Fuse Sizing - VFD Motor Control - Page 1
Forum section: electronics / projects

author: foetusmachine
author_rank: Contributor
timestamp: March 12, 2022, 10:28:51 pm
content: I'm putting together a variable speed drill press and I'd like to protect my VFD and motor, both of which are relatively expensive. In undertaking my research, I found a couple of resources online that suggest that 1.5x VFD input amperage is appropriate for this application. However, when I look at fuse specs (i'm using FF fuse data), it suggests that blow time at 100% rating is ~4 hours, and at 250% rating, the blow time is 0.2 seconds.If the input rating of the VFD is 4.9A, why should I not be using a 5A fuse and not 1.5x rated fuse i.e. 7-8A?Does it come down to the likely fault condition? i.e. is it most likely that the majority of faults will create high current draw and in such cases, the difference in trip time of a 5A fuse vs a 7A fuse is negligible?Any guidance would be appreciated.

author: langwadt
author_rank: Super Contributor
timestamp: March 12, 2022, 10:35:02 pm
content: a fuse is there to stop things catching fire and burning your house down, it is unlikely that it'll protect electronics.

author: foetusmachine
author_rank: Contributor
timestamp: March 12, 2022, 10:42:52 pm
content: Quote from: langwadt on March 12, 2022, 10:35:02 pmThanks. I think there's a contrary view out there that ultra fast blow fuses can contrubute to saving some of the electronics in the VFD when you compare the trip time to a circuit breaker for example.

author: uer166
author_rank: Super Contributor
timestamp: March 12, 2022, 10:55:31 pm
content: No fuse will save a FET or any modern power stage component, and if it needs to do that in the first place, the thing is broken by design. Fuses wear out if you operate it close to the rated value, it will nuisance-open eventually. Rate the fuse appropriately, treat is as a safety device only, and you wouldn't need to make it replaceable.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 13, 2022, 05:37:56 pm
content: Fuses are way to slow as has been pointed out. You can buy electonic breakers- expensive and usually a bit slow, or brew your own. eg LTC1153.A thermal monitor on your heatsinks are a good idea- they can predcict things going pearshaped before they go all the way.

author: nightfire
author_rank: Frequent Contributor
timestamp: March 13, 2022, 07:09:37 pm
content: Quote from: langwadt on March 12, 2022, 10:35:02 pmYes and no. Depending on the type of fuse and location, the primary use is to protect cabling, but there are special fuses on the market geared toward protecting sensitive equipment.These "Ultra-Rapid" fuses are used in big UPS systems (At my last job, we had a pair of Schneider/Silcon 320kVA UPS running in parallel) and as some internal protection they had some NH00 fuses that were intended for equipment protection.To the situation: I am not familiar with the load profile of the VFD used- here it would be interesting to know the following parameters:- Current drawn in normal operation- inrush current when powering up- load spikes during operation when changing speed or torque- prospective short-circuit currents in case of failureFor cable protection and to be safe against overload, some FF fuse is ok in terms of protecting against overload, but what happens in case of short circuit? Here the fuse has to be able to withstand the forces present, and some small FF fuses with clear glass body are not always up to the task. So proper selection has to be made.Due to the fact that fuses at rated current will ultimately age with time, and to account for small inrush current and load spikes (that will not warm up the cable) this fuse should be used at a bit less than rated current, so probably some traditional (F, or FF) 6,3A fuse can be used.To up the game, there are so called "UR" fuses. Just had a look at my favorite fuse manufacturer, and there are in fact lots of different types of fuses that are specifically designed to protect sensitive workloads. But, in this regard the short circuit currents or applicable currents in error scenarios have to be known.https://siba.de/de/produkte/fuse-detector/fuse-detector.html?fdf=Halbleiterschutz-Sich.einsatz

author: elekorsi
author_rank: Regular Contributor
timestamp: March 13, 2022, 07:21:04 pm
content: Just chose a fuse rating according to VFD manual (and use appropiate wire size from fuse to VFD).Motor is protected by VFD and not by the fuse. If there is a short on the motor, a good quality VFD will protect its self and trip.If input fuses blow, the VFD is allready broken as that would mean that most probably there is a short on the IGBT and that was the cause of blown fuses in the first place...

author: Siwastaja
author_rank: Super Contributor
timestamp: March 13, 2022, 07:21:51 pm
content: All you need is built-in in the VFD. Just wire according to manual; manual will define the maximum fuse rating for the input wiring.

