# Voltage and Power Relationship in Given Application



## major (Apr 4, 2008)

Hi ElvishWarrior,

WRT D&D ES-31B, see http://www.cloudelectric.com/inc/sdetail/169 

The 96 volt continuous rating is 12 hp. So at 72 volts, probably about 9 hp.

A lot of factors enter into motor ratings. But a large part is armature current. For series motors, current equals torque. A motor rated for 100 amps at 72 volts, is likely to have a 100 amp rating at 96. At the torque you get at 100 amps. The motor on 96 volts will have higher RPM than on 72 volts, so a higher power.

As to SepEx vs Series........Depends on what you need to do and available contollers. No clear choice as to one size fits all solution.

Regards,

major


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## TheSGC (Nov 15, 2007)

The lowest voltage you would want to reach 55 to 60 is 96 Volts. There is absolutely no way 72 volts will get you past 45 MPH unless it's a VW Bug, and that's pushing your luck. My Civic is going to be 96 volts with a realistc top speed of 60 MPH using a homemade controller. When I first started researching my project, I had a 72 volt system in mind, but further calculations should it just didn't have enough volts to get a mild 2500 LBs of Civic moving past 45 MPH. 

Lower voltages means less efficiency, and less range. Not to mention pulling a pickup at 72 volts might have some pretty poor acceleration making you wish you paid the extra $300 for the higher controller.


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## ElvishWarrior (Apr 10, 2008)

Those are definitely straight answers, but I'm still having trouble computing as to why the max sustainable power wouldn't be almost identical. Am I wrong that heating is what determines max sustainable power? Say you demanded maximum power consistently. Would it burn up, or just not do it?

That's sort of what I'm going for with the voltage. The max power would certainly be less, but would you risk damaging the motor if you ran it at its sustainable power at a lower voltage, for reasons other than maybe 5% less efficiency?

I'm almost on board with the idea that a given speed would require a given minimum voltage, but I've seen a number of motors listed for sale on E-bay as 30+ hp at 72V. (Like these two http://cgi.ebay.com/ws/eBayISAPI.dl...item=350047970517&_trksid=p3984.cWAT.m240.lVI and http://cgi.ebay.com/ws/eBayISAPI.dl...item=230243430518&_trksid=p3984.cWAT.m240.lVI)
Would my proposed project be more realistic using say a Warp 9, or one of these high power low voltage rated forklift or other motors?

As far as controller and battery issues, it's not about the cost difference in controllers as much as the choices, and with batteries it's certainly more about space and weight than about cost. A limited range is completely ok, 10 miles or even a little less will do it for me, although more is always better.

While a little off topic, I would like to go Sepex if at all possible for regenerative braking, but finding a motor has proven near impossible. Multiple motors are also a possibility if it enables use of reduced voltage. But somehow the idea of using twin Warp 9s at 72V seems like heresy. Using 2 or 3 PM motors has crossed my mind but the transmission losses from gearing them together would probably be severe. This motor would seem like a good choice to use in a dual motor setup, wiring the motors together in parallel, except that it doesn't seem to have a double shaft option.


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## frodus (Apr 12, 2008)

As far as power goes, volts is RPM, torque is current.

HP =









Where T = Torque (lbft), N = Speed (rpm)

So if you lower the volts, you lower the RPM and since N is speed, you would DECREASE the horsepower. Now, you COULD apply MORE current, and get the same HP out of it, but you need to look at the torque curve for the motors you're looking at. Also, motors can only handle so many amps.



As far as Sepex Both D&D and Advanced DC make sepex motors, but MOST of what they sell to the EV community are series because there are a wider voltage range of controllers, while sepex you're limited to just a couple manufacturers of controller. If you wanted to go above 84V, your choices are almost nill. Call both companies and tell them what you want, D&D has a little form you fill out, and their engineers get back to you pretty quick with a set of choices and cost.

Regen is great, but you might be better off throwing more lead in there, than you would be paying extra for a controller/motor combo that would match the size of vehicle you're driving.

I wouldn't go below 120V. thats only 10 batteries. it'l give you the short range you need, and also allow for highway speeds. I just don't see you getting to 55 with a motor that just doesn't have the ooomph.

There's no definitive answer on what controller/motor/battery combo you need, because with batteries are variables, same with controller and motor. Best thing to do is figure out how much horsepower you need to cruise along at 55mph. Then factor in distance. THEN you can play with motor numbers and see what one matches you the best (you can look at sepex, series at this point). Then, once you figure that out, figure out if there's a controller for it, and then finally, how many batteries you need. Work backwards.

do a search for "uve's electric vehicle calculator", that helped me a ton.


