# Motor Rating Question



## Beyonder (Sep 18, 2008)

I have some confusion here I need to clear up so hopefully someone can explain this for me :

The HP rating on the motors. For example I was looking at a 5HP motor ( not for an EV ), however the rating is:

5HP, 208/230VAC, 20.0 Full Load Amps, 3,450 Max RPM, 5/8 Shaft, 55LBS.

Here's the confusion, I read on here ( don't know where ) thatthe estimated torque of an Electric Motor is around its weight? If this is so, then using the formula Torque X RPM / 5252 = HP, the output should be 36.13HP at 3450 RPM...not the 5 its rated at?
The other half of my confusion here is ASHRAE refers to 1 HP as 550 Ft/lbs of torque and 745.7 Watts ( in the 2001 Handbook, Chapter 37 ). If this is the case, then 550 (Torque) X 3450 (RPM) / 5252 = 361.29 HP.

From what I've read this is more or less wrong, so the question is why and how? Is this maximum potential for this particular motor?


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## Georgia Tech (Dec 5, 2008)

No when we meant about weight, it is usually the heavier motors have more torque per amp as appossed to the lighter ones...thats all we ment


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## Georgia Tech (Dec 5, 2008)

Beyonder said:


> I have some confusion here I need to clear up so hopefully someone can explain this for me :
> 
> The HP rating on the motors. For example I was looking at a 5HP motor ( not for an EV ), however the rating is:
> 
> ...


It takes 1 horse power to move 550lbs 1 foot in one second.
Power is the change in energy divided by time. 
Now look at it from the electrical side the current is 20 amps and the volt is 230 volts...now Volts times amps equals power. 230volt X 20 amps makes 4600 watts. now estimate about 90% Eff. so take .90 X 4600 = 4140. Now it takes 746 watts to make one horspower from the electrical side.....4140/746 is 5.5 horsepower. 5.5 I might have been generous on the efficency maybe closer to 85% if its a 2 phase and closer to 90% if its a 3 phase...


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## Beyonder (Sep 18, 2008)

Georgia Tech said:


> No when we meant about weight, it is usually the heavier motors have more torque per amp as appossed to the lighter ones...thats all we ment


ok, that makes a little more sense. So, are you saying that the ASHRAE book is correct and that 1HP is 550 Ft Lbs & 745.7 Watts?

If this is the case then the second bit of math there says the 5HP motor should really be 361.29HP output? This is where I am having a problem. Why are they rating it for 5HP with a max 20 AMPS when the Torque X RMP / 5252 says it should be a lot more?
If this is correct and the output is that much more, is this divided by vehicle weight ( E.G. 361.29 / (4400 LBS / 550) = 45 HP produced in the vehicle)?


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## Georgia Tech (Dec 5, 2008)

Beyonder said:


> ok, that makes a little more sense. So, are you saying that the ASHRAE book is correct and that 1HP is 550 Ft Lbs & 745.7 Watts?
> 
> If this is the case then the second bit of math there says the 5HP motor should really be 361.29HP output? This is where I am having a problem. Why are they rating it for 5HP with a max 20 AMPS when the Torque X RMP / 5252 says it should be a lot more?
> If this is correct and the output is that much more, is this divided by vehicle weight ( E.G. 361.29 / (4400 LBS / 550) = 45 HP produced in the vehicle)?


 Check out my previuos response


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## Beyonder (Sep 18, 2008)

Georgia Tech said:


> Check out my previuos response


i posted at the same time as you..lol, reading your previous responce now


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## Beyonder (Sep 18, 2008)

Georgia Tech said:


> It takes 1 horse power to move 550lbs 1 foot in one second.
> Power is the change in energy divided by time.
> Now look at it from the electrical side the current is 20 amps and the volt is 230 volts...now Volts times amps equals power. 230volt X 20 amps makes 4600 watts. now estimate about 90% Eff. so take .90 X 4600 = 4140. Now it takes 746 watts to make one horspower from the electrical side.....4140/746 is 5.5 horsepower. 5.5 I might have been generous on the efficency maybe closer to 85% if its a 2 phase and closer to 90% if its a 3 phase...


Makes sense, so if i were to attempt to calculate the torque from the motor i mentioned the equation : HP X 5252 / RPM should work? In this case its 5.5HP X 5252 / 3450 = 8.37 for Torque. ?


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## Georgia Tech (Dec 5, 2008)

Beyonder said:


> Makes sense, so if i were to attempt to calculate the torque from the motor i mentioned the equation : HP X 5252 / RPM should work? In this case its 5.5HP X 5252 / 3450 = 8.37 for Torque. ?


yep thats it....Notice if you were to buy the 1750 RPM version of this motor, take a guess as to what this particular motor would look like compared to this one you have at 3450 RPM ......


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## Beyonder (Sep 18, 2008)

Georgia Tech said:


> yep thats it....Notice if you were to buy the 1750 RPM version of this motor, take a guess as to what this particular motor would look like compared to this one you have at 3450 RPM ......


 Ok, that clears up the confusion  thx.
Are you refering to the numbers of the physical size? The size for the 1800 RPM one is 115 LBS. Frame size is 184T compared to 56H for the 3450 RPM model ( not sure what the means exactly ), and its 369.99 compred to 299.00 for the 3450 RPM model.

So why would it weigh more? Thicker windings equalling more torque?


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## Georgia Tech (Dec 5, 2008)

Beyonder said:


> Ok, that clears up the confusion  thx.
> Are you refering to the numbers of the physical size? The size for the 1800 RPM one is 115 LBS. Frame size is 184T compared to 56H for the 3450 RPM model ( not sure what the means exactly ), and its 369.99 compred to 299.00 for the 3450 RPM model.


EXACTLY!! the motor with more torque is bigger and more expensive!



Beyonder said:


> So why would it weigh more? Thicker windings equalling more torque?


Well not nessecary thicker windings, but more windings!! Also more Lamitation or more magnetic material which inturn makes a physically bigger motor.
Thicker windings would make the motor have a higher power rating actually for instance double the thicknes with the same number of turns would maybe double its continous power to 10 hp, but again to take the bigger wire you need a bigger motor!


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## Beyonder (Sep 18, 2008)

Alright i understand now, thx


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