeter with the inrush current capability and do some testing on the vacuum (out of curiosity) and some other potential devices to tie into my system. I will follow-up on this thread with my results.
Hedges
I See Electromagnetic Fields!
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Sunny Island (my picture) is 120V, 5750W continuous. Spec is 180A surge for 60 milliseconds.
They run about $2200 brand new on eBay right now.
That should start the vacuum.
I still think if it is brush-type motor, it would pull down voltage of a generator (or less capable inverter that doesn't just give up) and still start, just take a second or so.
They run about $2200 brand new on eBay right now.
That should start the vacuum.
I still think if it is brush-type motor, it would pull down voltage of a generator (or less capable inverter that doesn't just give up) and still start, just take a second or so.
Thanks, Hedges. I will keep all this information for future reference. Like I said, this is my first system and I'm starting small with the intention of growing as I learn. But all this feedback helps. I do intend to purchase and use the ammeter to see just where the amps are with this vacuum. I will post my findings.
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