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## MrCrabs (Mar 7, 2008)

ElvishWarrior said:


> Those are definitely straight answers, but I'm still having trouble computing as to why the max sustainable power wouldn't be almost identical. Am I wrong that heating is what determines max sustainable power? Say you demanded maximum power consistently. Would it burn up, or just not do it?
> 
> That's sort of what I'm going for with the voltage. The max power would certainly be less, but would you risk damaging the motor if you ran it at its sustainable power at a lower voltage, for reasons other than maybe 5% less efficiency?



Motors have ratings for both voltage and current. They can withstand so much voltage before you risk arcing, this is where brush advance comes into play on series dc motors. You advance the brush timing to deal with a higher voltage, but reduce efficiency slightly. 
Motors can only withstand so much current before overheating. A motor can withstand X amps of current forever, assuming it is spinning fast enough for the internal fan to cool it off. They can also withstand Y amps for a certain amount of time before overheating.

When you apply a voltage to a motor, it starts to draw current, depending on the load applied to the motor. Then the motor starts turning, when the motor turns it generates a back voltage, or back emf. This reduces the current that flows thru the motor. To get it to spin faster you need to increase the voltage, then it will speed up, generating more back emf. Then you need to increase the voltage.....etc until you run out of battery pack voltage.

Think of a series motor controller like a simple switch. They connect and disconnect the battery pack from the motor very quickly, around 15000 times per second.
When starting out from a dead stop, the controller turns the motor on for a very short time, as the motor speeds up, the controller leaves the motor turned on for longer. Eventually you reach a point where the controller leaves the motor on, and the motor speed is now limited by the battery.


So the short answer is you can't run a motor at its peak power with a lower voltage, because you don't have enough voltage to get the peak current thru the motor.

And here is a link to the EV Calculator. http://www.evconvert.com/tools/evcalc/

When I enter the details for my potential conversion, my max speed in any gear is battery voltage limited (except first gear)

From your first ebay link: 72-120VDC, double shaft, *38.7 hp continuous at 120 vdc
*and your second ebay link it a 97 vdc motor, which is rated at 37hp, but at 72 vdc its only good for 27


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## 3dplane (Feb 27, 2008)

Hi Elvish!
May I add one more thing to consider regarding your power/heating question.
First off I think motors should be rated in Watts,but not exceeding certain amount of Amps and Volts.(then a burst rating).
If you sustain max rated power at a lower voltage that also means the amps are high(to sustain the max power)=(over)heating.Depending on the situation we really want that motor to spin at a happy rpm rather then bogged down and over amped. Barna.


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## ElvishWarrior (Apr 10, 2008)

I greatly appreciate your thorough explanation. There's one area that I'm trying to zoom in on for closure - Max sustainable power, as opposed to max power.

I'm completely on the same page that you can't get max power without max voltage. I'm familiar with how a series controller works, basically interrupting the voltage. What I'm getting at is what would keep you from getting the same sustainable power out of a motor at a lower voltage, besides efficiency? The difference as I've seen it is what the max power would be. The way I see it, with a lower voltage the max power would be closer to the max sustainable power. Ie you could still average out the same, but you wouldn't have the acceleration for short bursts. Even though a higher voltage would push more amps through, wouldn't it even out between a high voltage high current that's interrupted and a lower voltage, lower current that's less interrupted? I do understand that there'd be a loss in max output and that this would have an affect on max speed. But let's say that the voltage was low enough that the max power was the same as the rated sustainable power. Would it actually harm the motor to run it at that level, or would it just leave one lacking the performance they need?

Thanks very much for your patience with a fellow who should know this stuff from engineering school but still has a ways to go to understand the specifics of these motors.


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## TheSGC (Nov 15, 2007)

If you use less volts to get the same power (by using more AMPs) leads to shorter motor life span. AMPs create heat, no matter what the voltage is, if you drive the motor passed its rated AMPS, it will self destruct. 

So essentially motor are limited by AMPs more than watts or HP, hence the lower HP at lower volts.


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## ww321q (Mar 28, 2008)

Maybe take this as a question because I'm not sure . Isn't horsepower related to rpm , the higher the rpm the higher the horsepower for a given motor ? And the higher the voltage the higher the rpm . Also at higher voltage you advance the brushes for more power at higher rpm ? Saying that , I don't see the importance of getting the exact number ? After looking at almost all if not all the specs people put down on the EV Album and every where else I've done research , I still see similar car/motor/battery/controller setups with way different results .Almost like no mater what you think or speculate what will happen it's still a bit of a shot in the dark . Maybe because of other things we don't think of as much . Tires/ Third member ratio/front end alignment/driving style/ terrain/Temperature /ect...................J.W.


