What’s Inside that Surplus Center Bomb Hoist Motor? A Quick Break from BattleBots with Beyond Unboxing

Hey everybody! I’m back with a new episode of Beyond Unboxing focusing on a very unique piece of hardware that I just got way too curious about. In searching for a small American separately-excited (SepEx) motor, I remembered an item I saw on Surplus Center quite a while ago that they seemed to have a lot of trouble selling.

Huh. With no datasheet/manual/pinout available, the odd form factor, and shipping being extremely expensive due to it being heavy, I can see why I think they’ve only managed to sell 5? within the past year. To what other crazy dumbasses, I wondered. This motor definitely had at least 1 of the characteristics I was searching for – it was very American indeed, what with being American made for the American military to drop American bombs – and this will come into play later.

As for being sep-ex, I was at least very convinced it was a field-wound type motor, not permanent magnet. It seemed juuuuuust old enough that permanent magnets wouldn’t have been cheap in that size and not offered as much performance as a big series field winding in the required application – winching something. The multi-pin wiring harness coming out of the motor also indicated a wound-field type to me, since I couldn’t imagine this motor having an encoder or temperature sensor. Perhaps 2 of the pins were a load-holding brake, since hoist and all. But that still left at least three pins!

Not even mentioning the odd lobular shape of that gearbox of course – that got me even curious-er about what went on inside. Just looking at the thing, I counted at least 3 stages of gearing mentally, and with the weight of the whole assembly, there must be some seriously massive gears inside! So I went ahead and purchased one, just one, for merely 40% of the final checkout price as shipping this object was $75 alone. This better be really good…

Aand I’m glad to say that it WAS VERY GOOD INDEED, a journey into a vestige of the big & brute force style of American manufacturing where the product is an engineering textbook in physical form, and containing quite a few pleasant surprises. MURICA ONWARDS! Also, I managed to trace out the wiring for the motor, so there’s that.

First of all – They’re serious. It’s heavy. There was an attempt at shipping it correctly, for sure – the thing was firmly pallet-wrapped to a piece of thick particleboard on the inside of the box which had kinda survived transit. There were several holes punctured in the box also.

I wasn’t worried for it at all, obviously. I was worried more for what it took out on the way down the conveyor, like borderline expecting to pick the debris of someone elses’ Amazon sex toy order out of the motor fan grille level of worried. Luckily, my fears ended up unfounded.

It took… some effort to get this damn thing on the table. That’s an 18″ (0.5m) ruler next to it… the whole assembly is over 2ft long end to end. That’s a lot of little safety wire too, all neatly wound and tied off.

The motor has a nameplate on it showing it was made by the Steel Products Engineerng Division of the Kelsey-Hayes Wheel Company. Looking up the latter, I saw that they did indeed make many wheels, among other things – certainly during any of the 20th century war efforts they would have made military gear.

I back-searched the order number found on the motor and gearbox plate AF33(602)7109 and found some listings for a canceled defense specification from 1982 – so this would probably imply the units were manufactured well before then. Searching out that Mil-Spec MIL-H-25205A(1) showed that it was published as early as 1960. And searching the NSN (NATO Stock Number) 1730-203-8712, in the form of 1730-00-203-8712 yielded a 1963 date. So it could mean these things were built around the Vietnam War era.

Most of the info I could find was buried behind spammy paywall websites that aggregated mil-spec info. I wasn’t in the mood to research out the history of this damned thing, just to take it apart and see if I could use it for unintended purposes. I’m guessing after these were all decommissioned, they got stuffed in a warehouse in some recently-closed Air Force base before making it onto the surplus markets. Anybody who has more information on these, or if you’ve used one in real life, is welcome to tell me more about them! Would you recommend them to a friend?

The nameplate on the gearbox housing yields about the same information.  Based on my understanding from some light research, these were hitched to planes to lift bombs into position, but the planes did not carry them along for the ride. The numerous quick-release features and handles on it do imply this object was intended to be quickly moved in and out of position. I’m guessing the whole operation looked kind of like this, but I must balk at the unwieldiness of this device for such a purpose. Maybe 75 pounds is only heavy to me.

