Electrical In Front Of A Vfd, Between The Vfd And Motor?

I'm asking what are the basic best practices without going too hog wild on building an electrical panel. I'm starting with a mill that has been converted to a VFD drive. The motor is an inverter rated Leeson 3 hp 3 phase. The VFD is about a $400 unit, its decent.

But that's all there is electrical wise. No E-Stop button, no magnetic switch, no panel disconnect, fuse, or circuit breaker, just a power cord from the wall and a power cord to the forward/reverse switch and motor.

So what are peoples recommendations as a best practice in front of the VFD, and inbetween the VFD and the motor?

Let's start with the the enclosure depth, sounds like you need a 12 inch deep, and at least twice as big as the footprint of the VFD to get airflow clearance. I would consider mounting the VFD on the wall in free air, but away from the chips.

A service disconnect between the wall and the machine is fine, or just the wall plug is OK for that purpose.

You don't really need a contactor for the VFD input power, I have mine wired directly to the breaker in the wall panel, it's always on. Of course, you never have a contactor between the motor and VFD. VFDs don't like to be disconnected from the motor.

An E-stop circuit is a good idea. Wired into the control side of the VFD, you could also put a contactor ahead of the VFD if you like, also wired into the E-stop circuit.

I can whip up a schematic if you like.

Thanks Jim, the enclosure I ordered has about 9.5 inches of depth with the door. I need 8.5 inches to clear the VFD but 1 inch of that is the stupid cord that attaches to the front of the VFD out to the remote VFD control. Its 20x20x8 enclosure so plenty of air space and I plan to install a fan in the enclosure.

A schematic would be fantastic. So I'm thinking momentary power on button, latching relay/contactor, E-Stop button in front of the VFD. This VFD if it has power the fan is always ON. I'll need some method of turning the power off vs pulling the cord out of the wall or turning the breaker off. Breakers are not really meant to be on/off switches.

Update: So the motor draws only 7.2 amps but the amp hog VFD requires 26.4 amps single phase input dang! Fortunately I already have a 30 amp twist lock outlet wired to my service panel, 10 gauge romex, about a 4 foot run. My power cord from there to the machine is also heavy duty 10 gauge so I'm all set on this and no need for another breaker at the machine. My garage shop is such that no machine other than the lathe has a permanent location, I wheel them out into the middle when in use and wheel them off to the side out of my way when finished.

JimDawson said:

Of course, you never have a contactor between the motor and VFD. VFDs don't like to be disconnected from the motor.

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Hmmm but isn't that what the Forward/Reverse switch is doing? Its a 3 position switch, forward, stop, reverse. I'm fine with no contactor there I'm just curious.

coolidge said:

Hmmm but isn't that what the Forward/Reverse switch is doing? Its a 3 position switch, forward, stop, reverse. I'm fine with no contactor there I'm just curious.

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No your fwd rev switch should be connected to the controls of the vfd. They are low voltage low current signals so a cheap small switch will do.

I think your best bet for an emergency off should also connect to the vfd controls (can be part of your fwd rev switch if you program your vfd correctly). The reason I say this is that you can then program your vfd to brake the motor and stop quicker. If you put an emergency switch between the vfd and the motor (yes the vfd doesn't like this but we are talking about an emergency here) you will stop the motor but it will then coast to a stop.

Atunguyd from what I can see the 3 phase output from the VFD goes straight to the forward/stop/reverse barrel switch, and from there to the motor.

Another question...so I'll have 220vac power to the mill but I'll need 110vac for things like the X axis power feed. Is it considered a no-no to tap one leg of the 220 for that? Will that annoy the VFD? Of course I guess I'd have to add a breaker sized to the 110vac load maybe its easier to just run another 110vac supply.

It would help to provide the make and model of the VFD and as mentioned, review this. Although the VFD may be rated for an input of 26.4 amps, it does not necessarily draw all this power, it is dependent on the program settings. Also, since you are going from single phase to 3 phase, you would need to draw a bit over 3X the amperage to each motor leg to get the single phase input amperage. This also affects the wiring sizing into the VFD and to the motor. Fusing before the VFD would be 2 pole single phase breaker, the manual usually provide the recommended amperage.

The wiring as currently outlined is not correct, better to start over and wire correctly. You may or may not need latching relays for the directional inputs depending on the model/type of VFD, as some have maintained direction controls activated by a momentary switch. Simpler VFDs do not provide this function, I have made up dual ice cube relay controls to provide this function. This also may require a separate power source to drive the relays. I use a power disconnect before the VFD, you can add a fuse/breaker between the switch and the VFD. If you are connecting to a dedicated socket withe a 30A breaker, then should not need to fuse at the machine. I do pull 4 wire single phase to my machines, so I can pull off 1 leg (before the VFD, after the fuse) for 120V. That being said, I run all my feeds off of 240V, use 120V for the DRO and lights.

coolidge said:

Atunguyd from what I can see the 3 phase output from the VFD goes straight to the forward/stop/reverse barrel switch, and from there to the motor.

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If that's what it does, it ain't right. I don't think the VFD would live long under those conditions.