Motor Switches

[Ray LyonsParticipant @raylyons29267]

[Ray LyonsParticipant @raylyons29267]

A friend of mine has a fretsaw with a broken starter switch. It is one of those “push on push off ” no volt switches fitted to many small tools. When the start button is pressed, the motor starts but stops when released. I dont know a lot about these but it seems as if some kind of latching system has broken. I have tried trawling eBay but the only switches I could find were in excess of £40, making it too expensive to repair. It seems a pity to scrap the saw just for a replacement switch. Anyone know how to repair this type of switch or souce of a replacement would be appreciated

[michael howarth 1Participant @michaelhowarth1]

I have got a bandsaw and a pillar drill which were made with straightforward on off switches. If your saw is only for your own use, just fit a simple double pole on/off.

[Gray62Participant @gray62]

Warco sell a NVR switch for just under £30

Warco Switch

[Les Jones 1Participant @lesjones1]

Hi Ray,

There are a few possible causes of the problem with your NVR switch. Inside the unit will be a relay (contactor) with a number of contacts that close when the coil is energised. Probably two of these will break the live and neutral to the motor. There may be a third that holds the coil in or the coil could be fed from the same contacts that feed the motor. The stop button (Which is a normally closed contact) will be in series with the coil.

The start button may be a normally open push switch but more likely it will just push the relay closed. I think the most likely fault is that the stop button contacts are open circuit so check that they are not as a first step. The next most likely is the coil is open circuit so measure its resistance. I would expect it to be between about 100 ohms and a few thousand ohms. Pictures of the inside of the unit would be helpful so the internal wiring and contacts could be seen. (Or trace out the internal wiring and put that in your next post.)

If there any safety interlocks on the machine they will be normally closed contacts in series with the stop button.

 

Les

[Billy MillsParticipant @billymills]

Most recent far eastern no volt release switches are of very simple construction. It is basically a relay with a single normally open contact. Pushing the green button closes the contact, a coil connected across the output terminals then holds the contact closed until the stop button opens the contact.

 

It is most likely that the coil is open circuit. This can be caused by the output and input terminals being interchanged. The coil is then continuously connected to the mains but the armature will not pull in because the coil is only to hold the armature, not strong enough to pull it in. However because the coil current is much higher than it should be ( because of the much reduced inductance with the armature away from the coil) the coil overheats. The obvious clue is that initially the switch buzzes when the mains is applied but the motor does not run without the green button being pressed.

 

The terminals on input and output are the same size so it is worth checking that the switch is the right way around. As Les says the output pair will read a resistance of less than a few thousand ohms while the input pair will be open circuit. Found out about this issue when looking at a belt sander from a friend. CPC.CO.UK have a small range under “no volt release” for about £10 that look to be of better quality.

 

Billy.

 

 

[Peter G. ShawParticipant @peterg-shaw75338]

Hi Ray,

When the DOL/NVR switch on my lathe failed, like you I knew nothing about how they worked, and, just like you found the replacement cost rather exhorbitant so I set about learning how they worked. Here’s a summary:

 

The actual NVR portion is a relay, or contactor as electricians seem to prefer to call them, with four make contacts or switches. Three of the contacts are used to switch the three-phase mains supply through to the motor. The fourth contact is a holding hontact which simply parallels the ON switch. Note that apparently all DOL/NVR switches are made for three-phase working but have their internal wiring re-arranged for single-phase working.

 

Also included in mine at least, is a three-phase current monitor which monitors the current supplied to the motor, the idea being that as the motor becomes overloaded, so the current rises triggering the current monitor which then causes the disconnection of the power. Effectively this is a break contact. Note that htis is also a three-phase device which is re-arranged internally for single-phase working.

 

As Les has said, there may well be safety interlocks as well. All the safety interlocks are break contacts, ie if the interlock activates, then the contact breaks the circuit.

 

All the safety interlock break switches, the current monitor break switch, the OFF button switch (although this may be mechanical rather than electrical) and the relay/contactor hold contact are wired in series, hence for the relay/contactor to remain operated, all the break switches must be in the normal position and the relay/contactor hold contact must be operated. So, to initially operate the relay/contactor, the ON switch must be pressed. This contact shortcircuits the relay/contactor hold contact thus allowing the relay/contactor to operate, provided all the safety interlock switches, the current monitor switch and the OFF switch are in their correct unoperated position. The relay/contactor thus operates, and its hold contact takes over the function of the ON switch which is then released.

So, in your case, as the motor will start and run as long as the ON button is pressed in, then this suggests that the relay/contactor is working correctly in that it is operating and connecting the mains supply through to the motor. The fact that the motor stops when the ON button is released suggests that the relay/contactor hold circuit is not working as releasing the ON button is allowing the relay/contactor to release thus disconnecting the mains supply. I would therefore suggest that you need to investigate the holding circuit, ie are any of the safety interlocks out of adjustment? Is the current monitor tripping or out of adjustment? Is there an emergency stop button (which will be another break contact in series with the hold path) which has been tripped? Is the actual hold contact on the relay/contactor working correctly?

