Bad day connecting VFD to Southbend Lathe

Circuit / motor protection on 3PH motor with VFD

Connecting 2HP 240V 3PH motor to our 240V 1PH supply with an Automation Direct VFD rated 2HP 1 or 3PH. Using an existing 50A 240V 2 pole breaker with a welding receptacle we had already. Temporarily, I ran a #6-3 cord over to the panel until I can run a conduit overhead. As soon as the cord hits the panel, I switch to (3) #12 AWG THHN wires to the input on the drive, then (4) [T1, T2, T3 + GND] to the motor. With this setup the motor and the branch feeders are woefully unprotected, correct? Not knowing, my impulse is to add a 3-pole fuse block between the drive and the motor. Is this correct? Are you supposed to use special (like an I sq T, fast blow) fuse that will protect the drive better than a regular motor fuse? The Automation Direct catalog seemed to suggest the fusing went upstream of the drive (?) on the line side. I've looked at other (amateur like me) wired setups and see no fuses at all. There are tons of single phase motors in our shop with no protection of their own, but this lathe (Southbend with Louis Allis 2-speed 2-winding motor) originally had a really nice setup with Furnas starters / heater overloads. What's correct here? Thanks, John

my impulse is to add a 3-pole fuse block between the drive and the motor. Is this correct?

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No.

Connections between drive and motor should always be one continuous wire or cable. The drive is a code approved overload protection device for the motor, so no additional motor protection is needed.

With this setup the motor and the branch feeders are woefully unprotected, correct?

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The motor is protected by the VFD, but the 50A breaker will allow as much as 50A to flow through the #12 AWG conductors, which are capable of carrying only 20A. You either need to change the breaker to a 20A breaker, or run a new circuit protected by a 20A breaker (assuming the same #12 AWG wiring is used).

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Barry Milton

All motor circuits are required to have 3 basic elements, you can see the chart in the NEC article 430 near the beginning. They need 1) Short Circuit and Ground Fault protection, i.e. a circuit breaker or fuses (called a Short Circuit Protective Device or SCPD), 2) a disconnect means and 3) Running overload protection. Beyond that there are rules about MINIMUM conductor sizes and the protection thereof, but there is no rule against over sizing conductors. But when using VFDs, there are a few other issues:

A) Most all new modern VFDs are UL listed to provide the running thermal overload protection for the motor. Some are ALSO UL listed to provide the short circuit protection for the motor circuit (down stream only). You would have to check if those cheap little Chinese drives sold by AutomationDestruct are so listed, maybe not. If not you will have to add fuses or some other form of SCPD down stream.

B) Most VFDs are also UL listed behind LINE fuses, not circuit breakers. It's also true that most people are unaware of this or don't care and use breakers anyway. The NEC doesn't care either way, only that the unit is "listed for the intended use" and "INSTALLED AS PER MANUFACTURERS INSTRUCTIONS". So do exactly what the manual says and you are fine. Interpretation is risky but then again if you are not going to call for an inspection, that's up to you. Be aware however that if there is a fire cause by this unit, insurance inspectors love to find little loopholes to avoid payouts, and not following mfrs instructions on electrical equipment is one of their favorites. So if it were me, I would put in a Fused Disconnect Switch to feed the VFD with fuses sized to the maximum allowable in the VFD manual. Solves the disconnecting means requirement, the SCPD requirement and the VFD takes care of the 3 phase circuit down stream. If you can get #6 wired into the lugs of the switch, so much the better, that then becomes your junction box too.

By the way, you said "I switch to (3) #12 AWG THHN wires to the input on the drive, ..." By that I HOPE you are not connecting the Neutral to anything in the VFD! All it needs is the 2 hot lines and a ground. Also, #12 must be protected by a max. 20A SCPD.

As to "There are tons of single phase motors in our shop with no protection of their own...":
The rules still apply, but a lot of small 1 phase motors have built-in thermal protection switches, called "Klixons" that do the running overload protection, and a circuit breaker in the panelboard can be considered the SCPD if the wires are sized correctly. If they are plug-in, that is considered the "disconnect" but if hard wired, they are supposed to have a disconnect device. In some cases, if you see what looks like a toggle switch controlling the motor, that may be what's called a "Manual Motor Starter" and that is providing the necessary elements. 3 phase motors almost never have integral thermal protection because it would be required in all 3 phases (as opposed to only one in a 1 phase motor) so they become too big.

Thanks so much for the very helpful responses. I will add a fused disconnect with properly sized fuses (sounds like they need to be sized to the motor in this case). And point taken about the Chinese drive; came with the machine. It does seem to work pretty well though.