I need a cheap beefy adjustable speed motor setup for a DIY lathe I'm building. I was considering using a universal motor perhaps from a table saw or other source of large universal motor. I've been told that universal motors only operate at high speed and I know that they are almost exclusively used for high speed applications (table saws, angle grinders, dremel tools, blenders, etc) but I don't see why it couldn't be made to run at lower speed by controlling the power to it.
So my plan is to use an encoder or DC analog tach to read the speed of the motor and build a cricuit that would read the speed, compare it to setpoint, and use proportional control to adjust a phase angle firing triac/SSR to maintain a constant speed as the load changes.
In my mind, this seems like it would work. But there's the potential issue of my motor likely being designed to spin 20,000RPM and I'm trying to keep the speed down around 500RPM (or maybe more, if need be I can use mechanical gear reduction, but that only goes so far).
From what I understand about series wound motors, they are more or less "constant mechanical HP" - that is to say (possibly not 100% accurately) that as torque demand increases, RPM drops. So if I were to apply the maximum load that the motor could sustain without burning up and read the speed @ that load, I could call this my "maximum speed" (4000rpm for example)and design my circuit and my machine accordingly. At 75% of this load, I would need to use my phase angle firing circuit to deliver 75% of full load amps and therefore maintain 4000rpm. The purpose of this, would be that if I did not do it this way, I could find myself in a situation where as I'm increasing the load (say I'm running 6000RPM) and I hit an invisible wall where my motor can no longer maintain 6000RPM and the speed starts to fall off - that's not good.
So, do you think it can be done or what?
So my plan is to use an encoder or DC analog tach to read the speed of the motor and build a cricuit that would read the speed, compare it to setpoint, and use proportional control to adjust a phase angle firing triac/SSR to maintain a constant speed as the load changes.
In my mind, this seems like it would work. But there's the potential issue of my motor likely being designed to spin 20,000RPM and I'm trying to keep the speed down around 500RPM (or maybe more, if need be I can use mechanical gear reduction, but that only goes so far).
From what I understand about series wound motors, they are more or less "constant mechanical HP" - that is to say (possibly not 100% accurately) that as torque demand increases, RPM drops. So if I were to apply the maximum load that the motor could sustain without burning up and read the speed @ that load, I could call this my "maximum speed" (4000rpm for example)and design my circuit and my machine accordingly. At 75% of this load, I would need to use my phase angle firing circuit to deliver 75% of full load amps and therefore maintain 4000rpm. The purpose of this, would be that if I did not do it this way, I could find myself in a situation where as I'm increasing the load (say I'm running 6000RPM) and I hit an invisible wall where my motor can no longer maintain 6000RPM and the speed starts to fall off - that's not good.
So, do you think it can be done or what?