Hello, everyone. I have a friend that has offered me a 100HP VFD and the matching AC motor to use with it. I'm not remotely familiar with these, and I was wondering if it would be feasible to run it off of a battery pack for an EV conversion? Thanks!
Industrial VFD and AC motor
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Probably not. The Variable Frequency Drive puts out what the motor needs, but it is almost certainly designed for AC power input... probably 3-phase. That would mean one of two solutions would be required:
- a fixed-frequency inverter between the battery and the VFD (you might as well just toss the VFD and get a suitable DC-input 3-phase-output inverter), or
- modification of the VFD (which is probably not practical).
My friend said that the VFD takes a single phase input. I'll have it in hand on Sunday so I can look more closely, but that's what he tells me.
While single-phase is simpler than 3-phase, this also confirms an AC input. Same required solutions apply.
Gotcha. I had read that there is a DC bus input that is easily accessible on some VFDs. I guess I'll have to take a look and see on Sunday.
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That's right Dunc. Those variable frequency drives go AC to DC and then DC to variable frequency AC. In most cases you can connect DC power to the DC bus. In fact most have such connection points normally used for braking resistor banks. Often the contactors and fans are driven from AC mains power.
It'd be strange to have a 100 hp drive powered from single phase, and uncommon to see at 230 V. More likely 460-480 V 3ph. And the 100 hp motor likely is a half ton monster.
Regards,
major
If it's easily accessible, that would be a substantial head-start on the VFD modification.
True, there must a be DC stage, but that doesn't mean it's accessible or usable. And I agree that the voltage is important.
So there's reason to be optimistic about making the DC connection? Excellent
, but...There's a good example of a potential issue.
That's what I thought, and why I was expecting 3-phase. 100 hp or 75 kW at 240 V RMS is 313 A; in what installation would anyone run a single-phase circuit to support that current, instead of using 3-phase higher-voltage power?
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The reason an inverter is called an "Inverter" is because the challenging part is taking flat, positive DC, and creating negative (inverted) pulses from it. It inverts the positive voltage for the negative part of the sine wave (and keeps it positive for the positive part). If it didn't invert it, it would just be controlling the on/off pulsing of positive DC, which is what a DC controller is "Pulse, Width, Modulation", PWM.
A "VFD" is a "Variable Frequency Drive" which means that it can control, or vary, the frequency (how many times per second) of the switching between positive and inverted pulses. It could switch it many times, or few, or anything in between. A VFD is an inverter that isn't set to a fixed rate.
An inverter is needed from a battery pack because there is only positive voltage that way, and to create AC you need negative parts. If you already had negative parts, you wouldn't need an inverter, because you'd already have a cycling source of voltage that is positive/negative/positive/negative/etc. There's nothing for the inverter to "invert", it's already back and forth being inverted. In this case you may want to use a Transformer to Transform the voltage (which can only be done on AC), but no inverter. A transformer can make the AC voltage higher or lower, but it can't change how fast it's pulsing, the frequency.
So, a VFD presumes you have an AC source, but which needs to have its frequency changed which a transformer can't do. The way it accomplishes this is to take your AC source, then put it through a rectifier to flip the negative parts of the sine wave positive, then puts some capacitors on that to help fill in the gaps to more closely approximate the steady flat positive DC of a battery. Once it has that, it chops it up however fast it wants to and inverts every other pulse to negative so it resembles AC again, but AC at a different frequency.
...
So... a VFD has a couple extra components to first convert AC into DC before it uses it. It has to, that's how it works. The electronics won't work on AC, it'll mess them all up and confuse them.
So yes, it will have a DC bus. A bus is just a power rail of Positive and Ground. It will not be hard to find, follow the traces, it will be between the giant capacitors right by the input, after the big black rectifiers. The wires will be gigantic, probably the heaviest traces or wires on the board. It might not be "accessible" in terms of having output terminals, but, it's not going to be some sneaky hidden microscopic wire inside a 16 layer board, it'll be big huge traces, or, likely giant bus bars or cables. It'll be about as hard as finding the front seat on a car "Open the door, in front of the steering wheel, there you've found it."
Ignore the first part of the input, strap your batteries to the DC bus directly, and roll out.
A "VFD" is a "Variable Frequency Drive" which means that it can control, or vary, the frequency (how many times per second) of the switching between positive and inverted pulses. It could switch it many times, or few, or anything in between. A VFD is an inverter that isn't set to a fixed rate.
An inverter is needed from a battery pack because there is only positive voltage that way, and to create AC you need negative parts. If you already had negative parts, you wouldn't need an inverter, because you'd already have a cycling source of voltage that is positive/negative/positive/negative/etc. There's nothing for the inverter to "invert", it's already back and forth being inverted. In this case you may want to use a Transformer to Transform the voltage (which can only be done on AC), but no inverter. A transformer can make the AC voltage higher or lower, but it can't change how fast it's pulsing, the frequency.
So, a VFD presumes you have an AC source, but which needs to have its frequency changed which a transformer can't do. The way it accomplishes this is to take your AC source, then put it through a rectifier to flip the negative parts of the sine wave positive, then puts some capacitors on that to help fill in the gaps to more closely approximate the steady flat positive DC of a battery. Once it has that, it chops it up however fast it wants to and inverts every other pulse to negative so it resembles AC again, but AC at a different frequency.
