New GFI 110v outlets blow out TECO VFD

[tq60]

tq60, with 4-wire plugs, there is both 220V and 110V across the legs.

Only if wired that way...Some are 3 phase...Many 4 wire plugs but which one matters.

Main point is most common of the 4 wire types are the 50 amp range plug used for rv too.

The op may not have 240 vac available so they used the boosting vfd.

 

[Blackjackjacques]

Happily, that's not the case. GFCI is the recommended way to make-shock-safe a
two-wire (neutral and line) receptacle even when there is no ground wire available. There's little
labels supplied with the GFCI so you can tag those outlets as not-grounding.
So, a short extension cord, with a GFCI on the end, is a worthwhile precaution
if you have two-wire or ungrounded (or just uncertain) power outlets.

"Irrelevant" would have been a better word selection than inoperable since as you say the GFCI does permit shock safety for a 2-wire system. The GFCI is based on a simple differential amplifier that confirms that what current travels in one leg also occurs in the return leg, with the concept of "ground" an electric safety architectural presumption such that if there is a difference between the differential inputs, the current difference must have found another path, and that path is presumed to be ground since ground traditionally presents a potential difference with respect to the "hot" conductor. In an arrangement with an iso xfmr feeding a GFCI device, a body coming across an energized GFCI load conductor and ground would not cause leakage current flow as there is no ground reference, thus the device serves no purpose.

Not always !!
Isolation transformers are available in TWO different types, either with neutral/ground bonding
on the output side, or with neutral floating on the output side. If you want shock-safe
due to transformer isolation while working on live circuits, only the neutral-floating
type is going to work for you. This item TrippLite iso transformer has a 'N-G' output
limit of 0.5V, indicating that it has the neutral on the output bonded to ground on output (from
the third prong of the input wire). Hold the output LINE wire, touch a ground, and you get a shock.

It's confusing, and a little scary, that the 'isolation' situation is so poorly described. I've
tested both types of isolation transformers, they both DO have legitimate uses. A
three-prong outlet tester will 'fail' the isolation transformer output socket unless there's
a neutral-ground bond.

I think the industry has done a decent job in describing iso xfrmr connections and their function. While jumper configurations permit various needs, the general notions in the contexts of electrical safety, patient safety, stray current corrosion applications for boaters, etc understand the floating nature, benefits and limitations of these devices. The widespread understanding for ISO transformers is as I identified. However, in the testing realm, I understand how iso xfmrs may be used and special bonding features, particularly in regard to noise reduction.

 

[Dabbler]

Tq60: The Electrical code clearly describes the wiring for single phase twist lock plugs, which is the general topic here. Yes 3 phase is a possibility, and I correctly wire the output of my VFD to 4 wire plugs, but it is out of the scope of this thread. I use 20A twist locks for 50% of my shop connections, and standard 15A 110V duplex for the rest. In my jurisdiction, and coming soon to many others is the requirement for arc fault on all new wall sockets. Equipment manufacturers will take years to catch up with this electrical code requirement. My advice is a way for the OP to get his wiring to code, passed, and keep working without deception. This is important if he wants his insurance to remain intact.

I have no experience with any 50Amp twist lock.

 

[tq60]

You are correct but missed my point which agrees with your general statement...

Over the years we have seen and read of many odd ways of folks performing wiring that does not meet code then ask and defend the bad ways.

The mention of "4 wire plug" could mean anything without the code number, we use the numbers when needed for documentation but they are not etched in memory as other things are so not stating them here.

We have many 240 volt plugs in the shop that are the old dryer type that likely do not meet current code but considering neutral not needed they meet the function of L1, L2 and safety ground.

Knowing every township can have their own modified version of code for almost everything we usually suggest one possibility of a solution so the op can research it to understand it along with suggesting they follow up with local building department to develop plan that meets the local code.

 

[JPMacG]

This has been an interesting and educational discussion. I am an electrical engineer (microwave antennas) but have no background in power and am sadly lacking in this area. Thanks everyone!

 

[tq60]

This has been an interesting and educational discussion. I am an electrical engineer (microwave antennas) but have no background in power and am sadly lacking in this area. Thanks everyone!

Lots of differences between them!