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## frodus (Apr 12, 2008)

ww321q said:


> Maybe take this as a question because I'm not sure . Isn't horsepower related to rpm , the higher the rpm the higher the horsepower for a given motor ? And the higher the voltage the higher the rpm . Also at higher voltage you advance the brushes for more power at higher rpm ? Saying that , I don't see the importance of getting the exact number ? After looking at almost all if not all the specs people put down on the EV Album and every where else I've done research , I still see similar car/motor/battery/controller setups with way different results .Almost like no mater what you think or speculate what will happen it's still a bit of a shot in the dark . Maybe because of other things we don't think of as much . Tires/ Third member ratio/front end alignment/driving style/ terrain/Temperature /ect...................J.W.


reread all of post #5


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## ww321q (Mar 28, 2008)

frodus said:


> reread all of post #5


And? ..................J.W.


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## frodus (Apr 12, 2008)

ww321q said:


> And? ..................J.W.


oh, it was an explanation of the relationship between RPM, torque and horsepower 

But yeah, its best to keep the motor in its happy spot, you'll like the efficiency better, like the performance, and get much longer life out of it, because its not going to be SUCKING the amps just to drive under normal conditions.


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## ww321q (Mar 28, 2008)

I don't see the importance of getting the exact number ? After looking at almost all if not all the specs people put down on the EV Album and every where else I've done research , I still see similar car/motor/battery/controller setups with way different results .Almost like no mater what you think or speculate what will happen it's still a bit of a shot in the dark . Maybe because of other things we don't think of as much . Tires/ Third member ratio/front end alignment/driving style/ terrain/Temperature /ect..

that was the important part . not the part about calculating horse power
...........................J.W.


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## frodus (Apr 12, 2008)

ww321q said:


> I don't see the importance of getting the exact number ? After looking at almost all if not all the specs people put down on the EV Album and every where else I've done research , I still see similar car/motor/battery/controller setups with way different results .Almost like no mater what you think or speculate what will happen it's still a bit of a shot in the dark . Maybe because of other things we don't think of as much . Tires/ Third member ratio/front end alignment/driving style/ terrain/Temperature /ect..
> 
> that was the important part . not the part about calculating horse power
> ...........................J.W.


at this point alot of it is ballpark guestimate. There aren't tons of people putting a huge Advanced DC motor in a motorcycle, and the ones that did, are using all sorts of controllers, battery setups. range is all over the place. I just kinda decided to throw 144V at it (48-96V motor), limit the controller to only put out 72V max (I'm getting a proto of a kickass controller), and run it in its sweet spot.

Basically, kWh is what matters, both on the battery side, and the kW is what matters on the motor side. If you draw 500 kW/ mile, and you have a 10000 kWh pack, you can go 20 miles. Inside the controller, you may not at ANY time be actually giving the motor full pack voltage, even under full throttle. The answer is yes you can run the motor at lower voltages, just realize, performance, speed, horsepower all drop to a lower level.

Even with more current, you will never get higher speeds (unless you do some field weakening on the field windings of a series motor.... but thats another things all together).


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## ElvishWarrior (Apr 10, 2008)

TheSGC said:


> If you use less volts to get the same power (by using more AMPs) leads to shorter motor life span. AMPs create heat, no matter what the voltage is, if you drive the motor passed its rated AMPS, it will self destruct.
> 
> So essentially motor are limited by AMPs more than watts or HP, hence the lower HP at lower volts.


Ok, here's the thing I'm looking for a true or false on:
Doesn't the higher voltage drive higher amps through the motor, and that's where the higher power comes from? And thus wouldn't you run the same average current whether you run a high voltage with high interruption or a lower voltage with less interruption?

Or am I making assumptions that would relate to stall conditions? Does the generated EMF of a spinning motor change the dynamics such that a lower applied voltage would generate tons of heat for a minor voltage actually applied to the motor after subtracting generated EMF? That's what I was getting at when referring to efficiency. Was my 5% estimated difference way off, and the real number more like 20 or 30%, which is where the motor would give less power by creating less heat?


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## frodus (Apr 12, 2008)

ElvishWarrior said:


> Ok, here's the thing I'm looking for a true or false on:
> Doesn't the higher voltage drive higher amps through the motor, and that's where the higher power comes from? And thus wouldn't you run the same average current whether you run a high voltage with high interruption or a lower voltage with less interruption?
> 
> Or am I making assumptions that would relate to stall conditions? Does the generated EMF of a spinning motor change the dynamics such that a lower applied voltage would generate tons of heat for a minor voltage actually applied to the motor after subtracting generated EMF? That's what I was getting at when referring to efficiency. Was my 5% estimated difference way off, and the real number more like 20 or 30%, which is where the motor would give less power by creating less heat?


look at a torque curve, RPM, volts and amps aren't neccessarily related the same.