...
So... a VFD has a couple extra components to first convert AC into DC before it uses it. It has to, that's how it works. The electronics won't work on AC, it'll mess them all up and confuse them.
So yes, it will have a DC bus. A bus is just a power rail of Positive and Ground. It will not be hard to find, follow the traces, it will be between the giant capacitors right by the input, after the big black rectifiers. The wires will be gigantic, probably the heaviest traces or wires on the board. It might not be "accessible" in terms of having output terminals, but, it's not going to be some sneaky hidden microscopic wire inside a 16 layer board, it'll be big huge traces, or, likely giant bus bars or cables. It'll be about as hard as finding the front seat on a car "Open the door, in front of the steering wheel, there you've found it."
Ignore the first part of the input, strap your batteries to the DC bus directly, and roll out.
Bah, no it won't be.
I think it's likely that everyone involved in the conversation understands what an inverter is, what a Variable Frequency Drive is, and that an AC-powered VFD has a rectifier as a first stage.
The fact that the DC internal bus should be prominent is clear. Ability to safely and reasonably connect to it without any consequences is not so obvious. But I hope it does work out.
The fact that the DC internal bus should be prominent is clear. Ability to safely and reasonably connect to it without any consequences is not so obvious. But I hope it does work out.
I suspect that major was exaggerating a bit for emphasis, but an industrial motor rated for 100 horsepower (continuous, and likely with only air cooling) will likely be quite heavy and bulky by automotive standards.
jbman, please let us know how big this motor turns out to be.
That would be a roughly square-wave output; for an AC motor, you want a sinusoidal output (and actually three outputs each 120° in phase from each other), but the details of that are not important to this discussion (since the VFD does this already).
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I think it's quite obvious from several of the replies, that that is not true. There seem to be a number of people who do not understand the first thing about some of these., but worst of all, mistakenly think they do.
Also, it's nice to give soft introductions to other people who are reading and curious, not necessarily able to contribute on a technical level. Everyone starts somewhere.
If it wasn't obvious, I was explaining things from a beginner's standpoint. Yes of course, you're correct, there's more to it.
Sorry, I meant to say (but was not clear) that by now (and by several posts before this comment of mine) people in this conversation understand these terms... after several posts giving the meaning of "VFD" and then going through the components.
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Example, 1173 pounds.
https://www.weg.net/catalog/weg/US/en/Electric-Motors/AC-Motors---NEMA/Severe-Duty/TEFC-Cast-Iron/TEFC-W22-NEMA-Premium/W22-NEMA-Premium-Efficiency-100-HP-4P-404-5T-3Ph-208-230-460-380-V-60-Hz-IC411---TEFC---Foot-mounted/p/11355340
major
Edit: 2nd example 1335 lbs. See attached.
Attachments
Wow - so no exaggeration at all!
And the are running at what seems to me like bizarrely low voltage for the power level.
I suppose my expectations are biased by past exposure to large industrial motors which put out 3500 to 5000 HP, and probably not so heavy per horsepower... perhaps because they ran at much higher voltage.
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I was gonna say, "Yeah, you can cherrypick ones made of cast iron" but then I did some searching and, yeah, 1000 lbs is probably fair for an average.
I came across some that were 1500 lbs, and some that were only 500lbs.
Basically the motor is 500lbs (or less), and then add in the frame. Normally these are used in stationary places, bolted down to the floor, so they're purposefully built with a frame that outweighs the motor just for mass's sake. It's quieter and more stable with all that extra mass. If you're unlucky the frame and motor will be the same, but I think in most cases it's detachable.
I mean, 500lbs is what a V8 would weigh. It's not that stupidly large. A Tesla drive unit is 300lbs.
I agree that's a reason for the mass, but it's also a heat sink.
Since the engine doesn't need a huge battery, and has a far higher power output, it's not a straightforward comparison. And the 300 pounds for a Tesla drive unit includes the entire transaxle.
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I showed two of the first three off a Google search where the weight was readily noted on the specs. The third was 960 lbs IIRC. Please give the links or references to a few of those 500 lb motors that you mention. Thanks.
Also please elaborate on your detachable frame. Perhaps an example or two. I'm not aware of such so may be misunderstanding you.
But in a nutshell, when the OP says he's getting an industrial 100 hp motor, chances are it's a NEMA standard like the WEG or Baldor which I showed. Let's hope he posts the info.
Regards,
major
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Sorry, didn't make notes while searching specifically to give references. Didn't think anyone would care. One had a square cross section (not round), no fins, and was red. No base or anything. Form-factor-wise looked like a Nema23 with a disproportionately long midsection, but, obviously massively bigger, they didn't have anything else in the photo for scale. New off some industrial supply site.
I might be mistaken I suppose. I think almost all of them mentioned a cast iron mounting frame and showed feet, etc. I presume the motor body itself isn't cast iron. Wrong permeability/etc. The few industrial 3phase motors I've been around had the round-ish finned motor attached to a massive cup-shaped slab of a base. Bolted together and then usually a weld along the seam for good measure, but, not much of a weld, easily cut.
I'm curious too. Like, I'm presuming the guy offering it has seen the motor and has pictured it in a vehicle. It's not for example:
That, or if it is he's being joked with
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