The Sherline metal lathe is powered by a DC motor with a speed control on it. There are also two pulleys on both the motor shaft and the lathe spindle for different speed ranges. What I want to know is if the speed control lowers the torque as well as the speed. Does anyone know?
Thanks,
Rob
Almost invariably.
Here’s a chart of torque vs rpm for one of the Sherline motors.
What do you mean invariably? The chart shows the torque to be roughly inversely proportional to motor speed. Anyway, it is easy to understand how pulleys will increase torque while decreasing speed, but how does the speed control do it? I take it that it is not a rheostat.
Thanks,
Rob
Torque is related to current; speed is related to voltage. An ideal voltage source will drive an ideal DC motor at exactly the right torque to maintain the speed for a given voltage regardless of load. A speed-torque plot assumes a constant current. Of course there are no ideal voltage sources and motors cannot handle infinite current.
See here for example. They even go into detail about a typical DC motor sheet from Maxon.
The power supplies for mini lathes appear to have a power supply that adjust the current limit upward as voltage decreases. Here’s a little discussion of a non-sherline power supply. Doing this allows for better torque at low rpm, but naturally the implementation varies between manufacturers.
Is power from the wall constant voltage or constant current? Also, do these concepts hold true for other types of motors?
Thanks,
Rob
I think I am probably confusing more than I am helping, but maybe this will be instructive and give you something to think about next time you try and rush a cut 
What’s important is not what your wall is acting like (it is a voltage source, though) but what the circuit controlling the motor is acting like… which is also (almost certainly) a voltage source. Your main question is: “What I want to know is if the speed control lowers the torque as well as the speed.” It’s not clear to me what you’re asking, but yes, there is a relationship between speed and torque for some motor. It sounds like you’re saying if I decrease the speed I decrease the torque, but that’s not true. It’s the other way. And it also works both ways. Let me try to explain.
A motor is only so strong. It can move light things quickly or heavy things slowly. Furthermore, it can only do this continuously in a certain range of operation. Say you are trying to lathe some light material. You set the speed control to spin at some speed. There is almost no mechanical load on the motor (we said our part was light), so it spins close to the ideal speed (speed constant * voltage). Now you dial your tool in and start cutting the material. This makes it harder for the motor to turn the piece. What has to happen? You’ve just changed the operating point. Something has to give.
In the equations page of my link, here’s the important part: v[sub]m[/sub] = iR + v[sub]emf[/sub]. (The middle term only matters as we move from one steady state to another. Let’s just examine the end result of these moves and ignore this term.) The v[sub]m[/sub] is what’s coming out of our controller. It’s fixed. The v[sub]emf[/sub] is sort of like the resistance of the motor to speeding… the faster it goes, the greater this term is. But we just admit we’ve slowed it down. So if v[sub]emf[/sub] decreased, and v[sub]m[/sub] stayed the same, then the iR term must have increased. Meaning: more current. (R is not a variable.) Our torque went up! But this is just what we required by loading it with our cutting tool.
Now, consider this: your v[sub]m[/sub] is changing because you’re changing the speed. We know that as the speed increases, v[sub]emf[/sub] also increases by some proportion (our speed constant). This means the iR term has to decrease. But this is related to the torque. So our torque went down!
This is what the graph on my link shows. For some fixed voltage, we can load our motor at the cost of the speed at which it will turn for a given input voltage. But you can look at it another way, and that’s that we can change the speed at which we turn provided that we don’t need a specific torque. If we need a specific torque, then we can’t control the speed.
The ideal speed controller would allow us to monitor the instantaneous speed of the motor and could deliver infinite current. So it would set up an operating point at some voltage (corresponding to our desired speed) and see how fast the motor is spinning. If it is too slow, it increases the voltage until the desired speed is reached. Of course our motor would then have to be able to move any load. Practically, of course, this cannot happen, and this is the limiting cases of our curves: a motor can only do so much before it overheats or whatever. The other thing a practical controller can do is cheat by not giving great speed consistency when loaded. It will drive the motor with no load at some point, and monitor the current going out. Increases in current (as we learned above: someone applied a load) will cause some feedback and boost the motor voltage up to try to compensate for this. When the absolute speed is not super critical, this is fine. (I have a fine application note from Burr-Brown on this called “Control a DC Motor without Tachometer Feedback” by Bruce Trump.) Even if the motor is way overpowered for our application, our controller has its own limitations relative to the amount of power it can deliver (power = voltage * amperage).
So there are real limitations all around. Real motors with real controllers:
Let me give a little background. I was reading the Gingery metal shop from scrap series (if you are not familiar with this, it is a series of books that give instructions for how to build a machine shop from nothing w/o using precision tools (except a feeler gauge), starting with an aluminium melting furnace, followed by a 7"x12" lathe, a shaper, a milling machine and then a drill press) and when I got to the drill press book which details how to build a 12 speed drill press, I thought to myself, “it would be cool if I could make a variable speed version.” Making variable speed pulleys is a complicated task, but it occurred to me that Sherline lathes have this speed controler built in. But if that speed controller is just a rheostat, then we are lowering the current available to the motor and as a result, the speed and torque will go down, shouldn’t they? Is the speed controller just a rheostat? If not, what is it?
Thanks for your help,
Rob
No, it’s a power supply designed to control the speed of DC motors.
Sherline DC Motor and Speed Control Units:
Rather than get the $210 Sherline motor/controller, I’d start with a speed controller like this, and a suitable 3/4 horse or so surplus DC motor.
A rheostat will limit current in the circuit it is a part of, you are correct. It’s just not a part of the motor circuit. In your hypothetical, the controller would monitor the voltage across some element based on the rheostat (or the rheostat itself) and use that signal as a basis for how to control the motor. The rheostat would not otherwise have anything to do with the motor, much like a volume knob controls an amplifier, not the speaker. The amplifer controls the speaker.
As Squink notes, Sherline seems to be a charging a lot for a controller.