It is quite adjustable, however. The unpainted aluminum casting at the front pivots if you pull a pin, and can lock into one of six positions. The base has a pullable locking pin also, as well as four riveted shouldered sliders (two rivets visible above) so it was definitely designed to slide into something and lock in, then be removed quick.y


It also comes with accompanying toys – the tackle block and its accompanying 8-tooth ball-bearing-mounted #80 hard chrome plated sprocket, and its mating friend, an extendable wrench which fits into a cavity in the sprocket for when you need to give your bombs some manual assist. The fact that a #80 chain is the preferred hoisting medium still boggles me.

Let’s start taking this thing apart! First, there’s a spanner nut that retains the quick-release assembly which needs to be unscrewed. Absent the correct tool, I just used a needle-nose plier. The quick-release assembly has a 7-tooth #80 sprocket inside which has a splined bore for being driven by the gearbox.

There’s the drive sprocket. The chain enters through the vaguely H-shaped guide slot in the bottom of the quick release, and is fed into the sprocket and kept from losing contact by guides (and probably by virtue of being huge).

There’s a rotating cover of some sort which is retained by a pin – once the pin is driven out, the cover is removed and the sprocket is no longer being detained and is free to leave, thank you very much. Now you have a 7-tooth #80 sprocket to play with that fits on that specific type of splined shaft, which will be removed in due time!

I continued removing all the ‘jiggly things’ so they wouldn’t get in the way later. It has some other small clips and handles that all are held in by mildly press-fit pins.

Oh, and safety wire. Did I already mention all the safety wire? Everything is safety wire.  In beginning to remove the motor for my appraisal, I had to spend 5 minutes cutting and extracting all of the safety-wired screw heads!  I noticed the screws themselves were barely tight for the most part, with their only saving grace being safety wire. I assume this is not actually standard practice in the aviation industry…

All of the bolts are US threaded – they are generally 5/16″ thread, using a 1/2″ hex drive. Here we go! About to crack the motor off…

And let there be PEANUT BUTTER. There is a heavy grease packing inside which can only be described as peanut butter – it’s everywhere, and it smells straight up pre-EPA carcinogenic. You know that “Old Electrical Equipment Funk”? It’s that, but on anything it touches. I ended up digging a lot of it out with shop towels later and throwing it ot…. but part of me regrets it now and wants to keep the grease of the next one(s) I buy (if I do) in its own little jar for future generations to appreciate.

But here we see where I have already made wrong assumptions about this thing. I assumed it was a direct drive off the motor into the worm gearbox, or at most 1 stage of gearing. However, looking inside at this point, I see at least three stages of spur gears.  It’s interesting to note that I haven’t found a single molded plastic part yet – everything is rubber, phenolic, or metal.

Here is the motor by itself. I’m going to dig into this thing a little in order to find the pinout of the 5-pin circular military style connector, in the process discovering the type of winding it has – whether it’s series or shunt wound, or a seperately excited motor like I want.

First, four tiny safety wired!!!! screws need to be removed for the rear cover to pop off, which only houses a fan.

You may be wondering what that little “Westin Elevator Shaft” contraption is on the side. It’s a captive hex shaft which has a socket on the other end that mates to a hex stub in the spur stage of the gearbox. It seems like its purpose is to allow hand-cranking of the gearbox through the reduction the motor sees – presumably while your buddy is standing on the end of the manual tackle block tool also, so you can hoist bombs even when the power is out.

Another set of SAFETY WIRED!!! tiny screws and a shroud that covers the brushes is released. This doesn’t affect the brush holder – it only allows you to see into the ventilation holes. I’m guessing this motor might come in several flavors including open (like this) or enclosed/fan cooled (like it was before I cracked it open) depending on options.