author: foetusmachine
author_rank: Contributor
timestamp: March 13, 2022, 09:09:55 pm
content: Thank you everyone.With the benefit of this input, I feel like the fuse issue is a bit of a non-issue for me right now.Some interesting technologies such as the UR fuses noted, but these are outside the range of what i'm running (0.75kw) and overkill it would seem. Although I would be interested in reading more about this technology as it's promoted as being very high speed, able to "protect power semiconductors and DC circuits", "good resistance to cyclic loading" and "No downgrading of fuse characteristics over time". How this is achieved and what it means in reality is another story - there seems to be little information out there.I'll stick with the RCD/Breaker combo for now.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 15, 2022, 08:14:15 am
content: VFD, by design,isa highly sophisticated active electronic "fuse". It actively controls motor current (and hence, input current). So a fuse will not blow in any normal condition.Classical fuse is only required to prevent things from catching fire or melting in case the VFD* Has a serious, fundamental design or manufacturing error;* Is seriously abused (like large overvoltage applied to it, or possibly, output shorted if the VFD does not provide output short protection; most should nowadays.)In either case, fuse can't save the VFD, but it can save the motor, and it likely saves the house from burning down.And fuse is definitely needed. It's just that your house wiring already has the fuse, exactly for this purpose. VFD manual might have some conditions like maximum rating for that fuse (so that you are not allowed to connect a tiny VFD into a 100A circuit directly). Adding extra fuse of course does not hurt. To avoid false triggering, don't use excessively small size. 10A would work.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2022, 09:19:59 am
content: Quote from: uer166 on March 12, 2022, 10:55:31 pmSlight clarification, there are some components that can be fuse protected effectively; but only one is likely to be found in a typical small VFD (the input rectifier), and none of the active components (MOSFETs or IGBTs). And weakest links being what they are, it's the latter ones that matter.Quote from: foetusmachine on March 13, 2022, 09:09:55 pmMay find more info under the general term, "semiconductor fuse" -- so called because they act fast enough to manage to protect some types. Namely, semiconductors with low voltage drops and high surge ratings: basically just diodes, SCRs and TRIACs. Things used for general AC power control, rectification to DC (at the input of most electronics), and industrial power converters. (These devices can be used to switch many of the same loads that transistors are used on, but they act slower, so are typically only used as a last resort, at very high voltages and power levels where transistors are too bulky/expensive -- consider a VFD delivering 4.8kV, and about as many amperes, to a motor of ungodly horsepower.)The fuses are typically constructed something like, a parallel array of silver foils, punched in a diamond checkerboard pattern, and filled around with pure silica sand. The diamond cutouts causes current to neck down to narrow spots, which are able to heat up quickly; meanwhile the remaining diamond patches give wide area between necks, able to dissipate nominal heat into the sand filling. When melting and subsequent arcing occurs, the silica melts and vaporizes, the vapor quenching the arc while the bulk absorbs blast energy. Typical clearing times are as low as single to fractional milliseconds -- impressively fast for any kind of mechanical device. It's also much shorter than a mains cycle, meaning they can break the circuit at any time, without waiting for the AC cycle to cross zero -- which means they are often suitable for DC power as well (and usually rated as such). (Most fuses continue arcing during a full peak, until mains voltage falls to zero then reverses, the low point giving a chance for the arc to cool and extinguish; high voltage DC arcs are dangerously persistent, so this is a bit of a built-in safety feature of our AC system.)Note that, when faulting occurs, massive currents are drawn. For residential circuits, up to a few kA (breakers are typically rated to clear 10kA); for industrial circuits, up to 100kA, or large circuits even more (e.g. at substations or generators). This heats the fuse rapidly, causing it to clear quickly.Fuses don't act quickly at all, in general. Your average fuse will handle about twice rated load for some seconds before melting; it might be guaranteed between a minimum of a few seconds, and a maximum of several minutes! Fuses are not precision devices, and cannot be relied on for that kind of limiting or protection. They are, indeed, there to protect the wiring and such -- the wires in the walls will heat up much slower than the fuse, so aren't in any danger of starting a fire (hopefully, anyway!) by the time the fuse opens.Anyway, fault current. So, keep in mind, as current goes up, fuses open faster, and these semiconductor fuses can open in a ms or so, but that's at typical fault currents (~kA). However, transistors (MOSFETs, IGBTs, etc.) cannot switch into such currents: they are current limited devices, they don't handle faults nearly as boldly as their diode-ish kin. What'll happen is, instead of dropping a couple volts as in normal operation, the transistor sees full fault voltage -- DC supply or mains peak, say 320V or more -- at whatever maximum current it is able to draw, say 10s or 100s of A. It heats up very quickly, and in as little as 10-20µs for IGBTs, or maybe 100µs for generously sized MOSFETs, the silicon die surface begins to melt, at which point it can't turn off no matter how hard the control tries, and the game is over.Semiconductor fuses are sometimes still used with such devices -- in this case, they're not being used to protect the device, but to reduce the amount of carnage created. When a transistor dies in this manner, the die surface first melts, then the bondwires (inside the package) connecting it to the power source melt and vaporize, and within a few hundred microseconds, a hot, dense ball of plasma is building inside the package. Within a millisecond or so, the package goes off like a bullet, fragments of which may be propelled at dangerous speed. So, guards for shrapnel and arc flash are a good idea in the design of such equipment. (This is why we keep the covers on before throwing the switch...) Well, with a semi fuse halting that by the ~1ms mark, shrapnel may be avoided, as well as major arc flash damage to the circuit; repair may be possible, but mind that, during that miniature fireball, high voltage and current were applied to all terminals of the transistor -- usually destroying the drive circuitry too. (So you'd need to replace at least the power board, which will be most of the unit anyway, so... a unit like yours is probably disposable. Larger industrial kit can be worth making with more replaceable pieces, though.)Whereas with a conventional fuse, or breaker even: after the failed device explodes, the arc flash will continue between terminals (and anything else remotely conductive nearby, including circuit traces, component terminals, screws and nuts, etc.), until power is cut -- some 10s of ms later. In which time, huge swaths of circuitry can be charred and eroded, and the whole thing is a loss; let alone if the covers were open and someone happened to be standing beside it(!).Meanwhile, the advantage of transistors is their fast reaction time -- a well-designed control circuit can detect such a malfunction, and disable operation within microseconds. Maybe one transistor still fails, say due to overheating; but the other transistors around it can actually be saved, and maybe just a few components need to be replaced (transistor and driver?).The downside about complex electronics, of course; there's no way to tell what kinds of things they put into the design. It works in the average case, fine, but what happens under extreme conditions? And if those conditions should occur due to errors in the system itself (e.g. software control), how could those conditions be triggered and tested? Well, you can't know, in general. And even if you know about these things (like many of us here do), it's a huge amount of work to dig into the thing and figure out what it's doing. So it's just the same old crap shoot for anything you get these days -- if it works, it works; if it blows up, replace it (with a different brand, perhaps).(Just to be clear, arc flash isn't much of a hazard on residential circuits. Typically you'll get a flash and a pop, and that's it. Never hurts to wear PPE, of course. It's definitely a good safety lesson if you'll be working in industry; no need to be fearful, just respect it and follow protocol. Cheers!)Tim

