Hi @Beantown
I understand your desire to use technology, PLC, that you are familiar with. This is normal, but, it seems you are disregarding all the controls that the VFD has already built in so that a PLC can be added. The VFD is driven by some form of internal computer and is constantly sweeping the control inputs to make the desired changes. By the way, a PLC is not free, takes up extra space and power, as well as being something additional that can fail. If there were no VFD involved or you were trying to control multiple VFDs and motors I think I could see a reason to add this extra logic power. However, the lathe is a simple system. We only added the VFD to get to 3 phase, then because it had all of these extra functions we begin to add things like variable speed, electronic braking, proximity sensor, etc. By the way the transistors I used cost less than $0.10 each. Good capacitors cost more than small transistors. Relays are far more costly.
I am not familiar with the VFD you purchased, but I assume it has similar functions to the Hitachi. It would seem that to simply turn all of the signals off to the VFD is a bit like surrendering control of the system to what ever the VFD might happen to do. Also, there is a bit of concern as to what effects electrical noise might have on the VFD inputs when there are no signal being applied what so ever. You should definitely have some form of safety latch whether is is latch I designed, the relay system in the original equipment, or some other form that you come up with. Don't get lazy and think that nothing is ever going to go wrong. It WILL and you infinity want to be able to stop the machine easily and quickly. I have seen people get caught in machinery and it is not pretty and I can tell you people panic and do strange things in trying to stop a machine.
Shutting off all powere is one approach, but note that if you choose to have a proximity sensor, which is nice to have, you need to figure out how you would implement it and how to move the tooling away from it when it is putting out a "stop" signal.
You might want to remember that the inputs to the VFD in my design become active when a current is supplied into the control terminal. It is sensitive to current (it is not a voltage driven system). A couple of milli-amps into an input activates it. No current, or a current below the threshold, means the input is deactivated.
There are no solid state relays in my design, except for one used to turn the 220Vac off and on to the coolant pump. The one I used is way overkill and I used it simply because it is common and I had a couple of extras laying around. I did design a two bi-polar transistor latch (Q2 and Q3) into the design (we have been calling the safety latch). I use a few other bipolar transistors as interfaces between the mechanical switches of the lathe and the inputs to the VFD and to enable the effect of the safety latch. The VFD programing provides the logic and control. In the design there are a few diodes at places like the Jog line (labeled Jog-F and Jog -R) to prevent the Forward/Reverse signals from causing a Jog signal simultaneously. Likewise at the auto oil/coolant line diodes D1 and D2 provide a similar effect, but are simply providing a signal to Q6 via Q7 to turn on Q6 when the coolant switch is in the auto coolant position. I suppose you could say that Q4 and Q5 enable an AND function. So this is a very low level logic. If the safety latch is not active then neither Q4 nor Q5 can conduct and so the effectively AND the Latch condition with the signal coming from the mechanical switch. (Don't get confused by the colored LED-diodes. Other than the one at Q1 they are not needed. They are just there to light up and so inform the observer if there is current flowing in that path.)
By the way, the Hitachi VFD has an internal 24V dc supply to drive the control inputs. It's output current is limited (~100ma ?), but never specified in the documentation. I chose not to use it, but rather to use a small, cheap (~$15), external 1 Amp, 24Vdc supply. I did this primarily to ensure not to burn out the VFD's 24V supply. Features like digital displays and especially the proximity sensor require far more current/power than the VFD inputs. This supply is sufficient to also provide power to the lathe LED lamp (~0.5 Amp).
Dave L.
PS. I have a friend who is now implementing a PCB board to make putting my circuit together easier. He just sent off and got a board made and will be populating it and testing it in the future. Then he will probably redesign it to accommodation small changes like the connector arrangements. His plan is, once he is happy with it, to make it available to others. Amazingly it cost almost nothing to have the board made. He has a different lathe than the 1440 and he already has a relay style VFD conversion, but he is going to replace it with the solid state version primarily to conserve space.
