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Just a wee bit bigger motor
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Just a wee bit bigger motor

by keshka on Sat Apr 11, 2009 2:24 pm

I would like to control the speed and direction of a 90v DC 1/2hp motor. I have 120v AC available. :shock:

My little Arduino project is growing up!

One of the thoughts I had was to fire a triac via opto coupler at various points in the 60 cycle wave form, then run the result through a full wave bridge and to the motor. I would have to detect the zero crossing of the 120v for timing.

Or slightly diffrent, bridge first. Now I have pulsed DC that I perhaps could switch on and off with an SCR. I think it would cut off when the DC pulse went to 0 volts, not sure. Still would require detection of the 120v zero crossing.

Would a solid state relay help in the above ideas?

I am pretty good at electronics diagnostics and repair but I suck at design!


PS I did look into commercial controllers from the likes of Minarik Drives. They start at about $300 and go UP! (ouch!)

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Re: Just a wee bit bigger motor

by franklin97355 on Sat Apr 11, 2009 10:51 pm

I would go the route of creating 90vdc first and then pulsing that. How do you plan to reverse it on the fly? Giant H bridge?

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Re: Just a wee bit bigger motor

by keshka on Sun Apr 12, 2009 2:06 am

had something safe in mind....have little Arduino bring it to a stop then flip a DPDT relay.

good ole electromechanical :D

hmmm....if I turn it into more of a filtered DC I would have to use some heathy MOSFETs and would not need the zero cross detection and could fire at will. Only problem is MOSFETS make me kinda nervous :? Tried to use them 20 years ago and all I did was burn up lots of money and got frustrated, mayby it is time to try again.


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Re: Just a wee bit bigger motor

by Amberwolf on Sun Apr 12, 2009 2:22 am

Sounds like what you want is essentially a treadmill-type motor controller. The remains of the one I have here simply takes the 120VAC, rectifies it and filters it, then passes it on to a PWM slicing circuit controlled by the treadmill's main board. I haven't checked to be sure, but it was probably a standard half-bridge MOSFET setup.

It's what was originally designed to supply the 120VDC @ 21A to the motor that will eventually be on one of my cargo e-bikes. (all the electronics in the treadmill were damaged by it's prior owner letting it get very wet, probably rain from what I could tell, but the motor was in an area better covered against that and didn't get water in it).

Since all you need is 90VDC instead of 120VDC, just ensure that you never send full-width PWM thru the motor if you don't want to create a high-capacity regulated power supply for the motor end of things. Keep in mind that if you use MOSFETs you'll have to be sure you get ones that can handle at least 20-30% (I use a 50% rule) *above* the voltage you expect to have across them, so that inductive spikes and the like from the motor and power on/off transients don't demonstrate for you *why* they are oft-called Magically Operated Smoke and Fire Emitting Transistors. ;)

There may be treadmill power controller schematics out there; if so you could essentially copy that, except you will need a way to reverse the voltage--the H-bridge is a good way to do that but will alter the treadmill controller type of design significantly. They only need to go one direction, so have no reason to use the more complex ways of powering the motor.

EDIT: your post about the relay just showed up after I submitted the reply. :) So ignore the H-bridge stuff above, and just go with a treadmill-type controller.
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