# HP/NM - Volt/AMP DC question



## goglahey (Jul 8, 2009)

Hi

A while ago stumpled on a post on a racing site claiming that Hp=topspeed and NM=time between 0 and topspeedtime.

So more HP=faster_topspeed and more NM=fasterTime_between_0-topspeed.

So i was wondering if i had a 60hp~100vDC engine and lowered the voltage so it would match 20hp, would the NM at 20hp~XXvDC still be the same as 60hp~100vDC or will the NM drop with the Volt?

Also if Hp=Volt then will NM=AMP?

Regards 
Allan


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## major (Apr 4, 2008)

goglahey said:


> A while ago stumpled on a post on a racing site claiming that Hp=topspeed and NM=time between 0 and topspeedtime.
> 
> So more HP=faster_topspeed and more NM=fasterTime_between_0-topspeed.
> 
> ...


Hi Allan,

Hp=Volt....NOT. You do a good job at mixing units 

Top vehicle speed is dependent on the power it can deliver to the wheels. Period. The more power, the faster the top speed. I think few will argue that.

Acceleration is proportional to the torque at the vehicle wheels. As Newton put it, F = ma. So just use the rolling radius of the wheel to convert torque to force. And acceleration dictates the time required to change from one speed to another.

The same DC series motor will have a torque proportional to the current. It is not entirely linear, but always more torque for more current. And this is pretty much constant regardless of voltage. There is some difference due to rotational losses in the motor, but these are fairly minor.

The same DC series motor will have the RPM proportional to the applied voltage at a given load for much of its operating range. The higher the voltage, the higher the RPM.

The power output for that motor is the torque times the speed (RPM). The power input to that motor is voltage times current.

BTW, for my sake, please call an electric motor a motor, not an engine. Engines are those things which burn fuel and make stinky exhaust 

Hope that helps,

major


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## goglahey (Jul 8, 2009)

That cleared things up for me. I had i quite mixed up

Sorry about the engine bit, i meant motor 

Thanks 
Allan


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## goglahey (Jul 8, 2009)

Just a quick followup question. (sorry if im hard to get threw)

I looked at the agni 95 motor and the torque curve is almost the same at 12v as at 72v

The RPM increases with the volt by almost double from 12v to 72v.

So if we said the wheel size, and the given RPM at 12v makes the viechle have a top speed at 60mph and at 72v it would be 120mph.

Would the the 0-60mph be the same for 12v as 72v or would the increes of kw change this?

Thanks again
Allan


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## major (Apr 4, 2008)

goglahey said:


> I looked at the agni 95 motor and the torque curve is almost the same at 12v as at 72v


If you mean the torque current relationship, yes, this is practically independent of voltage and speed.



> The RPM increases with the volt by almost double from 12v to 72v.


I don't think so. For a particular torque (or current) load, the RPM increase will be proportional to the voltage ratio.



> So if we said the wheel size, and the given RPM at 12v makes the viechle have a top speed at 60mph and at 72v it would be 120mph.


I do not think that motor (or any motor) will propel the vehicle at 60 mph with 12 volts. The first thing you must do is calculate the power required for the condition of the vehicle. Then find that power value on the motor output curve. Then use the corresponding motor speed at that power point to figure the needed ratio for your wheel size. Do this and I think you will find the power required for 60 mph is well beyond the capability of a 12 volt motor.

Regards,

major


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## goglahey (Jul 8, 2009)

major said:


> I do not think that motor (or any motor) will propel the vehicle at 60 mph with 12 volts. The first thing you must do is calculate the power required for the condition of the vehicle. Then find that power value on the motor output curve. Then use the corresponding motor speed at that power point to figure the needed ratio for your wheel size. Do this and I think you will find the power required for 60 mph is well beyond the capability of a 12 volt motor.
> 
> Regards,
> 
> major


The 60mph was just a example, the only thing i wanted to know was if the 0 to what ever speed, took the same time with 12v as with 72v or if it is the kw that controlles 0 to whatever speed


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## major (Apr 4, 2008)

goglahey said:


> The 60mph was just a example, the only thing i wanted to know was if the 0 to what ever speed, took the same time with 12v as with 72v or if it is the kw that controlles 0 to whatever speed


It is all related as I tried to explain in post #2.


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