VFD wiring questions

[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

[Zachary Hoyt]

i'm planning to mount the VFD out of the way on the back of the bandsaw in a metal enclosure. To buy a NEMA 12 enclosure of the proper size the manufacturer recommends is $165, not much less than the VFD, but I found an old metal tool box on eBay that is the right size plus a bit and can be had for $30 with the shipping. I'll use some weather stripping to seal up the door edges if I can, and I think it should work.

[Malcolm McLeod]

Have not read all of these, but SO cord is fine for wiring: 2 cond +gnd for the supply to the VFD; 3-cond +gnd from VFD to motor. Be careful of enclosures. The VFD needs air flow to cool. If sealed in an enclosure, clearance from the VFD surface to the enclosure ‘walls’, the total surface area, and enclosure material are important to shedding heat.

Surge suppressors, filters, etc are largely unnecessary unless you are dealing with very long wire runs (>100’).