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

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

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.

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.