# Voltage is Speed?



## green caveman (Oct 2, 2009)

My teenage son really wants a car. I don't want another ICE hanging around and we're thinking, together, of converting one of the lightweight '80 pickups, a small Datsun, Nissan, Mazda or similar. (Saves him asking for gas money, and with a vested interest in the car it may make him a little more careful).

Since this is new to me, I've spent many hours reading the threads.

The forklift motor thread has a wealth of information, and it seems that using a forklift motor is a cost effective way to go. However, as I look at the motor specs. in that thread I notice that most are fairly slow, 1000RPM or so.

We want a top speed of about 50-55mph.

Since systems get much more expensive as the voltage increases, is it useful/possible, etc. to find a motor with a higher RPM and then convert something smaller, an Escort, Metro or similar.

It would seem to me that with this approach, you could use a 48V system rather than a 72V or higher. For example, a 48V 300A controller is available on EBay for under $200, but a 72V controller will run 600+ for the amperage required to run a truck.

What am I misunderstanding about motors and EV systems in general? I think that I understand that the 300A system would be a little sluggish, but would it reach the speed? Am I also correct in thinking that 72V/2000RPM would be a little slow in the truck?


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## everanger (Sep 7, 2009)

I'm still learning to but let me share my experience. i just comverted a 83 ford ranger. we wanted to try it cheap and see if it would work. we went with a 72volt system and the truck will go 45-50mph and decent accel. but the motor gets hot and we even burn up a set of brushes oops. so the 72 volt 10hp motor is going in to a sandrail.and we are upgradeing the ranger. one thing the motor company said was 10hp for every 1000 lbs. i dont think i have seen that on here but from the motor comany. the ranger with 12 batteries 2 strings of 6 at 72volts with driver was right at 2800lbs this why we overheated it. hope this helps judt dont under power it and hope for the best.the father son project is always a good idea


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## green caveman (Oct 2, 2009)

everanger,

What's the RPM/Voltage of the motor?

I'm actually quite impressed that the ranger came in at under 3000lbs - that's about our target figure. What was the initial weight?

It seems that big forklift motors are relatively inexpensive and not too uncommon, so I'm not too concerned about burning out the motor, especially since big controllers are expensive. I'm more worried about speed.

10HP is about ~7500W say 7.2 KW, so at 72V this would mean 100A, which sounds low. But then we always have peak, vs. 1min vs. continuous, etc.
[Calculating again, this is per 1000lbs, so we have 300A at 72V. I'm a little concerned about hills, we have lots of them!]


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## Guest (Oct 5, 2009)

Many of the larger 8" or 9" forklift motors are fine at higher speeds. The tags are continuous rating and usually for industrial specs. Almost all motors are just advanced and over volted and usually will handle most voltages we use. I use a 9" GE and run 96 volts and the motor will push my little VW Ghia to over 85 mph if I want. It does get hot at those speeds but runs fine. 

Pete


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## green caveman (Oct 2, 2009)

OK, now I'm clear that there is something I don't understand. The standard forklift motors say 1000RPM @ 36V. A typical motor curve is linear, or approximately so, so at 96V you have 96/36*1000 RPM, or ~2600 RPM. This would get a Geo Metro to about 49MPH. Assuming you have about the same gear ratios as a Metro (maybe a VERY bad assumption) how do we get to 85MPH?

What am I missing? Running a [email protected] motor at 72V should give you about 2000RPM. An ICE is usually running in the 4000RPM range at high vehicle speeds, so 2000RPM isn't going to give you much speed. Clearly, the experience of others is that I'm wrong. I'm just trying to understand why before I spend time converting a vehicle that I'll be unhappy with at the end.


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## everanger (Sep 7, 2009)

I dont have a tach so not sure on the rpm that is something we will address on the up grade for sure.while getting up to speed the motor was pulling real close to 300amps for id say over 30 secs. this is a bare bones truck no ac no ps no pb. i dont have the start weight . to me it seems the newwer the car the more it weights it looks like newwer ranger weight in at alot more.


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## green caveman (Oct 2, 2009)

I woke up with the thought on my mind that there is an obvious relationship to the load, which makes me wonder what the rating on the motor plate is actually good for. What's the effective difference between a motor rated at 1000rpm and one rated at 1750?

So, a motor rated at 1,000RPM will run faster, at lower loads, but I think I heard the suggestion that up above about 5,000RPM you might have problems.

This leaves me wondering about the donor truck. The old Datsun's are light (2300lbs or so) but have a 1.4L engine. I think that they have lower gearing, to compensated for the smaller engine, and a lower top speed with an ICE (I'm thinking of the metros which have a small engine, but which top out at about 75). With this in mind, it might be better to get a truck with a higher design top speed. Since I'm not likely to easily find the gear ratios (source anyone?), I suspect that a truck with a reasonable size engine (what's in the rangers 2.6L?) will have a higher top speed. So something a little heavier, but one which might more reasonably have been expected to see highway use with its ICE.

Anyone have any thoughts?

Thanks,


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## piotrsko (Dec 9, 2007)

get a haynes or chiltons manual for your vehicle, can be had at a local library, or internet for as little as $5.00 usd. They generally have most of the trans or diff specs.

rangers/mazda's from about 83 to 90 had either a 3.73 3.93 3.45 or 4.10 ratio with the 3.45 being in auto trans vehicles. I doubt the foriegn trucks were that much different. In my son's ranger, 2500 rpm in .8 OD with 3.73's is about 65 mph.


