PM-1440GT PLC\VFD Conversion? Stupid?

[Beantown]

Howdy! Now that my lathe and all the accessories are on the way (ETA Jan\Feb 2024) I'm starting to plan and source parts for the 3-phase conversion. As a result, I have been reverse engineering the OEM controls from pictures and threads from this forum. I think I have all the normal controls figured out, but I'm unsure on the following.

1. The foot brake. Is this operating the same was the e-stop circuit is killing power to the 24VAC controls? It's not shown in the manual like everything else is.

2. The gear cover interlock? I'm assuming it's using the e-stop circuit as well?

3. Proximity stops. It appears the most common method is to add a relay that kills power to the forward\reverse controls? Am I reading that correct?

I'm going a bit of a weird route with a Programmable Logic Controller. I know it's overkill and unconventional, but it will be fun and it will be far easier to wrap my mind around software logic rather than the relay logic. I am switching from physical relay logic to software controlled logic, but PLC runs pretty much every manufacturing plant in the world. In theory it should be extremely reliable?

I do plan to wire the e-stop to a physical "safety relay" that will control a contactor to kill 3-phase power to the motor. I'm not sure if there is a better way, but from how I'm seeing it that should be the surefire method and then I can still allow the VFD\PLC logic to brake the lathe. I've had relays and contactors get "stuck" on me, so they are not infallible either.

I am thinking I can do the proximity stop in PLC without concern. The operating time of a relay is milliseconds, and a PLC would be able to scan the entire logic code in microseconds.

I haven't purchased anything but the VFD up to now, but PLC doesn't seem like a dumb way to go? Or is it?

 

[Reddinr]

Hey Beantown! Here are my impressions.

If your foot brake is like the one on my Grizzly then it will be a mechanical brake that has brake shoes that provide friction to stop the motor quickly AND a switch to stop the motor. Very handy for quick start/stops. Love it.

I can guess that the gear cover interlock will stop the spindle motor.

If you use a PLC, be sure to not include it in the emergency circuit except you can provide an input to the PLC to "tell" the PLC that an EMO event has occurred.

Don't put a contactor between your future VFD and the motor. Opening the contactor when the motor is powered by the VFD can kill the VFD. It might be possible to use your brake with the VFD to slow the motor quickly. Depends on what your brake actually does on your machine.

So, years ago when I bought my lathe I had big plans for lots of modifications. After 8 years or so, I actually only did one. I added a DRO. I found that the lathe did pretty great things without any further bells and whistles. It patiently waited for me to get better at using it. To tell the truth, I use the DRO only sometimes. Things I didn't do that I thought I would: Add ELS, Build a taper jig, add speed control via VFD, add tachometer, "rewire" using PLC...

 

[Beantown]

Don't put a contactor between your future VFD and the motor. Opening the contactor when the motor is powered by the VFD can kill the VFD. It might be possible to use your brake with the VFD to slow the motor quickly. Depends on what your brake actually does on your machine.

Oh man, good point. I could see how that could be damaging to the VFD and it wouldn't allow the dynamic braking to work either. Thanks!

If you use a PLC, be sure to not include it in the emergency circuit except you can provide an input to the PLC to "tell" the PLC that an EMO event has occurred.

Yea, that is what I was originally planning to do, but then I was reading that OSHA doesn't allow you to do this. I guess they want the safety circuit to be physical. I don't really have to worry about OSHA, but I did want to do it as safe as possible. I'll have to think this one through a bit more.

So, years ago when I bought my lathe I had big plans for lots of modifications. After 8 years or so, I actually only did one. I added a DRO. I found that the lathe did pretty great things without any further bells and whistles. It patiently waited for me to get better at using it. To tell the truth, I use the DRO only sometimes. Things I didn't do that I thought I would: Add ELS, Build a taper jig, add speed control via VFD, add tachometer, "rewire" using PLC...

Yea, I didn't really need to go all gungho on mods just yet, but I need to convert to VFD either way as I bought a 3-phase lathe. I could do the easy route using the OEM contactors using the "basic" conversion, but that isn't any fun.

