Pm1340gt Lathe Basic Vfd Control Conversion Using The Stock Control Board And Switches

[smk-machine]

Thank you kindly for the welcome. This is a great forum!

 

[mksj]

Just to add what Jim mentioned, most newer 3 phase motors operate just fine on VFDs, and they are often operated beyond/below what is known as their base speed (usually 60Hz). The issue is the voltage switching spikes because the VFD creates a pseudo sine wave out of many little segments of the voltage being switched on and off. This causes some voltage overshoot in the motor cable and the motor windings. The breakdown of the insulation becomes more of an issue with higher voltage motors. Given that most of the 3 phase motors in our application are rate as 230/460V, and you use the lower volt setting, voltage wise you are running the motor very conservatively and very unlikely to see an insulation breakdown issue. This is a very nice technical description by ABB "Effects of AC Drives on Motor Insulation". https://library.e.abb.com/public/fec1a7b62d273351c12571b60056a0fd/voltstress.pdf

The other issue is one of cooling the motor at operating points above and below its designed operating speed. Most standard 3 Phase motors use a fan attached to the motor shaft to pull air over the motor known as Totally Enclosed Fan Cooled (TEFC), as such the efficiency/cooling can be an issue at low and high speeds. In general, they do just fine over a range of approximately 30-90Hz, but this is also load dependent. Inverter/Vector motors are usually Totally Enclosed Non Ventilated (TENV) or use an Electric Blower (TEBC). These type of motors have a much wider operating RPM range (usually about 10 fold on smaller motors) and higher short term overload capabilities. On mills and lathes, you will often see 2 mechanical speeds and the VFD covers the wider speed range. The motors are often oversized, to account for the Hp decrease when operating below their base speed. The insulation on inverter motors is usually rated for higher voltages and temperatures to insure longevity under continuous use at high loads, they also have a rated maximum speed of around 5000 RPM for a base speed motor of 1750 RPM.

On my mill the inverter TEBC motor operating range is 20-200Hz direct drive or with a 10:1 reduction gear. On my PM1340GT lathe I routinely operated the stock motor from 30-90 Hz and never had an issue with it's performance of cooling. I did switch it out to an inverter motor to give me a wider operating speed range so I would not need to do a belt change. The motor operates from 20-120Hz with the inverter overload at 180% for up to 1 minute. Running the VFD in sensorless vector mode, the RPM does not change with load.

 

[bench]

Hello Everybody,

I am learning lathe for hobby. I recently bought a Taiwanese lathe, LD-1216GH, which is basically identical to PM1340GT except for the bed size. Thanks to Mark(mksj) I converted it to run on a VFD (Hitachi WJ200) using the schemes described in this thread. Many thanks to Mark. One thing a bit different is that I added a DPDT mometary toggle switch to have the JOG function in both forward and reverse direction. Following pictures show how I wired the toggle switch. A DPDT switch has two columns of connectors and on the left column I wired p24 of the VFD to the center connector and input 3 to the top and bottom connectors. On the right column the center connector is connected to terminal 3 in the control box, and the top and bottom ones to the terminal 4 and 5, which activate the forward and reverse contacts, respectively. Now I need to add a proximity switch for carriage stop. Wish Mark would provide a scheme for it.

 

[mksj]

There are a number of ways to implement the Jog feature, but the basic VFD control conversion is designed to use the stock wiring and allow the use of a VFD to replicate the regular machine controls while adding the variable speed and programmed acceleration/deceleration of the VFD. It is a very simple conversion. The use of a proximity sensor that I use requires a DC control system (the stock PM1340GT is 24VAC), and involves a number of additional controls and safety interlocks to prevent the machine restart. My recommendation would be to go to a single relay design, but this requires a complete rewiring of the system, use of a WJ200 VFD (or a VFD that allows use of an external 24VDC power supply with source logic) and new switch gear. If using the WJ200 internal power supply it is limited to 100mA, so it is maxed out with one small relay (~70mA), one power LED (~15mA) and one proximity sensor (~15mA). One needs to be aware of the parts used in the build, so the power requirement is not exceeded. I have attached some designs for single relay systems in the attached file, some have been built other have not. So they are provided as a basic starting point, but provided as is and the user assumes all responsibility for their use. The designs are specifically for the Hitachi WJ200 VFD and will not work with other VFDs. All my newer VFD system builds use multiple relays, external 24VDC supply with 12 VDC step down converter, and more advanced VFD control and interlock safety systems because of the power limitations with the single relay design.

