# Proper forward / off / reverse switch for 2 hp DC motor



## caseyvand (Mar 13, 2013)

Hi....I'm installing a 2hp DC motor on a Bridgeport mill which is normally powered by 1 hp 3 phase power. I wanted to use the forward/reverse switch already on the mill but inside the cover of the switch its noted not to use on DC motors greater than 1/4 hp. These are pretty heavy duty switches and considering that most treadmills with this size of motor the wiring size seldom exceeds 14 gauge I'm surprized at the low DC hp recommened. Does anyone have knowledge what would be a suitable drum style switch to achieve both forward and reverse with 2hp DC motors?
Thanks.....Casey


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## British Steel (Mar 14, 2013)

It seems strange, I know, that the switch is down-rated so much for DC, but there are good reasons, on AC the arc between contacts is quenched 120 times a second so there's way less damage - on DC the arc lasts until the contacts are eventually far enough apart for it to run out of voltage, and a DC motor is an inductive load, just like an ignition coil you get much higher voltages out than in when it switches...

Have you thought about fitting hefty DC rated relays for motor current, using the original switch to feed the relay coils? That would leave the switch looking original, and would allow you to have a third relay for regenerative braking in there too? It would involve fitting a transformer to power the relay coils, but that and the relays could be hidden in the box for the DC drive electronics...

Dave H. (the other one)


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## JohnAspinall (Mar 14, 2013)

As I understand it, any given switch will have a lower rating (voltage and current) for DC than for AC.
That's because as the switch opens, you pull a teeny-tiny arc between the contacts.  The AC voltage goes to zero 120 times a second,
which gives that arc much less chance to grow, generate heat, and weld the contacts; much more chance to extinguish.


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## toolman49 (Mar 15, 2013)

Hi Casey,
I don't know what type of D.C. motor you are planning to use, but in my experience most treadmill motors, particularly those of Asian origin have somewhat exaggerated horsepower ratings (Bonsai horses perhaps?).
I run a Pacific Scientific 1.5 H.P. D.C. motor on my lathe with a KBMM controller, switching is by momentary push buttons  to latching control relays (for no volt drop out) which then control contactors rated at 20amps @ 200 volts D.C., there are 2 completely separate circuits for forward and reverse selected by the original rotary switch, the contactors are 3 phase with 1 contact to make & break the A.C. input to the KBMM, the other 2 make & break the D.C. to the motor (to protect the KBMM from back EMF) I have been running this setup for 2 1/2 years now with no problems.
Regards,
Martin


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## tigtorch (Mar 15, 2013)

I doubt you would have a problem with that drum switch but if you want to maintain the old drum switch look and be absolutely confident you won't harm it here is another vote for using a relay.


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## caseyvand (Mar 16, 2013)

Hi....The motor is a Leeson 2 HP DC powered by the circuit card from a treadmill. I will use my drum switch as is and just turn the power off when I need to change direction.....That will get me started...... and next I'll check out KB Electronics....they seem to have some really nice looking boards and I should be able to find a different circuit board which will be higher power than what I have right now.
Thanks for all your ideas and comments.


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