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## Tesseract (Sep 27, 2008)

green caveman said:


> OK, now I'm clear that there is something I don't understand. The standard forklift motors say 1000RPM @ 36V.
> ...
> What am I missing? Running a [email protected] motor at 72V should give you about 2000RPM....


You are missing the amps, or torque load. If you keep the torque (amps) the same but double the voltage then, yes, you will get approximately 2000 rpm out of that motor at 72V. Or 3000 rpm at 108V.

Conversely, if you demand less torque from the motor it will run at a higher RPM at a given voltage. Unload the motor completely and it will likely fling itself to pieces at much above 12V.


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## green caveman (Oct 2, 2009)

Tesseract said:


> You are missing the amps, or torque load. If you keep the torque (amps) the same but double the voltage then, yes, you will get approximately 2000 rpm out of that motor at 72V. Or 3000 rpm at 108V.


Thanks, that makes sense. So what is the number of the faceplate? Please excuse my ignorance, but, presumably, the motor design determines the RPM on the faceplate, say 36V 1000RPM, or 36V 1750RPM but without knowing the loading these values are not useful. Is there a standard load (seems unlikely!) or is that another value from the faceplate and is there any realistic way to compare any of this. (And do I really care??).


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## Guest (Oct 13, 2009)

What the motor sees is power. Some it uses to produce mechanical motion. Some it wastes as heat. When it reaches a certain temperature, usually as a function of current, it burns up.

Along the way, note that a DC motor is not only a motor, but a generator. And even when acting as a motor, it produces a counter voltage, usually termed "back electromotive force" or back EMF. This causes a declining torque curve as RPM increases, back EMF increases, and the difference between the applied voltage and back EMF narrows.

Virtually all ICE engine cars develop their peak horsepower at relatively high RPMS - typically 3600 rpm. In the case of the Porsche, or an MG, this is even higher 4600 rpm. So the gear sets are optimized for this.

Electric motors are quite different. This is why the initial acceleration is so exhilerating. At very low RPMs, where an ICE engine is almost useless, the electric motor makes its maximum torque. On a Netgain Warp 9, at 120 volts, this torque is great up to about 2400 rpm, and then begins to fall away. By 3600 rpm, even though the motor is rated at 5500 rpm before becoming a mortar round, there really isn't much torque coming out.

This is the problem with high speeds. As you raise the voltage, you do decrease the current, and you extend the torque curve further out the rpm band. This is why most EV's are centered around 144 volts.

Accelerating to 45-50 mph is doable with any of them, at any voltage basically. But beyond that, air resistance is just a MUCH bigger issue than most people can visualize. You're basically pushing a wall of compressed air down the road and it's a squared function. You have to double the power for a small increase in speed. 

Unfortunately, this need for power is upside down with the torque curve of an electric motor. Power declines with the increase in RPM, right when air resistance causes a need for more power.

BTW, my perusal of AC motor specs reveals nothing different. It does no good to have a 12000 rpm hand grenade limit on a motor if the torque curve starts its descent at 2800 rpm or in one case 2000 rpm.

MOSFETs are the darling of motor controllers. They are inexpensive, can be paralleled, and have incredibly low forward resistance when conducting. The problem is voltage. 200 volt MOSFETs are a relatively recent development, and yet pricey. Put those in a 96 volt environment and you have a lot of headroom for voltage spikes and transients. Put the same device in a 144 volt system and you can often blow them up just turning them on. 

Further, forklifts and golf carts have driven the development of these components and so the low voltages they can easily deal with at the low speeds they by definition travel, the volume production has served those markets. EV conversions essentially isn't a market at all - at least up to now. Ergo the $200 devices.

At 80 miles an hour, a 2200 lb Porsche is eating 540 battery amps at about 112 volts - 60 kW or 81 hp. At 50 mph, it's doting along at 110 amps. Five times the power for 30 mph.

It LOOKS aerodynamic, but the real numbers are not that good on a Porsche Speedster convertible. It makes a lot of drag.

Jack Rickard
http://evtv.me


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## green caveman (Oct 2, 2009)

Thanks for a great, simple explanation.



jrickard said:


> Virtually all ICE engine cars develop their peak horsepower at relatively high RPMS - typically 3600 rpm. In the case of the Porsche, or an MG, this is even higher 4600 rpm. So the gear sets are optimized for this.
> 
> Electric motors are quite different. This is why the initial acceleration is so exhilerating. At very low RPMs, where an ICE engine is almost useless, the electric motor makes its maximum torque.


At some point I would have thought that gearing up would have been a more cost effective solution.

There is a considerable, non-linear, cost to move from 48V to 144V (or even 72V to 144V), yet 48V would seem to deliver enough power to drive, say, a geo metro IF you could gear it up to increase the top speed. Metros are 4000RPM at 70MPH, add a 1:2 gear and now you're at 2000RPM for 70MPH, closer to the sweet spot of the motor. I would have thought that a gear box could have been introduced for less than the cost of the increased voltage, but I haven't seen much of this.


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