 

[B2]

Hi @Beantown

Your question issues:
1) Mechanical brake switch is essentially in series, but it is in series with the latching relay not just the 24Vdc supply.
2) Cover interlock also.
3) Proximity switch of VFD conversions also described below. It is a little different from the estop, cover interlock, and brake switch. The relay you mentioned ("latching relay") is very important, is used for most everything, and is fundamental to all safety features!

Did you happen to look at my Part 1 write up of how the original wiring of the original electronics of the PM1994GT works? Read the description or look at the schematic circuit diagram, Figure 2, page 14, located at the end of "PM 1440GT Part 1 FacOriglElecDescript DNL L910_1440.pdf" the first attachment at the link:

VFD conversion using solid state electronic components.

I will also attach just the schematic, Figure 2, page 14, below. If you want to actually see how the wires are physically connected then look at Figure 1, page 13, of this document.

In the original electronics there is a multiple contact safety "Latching Relay" which is required to have its contacts in the closed position so that there is conduction and so that the 24 volt powered is available to other parts of the lathe controls/relays. The foot brake, along with other safety components like the e-stop, cover interlock, etc are wired in series such that when the brake switch goes open it breaks the power connection to this relay and shuts down the 24 volts to all of the relays etc. Hence, the mechanics brake both applies friction to the spindle as well as turns the power off to the motor. In order to reset this relay after the brake is release the control lever must be moved to the Neutral switch position before being moved to a forward or reverse position. When the control switch is move to Neutral it supplies power to the Latching relay's coil causing it to be re-latched ON. (Hence, in my VFD conversion I put all of these series safety switches in series with my electronic safety latch. When any of these switches are opened then the electronic latch become unlatched shutting off the 24 volts to the control feeds to the VFD inputs.

Looking at the schematic you will see the brake switch near the left middle-bottom. 24Vdc power flows through it upward in the drawing. Next note the red line running to the Latching Relay. It connects to the most bottom used contact and out the other side. This is then connected back to the input of the relay coil at the top left of this relay. The feed back path provides the power to the latching relay to stay on even when the Neutral switch position is disconnected. Hence, the latch remains "Latched ON" and the contacts conducting until the power is removed from the relay by one of the safety switches. The brake switch that came with my PM1440GT is a Normally closed (NC) switch and I could use it with VFD my controls. (It was not clear in some of the HM comments about whether or not this switch could be NC or if it needed to be replaced with a NO version in the relay driven version of the VFD conversion.)

You might note that the even when the brake switch trips the latching relay off, the indicator light says on showing that there is still power available and the lathe can be made to run. However, it will not until the control switch has been moved to the Neutral position resitting the latch. On the other hand the e-stop and cover switches turn this light off and it remains of until the e-stop switch is reset (or the cover is replaced). Even after that the control switch must be placed in the Neutral position before the safety latch can be turned back on and the lathe can run. It might be pointed out that if the contacts on the latch relay were to weld shut, which is not an unheard of failure mode, the Latch could only be turned off by removing all power from the machine. Similarly if the latch relays became worn or dirty they might not conduct and the latch would NOT latch on. In which case the controls could never turn on the lathe even though the indicator lamp would be on!. There is a fuse in the circuit that would have a similar effect.

The VFD proximity switches cause the safety switch to deactivate. Hence, 24 volt cannot get to the VFD. It effect it operates is the same series style mode. In my solid state design the proximity switch also breaks the current in the latch circuit.

In the original wiring the Jog circuit also does not work to move the spindle when the latch is not active.

However, in many, if not all, of the relay based VFD conversions the Jog switch does not work when the Proximity switch is activated. Hence, a second push button switch is used to to over come this. This push button switch must be held down while the jog switch is activated to get the proximity switch out of range. I cannot recall now in those circuits if the latching relay must be reset first. I did this differently in my electronic VFD conversion. The Jog switch does not require a separate push button switch eliminating the two handed operation, but is still safe. This way you can still move out of the proximity switch range without touching the control levers. I considered this as nice convenience feature. YOu do not have to activate the latching relay inorder to back away from the proximity switch range. However, either approach works.

Dave L.

