Superchargers vs Turbo

[conozo]

This is a general question to all about the benefits and drawbacks of either or. I am curious why nobody has gone with a supercharger, they appear to be easier to install and manage especially since you dont have to make your own turbo manifold. I'm also curious why new cars now all have turbos and not superchargers? Is the gas mileage better with a turbo vs a supercharger?

[cygnus x-1]

This is a general question to all about the benefits and drawbacks of either or. I am curious why nobody has gone with a supercharger, they appear to be easier to install and manage especially since you dont have to make your own turbo manifold. I'm also curious why new cars now all have turbos and not superchargers? Is the gas mileage better with a turbo vs a supercharger?


There are new cars that come with superchargers, but more often turbos are used. Turbos tend to be more efficient because they make use of the waste heat from the exhaust. They are also easier to install generally because the only custom part needed is an exhaust manifold (those are usually easy to make). The down side to turbos is that they have some lag and tuning is more difficult.

Superchargers have no lag and are therefore somewhat easier to tune, but they aren't as efficient because they are powered directly from the crankshaft through a belt or gears. They are also larger and more difficult to adapt because they have to be belt or gear driven.

C|

[2oodoor]

And then there's the electric supercharger....
it does seem that by now somebody could make a reliable one, the hardware is out there in the industrial world

[lostforawhile]

someone did build a real electric one, but it's for short duration and requires three batteries, he rewound three starter motors, and connected them through planetary gearing to an eaton blower, it's good for short bursts while the batteries recharge, Thomas Knight Turbo Electric Supercharger - Tech Review - Turbo Magazine (http://www.turbomagazine.com/tech/0406tur_knight_turbo_electric_supercharger/) I dont know whatever happened to this project but it worked

[Accordtheory]

The reason why turbos see to be more popular that superchargers..hmm, well for one, look at the cost. A procharger, gear driven centrifugal compressor, is about 2x the cost of a typical turbo, if not more. And there is still fab work too, you have to build brackets and set up the belt drive, etc. Sometimes that is more difficult than the typical turbo manifold/header.

Then, you have the power potential. I think I remember reading somewhere many years ago that a procharger on a 5.0 mustang at 14lbs takes about 70hp to run at that boost. (This also means that you'd need a 70hp electric motor if you wanted to do this electrically..that would be a pretty big, heavy electric motor, wouldn't it?) I think the 5.0 was making something like 600hp at that point, but still. When you are octane limited or engine strength limited, as virtually all honda motors or small motors are, the turbo will put down considerably more power.


The reason I mentioned the procharger is those are really awesome superchargers. Centrifugal compressor like a turbo, but gear driven. Much more efficient than a roots blower, or twisted helical lobe style blower, like on my '02 gtp. But, they have a different powerband. The centrifugal compressor makes more and more boost as the motor revs. This would be fine for a motor that has good volumetric efficiency at low/mid rpm, but drops off as it revs, then the higher boost makes for a more linear powerband. But for a honda, or a motor that carries the torque well, the turbo will put down more area under the curve, ie, more average power, and be a clear win in a race. The roots blower does not do this with the boost like the centrifugal one does, but it is very boost limited, it does not compress the air as efficiently. OEMs like that type of supercharger for its response, more linear power, and ease of integration into a package that also doesn't cook everything under the hood like a turbo does. There are some applications that use that type of blower more aggressively though, I believe the Corvette ZR-1 is one of the them.

Fuel economy, well, the positive displacement blowers all have bypass valves, even though I don't know how much of a difference this really makes since the blower is after the throttle anyway, and the centrifugal blowers, I don't know. Block a hose on a vacuum cleaner and the motor speeds up, not bogs down. You still do have a little parasitic drag either way though. But my gtp gets about the same mpg as the non supercharged grand prix for example, so I think it's negligible. The turbo does create more backpressure, but again, negligible under steady cruise, since it's not steadily spooling.

But then again, I know some diesels do exactly that. I just bought an old dodge with a cummins in it, that thing whistles steadily going down the highway. I would think that would just waste energy, but I don't know. And what if the manifold pressure exceeds the drive pressure..? But I know jet engines are horribly inefficient at light /medium load. I should research VGT tuning more. With a VGT or VNT turbo, you can tune when/how it spools by controlling the size of the orifices in effect the exhaust goes through before hitting the turbine wheel, not like a wastegated turbo.