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## MrCrabs (Mar 7, 2008)

Here is a chart showing Torque, amps, eff, and rpm for 3 different voltages.

http://www.geocities.com/hempev/X91torque.gif

It shows 1 trace for current vs torque, because that is voltage independent. The RPM and HP depend on the voltage.
It shows that for any given HP, running at a higher voltage puts the motor closer to its peak eff. and for some HP, you need the higher voltage to get there.


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## ElvishWarrior (Apr 10, 2008)

If the 75% efficiency is correct, then that would answer my question on the effect of trying to get the max continuous power at lower voltage. Figure if the efficiency goes from 90% to 80%, that's twice as much energy that's turning into heat.

Thanks all for the very helpful information!


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## Dennis (Feb 25, 2008)

> So the short answer is you can't run a motor at its peak power with a lower voltage, because you don't have enough voltage to get the peak current thru the motor.
> 
> And here is a link to the EV Calculator. http://www.evconvert.com/tools/evcalc/
> 
> ...


All DC motors will have a peak power when loaded down to that level. It's just that with lower voltage the peak power will be a smaller value than with a higher voltage. Its not really a good idea to run a DC motor at peak power for long periods as it will be 50% efficient at peak power and thus you can get a very good idea of just how hot it will get. 




To everyone else: 

Why not use the formula ((E*I)/746)*efficiency. It's more useful for electric motors than the mechanical formula of (RPM*Torque)/5252......

Remember an electric motor's graph is a LOAD curve not a REV curve like that of a gasoline engine. So you have to keep in mind torque and horespower will be load dependent. Simply revving a DC electric motor to some X rpm will not follow what the graphs say.


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## major (Apr 4, 2008)

[/quote]

When I calculate efficiency from those points, I get 66.8 percent.

Regads,

major


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## major (Apr 4, 2008)

ElvishWarrior said:


> Those are definitely straight answers, but I'm still having trouble computing as to why the max sustainable power wouldn't be almost identical. Am I wrong that heating is what determines max sustainable power? .


Hi Elvish,

You're right, sort of. First off some motor terminology. Let's use continuous rated power. Where you can run the motor for days on end and it will not exceed rated temperature.

If you have a totally enclosed non-ventilated motor (TENV), then all the losses must be radiated by the outside surface to the ambient. So the looses determine the rating. Within a reasonable range of voltage and speed, the efficiency remains fairly constant, so increasing voltage and speed will not alter the continuous rated power. Just the higher speed will have less torque for the same rated power.

But, most motors used by EVers are ventilated fan cooled. So the higher voltage means higher speed and more air flow. So as one increases voltage and therefore RPM, the continuous rated power does increase. Within reasonable range, the torque or current rating stays about the same for ventilated fan cooled motors.

Hope this helps,

major


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## frodus (Apr 12, 2008)

major said:


> When I calculate efficiency from those points, I get 66.8 percent.
> 
> Regads,
> 
> major


Sorry, That was a borrowed image, I didn't create it, but it showed how to get some of the values off the curve.


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## ElvishWarrior (Apr 10, 2008)

Anyone know what one could expect from this motor here?
http://cgi.ebay.com/ws/eBayISAPI.dl...item=150239955263&_trksid=p3984.cWAT.m240.lVI

Listed at 28V and 1100A. Power conversion comes out to about 40hp max - efficiency.


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## frodus (Apr 12, 2008)

ElvishWarrior said:


> Anyone know what one could expect from this motor here?
> http://cgi.ebay.com/ws/eBayISAPI.dl...item=150239955263&_trksid=p3984.cWAT.m240.lVI
> 
> Listed at 28V and 1100A. Power conversion comes out to about 40hp max - efficiency.


also says 30volts max... so, I don't think that'd work, unless you're using a 28V system or something.


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## ElvishWarrior (Apr 10, 2008)

I would use whatever voltage would work. With that kind of power I suspect it would be useful for the lightest EV conversions at best, which wouldn't be what I'm considering at the moment. Or maybe a boat conversion?

Any theories on its tradeoffs, ie efficiency, torque, speed?


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## dataman19 (Oct 7, 2009)

Newbie question...
Aren't some of these efficiency concerns handled by the Motor Controller.
...
Are they not configurable to maximize motor efficiency over a varied range of pack voltage?
..
dataman19


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