# HP / Torque = Watts for New Guys!



## Tesseract (Sep 27, 2008)

LithiumaniacsEVRacing said:


> Most new builders do not know how to calculate torque/ HP from and electric motor, here are two websites that will help:
> 
> http://www.ajdesigner.com/phphorsepower/horsepower_equation_rotating_torque.php
> 
> http://www.supercircuits.com/resources/tools/Volts-Watts-Amps-Converter


Useful calculators! I'm a sucker for online calculators... not that I'm lazy or anything... 

One caveat, however, is that unless you know the motor's efficiency, the amount of torque calculated from electrical power and RPM will be overestimated. This is because any inefficiency in a series motor will mainly show up as a loss of RPM. You can guesstimate efficiency to be around 85% over most of the motor's "normal" current range, but you should be aware that efficiency can drop to shockingly low values at higher amperages (Lost V = I * R). IIRC from data posted to NEDRA a couple years ago, the old version of the WarP-9 drops down to around 65% at 1600A! 

Why this happens: most of the inefficiency in a series motor comes from voltage drops (ie - resistance) across the brushes, commutator, and windings, and since RPM is proportional to voltage, any reduction in voltage will result in a reduction in RPM (at a given torque loading, of course). Thus the torque remains the same but the RPM drops because of inefficiency.


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## LithiumaniacsEVRacing (Oct 9, 2010)

Tesseract said:


> Useful calculators! I'm a sucker for online calculators... not that I'm lazy or anything...
> 
> One caveat, however, is that unless you know the motor's efficiency, the amount of torque calculated from electrical power and RPM will be overestimated. This is because any inefficiency in a series motor will mainly show up as a loss of RPM. You can guesstimate efficiency to be around 85% over most of the motor's "normal" current range, but you should be aware that efficiency can drop to shockingly low values at higher amperages (Lost V = I * R). IIRC from data posted to NEDRA a couple years ago, the old version of the WarP-9 drops down to around 65% at 1600A!
> 
> Why this happens: most of the inefficiency in a series motor comes from voltage drops (ie - resistance) across the brushes, commutator, and windings, and since RPM is proportional to voltage, any reduction in voltage will result in a reduction in RPM (at a given torque loading, of course). Thus the torque remains the same but the RPM drops because of inefficiency.


Jeff,

Thank you for your expertise. Even with a drop of 85% these numbers are very impressive!


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## Yabert (Feb 7, 2010)

Do you miss the Tess point?
Peak power and peak inefficiency are linked. You need to forget nominal efficiency in a drag race.

200v x 2000A = 400Kw x 70% (a guess for a modified Warp 11 at 2000A) = 280Kw x 2 motors = 560 Kw or 750 hp.

Well, not to bad after all!


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## LithiumaniacsEVRacing (Oct 9, 2010)

Another great tool:

http://www.quartermilecalc.com/

Example:

1000 HP Motor
2300 Lbs Car 

will give you,

9.11 1/4 mile @ 167.52 mph!
_______________________________

http://www.hotrodpitstop.com/tool.php

Example 2:

1000 HP
2300 Lbs Car

could give you,

7.6 1/4 mile @ 177.3 mph
______________________________

Now most of these racing calculators are design to use HP with the torque factor from an ICE. When working with a high torque/ less HP motor, you can get the same answer, but your car must be geared for low rpm. These numbers depend greatly on your cars gearing!


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## LithiumaniacsEVRacing (Oct 9, 2010)

Gearing your car is the MOST important part of racing.

http://www.richmondgear.com/101032.html

Example:

Rear End Ratio: 3.00
Overdrive Ratio: .7
Tire Diameter: 33"
Mph in 1/4: 170
___________________
You will end the 1/4 mile @ 3635 rpm.


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## major (Apr 4, 2008)

LithiumaniacsEVRacing said:


> At 3000 rpm
> 600,000 watts can produce 804 HP & 1408 LBS Torque


You need to use proper units for torque. To some it would appear you don't know what you're talking about and just sticking numbers up here to impress others.



> Now that ratio can change depending on how you are building your "watts", more amps = more torque, more volts = more HP.


