Lathe Motor Amperage Rating

[Chanceywd]

I read the whole thread looking for the ones using the right formula for 3 ph wattage. Some folks think that they have 2 phase current in their single phase service and the VFD just adds a third one.
Service factor of motors comes into play if you are thinking of loading it near it's rated horsepower. Some of the motors in an effort to be "green" and lower power used have a lower service factor. You will find this on the id plate as S.F.

 

[Dakota_male]

I converted my Craftsman drill press to a 3 phase motor and VFD a couple of years ago, and love it. No more moving belts around, just dial in the speed I want and drill holes. I used a Teco 510 VFD and a Connex 1 HP 3 phase motor I picked up at a local motor repair shop, made a new control panel and added a tach display to it.
I'm looking at doing the same to my Jet B920N lathView attachment 780201e.

View attachment 780198View attachment 780200

I did the same as you with my 2hp milling and drill press and also my 2hp lathe. on the lathe I enjoy the soft start especially when cutting metric threads as I have to reverse leaving the split nut engaged When taking multiple cuts.

 

[strantor]

I read the whole thread looking for the ones using the right formula for 3 ph wattage. Some folks think that they have 2 phase current in their single phase service and the VFD just adds a third one.

Thanks for bringing this up. I think you are being generous in saying "some folks" - I would say "most folks." It is a pervasive misconception perpetuated by many sources online, even in industry whitepapers and in tutorials from respected sources such as:

This stems from the way it is taught in classrooms, especially in electronics curriculum. Students are taught to think of L1 and L2 as two separate sources 180 degrees out of phase, I think because this is most convenient terms in to explain the concept using only the fundamentals which have been previously learned by the time this topic is introduced. Those who don't complete the course of education or don't get a chance to apply it in the real world are deprived of the opportunity to realize that Santa Claus isn't real.

It is much better IMO to teach it as a single source which has been split in the middle. Because that's exactly what it is. You can make a split phase transformer out of an isolation transformer by scratching the enamel off the center winding and soldering a wire to it. This does not magically invert the polarity or shift the phase of half of the secondary. And if you connect that new 3rd wire to earth and call it neutral, still no magic happens. I have made a couple of videos on this topic:

 

[Chanceywd]

Thanks for bringing this up. I think you are being generous in saying "some folks" - I would say "most folks." It is a pervasive misconception perpetuated by many sources online, even in industry whitepapers and in tutorials from respected sources such as:

This stems from the way it is taught in classrooms, especially in electronics curriculum. Students are taught to think of L1 and L2 as two separate sources 180 degrees out of phase, I think because this is most convenient terms in to explain the concept using only the fundamentals which have been previously learned by the time this topic is introduced. Those who don't complete the course of education or don't get a chance to apply it in the real world are deprived of the opportunity to realize that Santa Claus isn't real.

It is much better IMO to teach it as a single source which has been split in the middle. Because that's exactly what it is. You can make a split phase transformer out of an isolation transformer by scratching the enamel off the center winding and soldering a wire to it. This does not magically invert the polarity or shift the phase of half of the secondary. And if you connect that new 3rd wire to earth and call it neutral, still no magic happens. I have made a couple of videos on this topic:

I worked 26 years in maintenance at a monofilament plant in mechanical and electrical. Took a few courses in high school on electric and at Boces. And some electronics and training on Drives ac and dc. No degrees. We had a lot of German equipment and it is often connected in a 220/380 y. That is another thing in how motors are wired. One course sticks out in that the guy stressed each phase is transformer a,b, or c. In a delta a phase is transformer wired from 1 phase to a second phase of the line. In Y it is a phase to neutral.
If you look on the pole feeding your houses you will see 1 transformer. If you see the one feeding a commercial place like Mcdonalds you see 3 for the 3 phase used there. An exception in our area is high leg on a Y line and you will see 1 lg and 1 small.
So all I am saying is there is a lot going on in that little mystery VFD that makes it work on single phase. I have a couple on my rockwell mill and rockwell drill press. And 1.73 figures in the math.

Bill

 

[LD1]

120/240 is still single phase....but the phase is split to two legs at half the voltage

Rarely do I see people screw up the wattage. Cause it's simple amps x voltage.

But yes, screwing up the fundamentals thinking there are 2-phases.

