For no good reason, I feel the need to correct some Internet lore related to this topic, some of which appeared on forums where they should know better. Most folks can and probably should just skip this post. But it bears on the problem of my lathe.
Lore: A motor draws power relative to the mechanical load. This is plain wrong. Power is not drawn, it is required, and then either delivered or not. Voltage is delivered, and resistance is what it is, though it is not constant, and the resistance is what draws current.
The static (or “DC”) resistance of a 120VAC induction motor coil is <1-3 ohms. The Dayton in this thread is 2 ohms resistance in each run coil with the motor not running, according to my Fluke tester (and 4 ohms for the two coils when wired in series for 240VAC operation). But when the motor speeds up, the apparent resistance increases because the impedance increases. Impedance is just like resistance but only applies at a frequency and is only meaningful with AC. This is called a reactive load, but it is measured in ohms and draws current just like simple resistance. If the rated run current at rated load is, say, 12 amps at 120 volts, as is typical for each of the two run coils in a 2-HP induction motor, the impedance (or “equivalent series resistance”) while running at full RPM load is 10 ohms, five times the static resistance. 120/10=12. This is why motors have such a high momentary starting peak current draw—the DC resistance is highest when the motor is not turning. 120 volts / 2 ohms = 60 amps, but just for a few milliseconds. (The specs for this motor say the “locked rotor” current draw is 58 amps.)
So, when a voltage is applied, the resistive load pulls current. NOT the mechanical load. Power is supplied, not drawn like current. If the supplied power is insufficient to drive the mechanical load, the motor will stall or not even start.
But if it stalls, that impedance drops back down to the static resistance and stays there. And THAT can draw locked-rotor current because the resistance is back down to 2 ohms per coil, which can overheat the coil wiring and melt the insulation, releasing the magic smoke, if the breaker doesn’t trip first (which it probably will). But if one turns it off immediately after stalling or refusing to start, it’s no worse than the typical startup current. If it runs, though, there’s no harm done except being weak. (Edited to add: The addition of the magnetic start-stop switch upstream from the drum switch would also provide overload protection, and would probably protect the motor even if the breaker doesn't trip--it's a good reason to add that.)
If 240VAC is supplied to a coil designed for 120, the resistance will be too low and it will draw current much higher than it was designed for. That will overheat it rather quickly, but not instantly. It’s particularly the case for starting coils, which are always run at 120VAC. If a dual-voltage motor is wired for 120 and run at 240, the capacitor may arc over and burn up (probably not), the starting coil will probably overheat first, and then the run coils will overheat if the breaker doesn’t trip first. The contacts on the centrifugal switch that disconnects the starting coil after starting may be damaged, but probably not. It’s the higher voltage with the given resistance that draws more current. Again, if the capacitor survives and the power is switched off immediately, probably the motor will survive unharmed.
If voltage sags from excess current draw for the size of the wire, the motor will slow, and that will decrease the impedance. The lower apparent resistance will draw more current to restore the synchronous speed, which will add heat. But running a 240VAC induction motor on 120 is NOT voltage sag caused by too much current. It’s a low supplied voltage that will draw less current and produce (much) less power.
If running a motor at half voltage caused it to burn up, my lathe would have been fried to a crisp half a century ago. The motor seems none the worse for having been run that way for decades, but I can’t imagine its users thought it was satisfactory.
I just had to get this off my chest. Maybe after someone reads on EEBLOG or similar that the mechanical load draws electrical power rather than requiring it they’ll work their way here and learn otherwise. A guy asked about running a 240VAC induction fan motor on 120 and several spouted the lore warning him of burning his house down and then harrumphed indignantly when others said it might work but probably not satisfactorily.
Rick “look, honey, someone is wrong in the Internet” Denney