This statement doesn't make sense. What ratio? Building your "watts"  "More volts = more HP"  Not necessarily.

I recommend new guys go to a reputable source to learn. The EVInformation http://www.diyelectriccar.com/forums/showthread.php?t=669 isn't perfect but is a good start.


Even the title for this thread is incorrect: *HP / Torque = Watts *


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## Tesseract (Sep 27, 2008)

major said:


> You need to use proper units for torque. To some it would appear you don't know what you're talking about and just sticking numbers up here to impress others.
> 
> 
> 
> ...


Tsk, major... you broke your rule and posted to a Ron thread...


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## major (Apr 4, 2008)

Tesseract said:


> Tsk, major... you broke your rule and posted to a Ron thread...


Yeah, bad physics bothers me more than Ron. BTW, thanks for quoting my post. Now I can't delete it


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## LithiumaniacsEVRacing (Oct 9, 2010)

major said:


> You need to use proper units for torque. To some it would appear you don't know what you're talking about and just sticking numbers up here to impress others.
> 
> 
> 
> ...


Mr. Motor Guru, thank you for your input.


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## LithiumaniacsEVRacing (Oct 9, 2010)

LithiumaniacsEVRacing said:


> Mr. Motor Guru, thank you for your input. So, the websites are not correct? My calculations are not correct?
> 
> 400 volts X 1500 amps = 600,000 watts
> 200 volts X 3000 amps = 600,000 watts
> ...


If your car is built with higher amps, than you will have more torque.
If your car is built with higher volts, than you will have more HP.

You can build your car to have your maximum power on the bottom end or top end. THAT IS A FACT! And I have proven it on the race track.


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## LithiumaniacsEVRacing (Oct 9, 2010)

major said:


> Yeah, bad physics bothers me more than Ron. BTW, thanks for quoting my post. Now I can't delete it


Yea, and bad attitude bothers me. So, keep your personal comments to yourself, if you would like to handle yourself like a professional than I have no problem discussing this with you.

I will see you at Power of DC this year, so any bad feelings you have against me you can tell me in person.


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## major (Apr 4, 2008)

LithiumaniacsEVRacing said:


> Yea, and bad attitude bothers me. So, keep your personal comments to yourself, if you would like to handle yourself like a professional than I have no problem discussing this with you.
> 
> I will see you at Power of DC this year, so any bad feelings you have against me you can tell me in person.


Ron,

Do me a couple of favors please.

1) Request the admin to delete this thread or correct the title. The equation is blatantly wrong. And it goes on to be sexist, guys.

2) Should we ever be in the vicinity, please leave me alone. I do not wish to meet you and have nothing more to say to you.

Regards,

major


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## LithiumaniacsEVRacing (Oct 9, 2010)

Read this page and decide if I made any mistakes in my calculations:

http://www.diyelectriccar.com/forums/showthread.php?t=6535

*The page states* "Notice also that if you increase the pressure in the pipe (or voltage in the wire), the flow (current) will increase. Therefore, your power also increases. Put another way, if you increase the pressure (voltage), you can deliver more water (electricity) with a smaller pipe (wire). That's why electric power transmission over any distance is done at higher voltages, not at the much lower voltages typically seen by end users."
__________________________________________________________________________________________________________

*Shouldn't it read*: Notice also that if you increase the flow in the pipe (current/torque) will increase. Therefore, your power also increases. Put another way, if you increase the pressure (voltage/HP), your power also increases.


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## electrabishi (Mar 11, 2008)

LithiumaniacsEVRacing said:


> Mr. Motor Guru, thank you for your input. So, the websites are not correct? My calculations are not correct?
> 
> 400 volts X 1500 amps = 600,000 watts (_*MORE TOP END HP)*
> _ 200 volts X 3000 amps = 600,000 watts (_*MORE BOTTOM END TORQUE)*_
> ...



Ron,
a little constructive criticism here, not trying to start any flame wars per our agreement. And in the interest of keeping this civil I'll try to explain what Major is talking about.