But a lot of it also comes down to the character of the car, the way you or the OEM wants it to drive. I think a procharger on a grand prix would be awesome, instant power all the way to the redline, without much in the way of drawbacks. At 3.8 liters, the motor is big enough so that it's not too octane or internally strength limited. But on a b18c, I think that would be a foolish choice, compared to something like the popular gt3076r turbo. And of course, a positive displacement blower would be an even more foolish choice, since it would not be able to efficiently boost at a level necessary to do anything with the honda motor.

[2oodoor]

Full throttle Electric Supercharger Build Thread - Scion FR-S Forum | Subaru BRZ Forum | Toyota 86 GT 86 Forum | AS1 Forum - FT86CLUB (http://www.ft86club.com/forums/showthread.php?t=39719)
this is pretty interesting but I think the parts could be sourced cheeper than 1500$
I don't think I agree about the 70hp tax on the mustang procharger, thats a lot! Definately wouldn't take a 70 hp electric motor ll youre being sarcastic im sure. It's tbe turbine that is key to moving air, like a leaf blower or one of thse super hard blowing hand driers..I don't even think air curtain motors or carwash driers are big hp motors. Rpm and current load would factor more so than hp rating for electrics.

[lostforawhile]

I dont know whatever happened to the jackson racing ones, I think they were discontinued, but well proven to work On honda, the main advantage is an almost flat power curve with no lag, http://image.hondatuningmagazine.com/f/features/htup_0907_1993_honda_prelude/38199365/htup-0907-11-o%2B1999-honda-prelude%2Bsupercharged-h22a.jpg

[Hauntd ca3]

i think 70hp parasitic drwa from a belt driven super charger for a 5 liter engine is quite reasonable, considering the blowers on tf rails use several hundred hp to drive.
the efficiency of a rootes, or helical type blower is typically very low due to belt/gearbox heat and friction
but since electric motors are 100% efficient, a procharger driven by a 2.5 kw planetary style motor would prob work very well, once you get round the 200 odd amp current demand .
but then again, once an electric motor is spinning, back emf limits current draw to less than 100 amps. a decent (re optima or similar ) slave battery just for the supercharger and an up rated altenator, somewhere round 150 amps would suffice for short bursts of full load of a motor capable of driving such a charger












Full throttle Electric Supercharger Build Thread - Scion FR-S Forum | Subaru BRZ Forum | Toyota 86 GT 86 Forum | AS1 Forum - FT86CLUB (http://www.ft86club.com/forums/showthread.php?t=39719)
this is pretty interesting but I think the parts could be sourced cheeper than 1500$
I don't think I agree about the 70hp tax on the mustang procharger, thats a lot! Definately wouldn't take a 70 hp electric motor ll youre being sarcastic im sure. It's tbe turbine that is key to moving air, like a leaf blower or one of thse super hard blowing hand driers..I don't even think air curtain motors or carwash driers are big hp motors. Rpm and current load would factor more so than hp rating for electrics.

[cygnus x-1]

If that 70HP is real, you aren't going to do that with any 12V DC motor. The simplistic conversion for HP to Watts is 550, meaning 1HP is 550W. 70HP is 38500W. At 12V that would be 3208 amps. Electric motors are also not 100% efficient. I think the better brushless DC motors can get into the 90% efficient range, but to operate at that kind of power they would need far higher voltage to minimize power loss due to resistance in the windings.

Anyway, electric supercharges are a neat idea but I don't see them ever becoming feasible for larger engines. The cost and complexity will be much higher than a mechanical compressor and they don't take advantage of the wasted power from the exhaust stream.

C|

[gp02a0083]

all that electric stuff for forced induction seems to be a gimick to me. The turbo would be a more "efficient" route to take. As mentioned SC's take power to make power, however application needs to be taken into account. Each forced induction method produces unique hp /Tq curves. With the newer turbos and a correctly designed and tuned system it almost eliminates lag making it even more appealing

[Accordtheory]

I don't think I agree about the 70hp tax on the mustang procharger, thats a lot! Definately wouldn't take a 70 hp electric motor ll youre being sarcastic im sure.

I'm confident that number is realistic, if not 100% accurate. Here's a thread talking about hp consumption of the prochargers. Procharger HP to Drive ??? - Yellow Bullet Forums (http://www.yellowbullet.com/forum/showthread.php?t=69420) Electric hand blowers, carwash blowers? They're not compressing the air. Those prochargers are efficient, but compressing the mass of air to the pressures they do takes hp, no way around it.

