# Understanding motor rated and peak power



## Ravishankar (Jan 26, 2009)

I have still not yet made an EV conversion. So an understanding rated and peak power of an electric motor is only theoretical to me ..

But practically, when I press the acceletator pedal I expect the power to flow in proportion..like in a normal ICE engine. But here comes the catch in electric motors..unlike ICE which is always rated to peak torque and power (which I think it is able to sustain if there is sufficient cooling), the electric motor does not sustain its peak but for 1 or 2 minutes. 

So I understand that until say 50% of throttle, I get rated performance from the motor controller, above that I get the peak performance. Is this correct ? 
Then in that case I would have to press the accelerator pedal to > 50% on a slope - very akward - unless the controller can detect grades. 

Also what about when I need sustained performance on a grade..like when moving up a mountain..? If I need the peak power for more than 90 seconds for example will the motor overheat (possiblly detected by the controller) and then cutoff or reduce to rated power ?? All these make me think that a rated power motor (sufficent for small grades), does not pass the test on sustained grades at all!! 

Well the real experience would be driving an EV, but I dont have one around . So could experienced people throw some light on this topic, please


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## ken will (Dec 19, 2009)

The reason you can not keep peak current going is temperature. 
The more current you are using the faster the controller and motor heat up. 
If you can use 100% For 2 seconds, you might be able to use 90% for 10 seconds, or 80% for 30 seconds, or 70% of peak power for maybe 4 minuets.


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## piotrsko (Dec 9, 2007)

a possible solution to your misunderstanding is: most motors used in American Ev's were originally designed for a forklift and were nominally rated at say 20 HP. When they are modified to generate 100 HP, they tend to get hot really fast. The ICE equivalent would be to power your car with a lawn mower engine, using nitromethane as a fuel in a ultra high compression set-up.

when you go up a hill, you down gear the car, it goes much slower, motor RPM goes up, more cooling, less amps used.


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## Ravishankar (Jan 26, 2009)

ken will said:


> The reason you can not keep peak current going is temperature.
> The more current you are using the faster the controller and motor heat up.
> If you can use 100% For 2 seconds, you might be able to use 90% for 10 seconds, or 80% for 30 seconds, or 70% of peak power for maybe 4 minuets.


Sure, I understand this. The motor is able to operate indefinitely in its rated power and speed range. If 10KW of power allows me 90kmph top speed on the flat, it may allow me 45kmph on a 20% incline. But when I want slightly more speed on an incline I press the accelerator pedal further and I get to say 70% of peak i.e 120% of rated. And this is sustained for say 2 minutes. But if I keep the pedal pressed for more than 2mins without my being aware of what is happenning, then what happens ? Does the controller detect the overheating and cutdown the power (which essentially means loss of power on a grade!) or phut! goes the motor and all hell breaks loose ??


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## ken will (Dec 19, 2009)

Ravishankar said:


> Does the controller detect the overheating and cutdown the power (which essentially means loss of power on a grade!) or phut! goes the motor and all hell breaks loose ??


It depends on your set up.
If your motor has a temperature sensor and the controller has the capability to lower the power to keep the motor or controller from burning up.

Some people have a temperature gauge and keep an eye on it, some have flashing red lights or a buzzer when the temp. gets high.

What ever works best for you.


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## Jonah Templeton (Aug 29, 2011)

Agreed with above, a separate temperature gauge is planned for my own kitbash project. Simple, but effective.


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## lithiumlogic (Aug 24, 2011)

I'm kind of looking at this from another end, trying to work out if a HPEV EV50 motor would offer enough power.

The car i learned to drive in had 60bhp @ 6200 rpm and a kerb weight of 695kg. It felt fairly nippy, but i never really went much above 4000rpm for fear of making it blow up. Even exceeding 3000 used to bring some sharp looks from the passenger seat.

These days i drive around in a turbo diesel with 86hp @ 4000rpm and a kurb weight of 1000kg. Getting it up to 3000rpm is drama free, but i don't like pushing the pedal all the way into the carpet because of the clouds of black smoke appearing in the rear mirror.

Next so these internal combustion engines, the EV50 looks really weedy, but i'm wondering how much of their theoretical performanance i'm actually using. The complete absence of drama, noise, and smoke means i can use all of the performance, and do so as often as i like.

I'm not a fast driver by any stretch but exiting a roundabout i want to get back up to cruising speed decently.

One other thing, the EV50 has a rather low output compared to DC motors of similar size and weight. I presume these limits are set by the rather low current (500Amp) and voltage (90V AC?) output limits on the curtis controller, rather than motor temperature, so with good cooling it can work hard.

The sternest test i could imagine is the M62 at Windy Hill, the UK's highest motorway. I remember going up this in my first car with 4 passengers, there is a 10% grade climbing from sea level to over 1200 feet. The climb seemed to go on forever, I was running a wide open throttle for 10-15 minutes and not exceeding 60mph. 

If i can climb that pass in an EV without burning anything up i'll be happy.


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