How would you control a BIG DC motor?

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

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! ;D


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

Search forum for “ac shield” :smiley:

(dig, dig, rummage, dig.....) ;)

I will try that!

I haven't had any experience with Triacs or SCRs at all. It would seem that you'd want to build a 90 VDC power supply. How much current do you need? What are you using the motor for?

90VDC is 120V single phase AC rectified and filtered with a big fat reservoir capacitor. That's why 90VDC is such a common industrial DC motor voltage. 1/2 HP is about 370W, so you can expect to draw a touch over four amps steady; rather more at motor start.

The easiest thing to use for controlling that kind of voltage/current is a power MOSFET rated for ~200V. 20 mOhm is an available on resistance for that kind of voltage, which would give a steady power dissipation in the MOSFET of 230 mW. Where to go from here depends heavily on what you want to do. So, do you just need on/off? Speed control? Direction control?

TY for the reply! I need to control the speed of the motor so if the cap gets me close to the 90v under load I can just fire the mosfet(s) with the PWM output from the Arduino via an opto isolator. Now comes the next hard part. WHICH cap, mosfet & opto isolator should I use. I am to inexperienced at reading speck sheets and marrying components together so it actually works. :-[ The ratind part is not so bad but something like the opto isolator, I don't know how to choose the perticular one I need to drive the perticular mosfet I select.

Hopefully, the switching rate of the Arduino is not so fast that it keeps the mosfet in the analoge mode and therefor, dissipate lots more power :o


You left out the MOSFET driver chip and the antiparalle diode. More on that later.

The filter cap needs to be rated for ~300V, and you will want a larger electrolytic capacitor. The capacitor in parallel with the inductance of the motor will form a low-pass filter -- size the cap such that the voltage ripple is acceptable -- remembering that the fundamental frequency you need to filter out is 120 Hz not 60 Hz.

Power mosfets used in PWM mode need a great deal of gate current drive in order to switch fast enough to avoid dissipating an unfortunate amount of heat. This can be provided with a mosfet gate driver. I suggest you read International Rectifier's AN-1084 Power MOSFET Basics ( Anything I could say on the subject here would probably be a (bad) paraphrase of it anyways, so you might as well go straight to the source.

You also need an anti-parallel diode across the motor terminals. This is because when you interrupt the current through an inductor (like an electric motor) you get a big voltage spike. The diode gives it somewhere to go that isn't straight through your mosfet. I usually choose a diode with twice the rated voltage and a rated current equal to that of the load. That's massively conservative for most applications, but diodes are cheap.

First thing to remember about optocouplers is that unless the ground and power of the two sides are separate, they are not actually isolated. Generally, they have an LED on the input side. You drive that just like you'd drive any other LED. Outputs vary, but the easiest to use are probably the ones with an NPN photo transistor output. You can then wire the output as an emitter follower (collector to V+, emitter grounded through a resistor, output attached to emitter) connected to the input of your gate driver circuit. Beyond that, just make sure it accomodates the switching speed and logic voltages you're using.

Thanks Rocketgeek! lots of good info there. I will check out the International Rectifier's app note as you suggested. And see! I did not know you needed a driver for the mosfet gate. Thought the opto isolator would be enough. Thank you again, prob. saved me from frying several expensive mosfets ::).

I had planned on using an anti-parallel diode. I always called it a "snubber". Back emf can be a bear! I plan on using a Fairchild FFPF10UP30STU.