[Accordtheory]

The simplistic conversion for HP to Watts is 550, meaning 1HP is 550W. |

That is actually not correct, the correct conversion is 746 watts. Horsepower - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Horsepower)

So yeah, electric supercharging at high power levels is not really a feasible concept. You would need an electrical system from an electric car, and then it'd make more sense to just dump that power into the electric drive motor instead.

[Accordtheory]

the efficiency of a rootes, or helical type blower is typically very low due to belt/gearbox heat and friction
but since electric motors are 100% efficient, a procharger driven by a 2.5 kw planetary style motor would prob work very well, once you get round the 200 odd amp current demand .
but then again, once an electric motor is spinning, back emf limits current draw to less than 100 amps.

I hope I'm not pissing you guys off trying to sound like I know everything here, because I know I don't. But I am more passionate about these subjects that virtually anything else in my life..except maybe fighting our evil, corrupt government, but that's a whole other story. But yeah, I do put a lot of time into researching these subjects.


Okay, first, the majority of inefficiency of a helical lobe/roots blower is not due to the belt drive/gearbox friction. How do I know that? Because those prochargers transfer a hell of a lot more hp through a belt drive and a gearbox, and their gearboxes spin that centrifugal compressor way up over the rpm of a roots blower, and still, that gearbox/belt hp loss is only a small fraction of the power the procharger consumes. The inefficiency shows up in a way you can easily measure, the temp of the charge air. I think the majority of inefficiency of the helical lobe/roots blower is that the more boost you run, the more the air leaks between the lobes of the rotor and the case. Eaton tried to improve this, or did improve this, by lining their rotors with teflon I believe, but still there has to be a gap there, the parts can't touch, and that tiny gap allows leakage, which is inefficiency.

Next, the only thing electrical that is 100% efficient is inefficiency itself, ie, heat. Years ago I worked at this food processing plant, they made cheese, lactose, whey, etc. It was all 304, 316 stainless, tig welded, touch screen computers everywhere, variable frequency drives and induction motors, several hundred million dollar facility. The technology there was awesome, but the place felt like a prison, so I had to quit. Anyway, the most efficient electric motors they ran were over 95% efficient, which I found to be absolutely impressive. Baldor high efficiency induction motors. But when you factor in the losses through the VFDs, wiring, etc, I doubt they were realistically running over 90% efficiency wise. I don't really know.

A procharger driven by a 2.5kw, or 3.35hp electric motor would not be able to do much of anything at all.

Back emf limiting current draw to 100 amps? All electric motors, controllers, and loads are not the same, 100 amps is an entirely arbitrary number, why would you say that? For example, the place I mentioned where I used to work, there was a 600hp electric motor running on 480V. The motor ran a huge fan that created a lower pressure in this giant series of tubes called "the evaporator", it was designed to remove water from a solution with lactose crystals. 600x746/480=932.5 amps, for example. Taking into account power factor, the amp draw was probably a little higher.

[Accordtheory]

I dont know whatever happened to the jackson racing ones, I think they were discontinued, but well proven to work On honda, the main advantage is an almost flat power curve with no lag

Yeah, just like any car with that style of blower. Instant response, linear power, but very boost limited, and expensive for what you get. Those kits were several grand, right? So in effect, you could run what, 6 or 8 lbs, making less hp than you would with a turbo at 6 or 8 lbs of course, at a higher cost. But with the turbo, intercooling is easier, and so is turning up the boost to the octane/internal part strength/compressor map efficiency limit, and then you're probably making 2x the hp of the jackson racing kit, right? Lol

[Hauntd ca3]

A procharger driven by a 2.5kw, or 3.35hp electric motor would not be able to do much of anything at all.

Back emf limiting current draw to 100 amps? All electric motors, controllers, and loads are not the same, 100 amps is an entirely arbitrary number, why would you say that? For example, the place I mentioned where I used to work, there was a 600hp electric motor running on 480V. The motor ran a huge fan that created a lower pressure in this giant series of tubes called "the evaporator", it was designed to remove water from a solution with lactose crystals. 600x746/480=932.5 amps, for example. Taking into account power factor, the amp draw was probably a little higher.

i used 100amps as an example, as thats about the average current draw of a 2.5 kw reduction gearbox type of starter once its turning the average 4 cylinder motor over.
it maybe a little more, but not much. you do have to remember though that the hp of an electric motor is calculated by the current draw at armature stall and not running speed
most starters free running on the test bench will only draw somewhere between 30 - 60 amps due to back emf limiting current.
using a planetary style of starter with a ratio of 3:1 , a centrifugal style sc should be able to be driven for short periods of time without to much hassle.

