In a POTD thread, I wrote this:
"(The lathe wasn’t happy with .030, which tells me something’s not right. I still need to rewire the single-phase motor for 240VAC. The power level seems low. It’s not the bearings in the lathe power train, which run free and are in spec.)"
I had put off digging into the wiring, because of the fear of making a mistake and burning up the motor, but I just had to get over that.
The motor in my South Bend 14-1/2 was originally a 540-volt, 25-Hz, 1410-RPM three-phase motor. Unless one is stealing and then stepping down power from the Amtrak Northeast Corridor catenary, that power source is not generally available, and when the lathe was sold by Beth Steel, either they or the buyer had to replace the motor with something that could be powered from the commercial grid. The motor they installed is a Dayton 5K482, which is a 2-HP, dual-voltage, single-phase, capacitor-start 1750-RPM induction motor.
The lathe has been operated on 120-VAC for the last 30+ years, and I assumed it was wired correctly for the low voltage. Uh, no.
Moment of explanation (skip ahead if you know this already): These induction motors use two coils fastened to the case to create fields on opposite sides of the magnetic core that is mounted onto the shaft. The fields are constantly changing with the AC, and pull at the core in a push-me, pull-you arrangement. But there's no way to get it started initially, because the fields are equal in their pull force and direction and only push me and pull you if the rotor is turning. So, the motor provides another two coils that are wired together, but in series with a large capacitor. The effect of the capacitor is to delay the waveform somewhat, and in doing that ensures that these coils pull the rotor a bit before the main run coils, which nudges the rotor enough to get it spinning. A nudge is all it takes and it's off to the races. The rotor will quickly spin up to to a speed that is related to the frequency of the supplied power. These starting coils are switched out of the circuit when the motor gets going, using a centrifugal switch.
If it spins too fast, the push-me-pull-you gets out of synch and the motor slows down. If it goes too slow, the push-me-pull-you works harder to try to get it in sync. Most such single-phase motors therefore spin at a nominal 1750 or 3500 RPMs, depending on the number of coils. For this Dayton, the run coils are really pairs of coils wired in series and opposite one another, making it a 4-pole motor with a theoretical synchronous speed of 1800 RPMs. These are not truly synchronous, in that they only attempt to lock into the frequency exactly, but there is always a slip angle as the motor gets pulled a bit under the sync speed by friction. But they are rated for their speed at full load, with the design drag creating the design slip.
These motors can be operated using 120VAC or 240VAC, single-phase in both cases. The two run coils can be wired in parallel and supplied by 120VAC. Or they can be wired in series and supplied by 240VAC. When wired for 240 VAC, the starting coil and capacitor is wired to one of the line powers on one end and on the other end to the point where the two coils are connected in series. The voltage drop across the two coils ensures that the voltage at the mid-point is 120VAC. When wired for 120VAC, the starting coil is wired to line on one end and neutral on the other, just like the run coils.
But to make the motor reversible, we have to nudge it forward or nudge it in reverse when it starts. We do that by changing the polarity of the starting coil. For 240VAC, forward means one end of the starting coil is on Line 1, and the other end is at the center point between the two run coils. For reverse, the other end of the starting coil is on Line 1, and the first end is moved to the center point.
For 120VAC, the starting coil is simply reversed between line and neutral.
It's the job of the drum switch wiring to route power to the run coils and to properly polarize the starting coil to obtain the desired direction.
Now, to my 14-1/2. I spent quite a bit of time studying how the lathe was wired, and made a discovery. The two run coils were wired for 120VAC in series. How can that be? It's plugged into 120VAC. But there it was--wires 2 and 3 (one end of each of the two run coils) were wired together in the box on the side of the motor. The line was provided to one end of one coil, and the neutral was sent to the other end of the series-connect run coils and also to the starting coil. The drum switch was not reversing the polarity of the starting coil, but rather it was reversing the polarity of the combined run coils with respect to the starting coil.
Because this seemed so wrong, I studied it a bit more. In fact, I drew several diagrams, traced wires, rang them out with continuity testing, etc. etc.
But there was no question: The two run coils were wired in series, but powered by 120VAC.
No wonder the lathe felt weak. The coils had half the voltage they expected--each coil was powered by 60 volts (that's how much drop there was to the point where they were connected in series). The starting circuit had 120 VAC because it was tied to the ends, so it worked. Normally, running a motor under voltage means that it draws more current, but that's because the resistance of the coils remains the same. Voltage = Current times Resistance. A given resistance will draw whatever current necessary to keep the equation in balance with the supplied voltage. But in this case, the resistance was doubled, because it was the resistance of both coils in series. So, it drew only half the current the coils were designed to carry and didn't burn up, thank goodness.
