Hello and Startrite

[practicalenglishteacher]

Hi

Hello to everyone on this forum. I’m not that much of a woodworker yet but gave been cutting my teeth repurposing some old oak flooring into tent pegs, restoring a sailing dinghy and such like.

I’ve recently picked up an ex-college Startrite 352 bandsaw, with 3 phase motor. After taking a bit of advice I’ve wired it up to 240v domestic 1 phase through a VFD/inverter. I changed the motor wiring from 400v star to 240v delta and it didn’t work using the original column mounted start/stop switches but worked fine wired direct to the motor.

After investigation and taking the cover off, the original contactor does work intermittently by pressing the switch but, with the cover back on, the button doesn’t seem to align with the switch on the contactor properly. It looks to be the original Danfoss unit though.

Does anybody have any experience of these switches - I wondering if the original unit can be serviced as it works intermittently? It hasn’t been used for at least 10 years so perhaps cleaning of the contacts will sort it? Are these switches strippable/restorable? Would there be any difference in the operation of the switch running 240v through an inverter rather than 400v 3 phase?

The whole thing works fine using the vfd straight to the motor with great cuts but I’d like to retain the original switches and foot brake/cut out for safety if I can - the run/stop buttons on the vfd being quite small. I know I could replace with a new contactor but don’t think the foot brake cut out switch would work if I did this.

Any advice appreciated.

Many thanks, Owen

 

[Sideways]

It is essential that your motor is wired directly to the VFD with no switching in between.
If you try to start stop the motor by disconnecting it from the VFD you'll fry the drive.

The existing NVR switch is now irrelevant. The VFD prrforms all the control. If you want to retain push buttons in the original position on the saw, that is fine but you must wire them to the appropriate control terminals of the VFD. Those will be low voltage circuits.
Industrial push buttons are standardised and most fit a 22mm dia hole and are generally referred to as M22 type. It would be easy to get a replacement red and green button with contacts to work with the VFD.

Generally, and simply, you want one start button which will be a Normally Open type. The circuit closes momentarily when you push the button. You can have as many stop switches as you like. These will be Normally Closed. You wire all the stop switches in series and pressing any one of them opens this circuit and the VFD will stop. Give yourself a simple red stop push button, a latching emergency stop button with a twist release, and the foot switch which will be a NC type with momentary open when you step on it.

Many / most VFDs will work with different types of control like rotary switches for run forward and run reverse, this lets them be adapted to the needs of the machine, but simple start stop push buttons are the most generic.

 

[ChaiLatte]

It is essential that your motor is wired directly to the VFD with no switching in between.
If you try to start stop the motor by disconnecting it from the VFD you'll fry the drive.

What you say was true for early generations of VFDs. The electronics have developed over the last 30 years* and that absolute prohibition has been watered down considerably.

Of all the motors in the world today running off VFDs, it is not plausible to suggest that if the cable between machine and VFD gets cut, the VFD will cease to function accompanied by the sound of sizzling bacon ("fry").

A more reasonable statement, as of today, for a brand name VFD, is that such behaviour is not recommended because it can shorten the life of the VFD. And that 'life' is measured in industrial terms, of multiple on-off cycles over a 40h working week. For a hobby user, switching the machine on four times over a weekend, the effect is practically insignificant.

It is good to give advice, but it is also necessary to retain some level of sensible perspective when that advice is given.

* I believe the components are called IGBTs, and modern ones are a lot more tolerant of back-EMF than earlier generations.

 

[Sideways]

@ChaiLatte
An old MD once told me "I'm paying for your opinion. So give it clearly and confidently." or words to that effect.
You are perfectly right that it may not fry the drive the first time or couple of times you do it, so yes, I'm guilty of dramatic license, but it's bad practice and unnecessary, so don't.
Also, we have no idea what the age or quality of the VFD in use is, so sensible advice to a novice user is surely to encourage best practice, not to encourage risk taking.

I'll give you two references:
The first is Schneider Electric as of 3 years ago.
"A disconnect can be used between the drive and the motor. This is general practice to assure that power does not reach the motor during maintenance and other non-operating times.
Note that the disconnect should not be opened or closed while the VFD is running (outputting voltage from T1,T2,T3). This is sometimes referred to as "slamming the motor." This can damage the output IGBT components of the drive and such damage is not covered under warranty. "

The second is nicely written by contributor jraef to the plctalk technical forum so I'll thank them for answering this same question in late 2020 and explaining why, while you are correct in what you say, this doesn't translate into good advice.
https://www.plctalk.net/threads/disconnecting-the-load-side-of-a-running-vfd.127566/

"As a set of contacts open while current is flowing, like with a contactor or disconnect, the current forms an arc across the gap until the air gap distance increases to where the dielectric is high enough to extinguish it. In an AC circuit that happens fairly quickly because the polarity is crossing zero twice per second to help out. With DC that doesn't happen. Although technically the PWM pulses coming from the VFD are DC, they too are going to be zero in between pulses, but that happens so fast that the DC arc is still there when the next pulse fires. But in the mean time because the pulses are turning on and off feeding an inductor (the motor), they make the air gap act like a capacitor, pumping up the voltage across the gap as it widens. That voltage is rising very fast and can quickly exceed the dv/dt rating of the transistors themselves, causing them to break down at their junction layers. Translate: fail.