[lostforawhile]

i used 100amps as an example, as thats about the average current draw of a 2.5 kw reduction gearbox type of starter once its turning the average 4 cylinder motor over.
it maybe a little more, but not much. you do have to remember though that the hp of an electric motor is calculated by the current draw at armature stall and not running speed
most starters free running on the test bench will only draw somewhere between 30 - 60 amps due to back emf limiting current.
using a planetary style of starter with a ratio of 3:1 , a centrifugal style sc should be able to be driven for short periods of time without to much hassle.

the guy who built the one with the three starter motors said he rewound them, due to overheating issues, I'm not sure how he decided to wind them as it was never specified

[2oodoor]

There's so many ways you could get there, and it would be reliable so don't sell short the concept by judging the gimicky prototypes going around. I fully expect to see electric sc on new cars in the upcoming years, maybe starting with small displacment motors such as in hybrids.

It is amazing how far cordless tools have come so the tech is around to make constant or durable duty motors that put out unbelievable power and do it a long time on one charge. Im refering to pro grade tools, Makita, Ridgid, Milwalkee...the brushless motors are outstanding and run cooler.

[lostforawhile]

There's so many ways you could get there, and it would be reliable so don't sell short the concept by judging the gimicky prototypes going around. I fully expect to see electric sc on new cars in the upcoming years, maybe starting with small displacment motors such as in hybrids.

It is amazing how far cordless tools have come so the tech is around to make constant or durable duty motors that put out unbelievable power and do it a long time on one charge. Im refering to pro grade tools, Makita, Ridgid, Milwalkee...the brushless motors are outstanding and run cooler.the technology going into the hybrid cars will eventually find it's way into the next generation of electric motors

[Vanilla Sky]

Toyota SC-14 from a Previa, mount where AC compressor used to, have fun.

[cygnus x-1]

That is actually not correct, the correct conversion is 746 watts. Horsepower - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Horsepower)

So yeah, electric supercharging at high power levels is not really a feasible concept. You would need an electrical system from an electric car, and then it'd make more sense to just dump that power into the electric drive motor instead.


Bah, you're right. I meant to say 750 but was thinking 550 for some reason.

That further proves the point. Now we're talking 52.2kW. FWIW, the Tesla Roadster uses either a 185kW or 215kW motor. I don't know how much that motor (and drive system) costs but I would bet even 1/4 of that is way more expensive than a mechanical compressor.

C|

[Accordtheory]

you do have to remember though that the hp of an electric motor is calculated by the current draw at armature stall and not running speed

I don't know how you'd possibly be able to calculate the hp of an electric motor by its stall speed power draw, since you wouldn't know how fast it's going to spin, or how much torque it will put out at that speed.

Not to mention, you're only thinking about one type of motor. When you look at the motors that are efficient, they're induction motors, and at that point, the hp generated is a function of the VFD running it. Same with the stall speed power draw.

[Accordtheory]

There's so many ways you could get there, and it would be reliable so don't sell short the concept by judging the gimicky prototypes going around. I fully expect to see electric sc on new cars in the upcoming years, maybe starting with small displacment motors such as in hybrids.


Audi is already working on it for their 3.0 tdi motor, here you go Audi brings its electric turbocharger closer to production with RS 5 TDI concept (http://www.gizmag.com/audi-electric-turbocharging-rs-5-tdi/32619/)

[Hauntd ca3]

I don't know how you'd possibly be able to calculate the hp of an electric motor by its stall speed power draw, since you wouldn't know how fast it's going to spin, or how much torque it will put out at that speed.



how do you think they calculate the hp rating of a starter motor?
simple volts x amps maths
stall speed is the only time that an electric motor is not producing a self induced voltage which opposes normal current flow ( back emf) limiting current flow.
induction motors included.
spin a magnet in a coil of wire, and a current is induced , its how altenators work, it just so happens that with an induction motor they are feeding the stator instead of the rotor.
an induction motor is just a brushless altenator with the stator being feed instead of the rotor

[Accordtheory]

how do you think they calculate the hp rating of a starter motor?
simple volts x amps maths


Maybe starter motors are rated like how you describe, by how much power they draw when stalled. But if so, that is meaningless, it does not reflect how much power the motor is able to put out at the shaft when it is spinning. The more you load it, the less efficient it is, all the way down to stall speed, which is 0%, since it is not turning.

