Tesla power output

[hoppy]

I read the other day that the Tesla Roadster's continuos rated power output is only 50 bhp while its widely quoted peak power is 185 bhp. I wish ebike sellers would quote both figures too! I wonder if the ratio stays about the same for all electric motors?

 

[Scottyf]

Because of Ohm's law it is pretty much linear so long as the resistance doesn't increase mentally with heat. So you can get some idea of power out put by figuring out the size of stators / magnets and copper fill.

 

[flecc]

It's nothing like as extreme a difference as that for our e-bike motors on which power is expressed in watts of course. Typically the net power is at 75% to 80% efficiency, so around that ratio of the gross power (consumption). They are also capable of running continuously at or near to full output without heat damage or other failure, continuous running regarded as the use of the entire battery content.

Most "legal" e-bikes have maximum consumptions between 400 and 600 watts, odd rare ones lower at a little over 300 watts and a few higher with peaks between 700 and 1000 watts. Hub motors appear throughout those powers, but mainstream crank drives are confined to around 400 or 500 watts peak to preserve the bike's transmission.

 

[mike killay]

I have never understood this. That a 250 watt motor can go over that limit. How and when does this happen?

 

[hoppy]

The crucial words are"continuously rated".Peak power seems to be 3 or 4 times as much. I wish someone would publish peak powers for the various ebike motors.

 

[flecc]

I have never understood this. That a 250 watt motor can go over that limit. How and when does this happen?

The 250 watt rating is notional, an e-bike that was only capable of 200 or 250 watts continuous would be useless. Originally back around the 1980s they were like that and failed to attract any attention, but by the 1990s manufacturers started to stretch the envelope. My own legal e-bike is capable of continuous net power output onto the road of over 550 watts. Most e-bikes can deliver 400 or more usable watts of power continuously.

The EN15194 tests show that testing laboratories recognise this and pass much higher than 250 watt power motors as legal. It's all a bit silly really and I hope the European parliament's wish to do away with power limits comes about, since it will do away with the pretext.

 

[flash]

The 250 watt rating is notional, an e-bike that was only capable of 200 or 250 watts continuous would be useless. Originally back around the 1980s they were like that and failed to attract any attention, but by the 1990s manufacturers started to stretch the envelope. My own legal e-bike is capable of continuous net power output onto the road of over 550 watts. Most e-bikes can deliver 400 or more usable watts of power continuously.

The EN15194 tests show that testing laboratories recognise this and pass much higher than 250 watt power motors as legal. It's all a bit silly really and I hope the European parliament's wish to do away with power limits comes about, since it will do away with the pretext.

See what you mean Flecc



Nikola Tesla sitting in his laboratory in Colorado Springs in December 1899.

 

[mike killay]

Thanks Flecc.
So when a certain manufacturer labels his 'off road' motor as 350 watt, he could just as well label it 250 watt?

 

[flecc]

Thanks Flecc.
So when a certain manufacturer labels his 'off road' motor as 350 watt, he could just as well label it 250 watt?

One scientist member did argue that an e-bikes motor power was whatever the manufacturer labelled it as! There is a degree of support for that, for example Heinzmann make both 200 watt kits for our market (600 watt peak) and also a range of higher powered kits. At one point for the higher powered market they were supplying 400 watt kits which were acknowledged to be exactly the same as those labelled 700 watts! Obviously a matter of markets, some jurisdictions have a 400 watt limit, others like the US federal and China have 700 watt limits, so they just use the appropriate label.

However, there seems to be a consensus that around double the legal limit is used by most makers.

Hoppy wished above that manufacturers would supply peak powers, but in fact the information they often give is sufficient for getting the continuous maximum. For example, one very well known and popular manufacturer of higher end bikes has stated he uses 16 Amp controllers now. So 16 times the 37 battery volts is 592 watts, the nominal gross power. At 75 to 80% efficiency the net power into the wheel will be some 440 to 470 watts, close to double the legal limit. When the battery is fully charged and delivering 39 volts under initial load the maximum net power onto the road could be just touching 500 watts.

So you can see that there is more information available than at first might be thought. Electric motors are a well understood technology with no secrets so they are all much the same for a given type. That enables us to draw quite reliable conclusions with little information.
.

 

[mike killay]

Thanks Flecc.
So, my understanding now is as follows:-
It is not the motor, but the controller that determines the actual wattage. Obviously a very small motor, badly overpowered will catch fire, but there is a range of outputs at which a given motor can perform before it overheats.
Is this correct?
Mike

 

[flecc]

That's correct Mike, and why in mentioning Heinzmann I referred to kits rather than motors. I didn't mention controllers for e-bikes since they come with the controller-motor pairing established.