# Fuses between VFD and motor?



## Clock work (May 21, 2017)

I've seen a lot of pix of people's builds and look at the recommendations for my VFD 10,000 times. I see no suggestions for fusing for the three phases but when I reason it out, it seems reasonable to add protection there. I do have fuses post-disconnect and upstream from the contactor. Thoughts? Thanks in advance. 

CW


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## JimDawson (May 21, 2017)

Ahead of the VFD the proper way to do it is to fuse, with fast acting SCR fuses designed to protect solid state devices , ahead of the contactor.  I have seen some installations with D curve breakers for protection ahead of the contactor. (EDIT: Which are not correct, because they are too slow acting)

Having said that, about the only place you find fusing is in professionally built control panels.  My router has that, but that panel was built for a commercial application.  My mill just has the breaker in the shop distribution panel for protection.

EDIT: Fusing between the VFD and the motor is not needed or allowed.  The VFD provides the circuit/motor protection.


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## Nogoingback (May 21, 2017)

In the case of my vfd, there is circuit protection built into the vfd itself.  The manual from your vfd manufacturer
should indicate if any other circuit protection is needed.


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## mksj (May 21, 2017)

If the VFD is used with a single motor, there is no fusing between the VFD and the motor, it is normally directly wired to the VFD. There protection circuitry in the VFD output to shut down the VFD should a fault occur. If one was to fuse the VFD output, this could damage the VFD if one of the fuses went open while running. There are cases where a VFD may be running multiple motors, and in this case there is usually a disconnects and fusing specific to each motor.  The VFD parameters are different running a single motor vs. multiple motors.

Most VFD manufactures recommend some form of input fusing/breaker (MCCB) which sized to the VFD. This is usually after the power disconnect. In a some installations a contactor may be used to either energize power to the machine or to provide a safety disconnect. Fusing may be before or after the contactor, but usually after the power disconnect. The type of cartridge fuse and fusing characteristics are often outlined by the manufacture and may vary as to their purpose. Cartridge fuses ratings are different between the fuse sizes, so say a 30A J class may be sized different for a smaller CC class fuse. When fuses protect a machine they may have a delay element to account for different loads being switched on/off, when protecting a specific component in the machine the fusing element characteristics may be different. Fuse manufactures usually recommend high speed cartridge fuses for use with VFDs, because it more closely matches the overload characteristics of the electronic semiconductor components it is protecting. To some degree they also can protect the VFD from high voltage input line spikes. With smaller VFDs, they are usually a toss if they have an electrical failure, but on larger costly units the components may be repairable.

There is some further discussion on this in this posting.
http://www.hobby-machinist.com/threads/vfd-enclosurers-wiring-schematics-and-components.55739/

Example of lathe schematic:


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## Clock work (May 22, 2017)

Thank you for all the expert replies! Sincerely appreciated. 

CW


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## Clock work (May 22, 2017)

Actually maybe I'm not done harvesting good ideas yet. The documentation for my VFD lists 30AF 40A for the breaker. I put a pair of 30A breakers in the panel. At the disconnect near the mill, I was intending to put a pair of edison base 30A fuses which are universally noted to be time-delay for edison based fuses. No problem... I haven't populated the mounting plate yet and can put a 2-pole holder for a pair of Class T fuses. When all this is complete, I will have throws $100-$120 to protect a $240 VFD. Am I missing something?


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## mksj (May 22, 2017)

Most hobbyist get by with just a panel breaker, usually a 30A 240VAC 2 pole ganged circuit breaker works fine and is sufficient for  most 2-3Hp VFDs. I do fine with a 2 pole 20A for a 2Hp VFD on my lathe and do not use high speed fuses (it was before I knew much about fusing VFDs, but I am fine with a breaker). I do not recommend GFI sockets/breakers unless required, as I have had numerous problems with VFDs tripping GFIs due to induced ground currents. Use of high speed cartridge fusing is optional, as mentioned if something lets go in a the smaller VFDs it is usually a toss. Still, if the cost is not significant to add some high speed fuses, it is what I recommend in addition to a panel breaker. This may limit additional damage to wiring and other components should something short or overload.