First the title of this thread is HP/Torque = Watts. This is a totally false statement and misleading to any new comers to which you are trying to appeal. HP and Watts are the same thing, they are both a measure of Power. The equation for Torque is:

T=(5252xHP)/RPM

.. or using common algebraic methods you can rearrange the formula to look like this:

HP=(TxRPM)/5252

This is truly the correct formula. Whatever website you got the other formula for the title of this thread is wrong and you should have words with them for publishing falsities.

Second it was mentioned to you about using the correct units. Torque is measured in ft-lbs or newton-meters. Not pounds or newtons, which is a common unit of force. One pound = 4.448 Newtons, of force. Torque is the measure of a force around a rotating center and a specific distance from that center. If I apply 1 pound of force with a lever arm or gear or sprocket a distance of 1 foot from the center, I have 1 ft-lb. I can apply 2 lbs at 6" from the center and still have 1 ft-lb. Or I can apply 1/2 lb at 2 ft from the center and still have 1 ft-lb. The amount of force in lbs you quote is irrelevant unless you also state in context what the distance from the center the force is acting. Saying you have 1800 lbs of torque is meaningless without many other details filled in. 1800 lbs of force acting around an 8" ring gear *could* give you 1200 lbs of accelerating force. Or 1800 lbs of force acting around a 10.25" ring gear *could* give you 1537.5 lbs of accelerating force. 1800 lbs pushing around a 3" chain sprocket will only give you 450 lbs of accelerating force.

And finally in your quote above is not necessarily true unless you qualify a few assumptions:

You say:
400 volts X 1500 amps = 600,000 watts (_*MORE TOP END HP)*
_ 200 volts X 3000 amps = 600,000 watts (_*MORE BOTTOM END TORQUE)*_

These are only true if you can maintain those voltage and current numbers on the motor itself.

And so if you carry your logic to the next level the problem will become obvious. Lets say you go to an extreme and build a battery pack that is

800 volts X 750 amps = still 600,000 watts ... or better yet
1600 volts X 375 amps = and yet still 600,000 watts

In any case you can imagine if you could actually see those numbers without limit from your controller on the motor, then you *may* see the power that is estimated.

But lets consider the 800V pack. You may well someday find a DC motor controller that will handle the 800V input and can put out the battery packs 750 amps, but a series wound DC motor will not handle that commutator voltage. So consider what your controller does to the battery pack parameters before you call those two packs equal. The packs may well be equal, but if the controller has to limit the commutator voltage to ~200 or ~300V or so and the pack is still putting out rated current. Then instead of 800V x 750A to your motor which you think is 600,000 watts, you'll in fact only have 200V X 750A into your motor which is only 150,000 Watts (or approximately 150 HP to the ground)

You are correct that more current through the motors will give you more torque. But you have to look at the current through the motors, and not the current from the pack. The only time those two numbers will be equal is if your controller is operating at 100% duty cycle. And if your pack sag voltage is higher than the allowable communtator voltage on your motor(s) then your controller will never reach 100% duty cycle.

Without making any comment on your quote copied above, as to whether it is true or not (one cannot make a statement to that effect without knowing the motor and controller parameters) I would say that for a newbie to believe that it is accurate would be misleading at best. 

Ron, you are on this list with some very intelligent people. I am not claiming to be one of them. And these topics have been discussed on this list many times and for many years before you got here. Some members have actually taken data provided by racers, with all the specifics needed to develop accurate models of the motors used. It is a fact that at least in the case of the older WarP9 motors that the efficiency is well down in the 60% to 65% range at rated comm voltage and rated controller current (for the specific case of a WarP 9 and Zilla controller). If you are running a different controller and a newer version of a WarP motor and actually want to know what your motors are doing, then I'd suggest asking what the "motor guru's" need to do the analysis for you. If you really wanted to help the community, this is information that everyone here wants to know and would be grateful to you if you could provide. Its not the comic book version of the formulas or the wild assumptions you are throwing out in the guise of helping the newbies that folks here appreciate. If the new guys use the assumptions you are stating here they may be very disappointed when their 600,000 watts really only turns out to be 250,000 watts or so.