But there were more a few that got the correct formula for 3-phase wattage.

Gotta be careful talking electric on a tractor forum though.... abbreviating 3-phase as 3ph can be misunderstood as 3-point-hitch

 

[Paul_M]

Thank you both for correcting me here. I thought I had a handle on wattage calculation from what I've read but it looks like I had just enough information to be dangerous as the three phase calculation is different! I appreciate it! This sort of good info from people willing to share is why I participate in these forums - I learn something new every time!

I might have missed this point in some other reply, but it's important: All kinds of motors draw more current during start-up. This is because motors generate "back EMF" in proportion to their speed. Back EMF opposes the supply voltage, thereby reducing supply current. If the motor doesn't come up to near its "synchronous speed", this "startup surge" continues. Motors that work against a significant load, and this includes most lathes because of their large inertial mass, can have a long startup surge. Startup surges can last long enough to pop breakers, and this is a consideration in sizing the wiring and the breakers.
Single-phase motors can have a particularly large startup surge. This is because their starting windings are connected to the supply during startup. The startup windings can draw as much or more current as the other stator windings. It is not unusual for single-phase motors to draw 3 or more times their full-load "run" amperage, which is often what is on the nameplate.
Startup surge current is often a limiting factor for VFD operation. Each VFD has an electronically limited ability to supply current which is generally proportional to its horsepower rating. When the motor "tries" to draw more current, the VFD throttles back the supply voltage to keep its output current within its safe output levels. If your VFD/motor combination fails to bring the motor up to the expected speed, this is the usual reason. And, this is the real reason why people say you need to choose a VFD with double or more the HP rating of the motor. In most cases, you can increase the "acceleration time" parameter of the VFD to a value that works, but you may be disappointed in how long it takes the motor to come up to speed.
The fact that the motor generates "back EMF" also accounts for a couple of VFD issues. If you suddenly stop or reverse the motor, the resultant surge of current back into the VFD can trigger its overload circuitry. A common workaround is to set "deceleration time" to a longer time, say 5 to 10 seconds. Again, you may be disappointed if you're used to having the motor immediately stop or change direction. The better way is to install a "braking resistor" (assuming your VFD has this capability" which can absorb the generated power and stop the motor more quickly.
A final note: most electric motors are very efficient, usually upwards of 90%. The same is true of VFDs. In many postings here, people assume that the differences they're seeing are due to efficiency differences. This is usually not the case. Most often, differences are related to either measurement problems or misuse of formulae for calculating power.
I designed VFDs, motors and transformers for many years, so this is not guesswork.

 

[coast40]

I picked up 2 free treadmills for the DC motors and controllers to use in my small milling machine and lathe. Based on Youtube and internet I can make them work. I might need to buy controllers to simplify the wiring.

 

[rScotty]

Thanks for bringing this up. I think you are being generous in saying "some folks" - I would say "most folks." It is a pervasive misconception perpetuated by many sources online, even in industry whitepapers and in tutorials from respected sources such as:

This stems from the way it is taught in classrooms, especially in electronics curriculum. Students are taught to think of L1 and L2 as two separate sources 180 degrees out of phase, I think because this is most convenient terms in to explain the concept using only the fundamentals which have been previously learned by the time this topic is introduced. Those who don't complete the course of education or don't get a chance to apply it in the real world are deprived of the opportunity to realize that Santa Claus isn't real.

It is much better IMO to teach it as a single source which has been split in the middle. Because that's exactly what it is. You can make a split phase transformer out of an isolation transformer by scratching the enamel off the center winding and soldering a wire to it. This does not magically invert the polarity or shift the phase of half of the secondary. And if you connect that new 3rd wire to earth and call it neutral, still no magic happens. I have made a couple of videos on this topic:

Is it just me or are these terrible videos? I sat down to enjoy them, and and quickly realized either they don't understand what they are talking about or can't explain it. Maybe both.
rScotty

 

[MTGreen]

Is it just me or are these terrible videos? I sat down to enjoy them, and and quickly realized either they don't understand what they are talking about or can't explain it. Maybe both.
rScotty

If you can't explain it simply, you don't understand it well enough.

Albert Einstein

 

[rScotty]

If you can't explain it simply, you don't understand it well enough.

Albert Einstein

Thank you, Albert.
rScotty