# VFD enclosurers, wiring schematics and components



## mksj (Feb 12, 2017)

I often get asked on what components to use in VFD cabinets how to wire 240VAC voltage. The attached files give some general recommendations, one should always follow local electrical code requirements and the VFD manuals recommendations as to wiring and fusing. The attached information is based on non-industrial settings, i.e. home shop installations. Since I am not an electrician, use this information at your own risk and check with your local electrician if you need specifics on your local code. Since this information has been asked on several PM lathes/mills I am parking it in the Precision Mathews section.

In the basic wiring schematic I usually recommend a main power disconnect at the VFD enclosure and some form of overload protection if you are not using a dedicated circuit with an appropriate sized breaker at the electrical panel.  The disconnect switches are usually 3 poles, you would use 2 poles for single phase.

Enclosures, I usually recommend a minimum of 14" tall, 12" wide and 8" deep for VFDs up to 3 Hp, up to 5 Hp you may want a 16" by 14" cabinet. The VFDs are very tall, and with a backing plate you loose height. A 6" deep cabinet will not fit most VFDs unless you cut the front panel and extended it through.  You may be able to pick up a cabinet on eBay at under $100, but you need to do a lot of looking and you might get lucky. Otherwise for a little more I recommend getting one from Automation Direct, usually a fiberglass reinforced plastic type. I recommend snap clasps, a clear window is nice but not necessary. Once the VFD is running, you do not need to look at it unless there is a fault.

Overload protection: fuses work quicker than breakers, and have a much lower fault current, they are usually used to protect the device. Breakers usually are used to protect the wiring. Even with a electrical panel breaker, machines/equipment are usually fused locally.  The usual arrangement with VFDs 3Hp and under is to use the larger J class type fuses, I prefer the finger safe fuse holders as opposed to the open type. Note that fuse holders are both specific to the class of fuse and they have specific ranges, so on J class you will see holders up to 30A, then over 30A to 60A, etc.  With VFDs you must use high speed fuses, not standard or dual element delayed type. These fuses are not inexpensive, but if you do not use the correct type size fuse it does you little good. Fusses are not interchangeable between class types, because they have different thermal characteristics and overload properties.  If space is limited, then one can use CC class fuse holder and high speed fuses, CC class fuse are NOT midget fuses. Some manufactures recommend a larger size fuse when going from something like a J class to a CC class, so check your manuals or call technical support.

Supplemental breakers/fuses. You may want to add supplemental breakers to protect other components and wiring  in the system. I often recommend a 3A supplemental breaker to protect the power going to the lathe cabinet, and to power a coolant pump contactor/relay if used.  A breaker will not protect a coolant pump from overload, only a direct short. If you want to protect the coolant pump from thermal overload (such as if the impeller gets locked), then you must use an appropriate sized thermal overload relay on the coolant contactor.  I often add a single gang 120VAC socket at the machine or a dual gang sockets. I use a 15A supplemental breaker to protect the wiring going to the socket(s), this also requires that a neutral wire is pulled to your VFD cabinet and then on to the 120VAC socket.

On cooling the VFD cabinet, with two vents, you can often get away with no auxiliary cabinet fan, but in this case I recommend that the VFD fan is set to run whenever the VFD is running the motor, there is usually a setting where it will run X minutes after the motor is stopped. If you add an auxiliary cabinet fan, I usually recommend mounting it on the bottom inlet with a filter so the fan blows into the cabinet (positive pressure). The fan is under the VFD, blows air over it and either out the top or side of the cabinet. With an auxiliary fan it turns on with the VFD cabinet power and runs continuously, the VFD fan is set to come on only if the heat sink goes beyond a fixed temperature. The advantage to an auxiliary fan is that the VFD fan will probably not turn on, the VFD fan (and components) will last longer, and an auxiliary fan is much cheaper to replace.

Braking resistor is sized to your VFD. I have never seen one get past luke warm.  They do have very high voltage on the wiring when braking is engaged, something like 380VDC. They can be mounted on the side of the cabinet or on the back panel.

If others have comments or recommendations, please add to this post.


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## pstemari (Feb 12, 2017)

Don't you get problems with the initial current inflow when using fast-blow fuses? The DC side of the VFD has some monstrous caps.