Now I know why the lathe would run on a 20-amp 120VAC circuit without popping the breaker. I figure it was delivering less then 1 HP, maybe much less because it wasn't saturating the electric field as needed by the magnetic core.
And that's why even the friction of running at top speed would often stall the motor. Frankly, I'm surprised it ran at all.
I rewired it to run at 240VAC, which means L1 to one end of Coil 1, and the other end connected to Coil 2. And then the other end of Coil 2 is connected to L2. I did not need to run L2 through the drum switch at all, because breaking the circuit at L1 stops all current flow and the motor stops. So, L2 is always connected. This is not strictly up to code, but it's common in these old designs. My lathe plugs into a wall receptacle that is within sight of the machine, so there's always a way to disconnect power completely. At some point in the future, I may wire a 240VAC magnetic switch with overheat protection in it, like I have on my 3-HP table saw.
I added a circuit with a 20-amp 240VAC receptacle to the shop, and now the total circuit length is about 60 feet of #12, instead of the 175 feet it was before.
Because I was only switching one leg of power, I had enough connections in the drum switch to reverse the polarity of the starting coil with respect to the center tap between the two run coils.
What a difference! I was able to tighten the belt up, and now the lathe positively jumps up to speed even with the belt positioned at the highest speed. My top speed went from somewhat below 1000 RPMs to a little above that threshold. The motor pulley was sized for the original motor, which turned at 1410 RPMs, so with this motor the lathe operates somewhat faster than the rated speeds. The motor speed, measured at the shaft, is 1789 RPMs, no matter what the belts are set to run. Before, top speed, even when the motor could be coaxed into developing full speed, never got above 1740 RPMs.
I feel like I have a new lathe.
I didn't have time to chuck up a chunk of steel and see what chips I could make, but I don't think I'm going to be limited to .030 depth of cut any more. Probably it will be closer to a quarter inch.
Rick "qualified electrically but still nervous when first throwing that switch" Denney
"(The lathe wasn’t happy with .030, which tells me something’s not right. I still need to rewire the single-phase motor for 240VAC. The power level seems low. It’s not the bearings in the lathe power train, which run free and are in spec.)"
I had put off digging into the wiring, because of the fear of making a mistake and burning up the motor, but I just had to get over that.
The motor in my South Bend 14-1/2 was originally a 540-volt, 25-Hz, 1410-RPM three-phase motor. Unless one is stealing and then stepping down power from the Amtrak Northeast Corridor catenary, that power source is not generally available, and when the lathe was sold by Beth Steel, either they or the buyer had to replace the motor with something that could be powered from the commercial grid. The motor they installed is a Dayton 5K482, which is a 2-HP, dual-voltage, single-phase, capacitor-start 1750-RPM induction motor.
The lathe has been operated on 120-VAC for the last 30+ years, and I assumed it was wired correctly for the low voltage. Uh, no.
Moment of explanation (skip ahead if you know this already): These induction motors use two coils fastened to the case to create fields on opposite sides of the magnetic core that is mounted onto the shaft. The fields are constantly changing with the AC, and pull at the core in a push-me, pull-you arrangement. But there's no way to get it started initially, because the fields are equal in their pull force and direction and only push me and pull you if the rotor is turning. So, the motor provides another two coils that are wired together, but in series with a large capacitor. The effect of the capacitor is to delay the waveform somewhat, and in doing that ensures that these coils pull the rotor a bit before the main run coils, which nudges the rotor enough to get it spinning. A nudge is all it takes and it's off to the races. The rotor will quickly spin up to to a speed that is related to the frequency of the supplied power. These starting coils are switched out of the circuit when the motor gets going, using a centrifugal switch.
If it spins too fast, the push-me-pull-you gets out of synch and the motor slows down. If it goes too slow, the push-me-pull-you works harder to try to get it in sync. Most such single-phase motors therefore spin at a nominal 1750 or 3500 RPMs, depending on the number of coils. For this Dayton, the run coils are really pairs of coils wired in series and opposite one another, making it a 4-pole motor with a theoretical synchronous speed of 1800 RPMs. These are not truly synchronous, in that they only attempt to lock into the frequency exactly, but there is always a slip angle as the motor gets pulled a bit under the sync speed by friction. But they are rated for their speed at full load, with the design drag creating the design slip.