author: Berni
author_rank: Super Contributor
timestamp: March 15, 2022, 12:47:27 pm
content: As others have said the VFD is supposed to protect itself.For the motor itself you don't have to worry about. There is nothing really to break in a async 3 phase motor. Overheating is the only thing that can for sure kill one, so this typically means it has to be overloaded. But the VFD is programed to deliver a certain max current to the motor, so as long as the VFD is configured correctly it should never feed the motor more juice than it can handle.(Note that this current has to be lower at lower speeds)

author: jrbass
author_rank: Newbie
timestamp: November 07, 2022, 04:17:12 am
content: So i'd like to say that "the VFD protects itself" is 100% incorrect. but i also immediately want to say it was my fault that i fried one.i disabled braking (for no good reason other than not understanding what it was for) and had the VFD on a switch that blew, and threw the breaker... the motor was spinning, and all that induction went from the motor back to the VFD and i saw smoke.This is the only reason i found this forum, cuz i'm wondering how to protect from future (idiot self) mistakes. Although i will never do what i did before, and maybe if a fuse isn't the solution, maybe a diode? so that the power cannot come back to the VFD if something happens?Also, i'm curious as to what power/amperage goes through the UVW wires... i have a 2.2kw spindle with a 110v VFD.Thank you, and hope to maybe help another learn from my ($130) mistake

author: WattsThat
author_rank: Frequent Contributor
timestamp: November 07, 2022, 05:48:33 am
content: As explained in the thread, fuses are there for when the drive fails to prevent shrapnel and fire, not to prevent failure of the VFD. It’s a common misconception.You attempted a “flying start”. Some drives can do it, if not the drive should trip on overcurrent, cheesy ones just go boom. If a motor is spinning, the VFD output frequency has to match the rotor frequency otherwise it looks like a short to the drive. In some drives it’s called a scan start.2.2KW at 120V in the US? That requires at least 20 amps from the ac supply and the motor current would be ~12 amps in each phase at rated load (2200/120/1.732).

author: Siwastaja
author_rank: Super Contributor
timestamp: November 07, 2022, 05:05:57 pm
content: Quote from: jrbass on November 07, 2022, 04:17:12 amOf course, it protects itself against certain specified set of things. For example, a VFD won't protect itself against meteor hitting it. Also, it won't protect from 20kV input voltage being applied instead of the expected 400VAC. And apparently, the VFD you had did not protect against input power loss plus spinning motor, which IMHO is quite crap, I would expect that protection to be a standard feature, but as long as the lack thereof is well documented in user manual (and the VFD is cheap to buy to reflect the lack of this protection), it's fine.But any even remotely decent VFD will limit motor current. That's the point. If you add a fuse,the only instance when the fuse will blow is when the VFD has already failed. I.e., the fuse will blow whenever the VFD fails to protect itself. Some people think fuse is a component used to "protect against overcurrent", and hence somehow related to limiting current, but that's exactly one thing which a VFD does fine; even a crappy, cheap one. No matter what kind of mechanical load you add, or stall the motor, the VFD always keeps the current within limits safe to itself. Failure to do so is not acceptable even to a cheap VFD. This is why we say fuse is for fire protection only; it can't save the VFD.