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.