You can pick up on eBay fuse holders for very little and also high speed fusing. I use the Limitron KTK-R CC 30A fast acting fuses just after the power disconnect on my mill with a 3Hp Yaskawa VFD, total cost of fuses and holder about $35. These are the main line fuses for the mill, I also pull off two 120VAC circuits in the cabinet which should have an additional 15A breaker/fuse for each circuit. Since my mill can run off of single or 3 phase input power, I use a 3 pole fuse holder and use L1 aand L3 for single phase.

http://www.ebay.com/itm/NEW-Fast-us...Fuse-Holder-Class-CC-30a-600vac-/262822996420
http://www.ebay.com/itm/LPSC002-Lit...nt-Fuseholder-for-Class-CC-Fuse-/222310948885
http://www.ebay.com/itm/6-NEW-Coope...-Fast-Acting-CLASS-CC-FUSE-600V-/252946282203


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## JimDawson (May 22, 2017)

I think you will be good either way.  The only time the line will see that kind of current is when you turn on the disconnect.  The inrush on the VFD is very high, but only lasts for a fraction of a second.  Just the feeder length will provide enough resistance to mitigate the inrush current by acting as a current limiting resistor.


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## Clock work (May 23, 2017)

Thanks guys... Again, really fantastic credible answers without the patina of internet BS. I (used to) design things for a living (high speed digital electronics stuff) and am a little embarrassed asking for help on things that involve electrons but I am very glad I did. I know from talking with others locally that the extra work to achieve an industrial standard in the final result is unnecessary for a personal system, but learning and implementing it (like moving it in to the basement..... safely) has been and is part of the satisfaction. My eBay fuses are on the way as is the non-rail holder from Grainger (clearance). Get this thing up and running... and get the pool restarted on when I finally rip a limb off playing with it


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## Clock work (May 23, 2017)

And oh... about in-rush... I am taking a pair of 1/2" cast-iron pillow block bearings (bearings removed from the frame) and wrapping the wires feeding the VFD around them a dozen or so times to isolate the noise a little. I used to go out on consults and "site audit" the precision measurement capability of electronics labs and the quality of the power would have a bearing (ROFL) on the achievable measurement precision of the instruments. Finding a piece of iron and wrapping the power supply cords around it would frequently produce observable improvements, so... doing it here too to smooth out spikes and make the antenna broadcasting the switching noise from the VFD shorter.


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## JimDawson (May 23, 2017)

Never seen that done before, but no reason it wouldn't work.  I like it!


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## wrmiller (May 23, 2017)

Clock work, I don't want to hijack your thread, but all this talk about fusing and stuff...

Does this mean I have to fuse the input to my VFDs for my mill and lathe? I have them both on 30a breakers, and then have a switch at the receptacle to turn on/off the power to the VFD. Seems to work just fine? But...


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## mksj (May 23, 2017)

I have had 240VAC wiring short in a box, and the breaker tripped, but some of the wire burned. If you take apart a high speed fuse, it is often some form of conductive metal strip  like silver between the caps with a series of holes punched in the strip. The narrow foil bridges will open very quickly based on current and heat, so you are most likely not going to have collateral damage is something goes wrong. The chance is small, but these type of fuses have saved my bacon on multiple occasions. 

The one thing I have on all my VFDs is a Line (EMI/RFI) Filter, which is a combination of inductors, resisters and capacitors. These shunt electrical noise to ground, and input inductors have a smoothing (dampening) affect on the incoming line voltage. Behind this on my garage sub panel I have an industrial surge suppressor and also one at the main breaker panel. We get a lot of lightening strikes in Arizona, I have had multiple neighbors who have had their electronics fried. The usual plug in surge suppressors do not have enough capacity to deal with any significant line surge.  

Most VFDs do have a shunt resistor between the input rectifiers and capacitor bank, the resistor is shorted after a short period on turn on. This helps reduce the input draw on turn on, and helps the longevity of the capacitors.


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