 

[bench]

Thank you so much. If I had to build from scratch I would pick up one of the designs but I would rather keep the current conversion and just add the proximity stop, and if required a DC power supply and a small relay. What I am thinking is connect the control output of a NO proximity sensor to a relay which switches a connection between P24 and an input of the VFD, and program the VFD input for STOP signal (code 21 on WJ200). With this when the sensor switch closes it would activate the relay which in turn issues a STOP signal to VFD. Would this work? And is an extra DC power supply required to run the proximity sensor or can it be run on the DC24V of the VFD? Or, is there a way to directly connect the proximity sensor output to VFD without going through the relay?

 

[mksj]

The problem is that you need to not have the system (usually controlled by the power relay) turn back on when you move the carriage away from the P sensor, and you need a bypass switch to be able to bypass the P sensor once it triggers. In the normal operation of the power relay, the spindle control must pass through the stop position to reset the relay and the relay has some form of electrical latching mechanism so it stays on once reset. I also use an additional mechanical switch should you override the P sensor and choose the wrong direction. I have done that on more than one occasion. The CODE 21 is a stop function only when using a 3 wire system with momentary switches, it is not a stop function in a 2 wire system. Most VFD emergency stop triggers put the VFD into an error state and prevent restart, but you then need to reset the VFD for it to accept any further commands. This is not a practical approach for this type of application.

The only way that "might" work is to interrupt the power to the DC relay in the stock control system. The stock control system uses a DC 24VDC power relay which is powered from the 24VAC going through a full wave bridge rectifier with its output connected to the coil of the relay. The problem is that this is not filtered DC but continuous 1/2 wave pulses, and the P sensor would pulse on and off as each waves goes to 0V. It would not work, in particular because of the high (fast switching) frequency of the sensor. There are some possible ways to smooth the DC, but it gets complicated and causes other problems. You would be much better off in my opinion to use a mechanical limit switch attached to the micrometer stop/adjustable rail that would act like an E-Stop. This is what is done for most drive systems in mills with motor drives. Trying something else could be unpredictable and very dangerous.

As I previously outlined, the single relay design is much simpler and easier to implement with an electronic sensor, then trying to do work arounds and patches on the original control system design. Many individuals have built the single relay system and it has worked very well. Contactors are not designed to conduct low level signals, so a simple single 4 pole electrical relay VFD system is the easiest to implement. You remove the contactors and stock power relay and mount a 4P relay on the board. Alternatively fabricate another board, and swap out the boards with everything intact.

 

[bench]

Yes, it is a patchwork and not as elegant as your designs, but I already have a proximity sensor (PFK1-BP-3H) and so keep thinking about utilizing it. I came with a simple idea of inserting a relay switch (NO) just before or after the E-stop in series and a momentary bypass switch (NO) accross the relay. The relay is connected to the proximity sensor (NC) as in the diagram below. In this wiring the proximity sensor plus the relay is essentially an E-stop. When the carriage approaches the proximity sensor the switch on the relay will open that stops the lathe. To return the carriage the bypass switch must stay closed until the sensor switch closes. How do you think?

 

[mksj]

That would would work, but it is 3 relays that are sequentially tripped , so you will have a slight delay of a few 100ms and may be a small variation in the stopping position. You can use a small switching universal switching power supply for the P sensor and additional relay, it can be 12 or 24VDC. They do make AC proximity sensors, but there switching times are much slower than the DC, they are usually are NO and there are voltage drop issues. The only advantage is it would not require a separate DC power supply and relay. I still feel the single relay design is very simple to implement and is a better overall solution.

 

[bench]

Thanks, Mark. I am glad for your confirmation. Yes, there would be a slight delay but more important will be consistency in stopping position, and hopefully the variation would be negligible.

 

[Mr.Miz]

I'm looking at doing this conversion with my Acer Dynamic 1340G. Would this be the right place to post pictures and questions for my conversion or should I open my own Form specifically for my Acer? For the most part I think I've got the conversion steps down I just want confirmation so I don't ruin anything.