# motor rating questions. hp is hp?



## Duncan (Dec 8, 2008)

Hi Colin
The motor is a thing to convert electrical power to mechanical power
The more power in - the more power out
Unfortunately they are not 100% efficient - the waste heat is what limits the power

Motors for industrial use are rated as continual (one hour) power - at that rpm - they can run forever with the heat being dissipated by the cooling
If you increase the rpm you will increase the cooling and hence the continuous power

A car does not use continuous power, it may take 20Kw to maintain a speed of 60mph
but you may want 100kw to accelerate

You cant use 100kw for long - you will break the speed limits
(my car gets to the max permissible speed in this country in about 10 seconds)

A motor that can convert 10Kw at 2000rpm may be able to convert 20kw at 4000rpm continuously 

Such a motor may be able to convert 200kw for 30 seconds or a bit longer before it overheats

A Warp9 is a 9 inch forklift motor with advanced brush timing (for higher rpm) and a nice coat of paint

So the demon is in the motor controller - it is the device that limits the power

AC motors get a bit more complicated - they will eventually be the best but at present they are either wimpy or expensive - or both


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## colinrickards (Mar 6, 2012)

Duncan said:


> Hi Colin
> The motor is a thing to convert electrical power to mechanical power
> The more power in - the more power out
> Unfortunately they are not 100% efficient - the waste heat is what limits the power
> ...


Thanks for the quick reply. I've decided on a DC motor for my build. Those are the ones I've been looking at.
So if I've got this right, ignore the HP and look at how many volts I can do for rpm and amps I can give it for desired torque? Or do I want to look for a kw rating on a second hand motor?
Or rather, get whatever motor, look for one that fits in the car give or take and design my limits of continuous run and max kw/acceleration around the rating of the motor? Or am I way off?

On the topics of volts and amps, how to i know how much more i can push it if say it's rated at 36v 10a, or whatever it may be. Or is it all dependent on cooling?
I'll have to research advancing motors, not something I've looked into yet.

I know these seem like some real newbie questions, but this is one area I'm very green on.


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## Duncan (Dec 8, 2008)

Ok
If you are going to use a "second hand motor" you need to read the fork lift motors thread 
(its stuck to the top of the Electric Motors section)

well worth reading

Crudely - you need a motor with four pairs of brushes that will weigh more than 40Kg

It will be 24v - 48v - and you will be able to use 150v+
The controller controls the voltage to the motor so you can use a 200v battery on a 24v motor

Select motor based on the car
Light weight car - 9 inch diameter motor - 40Kg
medium - 11 inch motor - 100Kg
heavy - 13 inch motor - 140Kg


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## colinrickards (Mar 6, 2012)

Duncan said:


> Ok
> If you are going to use a "second hand motor" you need to read the fork lift motors thread
> (its stuck to the top of the Electric Motors section)
> 
> ...


OK cool thanks. Maybe I'm over thinking it a bit.
I've actually got that thread open in another tab. Just reading it now. The one thing i didn't read before posting. Whoops.

I'm sure it's in the thread but before I get to that part, how is it that I can run a motor at 150+ volts that's rated for 36?
Any recommended technical reading you could suggest on the nitty gritty of motors that would apply to EV's so I can get myself up to speed and well versed on all of it?

I always like to know why I should use something and how it works when I'm building something. I could just go buy a kit that some company says should work and install it and drive it and be happy but then I'd know nothing about how it works. 
Just the thousands of newbie questions until I get to that point of comfort in my knowledge 

Thanks again on the quick replies.


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## Duncan (Dec 8, 2008)

Motor voltage

Your controller controls that
You use your foot to demand a certain current
1/2 throttle is 1/2 current
so my controller is 500 amps max - 1/2 would be 250amps

The controller increases the motor voltage until it gets to 250amps

At zero rpm 250amps = about 4v
So the controller does
4v out at 250amps = 150v in at 6.6amps 
as the motor rpm rises so does the "back EMF" - motor pushing back
1000rpm 
4v + 25v at 250amps = 150v in at 48.3amps
2000rpm
4v + 50v at 250amps = 150v in at 90amps

So motor voltage is a bit ........

There are some top limits set by;
Insulation, commutator design, brush materials
But read the tread - it goes into all that

The other reason that we can go higher is that we are "hot rodding" - we will put up with lower reliability than a forklift manufacturer

You don't think that a car engine rated at 150Hp would live if loaded like that all the time?
But a truck engine would


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## colinrickards (Mar 6, 2012)

Duncan said:


> Motor voltage
> 
> Your controller controls that
> You use your foot to demand a certain current
> ...


Hey now that makes the most sense out of anything I've read yet. Thanks so much for explaining it like that.
I'm reading through that thread now. Lots of information to digest in there but it's a good read.

Thanks again!


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## Ivansgarage (Sep 3, 2011)

Colin. Here is some reading on ac motors.. dc motors are antique. 

Ivan


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