Motor voltage, current and watts.

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deronmoped
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Motor voltage, current and watts.

OK, if motor voltage is increased, the amount of current used will go down with the same wattage output.
24 volt 400 watt motor= 16.7 amp draw.
36 volt 400 watt motor= 11.1 amp draw.

This only applies if you are still doing the same amount of work with the motor. Now on a bicycle that has a 24 volt 400 watt motor and you increase the voltage to 36, the added voltage will make the motor want to push the bicycle faster which equates to a higher wattage, what happens to the current, how high will it go? Would the motor want to draw the same amount of amps as it did at 24 volts?

24 volt 16.7 amp motor= 400 watts.
36 volt 16.7 amp motor= 600 watts.

Is this right?

Thanks, Deron.

LinkOfHyrule
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Re: Motor voltage, current and watts.

Assuming a controller is limiting the amps, it will draw the same max amperage during acceleration, but for a longer period of time, since the motor has to be spinning faster to give enough BEMF to start limiting the amps at higher voltage.

It'll also draw more at top speed, too, since the increased voltage alone isn't enough to overcome the exponentially higher wind resistance encountered.

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deronmoped
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Re: Motor voltage, current and watts.

Now without a controller limiting the maximum amps, what will the motor do?

Now when you increase the voltage to 36 the motor wants to spin faster, reaching a RPM it's happy at. It will draw as much current as it needs to get there and stay there, is this correct?

I'm guessing the design of the motor is what determines the RPM's it will operate at, at a certain voltage. The motor will only use the amount of current it needs to get there and stay there. If the motor holds together, the voltage times current will give you the amount of watts you are using.

My bike right now is: 24 volt 400 watt motor= 16.7 amps ____RPM's and 18 MPH.

What I'm trying to find out: 36 volt ____watt motor= ____amps, the new ___RPM the motor wants to run at and the new, higher ____MPH resistance the motor has to overcome. There is probably even a formula for this :)

Deron.

LinkOfHyrule
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Re: Motor voltage, current and watts.

Now without a controller limiting the maximum amps, what will the motor do?

Draw a crapload of current until the BEMF of the motor starts to cut down on the amps.

Now when you increase the voltage to 36 the motor wants to spin faster, reaching a RPM it's happy at. It will draw as much current as it needs to get there and stay there, is this correct?

Yah.

I'm guessing the design of the motor is what determines the RPM's it will operate at, at a certain voltage. The motor will only use the amount of current it needs to get there and stay there.

Not so much "needs" as "can". Assuming we're still talking about no controller, it's going to draw as much current as Ohm's law will allow at stall.

Here's a tidbit that might help you: The amount of current a motor draws for a given RPM can be figured out by calculating out how many volts of BEMF it's putting out for a given RPM, subtracting that from the battery voltage, and then using Ohm's law. So, if a 36V motor has 1Ω of resistance, it will draw 36A at stall. At 1/2 of it's max RPM (for 36V), it will be putting out 18V of BEMF. So, you get 18V / 1Ω = 18A.

(I think.)

Obviously, this is just theory, and various inefficiencies would come in to play in real life, but it should be pretty close.

My bike right now is: 24 volt 400 watt motor= 16.7 amps ____RPM's and 18 MPH.

What I'm trying to find out: 36 volt ____watt motor= ____amps, the new ___RPM the motor wants to run at and the new, higher ____MPH resistance the motor has to overcome. There is probably even a formula for this

Amp draw without a controller can be figured out if you measure the resistance of the motor. Increasing the voltage by 50% should also increase speed 50%, so it should go about 27mph. In real life, it will be a little less than 50%, so it will probably only go around 25mph.

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LinkOfHyrule
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Re: Motor voltage, current and watts.

Double post.

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LinkOfHyrule
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Re: Motor voltage, current and watts.

Wow, Link, you're really failing today. :/ Triple post.

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colin9876
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Re: Motor voltage, current and watts.

No one seems to have pointed out that if u increase the voltage you also increase the current draw (v=IR)

so re 24v motor 400w
put 36v thro it and the current will be about 24amps so you will be over 800watts

LinkOfHyrule
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Re: Motor voltage, current and watts.

Yeah, but only if you don't have a controller. And, assuming it's a brushless, you have to. Kind of silly to not have a controller even if it is a brushed.

Still, put enough voltage through it and the power level could be OVER NINE THOUSAAAAAAND! }:)

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