These motors can be operated using 120VAC or 240VAC, single-phase in both cases. The two run coils can be wired in parallel and supplied by 120VAC. Or they can be wired in series and supplied by 240VAC. When wired for 240 VAC, the starting coil and capacitor is wired to one of the line powers on one end and on the other end to the point where the two coils are connected in series. The voltage drop across the two coils ensures that the voltage at the mid-point is 120VAC. When wired for 120VAC, the starting coil is wired to line on one end and neutral on the other, just like the run coils.
But to make the motor reversible, we have to nudge it forward or nudge it in reverse when it starts. We do that by changing the polarity of the starting coil. For 240VAC, forward means one end of the starting coil is on Line 1, and the other end is at the center point between the two run coils. For reverse, the other end of the starting coil is on Line 1, and the first end is moved to the center point.
For 120VAC, the starting coil is simply reversed between line and neutral.
It's the job of the drum switch wiring to route power to the run coils and to properly polarize the starting coil to obtain the desired direction.
Now, to my 14-1/2. I spent quite a bit of time studying how the lathe was wired, and made a discovery. The two run coils were wired for 120VAC in series. How can that be? It's plugged into 120VAC. But there it was--wires 2 and 3 (one end of each of the two run coils) were wired together in the box on the side of the motor. The line was provided to one end of one coil, and the neutral was sent to the other end of the series-connect run coils and also to the starting coil. The drum switch was not reversing the polarity of the starting coil, but rather it was reversing the polarity of the combined run coils with respect to the starting coil.
Because this seemed so wrong, I studied it a bit more. In fact, I drew several diagrams, traced wires, rang them out with continuity testing, etc. etc.
But there was no question: The two run coils were wired in series, but powered by 120VAC.
No wonder the lathe felt weak. The coils had half the voltage they expected--each coil was powered by 60 volts (that's how much drop there was to the point where they were connected in series). The starting circuit had 120 VAC because it was tied to the ends, so it worked. Normally, running a motor under voltage means that it draws more current, but that's because the resistance of the coils remains the same. Voltage = Current times Resistance. A given resistance will draw whatever current necessary to keep the equation in balance with the supplied voltage. But in this case, the resistance was doubled, because it was the resistance of both coils in series. So, it drew only half the current the coils were designed to carry and didn't burn up, thank goodness.
Now I know why the lathe would run on a 20-amp 120VAC circuit without popping the breaker. I figure it was delivering less then 1 HP, maybe much less because it wasn't saturating the electric field as needed by the magnetic core.
And that's why even the friction of running at top speed would often stall the motor. Frankly, I'm surprised it ran at all.
I rewired it to run at 240VAC, which means L1 to one end of Coil 1, and the other end connected to Coil 2. And then the other end of Coil 2 is connected to L2. I did not need to run L2 through the drum switch at all, because breaking the circuit at L1 stops all current flow and the motor stops. So, L2 is always connected. This is not strictly up to code, but it's common in these old designs. My lathe plugs into a wall receptacle that is within sight of the machine, so there's always a way to disconnect power completely. At some point in the future, I may wire a 240VAC magnetic switch with overheat protection in it, like I have on my 3-HP table saw.
I added a circuit with a 20-amp 240VAC receptacle to the shop, and now the total circuit length is about 60 feet of #12, instead of the 175 feet it was before.
Because I was only switching one leg of power, I had enough connections in the drum switch to reverse the polarity of the starting coil with respect to the center tap between the two run coils.
What a difference! I was able to tighten the belt up, and now the lathe positively jumps up to speed even with the belt positioned at the highest speed. My top speed went from somewhat below 1000 RPMs to a little above that threshold. The motor pulley was sized for the original motor, which turned at 1410 RPMs, so with this motor the lathe operates somewhat faster than the rated speeds. The motor speed, measured at the shaft, is 1789 RPMs, no matter what the belts are set to run. Before, top speed, even when the motor could be coaxed into developing full speed, never got above 1740 RPMs.
I feel like I have a new lathe.
I didn't have time to chuck up a chunk of steel and see what chips I could make, but I don't think I'm going to be limited to .030 depth of cut any more. Probably it will be closer to a quarter inch.
Rick "qualified electrically but still nervous when first throwing that switch" Denney
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Good job and nice diagnostic work too!
