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



## coolidge (Mar 28, 2015)

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?


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## JimDawson (Mar 28, 2015)

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.


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## coolidge (Mar 28, 2015)

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.


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## coolidge (Mar 28, 2015)

JimDawson said:


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



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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## atunguyd (Mar 28, 2015)

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.


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.


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## coolidge (Mar 28, 2015)

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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## coolidge (Mar 28, 2015)

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.


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## atunguyd (Mar 28, 2015)

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.
> 
> /QUOTE]
> I can't see the image you posted, but I suspect that your system is not wired correctly. If you hit that switch while the vfd is providing power to the motor you run the risk of blowing your vfd. I suggest getting someone to help wire it correctly or consult the manual for the vfd.


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## mksj (Mar 28, 2015)

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.


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## JimDawson (Mar 28, 2015)

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.



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


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## coolidge (Mar 28, 2015)

JimDawson said:


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



Then its not right because that's exactly what it does. Okay I just spent the last couple of hours reading the instructions on this VFD. Its a *Danfoss VLT Microdrive FC 51 2.2 kW 3hp*.

Quite interesting, it has a Run Reverse function, I'm still looking to see if it provides inputs to switch forward/reverse remotely via an external switch vs navigating a menu on the control pad. It does support a remote 10k pot for controlling spindle speed. This particular unit comes with a built in pot (they offer a version without the pot). I'm not sure if I can disable the built in pot and wire up a remote pot via the menu, if need be I'll break out my electronics gear and perform surgery. If I can also control the forward/reverse from a remote low voltage/low current switch that will be great, I can get rid of the big ugly China barrel switch.


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## coolidge (Mar 28, 2015)

mksj said:


> 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.



Of course 4 wire. I'd have to re-work one of my two 220/30amp circuits and make up a 4 wire cord.

Question though, I never really thought about it but is a 220vac 30amp breaker really 15 amps per side? In other words if either one of the two legs exceeds 15 amps will the breaker trip? The reason I ask is ideally I'd want a 15 amp breaker protecting the 110v stuff, lights, etc. In a 4 wire configuration I'd be using one of the 220vac hots and the neutral to provide 110vac. The question is if something on the 110vac shorts when will the breaker trip, 30 amps or 15 amps?


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## wrmiller (Mar 28, 2015)

Interestingly, my CO (and I suspect all of their 12Zs) are wired exactly the same way.


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## coolidge (Mar 28, 2015)

Hey Bill welcome to the electrical party! In related news I have determined the Hobby Machinist forum is evil...my head is spinning with ways to spend money on this thanks to Jim and the boys. I'm only surprised DZ hasn't arrived to spend some more of my money.

I may fire up my CAD software and give my local CNC punch press shop pals a call next week to make me a pendant enclosure. There is limited space on the left side of the mill head, I have to keep clear of the spindle lock. Plus its in the line of fire of chips and coolant. I don't really want to be drilling holes in the side of it due to oil leaks. Here taking a lesson from CNC mills makes sense, move all that stuff out away from the mill head on a pendant.


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## chips&more (Mar 28, 2015)

Yes, I hear and read all over how not put any switches between the VFD and motor or risk blowing your VFD. Well about 20 years ago I bought an ACE VFD. The manual said nothing about the said warnings so I wired the VFD like it was 240V 3 phase in the shop. I just plug in 3 phase stuff and run it with switches on the raw 3 phase side of the VFD. I tap with instant reverse and everything. The VFD has performed flawlessly since I bought it. I do not know what an error looks like, it has never displayed an error code or smoked…Dave


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## wrmiller (Mar 28, 2015)

Ok, to be fair to CO here's the procedure used to start the spindle per their startup sheet:

Put the FWD/OFF/REV switch into whatever direction is desired. Then press the VFD's 'hand on' (manual mode) front panel button that activates the panel/applies power to the motor. THEN you turn the pot to what ever your target frequency is. To shut down is the reverse: turn the freq. pot to zero, press the 'off' button on the VFD's front panel and then (optionally) you can then turn the FWD/OFF/REV switch to off.

To be fair, I have forgotten all this and simply reached up for the big red switch and switched it to off while the VFD is in manual mode (which is what I've been doing for over a month now). When I told Paul about this he told me that I wasn't the first one to admit that, and that he has not had any reported VFD failures that may or may not be attributable to disengaging the motor while the VFD is on.

Probably the best solution for all this is to make up a panel that has a fwd/rev switch on it and a pot all wired to the VFD. Or...I could just leave the switch in the fwd position and use the VFD's front panel. Or just not worry about it and see what happens.


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## tmarks11 (Mar 28, 2015)

wrmiller19 said:


> Ok, to be fair to CO here's the procedure used to start the spindle per their startup sheet: Put the FWD/OFF/REV switch into whatever direction is desired. Then press the VFD's 'hand on' (manual mode) front panel button that activates the panel/applies power to the motor. ...



That would work.  Except it is a bad business practice on their part to do so.  It ignores the manufacturers instructions on installations.  It puts an expensive switch in the mix that is switching a fair amount of current.  I am very surprised, although CO/IH has always been a small company, and they might not have an electrician on their staff who knows better.  The funny thing is it would be easier AND CHEAPER for them to do it the right way.

Like the previous posters said, you use the low voltage contacts on the VFD to tell it what you would like to do.  And the computer in the VFD then decides to do what you asked and makes it happen. It is like magic.

Why do it that way rather then use a barrel switch?  It will make you motor last longer.  It will put less stress on your spindle bearings.  It will stress the VFD less.  It will eliminate the huge (3-4x running current) starting surge.