Mike


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## LithiumaniacsEVRacing (Oct 9, 2010)

electrabishi said:


> Ron,
> a little constructive criticism here, not trying to start any flame wars per our agreement. And in the interest of keeping this civil I'll try to explain what Major is talking about.
> 
> First the title of this thread is HP/Torque = Watts. This is a totally false statement and misleading to any new comers to which you are trying to appeal. HP and Watts are the same thing, they are both a measure of Power. The equation for Torque is:
> ...


Mike,

I thank you for taking the time to explain where my calculations went wrong. Remember I have been an ICE technician my entire life since 12 years old, I am still learning the difference in what you just explained. 

Your post will explain and create a good example of my mistakes in trying to use tools from ICE into numbers for electric motors. I can build a race car with my eyes closed, but still learning how to measure amps/ volts/ watts, etc.....I started this post looking for additional input, not trying to be Mr. Know it all.

I know my gearing was correct. 

Look forward to meeting you sometime in our 2012 Racing Season, and congratulations on your ASO status, I know it's not cheap.


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## EVfun (Mar 14, 2010)

LithiumaniacsEVRacing said:


> If your car is built with higher amps, than you will have more torque.
> If your car is built with higher volts, than you will have more HP.


More amps or more voltage will result in higher horsepower (well, until the commutator flashes over.) The formula HP = rpm * ft-lb / 5252 makes it clear why this is true.

More amps, all else being equal, will result in more torque.
More voltage, all else being equal, will result in a higher rpm.


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## electrabishi (Mar 11, 2008)

LithiumaniacsEVRacing said:


> If your car is built with higher amps, than you will have more torque.
> If your car is built with higher volts, than you will have more HP.





EVfun said:


> More amps or more voltage will result in higher horsepower (well, until the commutator flashes over.) The formula HP = rpm * ft-lb / 5252 makes it clear why this is true.
> 
> More amps, all else being equal, will result in more torque.
> More voltage, all else being equal, will result in a higher rpm.



I think the important thing to keep clear here is that we are talking about voltage and current through the motor, and not the battery pack. You can put a 360V and 4000 amp battery pack on a Zilla 2K, but at that input voltage the controller will never put out any more than 1600 amps to the motor. In fact if you are running a single motor you should never ever see 360V on the motor commutator either.

So that makes a 360v x 4000A = 1440KW pack only useful for 180V x 1600A = 288KW. In this case you cannot get the batteries power capability through the controller and so you are carrying extra power capacity that you cannot use. The trick is to find out the maximum power the controller can possibly provide, then design the pack to put out just that power under the particular voltage sag and current output conditions that you expect. And of course the Gearing comes into play because the voltage and current curves are going to depend on the RPM of the motor. So really its hard to just change one thing and keep all the other things equal.


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## LithiumaniacsEVRacing (Oct 9, 2010)

electrabishi said:


> I think the important thing to keep clear here is that we are talking about voltage and current through the motor, and not the battery pack. You can put a 360V and 4000 amp battery pack on a Zilla 2K, but at that input voltage the controller will never put out any more than 1600 amps to the motor. In fact if you are running a single motor you should never ever see 360V on the motor commutator either.
> 
> So that makes a 360v x 4000A = 1440KW pack only useful for 180V x 1600A = 288KW. In this case you cannot get the batteries power capability through the controller and so you are carrying extra power capacity that you cannot use. The trick is to find out the maximum power the controller can possibly provide, then design the pack to put out just that power under the particular voltage sag and current output conditions that you expect. And of course the Gearing comes into play because the voltage and current curves are going to depend on the RPM of the motor. So really its hard to just change one thing and keep all the other things equal.


When building a drag car or road course car, your power band must match your gearing, that is how you win races! 

In regards to designing a pack that provides extra power you will not use, is a good topic. You want your pack to supply enough amps and voltage with very little sag, so over building your pack (a little) has it advantages. The motor is also a huge factor, most motors cannot handle high voltages along with high amps. The technology is getting better and soon motors will be able to handle everything these new controllers can throw at them. Now, it's a balancing act with the controller's power settings. 

One example is my last drag car, we raced all season with 220 volts or less, this proves gearing your car correctly you can use the torque factor to win races! 10.08 in the 1/4 @ 128 mph ain't too shabby!


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