Issues about using a resistor...

[Toller]

The 120v VFD on my lathe burnt out and I replaced it with a 240v.  That is fine, but the tach is 120v.  It draws 22ma, so I figured it was fine to run it between one hot and the ground, but I was talked out of it.  For the moment I have it on its own 120v cord, but I hate that solution.  The tach is way too expensive to replace.

I thought that putting a 5500 ohm resistor in series with the tach between the two hots ought to work.  The same person who talked me out of my first solution says that resistors fail in a short, which would likely destroy the tach.  So I thought of a 3w LED bulb instead of a resistor; they must fail open, but I don't know if that 3w is reliable or consistent.

That made me think of a multiwire circuit; putting the tach in series with the resistor or led between the hots, but putting a wire between them going to the ground.  Unless something fails there should be no current going to the ground, but if the bulb or resistor either doesn't have the proper resistance or shorts, the tach would be protected by the ground. 

Any comments on this would be appreciated.  Just don't suggest using a transformer; aside from the expense, there is no place to put it.

[BeBuLamar]

Get a small transformer.

[strawberry]

resistors fail open
resistors with wrong voltage rating arc and burn until open
there is fusible resistors (when overloaded they safely blow open)
transformer or capacitor* dropper is more efficient

*X class capacitor

[Zero999]

Why did the person tell you not to connect the tach between hot and neutral? They're wrong.

A resistor isn't the right way to do this because the voltage drop across the resistor varies with the current.

[strawberry]

120V+120V

[macboy]

You absolutely can wire the tach between hot and neutral, but definitely not between hot and ground. Most (all?) 240V (120+120) sockets in North America will have a neutral. Some hard wired appliances like a central air conditioner may not have neutral. Of course this can vary by state or province.

[Zero999]

Oh, I misread. Yes, it can go between phase and neutral, not ground, which would be a bad idea. If there is no neutral available, then use a transformer.

[JustMeHere]

Yeah, ground is usually a guage smaller.  It's job is to trip breakers if there's an electrical fault.  It shouldn't be used as a current path.

[Zero999]

Yeah, ground is usually a guage smaller.  It's job is to trip breakers if there's an electrical fault.  It shouldn't be used as a current path.

The main reason why using ground as a current return path is the chassis will sit at mains potential, if it gets disconnected.

[james_s]

A resistor is definitely not what you want here, the draw is likely not constant.

In the US 120/240V system you can wire it between one of the hots and either ground or neutral and it will work fine. Using ground is not up to code and is not really the proper way of doing things but it does work. In fact until around 10-15 years ago it was the standard way accessories like the interior light in ovens and clothes dryers were wired. Until then these appliances typically did not have a neutral so they just used ground, it works fine and for small loads it's not going to cause problems. Personally that's how I'd do it.

[EPAIII]

If you don't have a neutral wire in the power cord, then you should use a transformer. You say 22 mA so that works out to a 2.53 VA transformer. By coincidence I just purchased one that is probably what you need:

Hammond 229 series
229A230
6 VA
Primary: 115/230 VAC
Secondary: 115/230 VAC
50 ma @ 115 VAC
Mouser pn: 546-229A230
$16.84 / 1

Be aware that there could be a question of the REFERENCE level in the two circuits (the VFD and the tachometer). It is possible that the VFD is operating between the two live lines with no connection to neutral. So one of the hot power leads may be the effective reference (sometimes called ground reference). While the 115 VAC, tach circuit would use only one of those hot lines and NEUTRAL which is grounded at the point where it enters the building. This means that there could easily be a 115 VAC difference between the reference or "ground" level of the two circuits. If that is so, then connecting a "ground" cable between the two may result in fireworks.

Using a transformer will completely isolate the tach circuit from the power line ground and allow it to operate with whatever reference ("ground") level that the VFD circuit uses. A well designed VFD would provide isolation (optical?) at the tach input. But you have no guarantee of that.

If you do not use the transformer, unless you absolutely know that the tach input is isolated, I would strongly recommend that the tach circuit be isolated from any grounded point (like the frame of the machine) when it is first powered up and a measurement be made from it's ground to an actual earth ground, like the frame of the machine or of the VFD. If you see 115 VAC or some other significant Voltage there, then connect a large value resistor across the meter leads, perhaps 100 KOhms and see if that Voltage comes down. Then try a smaller resistor, like 10 KOhms and see if it comes down even more. Then 1K, 100 Ohms, and 10 Ohms. By that point you should see almost zero Volts, indicating that there is not any great current flowing between the two circuits (tach and VFD) and know it is OK to install the tach on the machine. If the Voltage does not come down to near zero, then you have a problem and should use the transformer for sure.