Contactor for VFD

[Ulma Doctor]

An on off switch is all that is necessary for powering a VFD. Anything else is gymnastics and unnecessary

 

[David2011]

I like the 3 pole switch. Is the thermal overload relay recommended by Automation Direct also overkill?

 

[David2011]

I wouldn't interrupt the neutral, but that does look like a beefy switch

Agreed. Leviton offers a 2 pole 30A switch that also saves a few dollars. https://www.amazon.com/Leviton-MS30...&sprefix=motor+control+switch,aps,134&sr=8-60

 

[MikeInOr]

I don't see the need to interrupt the neutral, but that does look like a beefy switch. The two pole is only 20$

I couldn't think of any reason to interrupt the neutral either, then I couldn't think of any reason not to. For and extra $8.40 I would tend to go for complete isolation when the switch is turned off. But from a standard electrical standpoint the 2 pole should be plenty.

 

[MikeInOr]

I like the 3 pole switch. Is the thermal overload relay recommended by Automation Direct also overkill?

All of my VFD's will shut down long before a thermal overload would shut off the circuit. I do not have any thermal overloads on my 3 phase motors with VFD's. One of the settings on the VFD's I have is the current (amps) rating from the motor badge.

 

[mksj]

No thermal overload should be used on the input, an no need for a contactor. The latter is sometimes used in a latch circuit so the if the power drops out the VFD will not turn back on, it can also be used with certain redundant safety systems. Most VFD's have a program setting to prevent the VFD from going into a run mode on power up, even with 2 wire control. Typical power disconnect for an enclosure is a rotary power disconnect, the default configuration is usually 3 pole for 3 phase. You only use 2 poles for 240VAC, there is no neutral, and if there was it would not be disconnected by the power switch.

There is no output overload device used when a VFD is directly wired to the motor, the VFD is the overload device. Multiple motors run by a single VFD should have individual overload devices that can work with the VFD output. A breaker or fusing is sometimes used in a VFD enclosure for power to the VFD, this may help limit catastrophic damage, but the smaller VFD;s are typically a toss if they go. There are also cases where the input current overload device to the enclosure is high, and then you fuse/use a breaker for sub systems in the enclosure.

Another issue, and what I have seen as the most common failure mode in the VFD's I use is too rapid cycling of the power after the VFD is turned off. You should wait at least 5 minutes after the VFD is switched off before it is power back up. There is a current limiting circuit, typically and this fails with too rapid on/off/on cycling of the VFD.

 

[David2011]

These responses all sound reasonable. The questions were prompted by the VFD’s manual. The manufacturer recommended a contactor with a thermal overload relay. It’s hard to imagine overloading a home workshop mill to the point of tripping an overload relay. The cutting tool in my experience will break long before overloading the motor.

I was going to install a 2 pole fast blow fuse between the breaker and the VFD but I’m starting to think even that is overkill. The first breaker is 20A, 2 poles for everything that’s powered off of the electrical box. That will be 2 VFDs, the DRO, lighting and anything else that’s run with the mill. The 120V side will have an additional breaker.

 

[mksj]

I do add fusing to most of my VFD builds, but usually dealing with larger more expensive units or they are feeding something like a 2 or 3Hp VFD off of a 50A breaker. Also the theory that fast blow fuses may spare the amount of VFD damage, my take form some other installers is that a fast blow is probably not going to save your drive. They cost more and you also need to size them much larger to handle any surges. It is also brand and model specific, as well as the type of fuse and if you are using a DC choke. At a hobbyist level, match the panel breaker to the VFD recommendation, and this is for a 2 or 3 Hp single phase input VFD is a 30A breaker, although I have run 2 Hp on a 20A with not issues. I often use CC fusing/MCCB in the smaller VFD installs, in the larger sizes I typically use J class fusing. Fuse are typically Low-Peak dual element, as this decreases the size of the fuse vs. fast blow/semiconductor type. But if you are running everything off a 20A breaker I do not really see the need, unless specified by the VFD manufacturer.

The VFD manuals are designed to cover a wide range of installs from small to very large, as well as code/industrial requirements. The only time I usually see an input contactor used is in a power latch situation, and were there are remote buttons to energize the contactor/VFD. This also requires some always on power source either transformer or power supply. I also have not seen were they have indicated to use a thermal overload relay which would make little sense in this application.

 

[David2011]

A power latch is exactly why I was jumping through the hoops. A conventional switch eliminates the need. I’ll order a switch and be happy.