# DC Motor Speed.



## JRaef (Mar 23, 2009)

One thing to know is that treadmill mfrs are notorious for using "marketing values" for things like HP. So it may only be 1-1/2HP under very specific "peak" circumstances. That type of DC motor likely maintains a constant torque at any speed, so the "4700" is likely where it develops 1-1/2HP. How much torque your new use will require is going to be kind of a craps shoot though. All you can really do is try it out and if it works, you're good. 

Also though, treadmill motors were not built expecting to be exposed to a lot of dirt and grit, as in a sander...


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## acro (May 3, 2011)

Yea, I wondered about the optimism of their HP ratings. And yes, the motor is quite "Open". I am taking that into account and putting it in an enclosure of sorts with the flywheel and pulley sticking out one end.

But as you likely know, the cooling fan is built into the flywheel, which is also the original pulley, so that end of the enclosure is more open than I would like, I suppose.

We shall see. My load will largely be determined by the force with which I press the metal against the belt, and I am assuming leaving the flywheel in place will certainly help.


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## paulengr (Oct 8, 2017)

acro said:


> Yea, I wondered about the optimism of their HP ratings. And yes, the motor is quite "Open". I am taking that into account and putting it in an enclosure of sorts with the flywheel and pulley sticking out one end.
> 
> But as you likely know, the cooling fan is built into the flywheel, which is also the original pulley, so that end of the enclosure is more open than I would like, I suppose.
> 
> We shall see. My load will largely be determined by the force with which I press the metal against the belt, and I am assuming leaving the flywheel in place will certainly help.


No way you will get 1.5 HP...I will come back to that.

Do you have access to the fields and armature separate? Otherwise it's series or shunt (2 wire) and low performance.

Second an SCR is really a somewhere it blocks negative voltage except that it also blocks the forward bias condition too except when it is triggered. In a lamp dimmer they use two back to back to act as a wave chopper to give noisy but reduced "AC" out. A DC drive uses them in a rectifier configuration but the DC output feeds back to the controller that pulses them to get chopped DC out, then a filter cleans up the chopped DC to smooth it out. You're missing some steps so the DC motor sees chopped DC around 120x1.87 which is about 200 volts pulsed which will cause commutation problems at certain frequencies. Your meter on "DC" is filtering the output to give you estimated DC if it was filtered. A big run cap from a single phase motor or just buying a true DC drive such as one from KB electronics will do the trick but at this size a cheap AC drive from KB or Automation Direct and a cheap 3 phase 230 V motor will be about the same price and better performance and get away from contamination issues, and not fall apart from the make shift nature long term.


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## paulengr (Oct 8, 2017)

paulengr said:


> No way you will get 1.5 HP...I will come back to that.
> 
> Do you have access to the fields and armature separate? Otherwise it's series or shunt (2 wire) and low performance.
> 
> ...


Even adding the capacitor though...you are taking a drive efficiency performance hit twice so without going well over the 12.5 A limit of residential stuff you won't even get to the treadmill original ratings or even approach store bought belt sander torque.

That's why backing up and spending money up front for the VFD (get one with flux vector...same price as without these days) and the motor will be the difference between a home science experiment, store bought contractor grade, and industrial grade work horse. If you go VFD on the 110 single phase you top out at true 1-2 HP. If you go 220 the biggest ones are 3 HP which is better than any contractor brand belt sander. Figure your surface feet per minute and buy the right speed motor because torque goes up if speed goes down for the same HP motor. A 1 HP 900 RPM (8 pole) will be much larger and heavier than a 1 HP 1800 RPM (4 pole) motor because it is magnetically "geared down"...all the extra poles translate into more torque at a'll speeds below rated speed.

I've worked on water pumps with 1500 HP motors with 20 poles (360 RPM synchronous) that were essentially 4000 HP frames with an enormous WP2 air cooler on top that ran a 36" trash (slurry) pump directly at speed (no gearbox). Trust me it would NOT jam if a big rock wedged in it. The same trick works o smaller motors but people don't realize the cost savings and tremendous torque potential of more poles. 

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## acro (May 3, 2011)

It is just a cheap 2 wire motor. I have some caps around, I will dig them out and maybe use one across the motor leads. I do have a choke in series with the motor already.

I fully realize that a VFD and 3 phase motor would be a better option, but the electronics to allow me to use the motor I already had where under $20. And, I have room to re-power the sander later when this motor fails.


















I ended up using a 4.125" pulley - which is just layered plywood. I had a hole saw that was close, and then trued up the blank on my lathe and bored the center. I have not done anything yet to limit my speed electrically. I am considering installing a tachometer so that I can see the upper end of my range and just leaving it at that.


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## JRaef (Mar 23, 2009)

Nice. I especially love the tension adjustment mechanism. I suppose you have more time than money though, that's a lot of welding... I'm not a good enough welder to pull off something like that.


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## oliquir (Jan 13, 2011)

get a real dc drive (under 100$ form kb on ebay) and you will get better control speed/torque of motor or use the dc drive in the treadmil, it probably just use a 0-10v signals for speed and will self adjust


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## acro (May 3, 2011)

Well, the unfortunate thing with the treadmill controllers is the soft start feature. For safety, the motor would always have to start from zero speed after power up.

With a sander such as this, I prefer to set the speed and have a separate on/off switch. Returns to the set speed every time it is turned on.

Thanks for the idea on the DC drive.


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