"In a Variable Torque (VT) application, meaning a centrifugal pump or fan, the maximum turn-down ratio is 20:1, so 3Hz minimum, which is a pointless spec because a centrifugal machine will never be used at that low of a speed."

What about driving a reel of paper roll in a packaging facility, which is uncoiling?

Isn't that a variable torque, which could still be used at 3Hz

I don't see how your circuit works since you seem to be using one of the more obtuse input configurations. I also don't see an E-stop anywhere. Hope the machine can not be run with a human in any of it without an E-Stop.

Keith Cress
kcress - http://www.flaminsystems.com

My scheme is only showing the logic of the switching. I didn't include the E stop in the scheme since I knew it was easy to add it later, once I figured out the circuit logic.
I don't know what an obtuse configuration means?

Pin 1 needs to be held HI otherwise the VFD output to the motor is stopped.
Pin 2 needs a momentary HI input to initiate a RUN command
Pin 3 needs to be held HI to maintain a REVERSE RUN command

A depression of either limit switch, or any change in the changeover switch position, or pressing the STOP button, will immediately release Pin 1 from its HI state and stop the VFD output.
It seems pretty simple to me, and it works as expected.

(Obviously the limit switch on the FWD side, can't change the state of Pin 1, while the carriage is selected to go in the REV direction, and vice versa).

Maybe my VFD pin configuration isn't so normal?

Here it is.

@itsmoked I would like to stick with a more conventional layout, if indeed my scheme is a bit odd, but when I think about your description, I can't understand when digital Pin 1 (on your VFD - not mine) is high, then the alternative direction button is pressed, so that Pin 2 is also high, how does the VFD react to both Pin 1 and Pin 2 being high? Am I missing something? I think I must be.

In your VFD pictured above note the "run FWD" and "run REV". Those would be in lue of what I described as "Dig input 1" and "Dig input 2".

Your 24V would go to an E-STOP(NC) and from the other side of the E-STOP to UP Limit SW(NC) and from E-STOP to DOWN Limit SW(NC).

From the limit switches to their respective UP Button(NO) and DOWN Button(NO).
Then on to their respective Dig INPUTS "FWD" and "REV".

Now if any wire fails to the E-STOP nothing moves.
If any wire breaks between the E-STOP and the FWD or REV inputs (DIG2 or DIG3 on your VFD) the affected direction doesn't move making troubleshooting VERY simple.

On your drive you would wire 24V directly to DIG1.
Parameter wise you would set 2-wire control.

Keith Cress
kcress - http://www.flaminsystems.com

Thanks again for your detailed explanation. I get everything you say clearly, except one part. If the UP and DOWN buttons are (Momentary??)NO, how does a subsequent open limit switch trigger a STOP, since the UP and DOWN are open already, and the motor is in a running state? On the other hand if the UP and DOWN buttons are not momentary but maintained, what stops both being maintained simultaneously?

I was under the impression this was a button oriented operation.
So the operator would push a button until the desired up or down position was reached then release the button.

That would tend to be less cumbersome than pressing a latching up or down then waiting to hit a stop button at some correct point in travel. Also a latching situation can cause "unexpected movement". If a latching up or down is used the motion will continue until the limit is reached. If the limit is 'un-reached' because of, say, weight removed from a lift then since the latched direction is still in force the system can unexpectedly go into motion again.

A latched direction switch is not a good motion man-machine setup unless it's a continuous motion like a motor running a process. If you want a latching limited linear motion then it should be an electrically setup latch that self cancels when the limit is reached and definitely not a position latched switch.

Are all movements to the limit always?

Keith Cress
kcress - http://www.flaminsystems.com

So, it would seem that my obtuse arrangement isn't so cryptic after all and it serves my purpose well.
The need is.

The ability to go from fully up to fully down, and vice versa, at the press of a button, after selecting UP or DOWN on the changeover switch.
(No provision for someone to stand holding a button depressed for 5 minutes is required or feasible. I would class that as cumbersome).
The ability to control speed via a POT.
(There are no humans in the firing line).

That's all.

My initial scheme had one part I didn't like, i.e. if the wire to Pin 3 got severed, the travel direction would change, but there would have been no active limit switch in that new direction of travel.
I have modified my scheme so that the if the wire to Pin 3 gets severed, the VFD output will stop, since Pin 1 would no longer be held HI.
See below.

oops

I just discovered a bug in my last circuit.
It wouldn't work without a diode as shown below, so I prefer to revert to the previous double pole changeover switch rather than use the diode option.

Dear Mr. MRSSPOCK (Mechanical)(OP)19 Feb 22 08:51
"...I am considering using a VFD to operate a motor, which is linked by chains and sprockets to an elevating platform....The platform would need an upper and lower limit switch to prevent it crashing....I wish for the operator control panel to have just 3 buttons, UP, DOWN and STOP...."
1. Based on your requirement, I propose the control schematic without having a STOP push-button ; whether it suits your application for your consideration .
2. Operation procedure:
.1 switch on, motor does NOT move.
.2 Press UP push-button, the load moves up direction. Release the Up push-button, the load stops at any position. Continuously pressing the Up push-button, the load goes up to the upper limit and stops automatically; prevent it crashing.
.3 Press Down push-button, the load moves down direction. Release the Down push-button, the load stops at any position. Continuously pressing the Down push-button, the load goes down to the lower limit and stops automatically; prevent it crashing.
3. Pressing (Up+Down) both push-buttons together does NOT cause the motor to move. No damage either.
4. Press the E-stop (self latching) push-button stops the motor, irrespective of whether the motor is moving up or down.
5. Unlatch the E-stop push-button by [Twist or pull] as applicable.
Che Kuan Yau (Singapore)

• https://files.engineering.com/getfile.aspx?folder=74ded046-487d-4bc7-a5a4-7

@ che12345

Thanks for taking time to share your thoughts and your scheme.
My system needs a stop button, because holding the button in for long periods is not practical.

But I have created a circuit something like yours for a different job before.
I used a pendant with two buttons, and an E stop, so it made it very simple, because the two momentary buttons inside the pendant are configured, so that only one button can be pressed at one time. The mechanical systems inside prevents two buttons creating a contact simultaneously. That made the circuit very simple indeed. So this is a press and hold arrangement, with limit switches. See below

(P.S. If I just edit the scheme below to have two mutually exclusive latching switches for FWD and REV, instead of momentary NO, then it suits my other application also).

The pendant is a Schneider XACA2713