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*Lathe Modifications*
Jeff,
When I was at SWAT, I caught a glimpse of James Johnson's trailer mounted lathe, but not knowing about it, I did not go out to see it at close range. I recently read some information on the Hill Country Woodturners web site about him and his rig and now wish that I had taken the time to check it out.
My lathe that I will be converting to VFD is a Delta 1440 which currently has a mechanical variable speed drive. Other than the reliability of its variable Reeves drive system, it is a decent lathe, but because the pulleys are die-cast zinc, they tend to wear out rather quickly under moderate use (roughly 20 hours or more per month). Die-cast zinc is just not up to the task. Machined ductile cast iron would be much better.
After I rebuilt the mechanical drive (along with the rest of the headstock) two times, I decided that it could become a better machine by dumping the mechanical variable speed drive and going to something else. Of course, my engineering background in electronics and control system design pushed (or pulled) me in the direction of an electronically controlled drive system.
I was fortunate enough to find a 1.5 HP Baldor three-phase TEBC motor (model ZDM3584T-5) designed for vector-control drive on eBay and got it for less than $100 (it lists for more than $2100). I think that it was because it was too exotic for most applications. It has a shaft mounted optical encoder to provide both velocity and position feedback to the controller. It also has a fixed speed blower which is important for cooling at very slow speeds. I also got a Baldor ZD18H402-E vector controller on eBay for a bit more than $100 which, new would have been over $2400. I suppose that I got it so cheaply because it is a 460 VAC controller. But, this also meant that I needed to get a couple transformers to convert 240 VAC up to 480 VAC, which I also was able to find on eBay.
Since the motor weighs 60 pounds, the two transformers weigh 60 pounds each and the controller and other stuff will add another 50 pounds, the weight of my lathe will increase about 200 pounds even after subtracting the weight of the current 3/4 HP motor. I have not decided on a configuration to mount the motor and am considering placing it above the headstock so that its 60 pounds would not be cantilevered off the back side.
The new motor was selected to have twice the HP rating of the old one, not to make it a much more powerful machine, but so that a low speed torque could be produced that is comparable to the current low speed torque, but at a much lower minimum speed.
Although I have all of the stuff that I need, one of the biggest obstacles to completing the project is that I would not have a lathe for a few weeks. Since nothing is assembled yet, I do not have any pictures to share. Hopefully, I will soon. I may take pictures of the controller, motor, and transformers. I guess that I am a tinkerer and can't leave things well enough alone.
Postscript to Michael, you had written MOSFET circuits at least are programmable to provide the proper combination of voltage and frequency to provide maximum torque through the range of the motor. Exclusive of feedback. Brushless motors use this.
There is one essential ingredient in all of these things that brushless DC motors absolutely must have and which is rarely found on AC or DC motors that drive lathes and other such machines -- a means of velocity feedback, either an encoder or resolver. You usually don't see the encoder on brushless DC motors because they are integral to the motor design, but there nonetheless. Without direct measurement of motor speed there is no way to know within any reasonable confidence bounds, what its speed may be. Sometimes some stepper motors are used without a separate feedback device, but steppers are low torque devices which are not suitable for driving any kind of power machinery (the motor in a hard disk drive is a stepper). There are various types of MOSFET devices -- I had assumed that you were talking about power MOSFETs, but perhaps not. Usually DSP's which are MOSFET IC's are used in the controllers to perform the processing and if discrete power MOSFET's are used at all it would be for power switching and not significantly differednt than other devices such as IGBT's. Anyway, the signal processing and power switching are not performed in the same package.
Some of the mid-range AC motor controllers use what is called sensorless-vector control which is basically a look-up table function that estimates the motor speed based on frequency and current. They are satisfactory for many applications where precise speed is not important, but they suffer from slow loop response if there is a rapidly changing load torque or speed command and degraded at the low speed end. For a lathe, they are perfect, though.
