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• # How to get full HP from motors

Discussion in 'The main mechanical design forum' started by jawnn, Apr 4, 2012.

1. ### jawnnWell-Known Member

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Is may be a question for some one that knows about electric motor controllers.
Is there any way to get full HP from a motor other than proper gear reduction from peak efficiency RPM?
That is: if a motor is geared 8 to 1 at 3000 rpm and has 12â€ wheels, can it get full HP when climbing a hill at 20mph?

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3. ### mhjones12Well-Known Member

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In short, you can only get your max power by changing your wheel radius or gear ratio. I'll assume your talking about DC motors (I don't know much about AC motors so don't apply what I say here to AC without checking first). For a DC motor operating at a constant voltage, the peak power will only occur at one RPM. Since the power is the product of torque and speed, the power can be found by looking at the torque-speed curve for your motor. The power will be the area of a square with left and bottom sides at the axes and top right corner at the operating point on the curve. This is hard to describe in words, so go to http://lancet.mit.edu/motors/motors3.html and scroll down to the second to last row of pictures for some examples.

As you can see in the image, for an idealized torque-speed curve, the maximum power will occur at half the max RPM (or equivalently, half max torque). Assuming you have a set motor and set speed, you can only change your operating point on the curve by changing your wheel size or gear ratio. So ultimately, you have to change your wheel size or gear ratio so that you are operating at the center of the torque-speed curve.

Hope this helps.

4. ### jawnnWell-Known Member

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AC controlers

is there any magic that can be done with AC motor controlers in this respect?

5. ### DanaWell-Known Member

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Well, let's look at your numbers. 3000 rpm / 8 is 375 rpm at the wheels. With a 12" wheel, that's 1178 ft/min or 13.4 mph. Your motor would need to turn 4482 rpm to go 20mph. So you need to select a motor/controller combination that will work at that speed, or choose a different gear ratio.

What you should be doing is first figure out how much power you need for the hill climb, which is the sum of the power required to lift the vehicle's weight at that speed plus the power required to run it level at that speed (including any losses like friction and air resistance). Then select a motor that delivers that power, and only then choose the gearing to let the motor turn at the speed where it efficiently delivers that power.

6. ### mhjones12Well-Known Member

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Just wondering, where did the 3000 RPM spec come from? Is this the max speed (no load speed), rated speed, or something you've determined? The reason I ask is because given this thread and another thread of yours I've seen, it seems as though you're thinking of motors as constant speed devices. Actually, the motor will change RPM based on load (for a constant voltage).
Also, is there a reason you want the motor to operate at max power? As long as it makes it up the hill, does it matter?

7. ### AlbertSixNew Member

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Use a Torque Box. It will transform the full HP motor into any torque or any speed you want.
The Torque Box is made of one differential and one speed variator. Both are available for any auto store. Millions of cars already use a speed variator as part of their gearbox.
Good luck
Albert

8. ### The_InventorMember

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You can use an AC Motor Speed control, that will give you max tourque over the rate RPM speed range of the AC motor. A 3 phase AC motor at 480VAC, can run up to 6000RPM, at 60HZ depending on the windings, number of poles, and operating voltage. The speed of the AC motor is very voltage and frequency dependant. The tourqe of the motor is current dependant. A transmission of motor to wheels will give you a wider range of use.

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