 

[B2]

Hi

I haven't purchased anything but the VFD up to now, but PLC doesn't seem like a dumb way to go? Or is it

I think all of the features your are going to put into a PLC are already built into the Hitachi VFD. The hardest thing about the VFD conversion is not the logic or programing, but just simply the physical geometry, the connectors and running wires to and from the switches, sensors, potentiometer, displays etc. The VFD has about 8-9 inputs, called intelligent inputs, for taking care of all of these sorts of things. Then there is the internal program code to allow you to decide how you want to use these and what effect they have at the VFD power outputs. I provide a short description on how the VFD works in my Part 2. Open the pdf file, then you can either use the links to find part of the document or you can simply go to page 4 where there is a section called "C. Short VFD (Inverter) Description and Chosen Design Functions:". There you will find the various inputs and functionality describe. Or you can wade in really deep and read the Hitachi 600 page bloated manual, which is what I did!

By the way, on page 3, I listed the factory supplied lathe operational features and then I listed the VFD conversion additional features:

Original lathe features: (FP=located at the Lathe Front Panel)
On-Neutral-Off Control
Jog (forward direction only at 60Hz, high speed, FP)
Coolant Switch (Off-On, FP)
Active Safety Latching Relay (Neutral Control Position Latches On)
E-Stop (FP)
Cover Interlock (breaks relay latching)
Indicator Lamp (E-Stop & Cover Interlock enabled, breaks latching, FP)

Added lathe features:
Solid State Design (no-mechanical relays)
Input power: 220Vac single phase, 30Amp
VFD provides 3ph power variable frequency (a few Hertz to 400Hz)
Speed Control (Variable via Potentiometer, FP) 150
Frequency Preview (Digital Voltage Readout, FP, 0-10 volts)
Spindle RPM (Hall Effect, High Resolution Digital Readout, FP)
Jog: Forward & Reverse Momentary Switch (low speed, 6Hz, FP)
Integrated Electronic Braking Resistor (Small, 50 Ohm, 400 Watt)
Dual Rate Electronic Braking Switch (2 position switch, FP)
Automatic-Manual-Off Coolant Switch (3 position switch, FP)
Proximity Stop Sense Input (FP connector)
Proximity Stop Fail Safety Switch Input (FP connector)
Latch Ready Indicator Lamp (FP)
Manual Foot Brake & Power Loss Critical Safety Feature 160
Critical Safety Feature Reset Switch (FP)

There are even other things and logic that the VFD can do and apply, but does one need more on a stand alone lathe?

Dave L.

 

[Beantown]

Did you happen to look at my Part 1 write up of how the original wiring of the original electronics of the PM1994GT works?

Oh my, I read everything BUT part 1 to be honest. I will 100% read it.....it pretty much answers all of my questions and then some. Your thread has been unbelievably helpful getting to me to where I'm at today...I just missed the first lesson so that will fill in most of my gaps.

I think all of the features your are going to put into a PLC are already built into the Hitachi VFD.

I believe the Yaskawa GA500 I purchase has the same logic capability as well. You just assign input a function rather than writing the ladder logic. It only has 7-inputs, but honestly is should be enough. But, I was thinking with the PLC I could really customize things. Like having it control the worklight and coolant pump with various routines. I could push function button on an HMI screen and it would turn on the work light and set the coolant pump to activate with the spindle is running and silly stuff like that. I'm just overthinking it at this point. I might dial it back into reality and just use the VFD as you suggest.

Anyway, I just wanted to post a thank you in advance so you know your efforts to type this all up is appreciated and will be thoroughly read. It will just take me some time to really digest it. Hopefully I have time this evening. Since I'm an electronics hack I have to read it a few times and really study the schematic for things to click....but once they do I generally end up with a solid understanding.

 

[B2]

Thanks for the nice reply! I was a Professor/teacher/researcher before I retired a few years back, so yes, I try to document things so that all of my efforts might be useful to others. Simply put I get joy out of helping other to learn. However, my write ups tend to be on the long side. Also, while I have worked with my hands a lot, I did not have much experience at metal machining prior to retiring. So I appreciate all of the postings on HM. I still consider myself a newbie.