You do know they dyno electric motors too, right?


If you look at any other electric motor, like the motor on an air compressor, its hp rating is the amount of power available at the shaft, not input power. A 1hp electric motor is rated at 1hp because it can put out 746 watts, or 1hp, worth of power at the shaft. It is always going to pull more than 746 watts at that point, because no electric motor is 100% efficient, contrary to what you wrote earlier. If the nameplate on the motor says 1hp and 90% efficient, at a 1hp load it would actually be pulling about 829 watts.

[Accordtheory]

you do have to remember though that the hp of an electric motor is calculated by the current draw at armature stall and not running speed

The hp of an electric motor is not calculated by its current draw at any speed, IT IS A RATING OF THE POWER THE MOTOR CAN PUT OUT, NOT TAKE IN.

[lostforawhile]

Audi is already working on it for their 3.0 tdi motor, here you go Audi brings its electric turbocharger closer to production with RS 5 TDI concept (http://www.gizmag.com/audi-electric-turbocharging-rs-5-tdi/32619/)
it has a dedicated 48 volt electrical system for the tubo dependent on regenerative breaking, and it also has two standard turbo's, this just provides a small amount of boost while the other turbo's spool up

[Accordtheory]

Yeah, it is sort of a "band aid" thing, more than a separate setup, but still I have to give them credit for even going that far. I suspect as they actually come out, we'll be able to find technical info on that a little more easily. Like diagrams, cutaway pics, how much hp that electric motor actually produces, how much boost that little compressor makes, how long until the normal vgt turbos spool and take over, etc.

But here's what I'm looking at for my 2011 Golf tdi, the gtb2260vk. Darkside Developments - Darkside GTB Turbo Kit for 2.0 16v TDI PPD & Common Rail Engines (http://www.darksidedevelopments.co.uk/products/darkside-gtb-turbo-kit-for-2-0-16v-tdi-ppd-common-rail-engines.html) No electric turbo/electric supercharger bs for me! It'd make about 270whp, with enough diesel torque to totally destroy the oem dual mass flywheel, that's for sure. Too bad a pound sterling is about 1.7 us dollars, so that kit is about $3400, + shipping overseas...fuck...that's really not that bad!

I remember reading somewhere else that Audi was working on an "electric turbo", but they called it a turbo and not a supercharger because it still had a turbine wheel, it was just spooled early by the electric motor somehow. Tried to find that article but couldn't.

[Dr_Snooz]

So yeah, electric supercharging at high power levels is not really a feasible concept. You would need an electrical system from an electric car, and then it'd make more sense to just dump that power into the electric drive motor instead.

You raise an interesting point. If you can get a 90% efficient electric motor, why would you use it to boost a 30-40% efficient internal combustion engine? It's absurd. If we weren't subject to a ruthless oil oligopoly, we'd all be jetting around in ion pulsed skycars with onboard fusion generators. Instead, we're fiddling around, trying to make 19th century technology slightly more efficient.

[cygnus x-1]

You raise an interesting point. If you can get a 90% efficient electric motor, why would you use it to boost a 30-40% efficient internal combustion engine? It's absurd. If we weren't subject to a ruthless oil oligopoly, we'd all be jetting around in ion pulsed skycars with onboard fusion generators. Instead, we're fiddling around, trying to make 19th century technology slightly more efficient.


Because for now, liquid fuel (gasoline etc.) is still the most cost effective way to store energy (and the energy is free, in the sense that it is already in condensed form when we mine it). Batteries are getting better all the time, but still aren't efficient enough. And with batteries, the energy has to come from somewhere. That somewhere, as of yet doesn't really exist (at the levels we would need for electric cars).

C|

[Hauntd ca3]

The hp of an electric motor is not calculated by its current draw at any speed, IT IS A RATING OF THE POWER THE MOTOR CAN PUT OUT, NOT TAKE IN.

i dont know how they rate dc motors in the states, but automotive electricians rate max power at stall speed.

