Heavier duty VFD control

[Lee Alkureishi]

Hi all,

I currently use a VFD with my lathe. For the control box, I sourced an on-off-on switch and 10k potentiometer from radioshack, and wired it up with standard 18 gauge wires, which I then wrapped in a nylon sheath from a home centre.

I acquired a unisaw recently, and will be using the VFD with this also. I wired up an outlet on the output site of the VFD, and plugs to my 3-phase appliances. I've tested it, and it works well.

But, I feel that for safety's sake, I need to beef up the controls for use with the table saw. I've tried searching for heavier duty components, but this isn't my strong point and I figured it's best to ask advice from those in the know!

Can anyone suggest a good source for the following? Here's what I'm looking for:

1) A coiled cable with minimum 9 conductors, approx 6-8' length
2) A nice, solid, metal box to house my controls
3) More "solid" feeling 10k pot - the one from radioshack just feels a little flimsy
4) More "solid" feeling power control - see below.

I'm also looking for a good solution to the on-off problem: for the lathe, an on-off-on switch works well for reverse-off-forward. But, for the table saw it would be better to have a big, separate OFF button for safety. Can anyone suggest a way to accommodate this? I previously figured out the wiring for the existing controls, but only with the help of much googling!

Thanks in advance,

Lee

[Chris Parks]

I don't know what brand of VFD you have but the ones I have had any experience with can be controlled using a low voltage (12V) circuit into which any number of control points can be put. Like you I don't know the exact details but I think it was Alan Schaffter who put up a circuit design some time ago. I had one but have since lost it.

[Stephen Cherry]

http://www.factorymation.com/

For the saw, you can probably use the on-off switch that is came with. If you would like, post some pictures and we can see what you have.

[Lee Alkureishi]

Hi Chris,

Yes, it's a low voltage circuit. MY VFD is a huanyang.

Thanks for the headsup - I'll look at Alan's posts.

Best,

Lee

[ian maybury]

Not quite sure what you are doing Lee, but it's often the case (and definitely on the Yaskawa on my dust system) that the motor/device is connected directly to the VFD outputs using an appropriately sized cable for the peak amps - but that as Chris says that you control it by hooking up to the VFD's own control terminals which are normally 24V and very low current.

i.e. you by-pass the machine's own switches and controls. This can introduce further complexities on occasion in that it can by pass important safety or protective features on the machine for example.

The other issue is that unlike in the case of reduction gears where the torque increases in proportion to the amount by which the speed is reduced, the torque output from a VFD remains constant. Meaning that the HP also reduces with the RPM. Meaning that they can be prone to stalling on high torque loads at low speeds.

This can (a) mean that a VFD is not suitable for all types of machines, and (b) that (even if it's feasible) getting the machine set up to allow the use of a VFD can require a bit of figuring out. Fans and the like are very well suited to VFDs.

It's actually not very advisable to insert control devices (switches or whatever) between the VFD and the load - I'm not very clear on the technicalities, but basically it can pop some of the devices used to regulate its output.

It's also advisable to use a shielded cable (one with a braided stainless steel mesh or a foil sheath) for both power and control circuits as VFDs can generate a lot of electrical noise/interference which can (a) fed back into the mains, and (b) confuse some control devices.

The manual on your typical VFD can be fairly heavy going (huge amounts of material to wade through), but it's all in there. Some have a short form crib sheet version of a few pages available that covers most of the wiring and programming basics...

ian

[Lee Alkureishi]

Hi Ian,

Thanks for the input.

The control device is not between the VFD and the motor - it's on a separate, low voltage, circuit connecting the control box to the VFD. I understand the theoretical need for shielded cable also - part of my original post was asking about a source for a suitable cable. I didn't specify shielded cable, which was an oversight on my part.

To be clear: I already have a working control box for my VFD. I am interested in recreating this with better/heavier components, and in figuring out the best way to add in a big old "OFF" paddle. The manual is no help in that regard.

Stephen, thanks for the link to factorymation. They certainly have suitable control boxes, though they're a bit on the pricey side... The on-off switch that came with the saw is (I think) a 3-phase magnetic starter that I assumed wouldn't be suitable for use with the VFD. I'll try to post a picture of it later today. If it's suitable, I'm not averse to adding a pot and reversing switch to that box.

Thanks!

Lee

[Stephen Cherry]

Hi Ian,

Thanks for the input.

The control device is not between the VFD and the motor - it's on a separate, low voltage, circuit connecting the control box to the VFD. I understand the theoretical need for shielded cable also - part of my original post was asking about a source for a suitable cable. I didn't specify shielded cable, which was an oversight on my part.

To be clear: I already have a working control box for my VFD. I am interested in recreating this with better/heavier components, and in figuring out the best way to add in a big old "OFF" paddle. The manual is no help in that regard.

Stephen, thanks for the link to factorymation. They certainly have suitable control boxes, though they're a bit on the pricey side... The on-off switch that came with the saw is (I think) a 3-phase magnetic starter that I assumed wouldn't be suitable for use with the VFD. I'll try to post a picture of it later today. If it's suitable, I'm not averse to adding a pot and reversing switch to that box.