By the way, my VFD conversion included the auto oil feature. I have a 3 position switch on the control panel for oil/coolant: Off, Manual On, Auto On. It controls the pump that came with the machine and turn the pump on base upon the switch position when ever there is a Forward or Reverse Run process. However, I will admit that for almost any job I am doing I do not turn on this pump..... Oil every where! I should probably upgrade to a mist-coolant system. Maybe that would be less messy. I have flood coolant on my CNC Mill and when I use it I have quite a mess. But when I have a long job with a lot of cutting of steel I do use it.

 

[Beantown]

Oh man! This was super helpful! The latching concept makes perfect sense, but I never knew that is what it was using. It all makes so much sense now.

Thanks for the nice reply! I was a Professor/teacher/researcher before I retired a few years back, so yes, I try to document things so that all of my efforts might be useful to others. Simply put I get joy out of helping other to learn. However, my write ups tend to be on the long side. Also, while I have worked with my hands a lot, I did not have much experience at metal machining prior to retiring. So I appreciate all of the postings on HM. I still consider myself a newbie.

Well that certainly shows! I love all the information I can get, so I enjoy a long read. It's much appreciated.

The relay you mentioned ("latching relay") is very important, is used for most everything, and is fundamental to all safety features!

It could just be my ignorance, but the more I think about it the more I wonder why I would even need a safety latching relay in a VFD conversion? Just killing the 24V controls via a safety relay to the VFD doesn't mean it can't freak out and do something stupid, correct? The VFD I have has a "baselock" command that I can configure as NO or NC. The baselock command kills ALL inputs to the VFD and coasts the motor to a stop. No commands will work until that is enabled again. Couldn't I just wire my e-stop directly to that input? I could also wire the brake to this command as well, but it wouldn't latch like the e-stop would.....but I could bypass all the relays (solid-state or coil) with the PLC and do the latching in my logic code.

Added lathe features:
Solid State Design (no-mechanical relays)
Input power: 220Vac single phase, 30Amp
VFD provides 3ph power variable frequency (a few Hertz to 400Hz)
Speed Control (Variable via Potentiometer, FP) 150
Frequency Preview (Digital Voltage Readout, FP, 0-10 volts)
Spindle RPM (Hall Effect, High Resolution Digital Readout, FP)
Jog: Forward & Reverse Momentary Switch (low speed, 6Hz, FP)
Integrated Electronic Braking Resistor (Small, 50 Ohm, 400 Watt)
Dual Rate Electronic Braking Switch (2 position switch, FP)
Automatic-Manual-Off Coolant Switch (3 position switch, FP)
Proximity Stop Sense Input (FP connector)
Proximity Stop Fail Safety Switch Input (FP connector)
Latch Ready Indicator Lamp (FP)
Manual Foot Brake & Power Loss Critical Safety Feature 160
Critical Safety Feature Reset Switch (FP)

Although you are using the VFD for many of the features, it appears that you are using solid-state relays to create some basic logic. At least the for the safety circuit, dual mode coolant switch, etc. I could replicate all of that with the PLC I believe?

 

[Beantown]

It could just be my ignorance, but the more I think about it the more I wonder why I would even need a safety latching relay in a VFD conversion? Just killing the 24V controls via a safety relay to the VFD doesn't mean it can't freak out and do something stupid, correct? The VFD I have has a "baselock" command that I can configure as NO or NC. The baselock command kills ALL inputs to the VFD and coasts the motor to a stop. No commands will work until that is enabled again. Couldn't I just wire my e-stop directly to that input? I could also wire the brake to this command as well, but it wouldn't latch like the e-stop would.....but I could bypass all the relays (solid-state or coil) with the PLC and do the latching in my logic code.

I guess I thought about it more and I guess it wouldn't be very safe if I had a fault in my VFD....then I'm relying on my faulty VFD to stop the motor. Would there be anything wrong with having the e-stop kill the single-phase power to the VFD? I guess it wouldn't be any different than a power outage? Okay! Back to the safety relay.... LOL

 

[Larry$]

Okay! Back to the safety relay

???? I don't see a need for a "safety relay ." All the "safety switches" are wired in series with the pickup coil on the motor controller.