[Dr_Snooz]

Most motors I've seen have a Locked Rotor Amp (LRA) rating and a running amp rating. They are very different numbers and both are important.

[Hauntd ca3]

Most motors I've seen have a Locked Rotor Amp (LRA) rating and a running amp rating. They are very different numbers and both are important.

true, but for the general automotive use, the lra or stall rating is the most widely used. the running rating is usually a fair bit lower in my experience.
the most current draw is at startup speed and quickly drops off as armature speed rises, partly due to the lessing of the load once the electric motor has what ever its moving , moving.
a small 4 cylinder car will cause a load of up to 200 odd amps at engagement since the armature is close to stall trying to get things moving, but once moving load drops, current reduces and so does power

[Accordtheory]

You raise an interesting point. If you can get a 90% efficient electric motor, why would you use it to boost a 30-40% efficient internal combustion engine? It's absurd. If we weren't subject to a ruthless oil oligopoly, we'd all be jetting around in ion pulsed skycars with onboard fusion generators. Instead, we're fiddling around, trying to make 19th century technology slightly more efficient.

"Skycars with onboard fusion generators"? Well, I'll leave that engineering to you! You never know what's possible though, and our current generation of humans has only been engineering technology for a little over a hundred years, since the industrial revolution. People complain about not having flying cars, things like that, but to me it is insane how far we've come in such a short time. We went from steam engines to being able to blow up the entire planet with nuclear weapons in what, 50 years? Nowadays people interact more with technology than each other, it seems.

Wow, I really got off topic. Okay, so as soon as I posted what you quoted, I realized, well, a lot more could be said about that. Say with a little hybrid car gas motor, you could put 30hp into supercharging it, or 30 hp worth of electricity to the electric drive motor. 30hp to the supercharger could add another 200hp to the output of the gas motor. But to me a hybrid is really about efficiency. I would much rather always run the gas motor efficiently, at stoich, or lean, (keep in mind, diesels always run lean) and concentrate on building the electric part of the powertrain up a little more. It is also only a matter of time before capacitors of sufficient energy density are developed. Even those high speed flywheel storage systems can be efficient. To me an electric car with the right technology is far more appealing than our current technology, since it would allow very little penalty in efficiency with hard driving. But electric cars are a whole other thread..

[Oldblueaccord]

This is a general question to all about the benefits and drawbacks of either or. I am curious why nobody has gone with a supercharger, they appear to be easier to install and manage especially since you dont have to make your own turbo manifold. I'm also curious why new cars now all have turbos and not superchargers? Is the gas mileage better with a turbo vs a supercharger?

I looked into it some with a Jackson racing. They will sell the individual parts,manifold supercharger etc. For low boost I like there package.

OEM I think a lot of it is packing,room in the engine bay. Most of the hipo domestic production V8 are supercharged using a Eaton TVS supercharger.

TVS Supercharger - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/TVS_Supercharger)

It seems the smaller motor cars go with a turbo.

I think one of the drawbacks to a daily driven car with a turbo is the high under hood heat.


As far as the other posters the electric supercharger I mentioned here the last time this topic came out was this one.

Boosthead.com · Thomas Knight Turbos · Upgrades · Superchargers (http://www.boosthead.com/old/dyno.htm)

The other stuff about motor rating and efficiencies I cant really comment too much on since I'm not an EE I just play one on occasion in real life.
Some of you are mixing up AC and DC motors,terminology etc. Stateside ratings for motors are different then international ratings.

Electrical Motor Efficiency (http://www.engineeringtoolbox.com/electrical-motor-efficiency-d_655.html)

A VFD is a variable frequency drive. It varies the speed of an AC motor by manipulating the frequency.. I'm not sure what that has to do with a 12v DC system or even a 36volt system in the discussion.

[Accordtheory]

A VFD is a variable frequency drive. It varies the speed of an AC motor by manipulating the frequency.. I'm not sure what that has to do with a 12v DC system or even a 36volt system in the discussion.

Starting an engine is about the only semi legitimate application left for a brush style motor. Everything else can be better done with some type of brushless motor, permanent magnet or induction. The controllers all have a DC stage. If they run on ac, they rectify that into dc first, and then cut that up and reform it into whatever waveform the motor needs. They can also feed power back into the supply, like with regenerative braking. Technology FTW, brush style motors have been around for over 100 years, it's about time they become extinct.