Sent from my Nexus 6 using Tapatalk


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## mksj (Feb 12, 2017)

VFDs have inrush suppressors that prevent the current spikes typical of power supplies with large capacitors. Usually a PTC or a resistor between the diodes and the capacitors that is switched out after a specified period on the order of seconds, they also have a capacitor discharge mechanism.  This is also why VFDs can be damaged if the turn off and on too frequently. One should allow at least 4-5 minutes after a VFD is turned off to turn it back on.

The fast blow fuses are what are specified by both the VFD manufactures and the companies that sell them. They have very specific characteristics that allow some inrush current, but they are current limiting. This is a brief discussion on different fuses/breakers and their design characteristics.
http://www.cooperindustries.com/con...library/BUS_Ele_Tech_Lib_High_Speed_Fuses.pdf


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## pstemari (Feb 12, 2017)

I just reviewed what Hitachi put in their manual, and they specify "Class CC, G, J or R fuses or circuit breaker". They don't say anything about slow-blow vs instantaneous for fuses, but for circuit breakers they specifically say to use inverse time breakers, which are effectively slow-blow. Schneider specifically list both slow-blow and instantaneous fuses as acceptable for their drives. KB says either can be used but slow-blow is less apt to nuisance blowing.

http://www.schneider-electric.us/en/faqs/FA116067/
http://www.kbelectronics.com/tech_bulletins/fusing-app-info.htm

I did find some info about specialized fuses available now that attempt to limit internal VFD damage to a single component, but those appear to only be useful for larger devices.


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## mksj (Feb 12, 2017)

The fuse specification varies by VFD/fuse manufacturer and in most cases their specifications have more to do with maximum rating of the fuse/holder as opposed to the fuse type or characteristics. Although the WJ200 manual lists a number of fuse types that may be used, later in the manual, the current ratings are only for the J class fuses.  High speed fuses are designed specifically to protect electrical devices including VFDs. If you go to Automation Direct as to spare parts for VFDs, they specify only high speed fuses. You can use a different type of fuses, but you might as well use a breaker. In most cases the VFD will be toast, but a high speed fuse could protect from a power surge where a breaker would not.The fuse makers lead you to believe that the high speed fuses protect the devise, or at least limit damage.  The information is provided more as a starting point as options, but for my VFDs installs I have been using high speed fuse to offer an extra measure of protection and have had no cases where there has been a problem using them.
https://www.automationdirect.com/ad...es/AC_Drive_(VFD)_Spare_Parts_-a-_Accessories


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## pstemari (Feb 13, 2017)

Fuses of any sort will not protect against power line surges.  The current spike from a power line surge that melts the fuse is the result of the over-voltage damage to the drive. Surge protectors use a mixture of inductors and MOVs to limit the voltage. The high-speed fuses (e.g. Cooper class DFJ) supposedly can prevent a fried IGBT from rupturing and damaging adjacent components in the drive. That's beneficial if you have a big enough drive to make repair cost-effective.

http://www.cooperindustries.com/con...ic_Device_Protection_VFD_Drive_Protection.pdf


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## mksj (Feb 13, 2017)

I am well aware of spike suppressors in all various forms as I use several custom  industrial ones my house and others installs in multiple redundant fashions. But it is possible that if a voltage surge gets past a surge suppressor, which is common, then  less damage could occur to equipment if just the fuse is taken out before the device. Just about every VFD manual/schematic for mills and lathes shows the use of a fusable element  as required as opposed to optional, per the Durapulse VFDs "*Input fuses protect the AC drive from excessive input current due to line surges, short circuits, and ground faults*. *They are recommended for all installations and may be required for UL-listed installations*."  

I made it clear the if a VFD fails it is most likely not cost effective to have it repaired, all though I have heard of people who have repaired a fried tracing or component module. What I am trying to point out is what just about every fuse maker defines as the "appropriate type of fuse for this application".  If you read the application notes for fuses you find in almost every case that for variables speed drives a fast acting fuse is specified. 

Lttlefuse JLS Fast-Actingfuses - Provides fast-acting protection to equipment such as variable speed drives, rectifiers and other equipment containing surge-sensitive components.

