VFD wiring questions

[Zachary Hoyt]

I've been trying to figure out the wiring needs for the VFD to run the 3 phase motor on the bandsaw I've been posting about lately, and have run into a couple of general questions about VFD wiring that have gotten me pretty confused. I'm hoping someone here can steer me in the right direction.

Can I plug a 2 HP VFD into the existing 50 amp welder outlet in my shop? It is the only 240V circuit I have, though there is room in the panel to add a new one if I have to. I will unplug the VFD when I am out of the shop or not using it for a while to reduce the chances of lightning or other damage.

What kind of wire do I need to connect a 2 HP 240V VFD to a 1.5 HP 3 phase motor? What I've been reading seems to say that any kind of suitable wire can be used on the input side, but there is a lot being discussed about the output side and whether the wires need to be individually shielded to avoid interference, or whether they need to be in a properly grounded steel conduit, or what. The wire run from the VFD to the motor will be about 3 feet or less. On the drill press I set up with a VFD a few years ago I just used some stranded copper wire I had around, but that was only a 1 HP with 120V input and the VFD is right beside the motor. I'll be grateful for any advice about these matters.

[Terry Therneau]

Plugging it into the outlet should be okay for a quailty VFD. I power off the circuit to mine if I'm going on vacation, and have not lost the settings yet. That would be the annoying if it happened.
VFD to motor: I don't have shielding on any of mine and have had no trouble. (3 HP direct drive 26" Moak bandsaw, 5 HP dust collector, 1/3 HP 6" Yates jointer, 1 HP t Ekstom edge sander). All but the jointer are short cords. I know that there can be some high frequency harmonics in the VFD to motor line, so perhaps it could cause issues if I had something nearby that was sensitive to that?

Terry T.

[Bill Dufour]

Input #12 or #10. Output #12. Consider metal flex conduit to motor, probably not really needed. Maybe a fixture whip. A load reactor can be used between the vfd and motor to reduce interference generated. I would add a whole panel surge protector to the shop panel to prevent back EMF.
BillD

[Richard Coers]

Did you check that the 2hp vfd will handle the starting load?

[Bill Dufour]

Did you check that the 2hp vfd will handle the starting load?

Just slow down the start acceleration curve to keep amps under control. I would start at 3-5 seconds slow start. For most use never go below 3 second start or stop time.
BilL D.

[Steven Cooper2]

I'm just here to agree with what Bill D said. Use a reasonable start/stop acceleration and you should be good to go.

[Zachary Hoyt]

Thank you all for your help. I didn't think about whether the VFD would be big enough. My drill press has a 1 hp motor running off a 1 hp VFD and seems to be happy, but I have never run it for more than half an hour continuously. 3 or 5 second start is fine with me. A panel mounted surge protector sounds good, and I will aim to add one of them. I've only got 50 amps running to the shop subpanel. I could do flex conduit, it would just mean a slightly bigger hole. Thank you all very much.

[Bill Dufour]

I meant the metallic flex conduit from motor to VFD. I think that i the wiring portion that can cause radio interference.
Square D QO panels from like 1950 onward can replace one two-pole breaker slot with a whole panel surge protector. New cost around $150. I have bough a few from the bay for about $30. I assume other makers have similar for their panels or use a generic one that has to be wired in.
Bill D.
https://www.amazon.com/Surge-Protect...8aAhqCEALw_wcB

[Richard Coers]

Thank you all for your help. I didn't think about whether the VFD would be big enough. My drill press has a 1 hp motor running off a 1 hp VFD and seems to be happy, but I have never run it for more than half an hour continuously. 3 or 5 second start is fine with me. A panel mounted surge protector sounds good, and I will aim to add one of them. I've only got 50 amps running to the shop subpanel. I could do flex conduit, it would just mean a slightly bigger hole. Thank you all very much.

I just suggested looking at start load based on a static phase converter that turned into Mt Vesuvius on a big old iron bandsaw I had in the 80s. Blew the safety cardboard right out of a capacitor and filled the shop with smoke!

[Zachary Hoyt]

Thank you both. I think a phase converter is different in terms of sizing, but I don't actually know anything about them. It sounds scary when they blow up, for sure.

I will have to see what brand of panel I have in the shop, as I can't remember. The house has a QO panel that I bought for it, and the shop got the panel that came out of the house. It was under 20 years old but only a 100 amp, and I put 200 in the house. Could I put a QO surge protector in the house panel, or would I need a surge protector in the shop subpanel?

[Bill Dufour]

I would put one in both. The home one protects against lightening and vacuum cleaner sparking. The shop one will help protect the VFD and prevent all the. universal motors from damaging electronics in both locations.
Bill D

[Bill Howatt]

IMO, the house protector at the main entrance panel is the one that does the best work - because it has (or should have) a good ground. A protector in a sub-panel is likely to be grounded by the ground wire from the main panel and it will have in most cases a run of wire which means resistance and inductance diminishing the effect of the ground for higher frequency voltages like those created by a sparking universal motor. The inductance and capacitance in a run of wire will actually filter high-frequency spikes.

By all means put in whatever makes you feel comfortable but the cost of a second one to protect the VFD may be as expensive as a VFD if you need professional installation. A decent VFD likely has protection built-in since they are typically designed to be used in a non-residential environment but that may not be the case on every model.

[Bill Dufour]

Famous last words. Where I live and grew up we get almost no lighting strikes that touch ground. So most surges are caused inside the property by motors sparking and generating back EMF on shutdown or welding. Hence why I recommend the surge protector in the shop.
Of course a big line reactor will do almost the same thing.
Bill D

[Rod Sheridan]

Hi, the manual for your VFD will indicate the maximum size of over current protection required, probably 15 or 20 amperes maximum, a fused disconnect switch would be easiest

2hp at 208 volts 3 phase would be about 7 amperes, so 14 AWG wire would be fine.

The VFD will be fine for continuous operation unless you put it in a box with inadequate ventilation

Regards, Rod

[Bill Dufour]

Good idea to put the vfd up on a wall where it kinda stays out of the dust. Down inside the machine cabinet covered in sawdust is the worst place as far as overheating.
Bill D