I have a treadmill motor, and I'm thinking about dumping the MC-2100 board that came with it and running it off rectified 125V AC. I'm wondering how much ripple voltate I should tolerate in a DC motor. I'm planning to use it in a buffer.

I made a wild guess that 2V would be acceptable, and I came out with a capacitor value of about 500 uF.

 

100% ripple. It will work just fine but may hum a bit under high loads.

 

Thanks. Am I better off just going with a bridge rectifier and heat sink than adding a capacitor? I don't want to blow breakers charging up a big reservoir cap, but I am not in love with hum either.

 

How many horsies are in a treadmill motor? How many horsies do you think you need to buff stuff?
Personally, I find 1/2 HP to be plenty for grinding and buffing.
If you aren't getting near the maximum load, the motor will behave quite nicely.
Besides, how well can you hear it hum while paying attention to the buffing chores?

Probably an imaginary problem, just like the imaginary problem about blowing circuit breakers with 500 uf.

 

Let's see. From this I take away:

1. The cap isn't big enough to suck enough current to blow a breaker.

2. The hum probably won't be loud.

Thanks for the help.

The motor's continuous duty rating is 2 HP. I chose a 2 HP motor because it was the kind that was in the free treadmill I found in my neighbor's garbage. I think I need 1.5 HP, but 2 free HP are preferable to 1.5 HP that cost me money.

So I guess I'll get myself a bridge rectifier and a piece of aluminum.

 

The other versions, MC-60 etc and KB controllers ARE SCR bridge type controllers with not only 120hz ripple but phase chopped also.
If you have a straight bridge and large capacitor, you will have 160vdc feeding a 90vdc motor.
If running flat out on just a bridge alone you will not have an hum issue.
Make sure you remove the flywheel!
Max.

 

So the capacitor would actually be a problem.

I have an MC-60 on another motor, but the neighbor who threw out the treadmill was not thoughtful enough to provide another one.

I have a schematic for a simple control circuit to allow me to use the MC-2100, but it seems like a waste of time, since I'm going to want 5000-7000 SFM (somewhere around 3000 RPM at the buffer arbor) at all times.

 

The typical maximum for those motors is 3000rpm.
If running with a bridge flat out, you could set up a start circuit in half wave, then switch over to full wave.
Max.

 

I've ran similar motors with nothing more than a heavy duty light dimmer ahead of the rectifiers many times. Just make sure to use a dimmer that turns on by rotating the speed pot and not by pushing it that way the motor is always being started and stopped at minimum speed.

 

I guess the thing to do is get it set up with DC voltage somewhere in the 100V area, check the RPM's, and then figure out the size of the arbor pulley to give me 3000 arbor RPM. I assume (having no way to check) this thing is rated for continuous duty somewhere between 90V and 100V, so if I can arrange that, it will be running at a speed that won't destroy it in two weeks.

 

The motor runs at 840 RPM (Chinese tach reading) at 14.5 volts. That sort of surprises me, because it would extrapolate out to around 5500 RPM at the continuous duty voltage. I don't know how closely the motor speed varies with voltage, however.

 

There is a couple of way to find out, at least in the ball park, One is to apply 12vdc from an automotive battery and read the actual rpm then extrapolate for 3krpm.
The other is to back feed it with some means and read the generated DC at the rotated rpm, and again extrapolate.
Max.

 

That's pretty much what I did. I used a battery charger at 14.5 volts.

It's not a big deal if it insists on 5500 RPM. I can use a bigger pulley on the arbor.

 

5500 RPM is pretty common for most cheap treadmill motors. They can take it all day long as long as you don't go above their rated amp value for extended periods. Speed and input voltage wise if your motor has they near standard built in internal fan you can go nuts with the speed and not hurt one.

Years ago I took one to over 10K RPM once on rectified 240 VAC and it never did have a problem with it. Didn't even over heat running no load either. Real disappointment being I was trying to kill it by destructive failure.

 

So you and Max do not agree about maximum RPM's.

This motor has a fan on the cast iron pulley, which is why I want to use the original pulley. Last time I machined a pulley with vanes from aluminum. Big mess.

 

@Zero Potential You should find the max rpm on the motor plate, this is what is recommended by the manufacturer, you are free to exceed this at your own risk.
If your particular motor suffers because you exceed this, you live with the consequences.
Max.

 

Would you believe this thing doesn't even mention RPM's on the plate? It seems like anything goes with treadmill motors.

The casing will protect me from flying chunks, so I plan to go for the gusto.