What limits current draw on a motor

Shotgun

Shotgun

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I posted about the 3phase motor I got off a treadmill down below. 5 "treadmill" Hp == 1.5 actual Hp was the conclusion I came to after y'all educated me.

Now that I have the late-1942 era Sebastian in the workshop and setup, I'm taking a closer look at the motor it came with. Best I can tell, it is the original. At least it matches the drawings that the Navy produced for the lathe. This motor can be wired to run 115V or 220V. I went with 115V, 'cause that is what my shop is wired for, but the motor is name-plated for 25A at that voltage.

Have I just consigned myself to constantly resetting a 20A breaker? Or would that only happen if I'm pushing the system REALLY hard? What is the ultimate limit for power draw in a motor? IE, if I locked the shaft and switched it on, would the winding's resistance limit it to 25A, or is that what you'd measure right before the windings melt and all the smoke came out of it . . . which is what I believe will happen if you lock the shaft?

Safety: I wired the circuit with 12AWG, so I could just switch out for a 25A or even 30A breaker, and never have to worry about it. . . but, I wouldn't be any smarter at the end of the day if I did that.
 
Some motors will have a space on the nameplate marked LRA (locked rotor amps). A brief event with locked rotor would not likely damage a sound motor, but a repeated cycle of stops/starts might have some effect on the breaker and motor, I think it would be a good thing to increase the breaker size. Does the lathe have a clutch to allow the motor an easy start or is it direct connected?
 
I'd like to see the dataplate of that original motor- how many HP is it? 25 amp sounds more like a peak starting current rather than
a continuous running current.
Most 115 volt outlets are limited to either 15 or 20 amps in most homes. AC motors usually draw 5 to 6 times the run current during startup which may or may not trip the breaker- it depends
230 volts is recommended for any motor 1 HP and up
 
I would supply it with 220 requiring only 12.5 amps.
My guess is this a 5 hp motor with current ratings that high.
 
No clutch on the lathe, and it is rated at 2Hp. I've now verified that it will start up the lathe and move the carriage around without issue, but I only ran it for a few minutes, and not with the 80lb chuck on there. It is sweltering hot out there this evening.
 

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You might want to check the size of the breaker feeding that outlet- you may be able to take light cuts without tripping but not heavy cuts
The gauge of wire feeding that circuit limits you, otherwise you could slap in a 30 amp breaker and be done without creating a fire hazard
The voltage drop in the wire becomes more of an issue on 115 volts; as you draw more current the wire heats up and the breaker trips to
protect your house
Also, needless to say, more voltage drop in the wire means less voltage delivered to the motor, less power available
 
Last edited:
Shotgun said:
Safety: I wired the circuit with 12AWG, so I could just switch out for a 25A or even 30A breaker, and never have to worry about it. . . but, I wouldn't be any smarter at the end of the day if I did that.
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12AWG is generally only rated up to 20 Amps. The breaker is there to protect the wiring from overheating and causing a fire. I would not install a breaker larger than 20 Amps on that circuit.
 
I should be going to bed, instead I'm sitting here trying to decipher NEC code books. @talvare is correct. I will not be trying to solve any problems with a larger breaker. If it does trip, I'll extend the circuit left over when we replaced the electric kitchen stove for gas.

But, that leads to the next problem. These old motors don't seem to have an integrated ground. I guess I could anchor a green wire under one of the vent cover screws. But, that seems a just bit sketchy.

Man, powering this thing is turning into more trouble than cleaning it.
 
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