Power Supply for CNC
One of the first things that one needs to retrofit a Bridgeport Boss CNC machine is a power supply for the steppers. That is assuming that one has drives which I do in the form of Geckodrive G201s. The original Bridgeport power supply for the steppers put out 56 volts dc so most people use this voltage although some use 60 or even higher voltages. As far as current goes most people use 6-7 amps, 7 amps being the maximum that G201s will source.
While researching stepper power supplies I found a great reference on Gecko's website. http://www.geckodrive.com/upload/Step_motor_basics.pdf
This document not only shows how to wire the drives but gives a good example of a power supply with sample component values. Basically a power supply (unregulated) only needs a transformer putting out a desired voltage that is rectified by a full diode bridge and filtered by a big capacitor.
To get 56vac you need un-rectified 39vac. (56 v /1.414 = 39.6 v)
For a capacitor value C = (80000 uF x I) / V = 80000 uF x 5 amps / 56 volts = 7142 uF.
The closest standard value to 7100 is 6800.
For a bridge there are several high amperage bridges that are rated at a couple hundred volts. I found the MB352 made by Diodes Inc at Digikey that is rated 200 vac and 35 amps and only costs $4.50.
I designed a power supply using the suggested circuit but wanted some verification so I called Gecko tech support and had a very useful conversation with Mariss Freimanis from Geckodrive. After talking to Mariss I decided to run the motors at 56 volts and 6 amps. The motors are rated near/at 6 amps so giving myself 1 amp as a safety measure seems like a good idea.
It was suggested by Gecko that I first test my setup at 24v and about 5 amps. The drives are "hard to kill" at this voltage. After one maxes out the IPM at 24vdc the ratio of desired IPM is used to calculate the voltage needed compared to 24vdc. I will be happy with the IPM that I get at 56vdc which should be 150-180 IPM. I have decided to check out the drives and the motors at 24vdc first though however to get everything right.
It is suggested at CNCzone by a few people that one of the transformers (T1) in the control cabinet makes a great stepper power supply transformer. Also there are computer grade capacitors and a big heatsink in the computer cabinet that can be used. I did not have a computer cabinet so I had to provide my own.
T1 is a 240v transformer that provides 120v for the original setup. From the the CNCzone forum it is supposed to be rated at 1000va although I could not find this information on my own. The problem with T1 is that it does not have a tap at 39vac although a few people either rewind the secondary winding 0r cut a few windings out to make a 39 vac tap. The examples on CNCzone of this procedure have been with a 110vac input to the transformer.
It seems to me that using 120vac is not neccesery when you have 240 vac (which the machine needs to run anyway) already in the cabinet. The 240 is required if one is using a VFD to create the 208 three phase for the motor. Not having 3 phase in my shop this is a requirement for my installation. So with a single input of 240 vac I plan to run the VFD and T1 which will produce 120vac for the fans and the lube system (and coolant eventually) and 39vac for the stepper power supply.
According to the forum the secondary of T1 is 3 windings of 19 turns. So quickly I can estimate that 120vac/(3 x 19) turns or 120vac/57 turns or 2.10 vac / turn. So to get 39 vac 39vac / 2.10 vac/turn = 18.5 turns.
Hmmm... This is awful close to 1 winding. So if I put in a "tap" on the top of the second winding (or simply 19 turns away from X1) then I should get 39 vac. This is simple to test by plugging T1 into 240 and using a meter to look for a 39vac potential from X1 to one of the windings. By pressing (hard) the meter probe through the varnish on the winding you can find the magic spot and get the right voltage.
My test was a success but it was even easier than I thought. 39.6 vac (exactly) exists inside the core of the transformer but 40 vac (39.9) is on the top of the FIRST winding right below the terminals for X1 and X3. This is VERY simple. All I had to do was pull out the top winding and scrape off the varnish to solder on a new tap for the transformer. I made a new terminal from an extra contact from the 240 plug that I bought for the machine and connected it with a scrap of solid wire. The winding seemed to be 10awg which should handle the current just fine.
(Note that the schematics from Bridgeport show T1 as having multiple taps so there are probably multiple flavors of T1 and some may not be as easy to modify.)