Close to 40 years in radio including tractors to cell switches with a bit of microwave in the middle.

One of the least understood things due to lack of front end engineering...

Good luck!

 

[Glenn Brooks]

To answer a couple of questions. It’s a 15 amp circuit, and all three electrical fixtures are in line. In addition, another GFCI fixture is in line on the circuit betweeen the triplex box and the main. The grinder is a common 8” 3/4 hp Baldor pedestal model, in good condition. It runs perfectly well on the original legacy circuit, with the VFD installed and grounded to the frame and motor. Also the County requires GFCI on nearly all grade, or below grade locations, regardless of proximity to wet spaces.

To confirm, I called the King County electrical inspector and was informed basically, all newly installed fixtures need to be GFCI. This of course is a vastly expanded and altered definition of what the NEC actually requires. The inspector also stated GFCI works properly with all electrical equipment and detects ground faults, shorts, in electrical motors thus MY grinder must be defective, likely with an inadequate ground. Of course this is also uninformed nonsense, as VFD’s are widely known to NOT function with GFCI’s, hence the problem of tripping the circuit when NO SHORT HAZZARD EXISTS. mine certainly trips the GFCI as soon as it is turned on, and at no time with non GFCI circuits. But who am I to argue with such a knowledgeable guy.

 

[Bob S.]

To answer a couple of questions. It’s a 15 amp circuit, and all three electrical fixtures are in line. In addition, another GFCI fixture is in line on the circuit betweeen the triplex box and the main. The grinder is a common 8” 3/4 hp Baldor pedestal model, in good condition. It runs perfectly well on the original legacy circuit, with the VFD installed and grounded to the frame and motor. Also the County requires GFCI on nearly all grade, or below grade locations, regardless of proximity to wet spaces.

To confirm, I called the King County electrical inspector and was informed basically, all newly installed fixtures need to be GFCI. This of course is a vastly expanded and altered definition of what the NEC actually requires. The inspector also stated GFCI works properly with all electrical equipment and detects ground faults, shorts, in electrical motors thus MY grinder must be defective, likely with an inadequate ground. Of course this is also uninformed nonsense, as VFD’s are widely known to NOT function with GFCI’s, hence the problem of tripping the circuit when NO SHORT HAZZARD EXISTS. mine certainly trips the GFCI as soon as it is turned on, and at no time with non GFCI circuits. But who am I to argue with such a knowledgeable guy.

Glenn I have the same VFD as you plugged into a GFI protected outlet it runs my old Delta drill press and works without tripping. Am I just lucky?

 

[whitmore]

Glenn I have the same VFD as you plugged into a GFI protected outlet it runs my old Delta drill press and works without tripping. Am I just lucky?

The GFI is probably sensing a 'fault current' because of the RF filtering in the VFD, but the GFI trip point
is set at (5 mA?) and it is common practice to steal a little current (like, one-third of the trip current)
in some appliances. For RF filtering purposes, a small capacitor on the line/ground will take
acceptable 60 Hz current, below the GFI trip point, BUT will also pass some switching current
at high frequency. Other appliances ditto pass small currents.

So, the VFD, and the load on the VFD, and the filter detailed characteristics, and the capacitance from line
to ground of everything ELSE on the circuit, and the high frequency sensitivity of the particular GFI module,
all combine to determine whether it trips. The production year/month of a VFD might
determine whether the RF filters put 1.5 mA to ground, or 3.5 mA. Perfectly good devices,
no shock or fire safety problems at all, can be incompatible with the GFI.

This suggests that a breaker-box GFI (one unit protecting a half dozen outlets) is more likely
to trip than an end-node GFI (one unit protecting two outlets). It also suggests that a single
component (the powerline filter) of the VFD can be the only problem, and those can be replaced
fairly easily. Me, i'd see if it was possible to clamp a ferrite common mode choke on the
line/neutral wires inside the VFD. Or, ask the manufacturer of the VFD.

 

[Dabbler]

Glen, without taking a meter an scope out to your garage, I can't offer much more. I'm an engineer, but I consult with an RF engineer when I have weird problems... You might consider asking for a little advice from an EE to help you out with the inspector, however it seems the only solution that will satisfy the inspector is wire a completely different circuit.