Why does higher voltage produce better torque from ...

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It is common to see a small stepper motor being driven from 12V or 24V, but it only has 5Ω coil resistance and is rated for 1.5A. Ohm's law tells us that 5*1.5 = 7.5V, so 7.5V should be the maximum input voltage limit. So how does it not catch fire?!

The answer is that the driver is PWM-limiting the current. It turns the driving voltage on and monitors the current. When the current exceeds 1.5A, it turns the voltage off and waits. It repeats this thousands of times per second. The net result is that the pulsed voltage averages to around 7.5V, and the current averages to 1.5A - within limits.

The advantage of using these higher voltages is the rate at which the current ramps. Higher voltages cause the coils to charge faster, which in turn means two things:

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1. More time for the selected pole to interact with the moving stator (more torque)
2. Higher usable top speed, because the coils physically charge faster.

This is especially important for steppers, as their torque rapidly dwindles at higher speeds. If you need torque from steppers at high speeds, a higher-voltage source is needed.

There are limits though, and you may find that a different type of motor would be better-suited to such an application (like a servo motor.)

As a general rule, particularly applicable to direct drive motors, a direct drive motor can do higher and higher wattages where those wattages are derived more from the factor of voltage than from the factor of amperage.

This line of thinking underlines a general philosophy which holds that it is better to acquire the required wattage by using high voltages and low amperages. Hence the expression, "Volt up, Gear Down" {ie go for high voltage in a small wheel build eg 24", 20"}

Most DD motors can handle the 72v. Not all of them have a winding, or a lacing, or a wiring, that is 60/70/80 Amp friendly.

Example: When I was a noob, I bought a direct-drive motor off ebay. Here are its characteristics:

1. 8*8 Winding. Excellent. Won't need high amps, can handle all the voltage you fling at it, within reason.

2. 26" wheel build. Not good for high draw applications.

3. Long, skinny phase wires. Again, forget about high amps.

What was the best scenario for that motor? Run it at 72v 30A in a 26" wheel, or re-lace it into a 20" rim, rewire it with some 14G wires, and run it at 60amps for some fun.

There are many matters to consider. Terrain. Top speed. Size of the finished wheel. Riding style. Weight of the individual. All of these will dictate how your setup should be, or how well your existing setup will perform.

If you have an X5305 in a 20" rim, then do what you please. Pump in all the amps you want. If you have an X5302 in a 28" wheel, then better to use higher voltage, and much much lower amps.

However, the matter becomes complicated once the user moves beyond the 100v mark, where issues of reliability and safety, not stemming from amperage, begin to present themselves irrespective of the above factors. This is not to say that the issues mentioned above will disappear, but that new problems will present themselves anyway.

Specifically, the matter of controller-reliability, efficiency etc. Moving from 4110 Mosfets to 4115 Mosfets presents its own issues.

With geared motors, most of them "dislike" both voltages and amperages which go beyond 50 units.

Running MAC motors at 72v is asking for trouble, whereas the same voltage would be nothing to an X5 or a 9C.

I read here on this forum that 55v 50A for some of the original BMC geared motors was pushing the boundaries.

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