My scenario:

  • Installing new VFD (1phase 120v input with 3 phase output), 3 phase motor to Lathe
  • I want to install a disconnect or on/off switch between the house 120v plug outlet and the VFD input. So I'll plug the VFD into the 120v wall outlet, but I want a on/off or disconnect switch between plug and VFD so I can shut off power to lathe completely.

I'm looking at switches similar to this and to get over 20A, everything switch seems to be 3 pole. Katko KU325

I'm going to need to figure out which is neutral in the power cord, right?

How can I make the 3 pole switch work?

  • If your input is "Over 20 amps" at 120V you're going to need a non-standard plug (Such as a NEMA 5-30) to run off 120VAC, not to mention a non-standard circuit (10 gauge copper) to run the receptacle for that plug. At that point, it's often easier to move to a 240V input... – Ecnerwal Dec 9 '16 at 19:05
  • Just a note It would be helpful to know the 120V current demand. As a safety caution Some VFD's I work on take up to 3 minutes to discharge not only the big ones some with fractional HP motors take longer than the big 500HP drives for the internal rail voltage to bleed off . a local disconnect on the 3 phase supply to the lathe is the safest way for lockout reasons. – Ed Beal Dec 9 '16 at 20:10
  • ...and many VFDs will die a horrible death if you interrupt the connection to the motor without first shutting down the drive, or power them up without a motor connected. Read the fine manual to see what applies to the particular one in question, as usual... – Ecnerwal Dec 9 '16 at 20:14
  • Thanks for the info Ed and Ecnerwal. Sorry I should have provided complete information. The lathe is 1/2 hp but i'm oversizing the VFD a bit at 1 hp. The VFD manual says it is good for 19A on input current. The breaker on the outlet is 15A. Are homes with 15A & 20A breakers on 120v outlets have the same size wire? Could I just put a 20A breaker on this outlet or will it require running a new wire to the outlet? Thanks again for the help. – JP4 Dec 10 '16 at 22:59

You could use one contact, two, or all three.

If using more than one, you could put them in parallel to reduce amperage through each one, which would increase service life (however if any stick "on", you cannot turn the device off). Or you could wire them in series for a higher capacity to interrupt voltage (but if any stick "off", you cannot turn the device on). The latter is a better fail-safe.

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  • All the 3 phase VFD's I use require a balanced motor loas IE 3 phase motors and will not work with a single phase or unballanced load. – Ed Beal Dec 9 '16 at 19:47
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    Read more closely what OP is asking. He wants to switch the 1-phase input to the VFD, not the 3-phase output. Yes you are correct, if switching a 3-phase output, you use one pole per phase. – Harper - Reinstate Monica Dec 9 '16 at 19:51

The hot (line) from your 120Volt supply will connect to any one of the three poles in your disconnect switch (unless it has guidance for a particular pole for single-pole use in its labeling)

The neutral and ground connections pass through, with the ground connection bonded to the case, assuming it's a metal-cased switch.

As commented, you may have some issues supplying more than 20 amps of 120V without some custom wiring, and it's often easier (for the many VFDs that will happily take 240VAC input - or swap yours if it's 120VAC-only input) to switch to 240VAC if you are facing custom wiring anyway.

In a standard line cord, neutral is White (or gray) (USA Standard) or Blue (IEC standard, not uncommon on imported equipment even if intended for the USA market.)

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  • Thanks Ecnerwal. I think I understand what you're saying about the switch. The 120v outlet i plan on using has a 15A breaker, will it be ok to just put in a 20 A breaker. I have plenty of 20A breakers on other 120v outlets? – JP4 Dec 10 '16 at 22:53
  • First, you said more than 20 amps initially - I see now you've revised to 19A max - second, if the circuit is wired with 14Ga wire, no, you cannot change to a 20A breaker. If the circuit is wired (the WHOLE circuit) with 12Ga (copper) wire, you can. On the third hand you could see if running the smaller motor means the VFD does not actually draw enough to trip the 15A breaker. – Ecnerwal Dec 11 '16 at 0:35
  • I think I'll give it a try Ecnerwal with the 15A. Theax of the 1hp VFD is 19A input. But 3 phase motor is listed at 2A/1 which I assume is 2A per phase, which would be 6A. – JP4 Dec 11 '16 at 4:34
  • Opinion - If you are going to cord and plug the thing into an outlet, just unplug it when not in use for a "positive off" rather than buying a switch just for that purpose. Your in-use control would be via the VFD anyway, so I don't see the switch buying you much of anything. The VFD will draw more than 6A (it's not 100% efficient) but likely it will either work, or it will blow the breaker while charging its capacitors at start-up-time (which current may greatly exceed the "in-use" current, briefly.) If you are looking for emergency stop - those should be tied into the VFD. – Ecnerwal Dec 11 '16 at 15:01

With 3 phase the neutral is not needed but an equipment ground is needed. Other than fuel dispensing the ground is not opened for most normal installs. The switch you linked would make a convenient local disconnect. Note some VFD's when disconnected will fault on start if the VFD output was active when disconnected (requiring a reset at the VFD).I originally read this as a safety disconnect at the motor using the 3 phase outputs of the VFD. When using a 20A or larger 120V disconnect only the hot lead is needed to be opened many times a 240V disconnect is used with only 1 leg and there are many of these available with 30-60 amp ratings at most big box stores.

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  • Thanks Ed. You are correct I need to verify that the VFD can will retain settings after being disconnected from supply power. Can you tell me how the 1 phase 120v power cable will be wired to the 3 pole switch? – JP4 Dec 9 '16 at 18:16
  • The neutral is certainly needed since the only scenario that makes sense with the question is a 1-phase in, 3 phase out VFD. – Ecnerwal Dec 9 '16 at 19:00

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