# A simple RPM formula .

1344 Views 11 Replies 4 Participants Last post by  Bird of Prey
To find the formula for Horsepower is simple .... volts x amps =watts /746 = HP that is simple and easy . Now the hard one , what is the simple formula for the RPM's of a DC electric motor ? The RPM of a AC motor is... volts x hertz = unloaded RPM . But what is the RPM of a DC motor ? Loaded motor or unloaded motor . Just in case , in general you can see the poles in a DC motor . Most of our motors are 4 pole but they are painted , no tags , and over volted . The controllers are mainly 120 volts and 480 amps or 142 volts and 900 amps . The formula should work on any motor , from 165 MPH DC Plasma to my monster road course car to a scooter .
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#### galderdi

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On the surface I agree with the formula volts x amps =watts /746 = HP

However it requires further explanation otherwise inexperienced people will mis interpret it.
It needs a statement to say the volts and amps used as inputs to the formula need to be at the peak of the motor's power (generally somwhere mid rev range). The volts rise and the amps drop as the motor revs rise.

If there is a system that sees maximum amps and volts at the same instant then the simple formula would be accurate. But its not the case for most systems.

For example my maximum volts are 172v while my maximum amps are 1000amps. Applying the simple formula would indicate my motor is 230HP. This is not the case. The peak for my motor is actually 100HP because the amps drop off as the voltage rises.
I think a lot of people use the simple formula and then quote the HP making their system sound more impressive than it really is.

#### Bird of Prey

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#### kennybobby

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The simple formula for the RPM of any motor for which you know the power in horsepower (HP) and torque (TQ) in ft-lbs, RPM = HP x 5252 / TQ.

i don't know where you got your AC motor speed formula, but it is not correct.

#### Duncan

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Hi Galderdi

I think you are wrong

What I think will happen is that the current will stay the same (the controller is in charge) until the controller is 100% and then as the rpm rises the current will drop

So (making the numbers up) - but I think your 9 inch motor will be about like this

500 rpm - 1000 amps - 22 Volts (motor) - 22Kw
1000 rpm - 1000 amps - 44 Volts (motor) - 44Kw
1500 rpm - 1000 amps - 66 Volts (motor) - 66Kw
2000 rpm - 1000 amps - 88 Volts (motor) - 88Kw
3000 rpm - 1000 amps - 110 Volts (motor) - 110Kw
3500 rpm - 1000 amps - 132 Volts (motor) - 132Kw

4000 rpm - 1000 amps - 176 Volts (motor) - 176Kw --- Nope current has to drop
4000 rpm - 970 amps - 172 Volts (motor and battery) - 167 Kw

4500 rpm - 870 amps - 172 Volts (motor and battery) - 149 Kw
5000 rpm - 780 amps - 172 Volts (motor and battery) - 134 Kw

This assumes that your controller does not limit motor voltage and just keeps on going until it is 100% - and then stays there

You will also have to factor in sag - at 1000 amps there will be lots!

#### galderdi

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OK, thanks Duncan.

It's not what I am seeing though.

I generally leave the battery current setting on the controller at 500amps to protect the Chevy volt cells. I had read somewhere that this is the factory limit for those cells. So maybe that is why my controller is forced to juggle the amps and volts. It can only work within the 86KW the batteries are providing. If I were game to turn the battery amps up I might get closer to your numbers.......If only I knew what the volt cells can really handle. Time for some more research.

#### Duncan

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Hi Galderdi

Adding your 500 amp battery limit - this is what I think it will look like

500 rpm - 1000 amps - 22 Volts (motor) - 22Kw - 127 Amps battery
1000 rpm - 1000 amps - 44 Volts (motor) - 44Kw - 254 Amps battery
1500 rpm - 1000 amps - 66 Volts (motor) - 66Kw - 383 Amps battery
2000 rpm - 1000 amps - 88 Volts (motor) - 88Kw - 508 Amps battery - so now limits
3000 rpm - 880 amps - 100 Volts (motor) - 88Kw- 508 Amps battery
3500 rpm - 800 amps - 110 Volts (motor) - 88Kw- 508 Amps battery
4000 rpm - 650 amps - 135 Volts (motor) - 88Kw -508 Amps battery-
4500 rpm - 508 amps - 172 Volts (motor and battery) - 88Kw- 508 Amps battery
5000 rpm - 400 amps - 172 Volts (motor and battery) - 68Kw - 400 Amps battery

My theory was that I didn't need to limit battery Amps as I would only be there for a couple of seconds - and besides my controller did not have limiting battery amps as an option!!

I am still using the Chevy Volt pack splitter thing - which I have been told has a fuse in it

With your car you will only be using above the 500 amps at full power above about 2000rpm
(From my very rough guesses)

With my car at full throttle - set to 1200 amps

500 rpm - 343 Amps
1000 rpm - 686 Amps
1500 rpm - 1029 Amps
2000 rpm - 960 Amps
2500 rpm - 760 Amps
3000 rpm - 670 Amps
3500 rpm - 600 Amps
4000 rpm - 480 Amps
4500 rpm - 400 Amps - end of 1/8th mile - 9.2 seconds
5000 rpm - 350 Amps
5500 rpm - 300 Amps
6000 rpm - 250 Amps

So with a full power 1200 amps blast I'm only breaking the 500 amps from 1000 rpm to about 3700 rpm - maybe 6 seconds?

Cross fingers I don't seem to have hurt my cells so far

#### galderdi

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Yes that is quite close to what I get.

Any ideas what current you have actually been pulling from your batteries during the 6 seconds or so?

Have you had any noticable heat from the batteries after such a run?

Yes I had heard the packs have fuses between them.

The only information I can see is that they are limited from the factory at about 430 amps (but for what time???)

#### Duncan

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Hi Galderdi

I think that I'm pulling close to 1000 amps - but not for very long
I believe that it ramps up from below the 500 amps - up to the 1000 amps and then back down below the 500 - over the six seconds - probably less - above 900 amps for maybe a second?

I have not noticed any heat in the batteries - my controller gets warm - but that heat is transferred to the batteries

I suspect that they have the fuse to blow at 430 amps continous - that is about a 50% overload -

The fuse (and batteries) should be able to take about 300 amps continuous - and to do that with no real chance of blowing even in the Arizona dessert

I would have thought 430 amps continuous would have been about right

So maybe 830 amps for 1 minute?

I will let you know when/if I blow the fuse (or the batteries)

#### galderdi

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Oh my!!! You have spiked my interest.

Even if I keep it conservative at move the limit to say 650amps, that is about an extra 25KW. That will make a big difference for me. Combined with all the other improvements my new car should be a weapon......Till I crash into a wall.

#### Bird of Prey

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So my controller and battery's at max charge is at 139 volts and 850 amps , after stomping on it , my volts went to 130 volts . The most rpms I can get is 3500 rpm ? It can take (not using max voltage ) 144 volts and 900 amps at max . So my max unloaded rpm is 8,500 RPM maybe ? What is loaded , roughly ?

#### Bird of Prey

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tq(lbs ft)/5252x Hp =RPM , is that right ? If so , I am on target . I just need more motors for top speed under load right ?

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