I understand your desire to use technology, PLC, that you are familiar with. This is normal, but, it seems you are disregarding all the controls that the VFD has already built in so that a PLC can be added. The VFD is driven by some form of internal computer and is constantly sweeping the control inputs to make the desired changes. By the way, a PLC is not free, takes up extra space and power, as well as being something additional that can fail. If there were no VFD involved or you were trying to control multiple VFDs and motors I think I could see a reason to add this extra logic power. However, the lathe is a simple system. We only added the VFD to get to 3 phase, then because it had all of these extra functions we begin to add things like variable speed, electronic braking, proximity sensor, etc. By the way the transistors I used cost less than $0.10 each. Good capacitors cost more than small transistors. Relays are far more costly.
I am not familiar with the VFD you purchased, but I assume it has similar functions to the Hitachi. It would seem that to simply turn all of the signals off to the VFD is a bit like surrendering control of the system to what ever the VFD might happen to do. Also, there is a bit of concern as to what effects electrical noise might have on the VFD inputs when there are no signal being applied what so ever. You should definitely have some form of safety latch whether is is latch I designed, the relay system in the original equipment, or some other form that you come up with. Don't get lazy and think that nothing is ever going to go wrong. It WILL and you infinity want to be able to stop the machine easily and quickly. I have seen people get caught in machinery and it is not pretty and I can tell you people panic and do strange things in trying to stop a machine.
Shutting off all powere is one approach, but note that if you choose to have a proximity sensor, which is nice to have, you need to figure out how you would implement it and how to move the tooling away from it when it is putting out a "stop" signal.
You might want to remember that the inputs to the VFD in my design become active when a current is supplied into the control terminal. It is sensitive to current (it is not a voltage driven system). A couple of milli-amps into an input activates it. No current, or a current below the threshold, means the input is deactivated.
There are no solid state relays in my design, except for one used to turn the 220Vac off and on to the coolant pump. The one I used is way overkill and I used it simply because it is common and I had a couple of extras laying around. I did design a two bi-polar transistor latch (Q2 and Q3) into the design (we have been calling the safety latch). I use a few other bipolar transistors as interfaces between the mechanical switches of the lathe and the inputs to the VFD and to enable the effect of the safety latch. The VFD programing provides the logic and control. In the design there are a few diodes at places like the Jog line (labeled Jog-F and Jog -R) to prevent the Forward/Reverse signals from causing a Jog signal simultaneously. Likewise at the auto oil/coolant line diodes D1 and D2 provide a similar effect, but are simply providing a signal to Q6 via Q7 to turn on Q6 when the coolant switch is in the auto coolant position. I suppose you could say that Q4 and Q5 enable an AND function. So this is a very low level logic. If the safety latch is not active then neither Q4 nor Q5 can conduct and so the effectively AND the Latch condition with the signal coming from the mechanical switch. (Don't get confused by the colored LED-diodes. Other than the one at Q1 they are not needed. They are just there to light up and so inform the observer if there is current flowing in that path.)
By the way, the Hitachi VFD has an internal 24V dc supply to drive the control inputs. It's output current is limited (~100ma ?), but never specified in the documentation. I chose not to use it, but rather to use a small, cheap (~$15), external 1 Amp, 24Vdc supply. I did this primarily to ensure not to burn out the VFD's 24V supply. Features like digital displays and especially the proximity sensor require far more current/power than the VFD inputs. This supply is sufficient to also provide power to the lathe LED lamp (~0.5 Amp).
Dave L.
PS. I have a friend who is now implementing a PCB board to make putting my circuit together easier. He just sent off and got a board made and will be populating it and testing it in the future. Then he will probably redesign it to accommodation small changes like the connector arrangements. His plan is, once he is happy with it, to make it available to others. Amazingly it cost almost nothing to have the board made. He has a different lathe than the 1440 and he already has a relay style VFD conversion, but he is going to replace it with the solid state version primarily to conserve space.