 

If you have a multimeter with an ohms range, it should be possible to check the hold path for continuity (disconnect the mains first) as there should be zero ohms around the path.

 

Despite the fact that I knew nothing about these devices when mine failed, I found that they are really quite simple circuits, and in fact, as it was the relay/contactor coil which had failed on mine, I found that I could easily replace it with a standard three-contact mains relay and press button, all for the princely sum of less than £20.

 

Regards,

 

Peter G. Shaw

[TerrydParticipant @terryd72465]

Hi Ray,

 

I have a problem like yours on a tile saw. When I inspected teh switch it was the coil on the relay which had burnt out. At about £35 for a new saw it hardly seemed worthwhile repairing but I need to thank Billy for pointing me to CPC as they have a suitable switch for just over a tenner here. Just not worth repairing the old one with all the fuss and time that involves. That’s my solution anyway.

 

Thanks Billy,

 

Best regards

 

Terry

[Ray LyonsParticipant @raylyons29267]

Thanks All,

 

I went over to see my friend this morning and he has the builders in so we wont be able to get into his shed until next week now. I have printed off all your replies and will take them with me when dismantling the switch. Now almost sure that we can find a fix to get it going again. I hate scrapping good machines when perhaps a small repair is all that is neccessary.

I will let you know how I get on later.

 

Ray

[Clive HartlandParticipant @clivehartland94829]

Another thought is that as its a fret saw that the switch has a build up of dust on the contacts or the moving parts, If you can try blowing it out with compressed air and see if it makes a difference.

My small bandsaw has the same problem which I correct using a contact cleaner and it clears it of wood dust!

 

Clive

[Ray LyonsParticipant @raylyons29267]

Billy and Terry,

 

Did’t know that CPC stocked this type of switch. Used the link Terry set up to print off the whole range. I will take that with me next week and even if we replace the switch, I will still take the defective one apart just to see how it is supposed to work.

 

Regards,

 

Ray

[Billy MillsParticipant @billymills]

Thanks for the Thanks! Would like to see a THREE PHASE FRETSAW!!! Some BEAST!!

Use CPC quite a bit but it’s difficult staying in contact with suppliers with massive ranges such as CPC, Farnell and RS. Fortunate that they have searchable websites.

 

£30 is a massive amount if the NVR switch is like the one on my friend’s machine. Cheap Router tables, belt sanders and drills tend to have the grotty plastic jobs which are at the other end of the Universe to the Proper Job as described by Peter.

 

CPC also have some good prices on cables and connectors including a good range of 3 phase tackle.

 

Incidentally it isn’t dust in the contacts as the machine STARTS, it can only be an open coil.

 

 

Billy.

 

 

[Les Jones 1Participant @lesjones1]

Hi all.

Today I had a look at the NVR switch that is fitted a router table I bought from Aldi.

It is double pole. The on button pushes the relay closed. The stop button does not have a set of contact in series with the coil as I expected. It just pushes the armature away from the coil with a lever.

 

Les.

[Sub MandrelParticipant @submandrel]

I have to eat humble pie today.

 

I have got my lathe working with the hoover motor, at just two speeds, about 250 and 500 rpm, using the high-low gearing, which will do for finishing the job I was on and making a proper pair of stepped pulleys.

 

But i don’t want to work without emergency stop and NVR. So I pillaged the controller box for its relays and and connectors and stop switch, added a double pole start switch and made an NVR circuit on veroboard. A check with the board showed my estimate of a 15K rsitor to drop the voltage for the relay coils was spot on.

 

All very nice, and checked perfectly with the DVM.

 

In practice comlete disater – the relays buzz and nothing happens. I’m assuming my stupid error was trying to run the 24VDC relay coils off AC?

 

Has anyone tried this and will using a bridge rectifier, but no smoothing capacitor work?

 

Thanks

 

Neil

[Les Jones 1Participant @lesjones1]

Hi Neil,

I think it will have to trial and error on this. I would try it without the capacitor. If the inductance of the relay is high enough it will smooth the current ripple it self. I suspect the relays will be quite small so they probably will need a capacitor.

 

Les.

[Peter G. ShawParticipant @peterg-shaw75338]

Neil,

 

Provided your power requirements are suitable, have a look at the RS Components offerings (Plug-in 230V Relays 3PCO) and the Maplin equivalent (much cheaper).

 

Provided that your power and switching requirements are within the relay’s capability, you will find the RS one entirely satisfactory. I used one a few years ago to replace the contactor in my DOL switch when it failed.

 

There is a slight problem in that there is an inductive power limitation which I only found out about when I received the relay, but depending on your requirements, may not be an issue. In my case this limit is less than my lathe power, but under normal circumstances is irrelevant as power switching is not done by the relay.

 

Regards,

 

Peter G. Shaw

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