While researching stepper power supplies I found a great reference on Gecko's website. http://www.geckodrive.com/upload/Step_motor_basics.pdf
This document not only shows how to wire the drives but gives a good example of a power supply with sample component values. Basically a power supply (unregulated) only needs a transformer putting out a desired voltage that is rectified by a full diode bridge and filtered by a big capacitor.
To get 56vac you need un-rectified 39vac. (56 v /1.414 = 39.6 v)
For a capacitor value C = (80000 uF x I) / V = 80000 uF x 5 amps / 56 volts = 7142 uF.
The closest standard value to 7100 is 6800.
For a bridge there are several high amperage bridges that are rated at a couple hundred volts. I found the MB352 made by Diodes Inc at Digikey that is rated 200 vac and 35 amps and only costs $4.50.
I designed a power supply using the suggested circuit but wanted some verification so I called Gecko tech support and had a very useful conversation with Mariss Freimanis from Geckodrive. After talking to Mariss I decided to run the motors at 56 volts and 6 amps. The motors are rated near/at 6 amps so giving myself 1 amp as a safety measure seems like a good idea.
It was suggested by Gecko that I first test my setup at 24v and about 5 amps. The drives are "hard to kill" at this voltage. After one maxes out the IPM at 24vdc the ratio of desired IPM is used to calculate the voltage needed compared to 24vdc. I will be happy with the IPM that I get at 56vdc which should be 150-180 IPM. I have decided to check out the drives and the motors at 24vdc first though however to get everything right.
It is suggested at CNCzone by a few people that one of the transformers (T1) in the control cabinet makes a great stepper power supply transformer. Also there are computer grade capacitors and a big heatsink in the computer cabinet that can be used. I did not have a computer cabinet so I had to provide my own.
T1 is a 240v transformer that provides 120v for the original setup. From the the CNCzone forum it is supposed to be rated at 1000va although I could not find this information on my own. The problem with T1 is that it does not have a tap at 39vac although a few people either rewind the secondary winding 0r cut a few windings out to make a 39 vac tap. The examples on CNCzone of this procedure have been with a 110vac input to the transformer.
It seems to me that using 120vac is not neccesery when you have 240 vac (which the machine needs to run anyway) already in the cabinet. The 240 is required if one is using a VFD to create the 208 three phase for the motor. Not having 3 phase in my shop this is a requirement for my installation. So with a single input of 240 vac I plan to run the VFD and T1 which will produce 120vac for the fans and the lube system (and coolant eventually) and 39vac for the stepper power supply.
According to the forum the secondary of T1 is 3 windings of 19 turns. So quickly I can estimate that 120vac/(3 x 19) turns or 120vac/57 turns or 2.10 vac / turn. So to get 39 vac 39vac / 2.10 vac/turn = 18.5 turns.
Hmmm... This is awful close to 1 winding. So if I put in a "tap" on the top of the second winding (or simply 19 turns away from X1) then I should get 39 vac. This is simple to test by plugging T1 into 240 and using a meter to look for a 39vac potential from X1 to one of the windings. By pressing (hard) the meter probe through the varnish on the winding you can find the magic spot and get the right voltage.
My test was a success but it was even easier than I thought. 39.6 vac (exactly) exists inside the core of the transformer but 40 vac (39.9) is on the top of the FIRST winding right below the terminals for X1 and X3. This is VERY simple. All I had to do was pull out the top winding and scrape off the varnish to solder on a new tap for the transformer. I made a new terminal from an extra contact from the 240 plug that I bought for the machine and connected it with a scrap of solid wire. The winding seemed to be 10awg which should handle the current just fine.
(Note that the schematics from Bridgeport show T1 as having multiple taps so there are probably multiple flavors of T1 and some may not be as easy to modify.)
Comments
Are you still happy with the original bridgeport power supply? I can't figure out why people tear them out and buy new power supplies.
I would love to converse more about your retrofit if you have the time. I am also looking for the wiring diagrams and any printed manuals for the machine.
Matthew
But I agree, to reduce cost there is no better power supply than one that is basically free. And it works fine. Never had a problem with power being available for the steppers. However, I do not run my machine fast as I am still too much a machining newbie.