VFD wiring

Musky_Hunter

Musky_Hunter

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I wired my VFD and have my mill up an running. Do I still need the reversing switch on the left side of the head or can I just wire the motor direct to the VFD since it does the reversing?
 
You wire the VFD motor outputs directly to the motor. You use the VFD low voltage inputs through a separate switch to activate the run commands and motor direction. There is both what is called 2 wire control (which requires a continuous connection for the run direction), or 3 wire control which uses momentary switch for the motor on/off and a separate sustained 3rd input for reverse. I often use the switch gear below for the low voltage inputs, do not use the old rotary switch which is designed for high voltage switching.
 
mksj said:
You wire the VFD motor outputs directly to the motor. You use the VFD low voltage inputs through a separate switch to activate the run commands and motor direction. There is both what is called 2 wire control (which requires a continuous connection for the run direction), or 3 wire control which uses momentary switch for the motor on/off and a separate sustained 3rd input for reverse. I often use the switch gear below for the low voltage inputs, do not use the old rotary switch which is designed for high voltage switching.
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Oh I like switches. Thanks for the link. So it's not hurting anything to use the start/stop and reverse functions built into the VFD?
 
Musky_Hunter said:
Oh I like switches. Thanks for the link. So it's not hurting anything to use the start/stop and reverse functions built into the VFD?
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Correct, using the VFD functions is preferred. Using the old high voltage reversing switch can easily damage the VFD if the VFD/motor is running when that switch is used.
 
rabler said:
Correct, using the VFD functions is preferred. Using the old high voltage reversing switch can easily damage the VFD if the VFD/motor is running when that switch is used.
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Ok then I am going to remove that switch and just wire straight to the motor. Is it okay to leave the VFD on all the time or should that be on a switch?
 
Musky_Hunter said:
Ok then I am going to remove that switch and just wire straight to the motor. Is it okay to leave the VFD on all the time or should that be on a switch?
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Leaving the VFD on all the time should be OK, but I prefer to put them on a switch. That way, if a power surge such as a nearby powerline lightning strike occurs while you're not using it, the VFD is disconnected and unlikely to be damaged. Of course I knew people who unplugged their television when not watching it for the same reason, I never do that and my flat screen is more expensive than my VFD.
 
I leave mine on, never had a problem. I doubt a switch would help with a lightning strike, it's already jumping miles in the air, the few mm in a switch isn't going to slow it down. That said, most surges would likely be stopped with one. If you have a lot of surges, it might be wise to consider a whole home surge suppressor in the main panel.
 
ttabbal said:
I doubt a switch would help with a lightning strike, it's already jumping miles in the air, the few mm in a switch isn't going to slow it down
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Electrical transient analysis of surges is definitely a complex issue. Chances are pretty good that the closed switches (or the VFD without a switch) will provide enough low impedances paths to prevent hitting breakdown voltage across an open switch. If you get a lightning strike on the powerline on your side of the transformer, then all bets are off. Lightning strikes are also a very good reason you want single point grounding on your home electrical system. I don't know how many shops I've seen that are home wired with a separate ground rod for the shop. If lightning strikes ground anywhere in the vicinity, there is a huge potential across the two ground rods, and some hideous induced currents. Better to have only a single ground.

When I worked at Lockheed's antenna lab, we used to have a lightning generator, a bunch of power line capacitors charged in parallel and discharged in series. Was used to check C-130's wing tank covers, the original design was susceptible to internal arcing if lightning hit the plane. Internal arcing inside a fuel tank = bad. They filled a fake bladder behind the cover with propane to test various arc suppression methods. Lots of fun when one failed. We also used it for demonstrating that surge suppressors (MOVs in particular) can fail rather violently when over stressed.
 
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