I have a cnc milling center that I want to run the coolant pumps off a VFD. I wanted to connect a relay to the outlet that the machine supplies power to so that once energized the contacts would close and turn on the VFD. The problem is that the cnc machine uses some type of thyristor to turn on the 220Vac. If I connect a 240Vac relay coil (11K ohm) the relay is always on do to what I think is stray leakage current (with no load connected my meter reads 240 with the coolant turned on or off). Is there a easy way to bleed this off without needing a high wattage resister in parallel with the relays coil?

 

What is the voltage across the relay coil when the CNC machine is off?

 

Try an isolation transformer if it is bleed from a snubber circuit the primary will probably sink it without coupling anything significant to the secondary. In fact thinking about it you could simply step it down to 24V, with a transformer, and use a low voltage relay.
have you measured the current through the pump, or a lamp, when the socket is off?
Are you sure that it is the live being switched?, it could be nutral, although it shouldnt be.
What voltage is the pump?

If the transformer doest work and you cant access whatever control voltage is switching the output the only other thing I can suggest is attaching a load, perhaps a lamp or a relitively high wattage resistor, and sensing the current rather than the voltage.
You can buy current sensitive relays in many forms with adjustable thresholds, they often get called trip-amps, but control relay, safety relay and monitoring relay are better terms with the latter being your best bet for a none speciffic device designed for general use as opposed to some speciffic function.
http://uk.rs-online.com/web/c/automation-control-gear/control-relays/monitoring-relays/

Not too hard to build either... Electronics, obviously, literally hundreds of ways, but a simple bridge rectifier will take your AC current and give you DC current which, once smoothed, could pull a solenoid or close a reed switch placed in a few turns of wire.
It all depends on how much you want to play and what you have to spend...
Me I would try a small transformer and a standard relay first and go with a current sensitive control relay if that failed.
Of course I would also establish that the supply wasnt faulty.

I wouldnt recomend feeding your VFD with this supply, give it a good supply directly from a plug or an isolator and just manipulate its control inputs with volt free contcats.

Al

 

Db has a good suggestion.
Try an incandescent lamp across the relay coil and see if it lights.
A small bulb may be all you need to turn off the relay.

 

What is the voltage across the relay coil when the CNC machine is off?

The voltage is ~240vac with the coolant on or off. The main problem is that I have a 3 phase pump so I need the VFD. The machine switches 240vac single phase. I wanted to use the machine output to run the relay. This way I could connect the relay contacts to the VFD digital input to command it on. There are lots of different solutions to this. The step down transformer and lower voltage relay came to mind but I was hoping there was a simpler way that I overlooked. A bulb is a good call but I don't have any 240 volt one laying around.

 

I don't have any 240 volt one laying around.

Two identical 120V bulbs in series, just for a test to see if that allows the relay to work.

 

How big is the pump?
have you considdered using capacitors to run it from a single phase, you will not get much starting torque but then you shouldnt need it for a pump...

If the pump is 110 delta then you can use two of its windings in seriese and the third as a start/run winding...
you will not get the rated power of the motor but it will be happy enough at 60/70%, provided your load isnt too high.

We run 110 kW motors as genedrators, derated by 25%, on single phase supplies all be it at 460V
The connection is called C2C which is fairly good for generation but has no inherrent direction, at least not enough to start.
However if you fiddle with the cap value and winding configuration single phase starting is definatly possible with a few caviats.

Happy to draw you a pic if you want to have a go at it, wiring is simple and caps are cheap.

Al

Al