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If you are familiar with dental micromotors, there are generally two kinds of them in foot pedals: simple switch or variable speed (like throttle.)

The simple one acts like a push button to be manually operated over and over again at very high speed (above 30,000 RPM for brushed motor handpieces and 70,000-90,000 RPM for brushless ones) without any problem.

I was wonder how to reduce the inrush current of a 12V 10A DC motor which I want to use to build a tool like a micromotor (using switch mode power supply.)

A couple of suggestions popped out but all of them is to lower the RPM (=low torque) to reduce the huge initial current; the other thing is giving the motor time to develop speed is good for long life and efficiency but we need high torque at moment to carve agates, so when there is a tool which can do it, there must be a solution for it. When we adjust the RPM on the control box in hand mode on 100% speed add up with the dial, so that's what a PWM speed controller does, but when we switch it to foot mode, each time we put the pedal down motor will start at the wanted speed.

I don't know how to do it without damaging the power supply or the motor, so if you know how or have ideas about, that would be helpful.

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  • \$\begingroup\$ On one hand: adding a current-limiter to your 12V supply can limit the initial current surge, while not limiting current once the motor is up-to-speed. On the other hand, a 30,000 rpm motor has a long way to go from idle, and may not start when starved of current from a current-limiter. If the (brushed) motor does start, it would accelerate optimally at a rate determined by the current limiter. \$\endgroup\$
    – glen_geek
    Nov 8 '20 at 13:16
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The PWM speed controller could be doing a soft-start every single time you push the pedal, just one that is too fast for you to perceive.

The other thing is giving the motor time to develop speed is good for long life and efficiency but we need high torque at moment to carve agates,

Do you actually apply the micromotor to your workpiece prior to pressing the pedal? Or crash your microtool under speed into the work piece? I doubt it.

Instead, I bet you press the pedal to turn the tool on and then gradually move it into the workpiece. That means the motor is only starting up under the torque due to its own inertia rather than high torque due to load. And the fact you gradually pressure to the workpiece means that the load torque is introduced over a period of time (and having inertia already built up helps too).

In which case, it might not even need a soft start to prevent current inrush.

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