The VFD is designed to ramp up and down speeds, rather than instantly try to change them as the barrel switch would do.  When you go to reverse the mill, it ramps down, reverses, and ramps back up.  Much less stress. 

You could use that big honking barrel switch for the low voltage reversal command if you like.  I personally would spend $6 for a new switch, that is more compact and fit the controls in a handy pendant attached to the side of the mill head.


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## wrmiller (Mar 28, 2015)

Interesting. My motor/spindle doesn't 'surge'. It takes (and is programmed to do so) 3 seconds to reach target frequency/speed. So how does the VFD control this when I'm simply switching on the VFD's output?


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## tmarks11 (Mar 28, 2015)

wrmiller19 said:


> Interesting. My motor/spindle doesn't 'surge'. It takes (and is programmed to do so) 3 seconds to reach target frequency/speed. So how does the VFD control this when I'm simply switching on the VFD's output?


I was imprecise.

I am guessing that CO has wired it up so the on/off switch provides a low voltage signal to tell the VFD to start-up, and the VFD (as you said) is set-up with a 3 sec ramp-up to get to full speed.  The VFD also prevents huge surges during motor reversal by controlling ramp-up of voltage and current when it swaps two phases.

If your VFD is always running, and your on/off switch is between the VFD and the motor, than you would see a large current surge when you started the motor (if you had an ammeter on it)... and your VFD would probably generate a fault (excessive Full Load Amps) and shut-down to protect itself.

As long as the VFD is allowed to control it's on/off/reverse cycle on its own, it avoids excessive current surges.  The VFD prevents startup surge by ramping up both current and voltage slowly, minimizing the huge differential voltage usually placed across the windings at startup.

Using the barrel switch to switch motor directions with the VFD energized could result in current surges in excess of 5x run current... not good for the transistors in the VFD, as you are exceeding their ratings many-fold, which could lead to early death.  Your VFD will probably (again) generate a fault (excessive Full Load Amps) and shut-down to protect itself.

DISCLAIMER: I have never switched the power between a VFD and motor, not wanting to brick a $250 VFD.  So it is possible that the VFD won't fault on FLA if you start it that way (maybe there is a time delay).  The enemy of transistors is heat; current passing through the transistor generates heat.  However, short durations starting surge might not heat it up enough to fry a transistor.  I don't know, and I am not curious enough to risk $250 to find out.


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## mksj (Mar 28, 2015)

The purpose of the VFD is to control the motor.  The Danfoss manual is quite straight forward on how to setup the VFD, you can set it up with remote low voltage controls for direction and a 1K ohm pot for speed control. Just remove the current high voltage direction switch, connect a small control wire cable to the low voltage controls, and it will work as it is suppose to. Add an E-stop. There is a provision on the FC51 to remotely mount the complete control panel, so even simpler, just pop it out, get a cable and relocate to the mill head. You have a good VFD and a nice motor, why waste the programmable features of the VFD.

http://www.danfoss.com/NR/rdonlyres...-8B2D62FCEE9E/0/FC51_DesignGuide_MG02K302.pdf
http://www.danfoss.com/NR/rdonlyres...3-1CA240969B4E/0/FC51_QuickGuide_MG02B902.pdf

The recommended fusing for this model is 50A, but 30A may work. You can separately fuse the 120V/240V  branch circuit/if you pull power to other devices. This is commonly seen in the control boxes of larger mills/lathes. A dual pole 240V breakers with a single breaker bar, will trip if either side is overloaded.


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## itsme_Bernie (Mar 28, 2015)

No magnetic switches between the VFD and motor.  A three position switch is fine, just don't switch it while it is running or even turning.  
I have my VFD hanging on a hook next to whatever machine I am using.  I use twist lock connectors so I won't blow it.  No enclosure for 10 years now, no problems.

Bernie


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## JimDawson (Mar 28, 2015)

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.



I'm a little late getting back to the party.  Well that seem like that is exactly what the switch is doing, unconventional at best.  I have never seen that done before, and it goes against every VFD manufacture's recommendations that I have ever read.

Here is a simple schematic for the power contactor circuit.  If you don't have a neutral on your plug, this could be wired across the 240V line with a 240V coil relay.  The Automation Direct relay part number shown is a 120V coil.


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## JimDawson (Mar 28, 2015)

tmarks11 said:


> DISCLAIMER: I have never switched the power between a VFD and motor, not wanting to brick a $250 VFD. So it is possible that the VFD won't fault on FLA if you start it that way (maybe there is a time delay). The enemy of transistors is heat; current passing through the transistor generates heat. However, short durations starting surge might not heat it up enough to fry a transistor. I don't know, and I am not curious enough to risk $250 to find out.



I agree, maybe some day I'll have a spare to try that with.  I'm curious as to what would happen.


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## wrmiller (Mar 29, 2015)

My Danfoss control panel is located next to the DRO. It was on the head but I don't like electronics getting abused like that.


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## coolidge (Mar 29, 2015)

Bill I'm replacing that Danfoss 10k pot. Its rather pathetic imo, it only has about 180 degrees of rotation, zero resistance to turning, and the knob is tiny. I found trying to adjust the speed pretty annoying. I'm not going multi-turn or anything but at least a single turn 270 degrees. And ideally I would put that control panel back on the main unit vs having it remote mounted out front.


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## tmarks11 (Mar 29, 2015)

JimDawson said:


> I agree, maybe some day I'll have a spare to try that with.  I'm curious as to what would happen.


It should just fault on exceeding FLA limits, not fry a transistor.  But since it is just as easy to hook up the VFD the right way (instead of the wrong way), it never really seemed worth experimenting to find out,


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