Ferraz Shawmut HSJ fusing -  Most VFD failures occur from over voltage and transient voltage conditions. Left unprotected these leads to component failure and melting of the bonding wires. Only high speed fuse can prevent this. Mersen has developed an innovative solution to this problem, the UL listed Class J Fuse (HSJ) designed specifically to fit the operating parameters of drives and soft-starters, and protect the internal power electronic devices. The HSJ combines the following features:
• Semiconductor protection (very low I2t)
• Compliance to NEC (branch circuit protection)
• Mirrors operating characteristics of electronic motor controllers
• Capable of protecting overloads down to 135% of fuse rating

Eaton Class J high speed fuses - for full range protection Bussmann series full range DFJ high speed, current-limiting fuses provide overload and short-circuit protection for variable speed drives and electric controllers, and meet NEC® branch circuit protection requirements. The DFJdrive fuse has the lowest I2t of any UL branch circuit fuse in the industry to protect powersemiconductor devices that utilize diodes, GTOs, SCRs and SSRs.

*AC Drives FAQ (part 4): Why semiconductor (or high-speed fuses) are used on a variable-speed drive (VSD, VFD) application*
Semiconductor fuses are a type of power-limiting fuse using specifically-shaped silver elements in a silicon/sand environment. Because of their power-limiting characteristics, they can protect some semiconductor devices from excess power. The power-limiting characteristic of semiconductor fuses is also useful for reducing the amount of damage done by an arcing fault; hence,the fault current is effectively reduced.

I am just indicating what is recommend for this application, not debating if a different fuse type can be used. The concept of a fuse is it limits a whole host of different failure modes, a high speed fuse closer approximates the maximum specifications of a device and is more likely to go open faster in a fault situation in comparison to fuses that are designed for induction or motor loads. This may intern prevent or limit the down stream damage.

What I have recommended, is very simple rater than dance around the fire, call the VFD manufacture and see what they recommend. Some machines have transformers and or non VFD driven motors, they use different fuses at the machine input and to protect the individual components of the system. I tend to be conservative on safety, but also using the specified devices as outlined by the manufacturer, As I mentioned, there are local codes, so consult an electrician to hopefully get a more definitive answer.  If I have space for a fuse block, I use high speed fuse that are specified for the VFD application in the particular fuse rating/type post by the manufacturer. 

It is also fully acceptable to use an electrical panel breaker sized to a particular device and negate the whole fuse issue. I do this on my lathe, but it may not meet the electrical code requirements.


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## uncle harry (Feb 18, 2017)

mksj said:


> I often get asked on what components to use in VFD cabinets how to wire 240VAC voltage. The attached files give some general recommendations, one should always follow local electrical code requirements and the VFD manuals recommendations as to wiring and fusing. The attached information is based on non-industrial settings, i.e. home shop installations. Since I am not an electrician, use this information at your own risk and check with your local electrician if you need specifics on your local code. Since this information has been asked on several PM lathes/mills I am parking it in the Precision Mathews section.
> 
> In the basic wiring schematic I usually recommend a main power disconnect at the VFD enclosure and some form of overload protection if you are not using a dedicated circuit with an appropriate sized breaker at the electrical panel.  The disconnect switches are usually 3 poles, you would use 2 poles for single phase.
> 
> ...



Thanks for your additional info regarding VFD applications especially with regard to configuration of enclosures.  I have 3 VFDs installed and have employed most of the concepts that you have described. A great source for non-electrical hobbyists.


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## WHKAMP (Feb 18, 2017)

First let me thank Mark for his unselfish help to all budding and needy VFD folks. He was of great help to me when I began my first VFD project.

I want to point out that there are VFD’s that do not have an integral heat sink but rather use a thick aluminum plate to mate with a separately supplied heat sink (using thermal compound). These units are thinner and I have found if the VFD is mounted on full sized aluminum back plate within the electronics enclosure with fan cooling, temperature rise has not been a problem.

Using the thinner VFD makes finding an affordable enclosure much easier and facilitates where to mount the unit.

In the picture the smaller .2KW VFD with sink is mounted next to a 2.2KW unit without integral sink. You can see they are about the same height.  Cabinet size for this installation was 6 ½” x 16” x 16” (a Hoffman cabinet)


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## uncle harry (Feb 20, 2017)

Testing account


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## jer (Apr 17, 2017)

Thanks, good info.


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