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Potentiometer to control DC motor speed Answered

I have a 12 volt fan in my RV. It has a 1.6 amp DC motor.
Would a 2 watt, 350 ohm potentiometer be suitable to use as a variable speed control?
If not, what potentiometer would I need?

14 Replies

user
gaugeguy (author)2015-01-09

Here is a video. Sorry the table shakes too much at high speed.

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user
gaugeguy (author)2015-01-07

steveastrouk I made a significant error. My rheostat is 25 ohms, 3 watts; NOT 25K ohms. I apologize for that.

I performed a second test.; this time with the fan blade installed (which I should have done before).

I used the same two terminals as before.

Using the aforementioned pot, I could regulate the fan rpm from about 1200 rpm through 250 rpm.

This time, however, the pot heated up significantly at any speed except high. After 5 minutes it was too hot to touch. If I made the proper connections using all three terminals of the pot, it gets even hotter; even at max speed. So this looks like a no-go to me.

I am very familiar with basic electricity as I was a mechanic for 45 years, I have rebuilt alternators, generators and regulators, re-wired instrument panels and so on. So please don't take me for dummy.

iceng, you speak of "one to six 50 volt 3 amp 6A1-T diodes in series with the motor", but how are they switched to vary the motor speed?

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user
steveastrouk (author)gaugeguy2015-01-08

What you will also find, is if you leave the motor at the lowest speed, with a pot, switch it off and restart, your motor is likely stalled, and burns out.

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user
rickharris (author)2015-01-07

1.6 amps x 12 volts is 19.2 watts

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user
steveastrouk (author)rickharris2015-01-07

and therefore at half speed, one could reckon on the pot dissipating 10W. I don't understand how the OP got results with a 25K pot !

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user
rickharris (author)steveastrouk2015-01-08
user
gaugeguy (author)2015-01-07

Got it! Thank you very much.

steveastrouk I will order your suggested motor speed control from eBay as a backup.

Thank you to all

I will get back with results.

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user
gaugeguy (author)2015-01-07

OK. So I need a rotary switch?

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user
iceng (author)gaugeguy2015-01-07

Yes, a 3 or 5 amp to handle the start current surge, be sure to plan for one more position which will be your OFF and should be next to the high speed..

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user
iceng (author)2015-01-07

Motor Resistance is R = V / I = 12 / 1.6 = 7.5 Ohms

Motor Power = V x I = 12 x 1.6 = 19.2 Watts

No you cannot use a mere 2 W 350 Ohm pot.

You could put one to six 50 volt 3 amp 6A1-T diodes in series with the motor

http://www.jameco.com/webapp/wcs/stores/servlet/Pr...

Each diode will lower the voltage about 0.7 volts slowing the fan in gradual steps and they cost $ 0.25 each.

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user
iceng (author)iceng2015-01-07

Each diode reduces the voltage to the fan motor.

12v, 11.3v, 10.6v, 9.9v, 9.2v, 8.5v, 7.8v which reduces the speed

If you want a greater step change put two diodes in series for each step.

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user
gaugeguy (author)2015-01-06

I appreciate your feedback. Thank you.

I tested the motor with an old 25k ohm rheostat (pot) that I had laying around. Using the center pin and pin #3, it varied the RPMs quite nicely. I let it run for a couple of minutes while at low speed and then many speeds. It seemed to be just fine. No heat at all that I could detect.

However, I need one that has a on/off switch in it. I have a very limited amount of space (the size of a 2" cube) in which to install a controller. Pretty sure I am going to need a pot of some kind, but I have read they may cause motor overheating at lower speed settings.

I have my eyes on a Radio Shack 10K-Ohm Audio Control Potentiometer with SPST Switch.

If I can't keep it really simple, I will just install an on/off switch.

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user
steveastrouk (author)gaugeguy2015-01-06

25K ? Really ? Did you try starting it at minimum speed too ?

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user
steveastrouk (author)2015-01-06

Using a pot won't generally work. You really need a little fan controller, like this one I found on Ebay

http://www.ebay.com/itm/Top-12V-36V-3A-DC-Motor-Speed-Control-PWM-HHO-RC-Controller-24V-12V/131267025868?_trksid=p2054897.c100204.m3164&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140407115239%26meid%3D4b10d52de3ce46b6b6b5ffe5cabf934e%26pid%3D100204%26prg%3D20140407115239%26rk%3D8%26rkt%3D30%26sd%3D261493172651

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