Hi all, in a nutshell I'm trying not to kill my laptop PS. I'm using a old 18V Milwaukee drill motor to power a rock tumbler. I'm using the original trigger switch from the drill as well. I would like to have the trigger tie wrapped to the full "on" position to prevent having to use it repeatedly however the motor draws too much current for the 18V 4.7 amp laptop PS to handle. If I start the motor at half power, via the trigger switch, all is fine but the switch is the weak link. (I already killed one from repeated cycling during prototyping) Is there a simple circuit I could build using capacitors perhaps, that would allow the motor to start at max power? Secondarily, I thought I read somewhere that power supplies can be damaged from the back EMF generated when power is cut off and I'm hoping to solve that issue as well. Am I way off base here or do I just need to pony up and buy a supply that can handle the start current( ~30amps I think) of the motor? Any advice or suggestions very much appreciated.

Andrew

 

Personally I would source a supply that is able to ~comfortably~ deliver the motor requirements, which can be much more than it's normal running current. as startup is a considerable load.

If you want to continue with your small power supply and regulate the current in the same manner as the trigger, it will be by PWM control to limit power delivery, simple enough to implement by homebrewing it yourself, but old drills are easy enough to source since the batteries or the motors die but the triggers are often fine.

Motors are electrically very noisy so some protection against spiking and back emf would be wise.

 

A more powerful motors shouldn’t be started with max power. Even if your power supply is oversized the motor brushes will hurts.
If you need to overcome a starting torque because of initial shaft loading it is good to use a mechanical clutch.
If your supply isn’t able to deliver enough power to handle initial torque (load) you can help yourself with flywheel (accumulate energy to it with no load) and then clutching to load.

 

Use a speed controller with soft start, e.g.

Soft start speed controller

 

That Motor isn't going to last very long running for days at a time.

I would recommend a large AC Shaded-Pole-Motor, like maybe from a cheap ~$20.oo Box-Fan.
It will run silently for decades with only occasional oiling of the bearings.

Available at any Walmart.
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I second LowQCab’s suggestion. Those drill motors are designed to run intermittently only.

 

I would recommend a large AC Shaded-Pole-Motor, like maybe from a cheap ~$20.oo Box-Fan.

Those have low starting torque so may not be adequate for running that rock tumbler.

 

Rock-Tumblers run at relatively low RPMs, something like less than maybe ~200-Rpm.

The Gear / Belt / Roller-Shaft, Gearing-Reduction, that is automatically required on any Rock-Tumbler
provides a Torque-Multiplication of at least ~100X, and possibly even as much as ~200X or more.

Unless you're talking about spinning something like a ~150-pound 55-Gallon Steel-Drum,
adequate Torque is not going to be an issue with a cheap Box-Fan-Motor.
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Personally I would source a supply that is able to ~comfortably~ deliver the motor requirements, which can be much more than it's normal running current. as startup is a considerable load.

If you want to continue with your small power supply and regulate the current in the same manner as the trigger, it will be by PWM control to limit power delivery, simple enough to implement by homebrewing it yourself, but old drills are easy enough to source since the batteries or the motors die but the triggers are often fine.

Motors are electrically very noisy so some protection against spiking and back emf would be wise.

Thanks for the reply. I think startup requires at least 30 amps from what I recall. I do have 3-4 drill motors to use. Hoping to use what I have on hand before buying a power supply.

 

Thanks for the reply. I think startup requires at least 30 amps from what I recall. I do have 3-4 drill motors to use. Hoping to use what I have on hand before buying a power supply.

I appreciate all the responses. I will look into that Michal. Thanks for the link Bob. That just might work. Thanks LowQ and schmitt. I should add that this drill motor spins quite quickly at full power. My solution was to print a gear reduction setup which works well. I agree these type motors aren't made for this situation. I just have a few of them laying around. I like the fan idea. I have a couple of those as well.
As far as the back EMF...is that something I need to worry about with this style PS? Thanks again all.

Andrew

 

I would not ever, at any time, under any condition risk my computer supply to power a motor., unless te computer was in such a broken condition that repair is impossible. In addition, as the others have already stated, a DC brushed motor is the wrong choice for a rock tumbler that is supposed to run for many days, Rock tumbling is measured in weeks, not hours, and a drill motor lifetime is much less than that, especially a battery drill motor.