# Understanding motor power in kw



## Squidlings (Aug 1, 2019)

Hi,

Im new but i kinda get that the kw of a motor is the number of kilowatts it can use. 

I have a 1000w ebike which is 1 kw right?

The nissian leaf has a 110kw motor which i wouldnt say is a high performance car. 

All the ev kits appear to be 15-25kw 

Im so confused how a leaf has such a big kw rating? 

Just looked at a tesla model x 100d has a 311kw motor. 

So are all these kits rated at 10-20kw going to deliver terrible slow performance?


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## Duncan (Dec 8, 2008)

Hi
YES those kits will have you being overtaken by cyclists

But the motor is a converter - it will convert as much (or little) power as the controller feeds it
And for a short time that can be a LOT

My Hitachi motor is "rated" at 10 kW - and that is what the forklift controller feeds it - it will run forever at that rating 

I am feeding it with up to 400 kW - but only for a few seconds and it's fine

Cooling is the usual limit - so you can usually increase the revs and increase the power that way without a problem

My motor will see the same heat loads at 10 kW and 1400 rpm as it will see at 30 kW and 4200 rpm


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## brian_ (Feb 7, 2017)

Squidlings said:


> Im new but i kinda get that the kw of a motor is the number of kilowatts it can use.
> 
> I have a 1000w ebike which is 1 kw right?


Close... it should be the number of kilowatts of power it can produce (turning the output shaft). Yes, a kilowatt is one thousand watts.



Squidlings said:


> The nissian leaf has a 110kw motor which i wouldnt say is a high performance car.
> 
> All the ev kits appear to be 15-25kw
> 
> ...


110 kW is 148 hp... not particularly high-performance (especially for the mass of the car), but most drivers will never use that much power. In addition, the rated power is available over a broad range of the car's speed range, so an engine would require a higher peak power to get the same performance, unless the car has a continuously variable transmission.

That's the current Leaf rating; before last year, it was only 80 kW, and that was adequate for most drivers.

The Leaf, like any modern EV, has liquid cooling of at least the stator (some also directly liquid-cool the rotor), with a computer-controlled coolant pump, a substantial radiator for the coolant, and thermostatically controlled fan on the cooler. A typical brushed DC motor has a cheap stamped steel fan wheel stuck on the end of the shaft, if you're lucky.

Tesla's ratings are... interesting. That rating will be for a short period, perhaps only several seconds, before overheating reduces the power which the controller will allow. Tesla is much less conservative than Nissan, so they allow higher motor current as long as they think they can get away with it.

To help understand the importance of cooling, consider an example of a motor being fed 100 kW of power and running at 90% efficiency: it would produce 90 kW of output power and 10 kW of heat. That's like running seven typical plug-in portable room heaters at full power, and putting all of that heat into the motor case.



Squidlings said:


> So are all these kits rated at 10-20kw going to deliver terrible slow performance?


Yes, unless the motor is pushed much harder (higher torque by pushing more current and/or higher speed enabled with more voltage) than the conditions on which the ratings are based.


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## brian_ (Feb 7, 2017)

Duncan said:


> My motor will see the same heat loads at 10 kW and 1400 rpm as it will see at 30 kW and 4200 rpm


Duncan, are you saying that your motor dissipates only one-third as much heat per unit output at 4200 rpm as it does at 1400 rpm? For instance, if it is at 90% efficiency at 4200 rpm, so it is dissipating 3.3 kW to produce 30 kW, then at 1400 rpm it would be 75% efficient if dissipating the same 3.3 kW while producing only 10 kW.

That makes sense, assuming nearly ideal motor characteristics and so a constant relationship between torque (identical at 1400 rpm and 4200 in this example), current (the same at the two speeds because the current is the same), and the power dissipated due to winding resistance (the same current through the same resistance at the two speeds).


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## Squidlings (Aug 1, 2019)

Ahhh right thx. That all helps loads. Especially the heater analogy. Those heaters get hot.

So the controller feeds ( volts * amps = watts.) Power To the motor providing the motor can take the load and the battery feeds the controller providing it can feed the load. 

Is it possible to use copper pipes and run them around the forklift motor to liquid cool? Im surr one of u guys has tired 😜


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## Squidlings (Aug 1, 2019)

Duncan - 400kw for a few seconds sounds like chaos 😜 what kind of performance do u get from this? Even if it is for a few seconds that sounds ace. Especially in a fork lift. 

What kind of controller do you have?


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## Squidlings (Aug 1, 2019)

Duncan...so a rated 10kw motor is really a powerful beast provided you dont melt it. I like the idea that the motor converts what you feed it. Thats helped me build a picture in my mind. 

Battery is food > controller is spoon > motor is baby


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## Squidlings (Aug 1, 2019)

Brian you are super technical 😜 i think duncan understands where im at. However im starting to understand you more and more as im digesting all these new ideas and i really appreciate your help and knowledge.

Why does it have the same exposure to heat at 30kw? Is it because of the speed the rotor turns creates air flow and the air flow cooling effext from the moving car?


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## Squidlings (Aug 1, 2019)

Brian...yes i see now through wind resistance. 🙂 interesting stuff.


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## Squidlings (Aug 1, 2019)

Duncan...im trying to work out the math on your 400kw feeding. How many volts is your battery? This setup sounds insane.


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## Squidlings (Aug 1, 2019)

Im making it a 320v * 1200 amp controller? Wtf? Thats crazy.


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## brian_ (Feb 7, 2017)

Squidlings said:


> Why does it have the same exposure to heat at 30kw? Is it because of the speed the rotor turns creates air flow and the air flow cooling effext from the moving car?


No, the amount of heat (actually rate of heat production) is the same in Duncan's case at the two power levels because the heat all results from current going through the resistance of the windings. In his example he is using the same current at two different speeds, so the same amount of heat is produced. How that heat is taken away from the motor wasn't mentioned.


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## Duncan (Dec 8, 2008)

Hi Squid
This is my "Device"
https://www.diyelectriccar.com/foru...dubious-device-44370p15.html?highlight=duncan

I'm running most of a Chevy Volt Battery - 84S - which I run from 300v to 340v and a P&S 1400 amp controller set to 1200 amps

I got a "fine" at our last 1/8th mile drags - apparently there is a club rule about beating ALL the supercharged V8's

7.90 seconds and 93 mph is not bad for the 1/8th


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