Most modern IIGBT drives now have protection circuits on the outputs (flyback diodes, etc.) that will suppress this capacitive action. So opening once in a great while, like in a emergency, is unlikely to cause damage any more, however the potential damage is also incremental. So if the mfrs said that it's not immediately fatal to open a contact down stream, someone would immediately abuse the concept and blow up some drives, then say "Well you didn't tell me not to!" or "Define once in a great while." So instead, they leave the admonition in saying "Don't do it" as the default position. In addition, you don't know for SURE whether your particular VFD has or doesn't have that protection; some older and / or cheaper ones don't, so it's best not to press your luck."

 

[practicalenglishteacher]

That’s really helpful, thanks. The easiest way to start with would seem to just use the supplied parameters table and connect to one of these X terminals and select function 7. I’ll have to work out a different solution to connect a stop switch to the foot pedal.

 

[Fitzroy]

I wired a 352 to a VFD and used the original start stop button and door interlocks.

The 352 switch and interlocks are a mechanical linkage. The start/run button closes puts a voltage across the coil in the he Danfoss unit, this coil then pulls the main contacts closed and the machine runs. The coil also closes a secondary circuit which then continues to provide voltage and holds the coil closed. When a door is open, the foot brake is pressed, or the stop button is pressed this results in the secondary circuit being opened, the coil then deenergizes and the main contacts open. These elements are all mechanically linked and depress the stop button on the danfoss unit when operated.

As you state your VFD is connected directly to the motor and you need to use the above system to send a run or stop command to the VFD. This should be simple as all you need to do is wire a circuit through one of the spare lines on the Danfoss unit (as you've disconnected the power circuits). In theory you push start button and this closes the start/run circuit controlling the VFD and it runs, when you push stop this circuit becomes open and the VFD stops the motor.

However, the problem is that the coil in the Danfoss unit holding everything shut is designed for 400v, all you can feed it is 240v which is insufficient to hold the coil shut. When you push the start button it likely buzzes but will not latch closed. I think you can get a 240v unit that will directly replace your 400v unit, it was a few years back and I can't remember if I could't find one or it was too expensive. I bought a cheap 240v relay/coil from ebay (£10) and wired it in as an alternative. The relay simply closes the start/run control circuit to the VFD, it does not close the power circuit so it does not need to be a high amp device like the Danfoss is.

Here is me bench testing the set up, the Danfoss unit is just acting as a switch. When you push the start button it presses on the silver strip closing the circuit that energises the relay and closes the start/run circuit to the VFD. When you push the stop button it presses on teh small red/brown button on the bottom right corner of the Danfoss unit, which opens the circuit holding hte relay closed, this opens the start/run control circuit on the VFD.

Relay installed on the saw, you can see it hiding on the left.

Hope that helps, and of course stay safe, 240V can kill you!

Fitz

 

[Sideways]

Direct On Line starters as built into many machines can be adapted as Fitz has done above.
They are a bit annoying once you have incorporated a VFD as you need nothing but the simple momentary contacts, the relay is entitely redundant. For cheapness, the DOL starter is usually built without switches and just mechanical actuators that push on the contacts of the relay.

For the cleanest electrical installation, and subject to there being enough space, I would remove and discard the relay (contactor) and associated thermal overload relay and simply fit 2 new industrial pushbuttons in the holes where the start and stop buttons sit. They are quite bulky so need space around and behind.

For any one unfamiliar, industrial switches are robust and designed for maintenance as they may be hard used over their life. Standard is something that screw fixes into a 22mm dia hole in a panel.

Buttons are usually modular so the coloured front part can be swapped or replaced, a section in the middle, and lastly modular contact blocks so you can choose Normally Open (NO) for a start button or Normally Closed (NC) for a stop button. It is common that either one, two or three contact blocks can be clipped onto the back of a unit like this according to need.

There are many manufacturers but a couple of ranges that I've used before are

Schneider Harmony XB4

Eaton Moeller M22

Use these to understand how this sort of thing fits together then look for cheaper equivalents as the proper stuff costs £20+ a switch at retail.

The Schneider button goes through the panel, metal frame attaches to this with a 1/4 turn bayonet action, the grub screw is tightened and causes the frame to press on the back of the panel at three points and pull the button tight. It's very secure and and can't unscrew over time as a switch is turned back and forth. Lastly 1 to 3 contact blocks are clipped on.

The Eaton ones have a threaded stem that goes through the panel, locknut from behind, then clip on the carrier and clip in the contact blocks. As with the Schneider, NC blocks are typically used with stop switches so are coloured red, NO blocks are used with start switches so coloured green.

All have light modules that can be fitted among the contact blocks in case you want your switches to light up.

 

[practicalenglishteacher]

That’s really helpful, thanks, and makes sense - the original start switch would work sometimes if pressed in and held but the coil rarely had enough power to hold it in.

I figured the easiest option would be to use separate start / stop switches in the original positions. The stop one was simple with it just being connected up to VFD input terminal X3 and function 7 ‘External Stop Input’ selected. With the ‘stopping freely’ instead of ‘slowing down to stop’ option also selected I can still use the foot brake.

The external start button has me stumped though. On the attached diagram I connected it to ground and terminal X1 with (3) Forward Control selected. Doesn’t work. I’ve looked through the other options and can’t find anything for ‘External Run’ input. I can do without it but it is a bit annoying.