Enquiry into functionality of NVR (No voltage release) switches for 3 phase and single phase machine tools

John Fletcher 1/Ted (Which is it? John ? or Ted?)

I hear and understand what you say about the operating procedure, eg select direction, then use DOL switch.

So, in my case the FWD-OFF-REV switch is mounted on the front of the headstock whilst the DOL switch is mounted to the right of, behind , and level with the tailstock. Are you really suggesting that I should first select the direction and then stretch over or around the tailstock to activate the DOL switch? On the grounds of safety alone that simply does not make sense.

Furthermore, that also implies that in order to stop the motor, I should stretch over or around the tailstock again in order to operate the "OFF" button on the DOL switch, before placing the direction switch in the OFF position. It also implies that in order to reverse the motor, I should use the DOL switch to stop the motor, move the FWD-OFF-REV switch to the REV position, then reactivate the DOL switch.

Given the relative position of the switches, I don't think so, although to be fair, the latest version of my lathe does have a DOL switch in a much more usable position. I have also looked through my operating manual which specifically describes the FWD-OFF-REV switch as "Lever for starting and stopping....". The DOL switch buttons are described as power supply ON & OFF.

I would venture to suggest that the machine manufacturer will be, or should be, well aware that users will use the most convenient operating procedure, hence he will have, or should have, provided the FWD-OFF-REV switch with adequately rated contacts as otherwise he may be inundated with complaints.

Regards,

Peter G. Shaw

Hi. I have to agree with Peter, as he knows I have the same model lathe and the foward/reverse switch has been used countless number of times without any problems to the switch. Under the heading of Electrical equipment on this lathe's operation manual and a sub heading of Main eliments; "SB3 - cam-operated switch nnr-20 for switching and reversing the motor". Maybe they just made them better than the Dewhirst ones.

Regards Nick.

Edited By Nicholas Farr on 25/04/2013 21:52:55

I experienced the 50Hz arm to arm ripple years ago, when electric drills had metal casings. Drill in one hand, unplugging it with the other. The back came off the 13A plug, with the rest remaining in the socket, and my fingers brushed across the fuseholder. Ironically, had the drill not been earthed, It would have been less memorable. Though everything in the shop runs through an RCD, that event made me careful - I use switched sockets and turn them off before unplugging, in case my other hand is resting on something which is earthed.

Getting vaguely back on topic, I have made one or two NVR switches for use on single phase. It's not hard, and you can put as many conveniently situated push-to-break emergency stop switches as you like in series with the big red one on the switch housing. I toyed with the idea of a push-to-make "dead man's footswitch" in series, so that stepping back from a machine would shut it down.

Andy.

Posted by Les Jones 1 on 17/04/2013 20:58:27:

The very cheap NVRs do not have a separate stop and start button. On these the start button just pushes the relay closed and the stop button forces the armature away from the coil.

Les.

Les, I was not aware of this - thanks for this info.

Posted by Weldsol on 25/04/2013 14:56:25:

I have used these units so I have start/stop and E stop

**LINK**

Paul

Hi Paul,

With the info above from Les Jones in mind, do you know if the Axminster one has contacts for the buttons or is it a mechanical push button as Les says?

I am in New Zealand and some of that stuff is so expensive here that, as ridiculus as it seems, it is sometimes cheaper to buy it from the UK and pay postage. In the case of this Axminster unit, it certainly is but I would be very interested to know what the insides of it are like.

Thanks, Stephen Sandford.

Hi Stephan,
The type of NVR I describe without separate stop and start buttons is fitted to my Chester DB10G lathe. Without power to the lathe if you hold ether the stop or start button in then try to press the other one you can feel the first one being pushed out. The connection to one end of the coil is accessible so further emergency stop buttons or interlocks can be connected in seies with the coil.

Les.