Bill Boehme
Jeff,
When I was at SWAT, I caught a glimpse of James Johnson's trailer mounted lathe, but not knowing about it, I did not go out to see it at close range. I recently read some information on the Hill Country Woodturners web site about him and his rig and now wish that I had taken the time to check it out.
My lathe that I will be converting to VFD is a Delta 1440 which currently has a mechanical variable speed drive. Other than the reliability of its variable Reeves drive system, it is a decent lathe, but because the pulleys are die-cast zinc, they tend to wear out rather quickly under moderate use (roughly 20 hours or more per month). Die-cast zinc is just not up to the task. Machined ductile cast iron would be much better.
After I rebuilt the mechanical drive (along with the rest of the headstock) two times, I decided that it could become a better machine by dumping the mechanical variable speed drive and going to something else. Of course, my engineering background in electronics and control system design pushed (or pulled) me in the direction of an electronically controlled drive system.
I was fortunate enough to find a 1.5 HP Baldor three-phase TEBC motor (model ZDM3584T-5) designed for vector-control drive on eBay and got it for less than $100 (it lists for more than $2100). I think that it was because it was too exotic for most applications. It has a shaft mounted optical encoder to provide both velocity and position feedback to the controller. It also has a fixed speed blower which is important for cooling at very slow speeds. I also got a Baldor ZD18H402-E vector controller on eBay for a bit more than $100 which, new would have been over $2400. I suppose that I got it so cheaply because it is a 460 VAC controller. But, this also meant that I needed to get a couple transformers to convert 240 VAC up to 480 VAC, which I also was able to find on eBay.
Since the motor weighs 60 pounds, the two transformers weigh 60 pounds each and the controller and other stuff will add another 50 pounds, the weight of my lathe will increase about 200 pounds even after subtracting the weight of the current 3/4 HP motor. I have not decided on a configuration to mount the motor and am considering placing it above the headstock so that its 60 pounds would not be cantilevered off the back side.
The new motor was selected to have twice the HP rating of the old one, not to make it a much more powerful machine, but so that a low speed torque could be produced that is comparable to the current low speed torque, but at a much lower minimum speed.
Although I have all of the stuff that I need, one of the biggest obstacles to completing the project is that I would not have a lathe for a few weeks. Since nothing is assembled yet, I do not have any pictures to share. Hopefully, I will soon. I may take pictures of the controller, motor, and transformers. I guess that I am a tinkerer and can't leave things well enough alone.
Postscript to Michael, you had written MOSFET circuits at least are programmable to provide the proper combination of voltage and frequency to provide maximum torque through the range of the motor. Exclusive of feedback. Brushless motors use this.
There is one essential ingredient in all of these things that brushless DC motors absolutely must have and which is rarely found on AC or DC motors that drive lathes and other such machines -- a means of velocity feedback, either an encoder or resolver. You usually don't see the encoder on brushless DC motors because they are integral to the motor design, but there nonetheless. Without direct measurement of motor speed there is no way to know within any reasonable confidence bounds, what its speed may be. Sometimes some stepper motors are used without a separate feedback device, but steppers are low torque devices which are not suitable for driving any kind of power machinery (the motor in a hard disk drive is a stepper). There are various types of MOSFET devices -- I had assumed that you were talking about power MOSFETs, but perhaps not. Usually DSP's which are MOSFET IC's are used in the controllers to perform the processing and if discrete power MOSFET's are used at all it would be for power switching and not significantly differednt than other devices such as IGBT's. Anyway, the signal processing and power switching are not performed in the same package.
Some of the mid-range AC motor controllers use what is called sensorless-vector control which is basically a look-up table function that estimates the motor speed based on frequency and current. They are satisfactory for many applications where precise speed is not important, but they suffer from slow loop response if there is a rapidly changing load torque or speed command and degraded at the low speed end. For a lathe, they are perfect, though.
Bill Boehme