Thanks!

Lee

Usually you can use the three phase mag starter to supply a run permit to the drive, if you post a few pictures I'd be happy to take a look and advise. Also, if you use sheilded cable, one thing to remember is to ground only one side of the shield.

[Chris Parks]

If it were me I would use a low voltage circuit and the original switch on the saw to control that circuit, it is only a switch there is nothing special about it.

[Stephen Cherry]

fleabay also has this sort of thing.

[ian maybury]

It's hard to advise specifically Lee as there really isn't any alternative to reading the manual which will specify the cable sizes, shielding, earthing etc to use for power and controls. Also the type of terminals, typically the size of crimped fork and/or pin variety to ensure a reliable connection.

Good grounding/earthing of the VFD and the cables is very important too.

US regs may be different, but on my VFD hooking up an E stop is just a case of removing a jumper from a designated pair of terminals, and running a wire to to replace it through an e-stop button located where you want it. (up to some distance limit) That will stop the drive when actuated (check the manual for whether this should be normally open or normally closed), but depending on the regs you may need to wire in a fancier variety of 'safety relay' that forces a full reset of the VFD to get going again. (this is required in the EU in an industrial situation) This might need some help from an electrician as it won't necessarily be covered in the manual.

Start/stop and speed controls can very similarly be wired through other designated pairs of terminals.

The control cables on the Yaskawa are required to carry only 24V/10mA so something of the order of 0.5mm2 (shielded and earthed) is recommended - but again check your own manual. PVC insulation and switch boxes are fine for this - use proper dust/splash proof glands for cable entries.

Power cables are typically quite a lot heavier than is typical for a given size of motor/load, also shielded and earthed - VFDs don't like voltage drops for some reason. The advantage depending on code is that the shielding effectively armours the cables, so rigid conduit isn't needed. (at 230V anyway) See manual again.

The big installation issue that many seem to sidestep is that VFDs are often intended to be mounted inside a cabinet. This because the shielding of the terminals isn't always complete. A kid or whoever could conceivably poke in a bit of wire or something, and touch something live.

If you go this route you again need correctly sized sealing glands at the cable entries, and need to (per the manual) earth the cabinet/VFD mounting baseplate. The unfortunate effect of a cabinet is that it'll probably need lots of correctly positioned air vents per the manual (you can buy pop-in plastic ones) so that the cooling fan on the VFD has access to cool air, and so the warm air is vented out the top. It may also need a cut out or a window correctly positioned over the VFD so the touch pad and screen can be accessed/seen without opening the cabinet.

It sounds straight forward enough when described, but in truth digging the information out of the manual, buying the parts, building the cabinet and wiring everything is a sizeable enough job unless you have a lot of experience - it's not the sort of thing you do in an evening.

Sorry I can't be more specific...

ian

Here's photos of the VFD cabinet and control box on my dust system - the e-stop is below the box, the box includes a manual control switch for the fan plus a parallel remote radio operated relay wired in a control loop through the forward start/stop terminals:

vfd in enclosure.jpgvfd enclosure, closed.jpgdust system rf & manual controls.jpg

[Mike Heidrick]

Hoffman makes steel boxes you can drill/cut to your liking.

An Estop has NC contacts - run them between your on/off switch on and the contacts you are using on your VFD.

Check out an Igus catalog and see if they have the multi conductor cable you need.

[John Lifer]

I used standard industrial Allen Bradley on off switch for my lathe. If you have simple stop switch on your saw, you should be able to use that. Otherwise get a estop from other source. I'd NOT put the potentiometer on the TS, no need that I can see to try and reduce speeds. Pretty good wiring diagrams in archives on the OWWM.org site for VFDs.

[Tom Stenzel]

Like Mike wrote, emergency stops must be normally closed. If a normally closed E-stop button fails the drive won't start. If a normally open E-stop button fails it won't stop the drive.

Are both of the motors close enough in their ratings so the drive can be programmed for a single current limit and protect either motor?

-Tom Stenzel

[Stephen Cherry]

Like Mike wrote, emergency stops must be normally closed. If a normally closed E-stop button fails the drive won't start. If a normally open E-stop button fails it won't stop the drive.

Are both of the motors close enough in their ratings so the drive can be programmed for a single current limit and protect either motor?

-Tom Stenzel

I think the hitachis are programmable for multiple current limits

[Dick Strauss]

Lee,
In my home shop I've used multi-conductor 18g sprinkler cable wire for the external VFD control circuits. They are not shielded but I don't think it is necessary in a typical home environment with very short runs between the VFD and the controller and few sources of interference. I do use one wire to ground the control box though.

Ian makes a very good point! You don't want to disconnect the load (motor) directly from the outputs of the VFD without running the risk of damage (at least for the units that I have). You can use the low voltage contacts in the mag switch to control the VFD on/off, but the motor must be wired directly to the plug/VFD.