The fan is retained by a single nut, and removing it exposes the part of the motor which I suspect was designed last.  So here’s what’s going on:

  • The outer ring of 4 nuts hold the whole endcap onto the tie rods that run the length of the motor. These are structural to the motor. They’re #10 thread, so a 3/8″ hex drive, except one by the Westin Elevator Shaft which is a “thin pattern” hex nut using a 9/32″ hex drive, probably since there isn’t enough space for a bigger nut.
  • The inner ring of 4 nuts retain the brush holder onto the endcap, and are #8-32 nuts, so a 11/32″ hex drive.

So there you go: 3 tools to do this operation, all within a few 32nds of each other. I was confused as hell about what was just painted over or not and if i was really seeing things or there were #8s and #10s in close proximity. ‘Murica


While I was undoing the endcap, I also unfastened the giant die-cast junction box and unscrewed the circular connector to try and find out if there were obvious armature/field wires. They all disappear into the motorial abyss, so I’d need to keep exploring.

Continuing with the theme of “no plastic anywhere”, all of the wire in this thing has braided insulation!

So I’m a wee bit confused on the order of operations needed to assemble this thing. I clearly did it the wrong way, which is to remove all of the endcap nuts at once and yank. The inner ring of nuts retains the brush holder and also locks it in a certain brush timing, which I will not be able to recover exactly.

What I think is the correct way to disassemble the motor appears to be removing the tie rod nuts (outer ring of nuts) and then removing both the armature and the brush/endcap assembly at the same time. That way, the brush holder isn’t disturbed, and since it’s a wound-field motor, you’re also not fighting magnetism to do so. Something to keep in mind for next time! I’ll show how to disengage the armature in a minute.

This brush setup is quite something. The brushes themselves are circular arc shaped and they pivot on little arms, instead of the traditional inline coil spring setup. Wonder why they did it this way? You potentially get more brush life from the small amount of space the other springing methods would take up, I suppose.

To pop the armature out, use a thin 1/16″ pin punch to drive the pin out of the pinion side. The pinion, it turns out, lives on another spline and the pin’s only for axial placement.

Then you yank. That’s a real pretty armature – it’s wound almost like a starter motor (which I suppose it will share a lot of intermittent-duty high-power lineage with).

Of note, it has a large flat steel faceplate. This is related to the next photo:

First of all, field windings and brush terminations! This allowed me to back-trace much of the 5-pin connector. I determined that the motor was indeed a separately-excited (Sepex) motor!

You see the fibery-looking pad at the bottom? That’s a phenolic brake pad. It’s spring loaded upwards naturally, and you pull the little pin on the right to engage/disengage it. It mates with the steel disc that is on the armature. When it’s engaged, the motor shaft is hard but not impossible to turn. It’s likely a load-holding brake for the motor and is there more as an extra precaution – unless that worm gear stage is very high helix angle, I can’t imagine the motor contributing all that much to load-holding versus the worm gear.

Repeat after me: “Reassembly is the opposite of disassembly.” I discovered that my “correct way” of disassembling the motor was in fact not going to be possible – there’s two attachments to the brush holder that are screw-in and must be obviously done so while the brush holder is not mounted to the endcap!

So the way I did this reassembly was armature, then brush holder, then the two screw connections, and then the endcap (then thereafter the fan shroud and so no). As for how the factory made sure the brush timing is correct…. hell if I know. I did my best by visually inspecting it through the vent holes.

You know what? Just don’t take the motor apart and take my word for it.

Since the motor was really my agenda, I decided to do some basic characterization of the motor. For my sepex application, I would like to know the field resistance, armature resistance, and ideally the magnetization curves of the motor – no-load voltage (out) versus field current for several different input speeds.

The problem was I would have needed a controlled way to spin the motor up to around 10,000 RPM, so I didn’t get any of the magnetization curve data for the time being. I found out that on its native stated specifications – 28 volts applied to both field and armature – it wanted to draw 6 amps spinning no-load at 9,200 RPM. For such a big motor, 9,200 is really fast…. but the 8,000 RPM @ 44A specification on Surplus Center made more sense.

I also solved the armature resistance to be 0.04 ohms and the field resistance to be 10.1 ohms. This is one hell of a motor – albeit only for a short period of time, which sounds perfect.

all together now… what the fuck is he building now that needs this specific motor? doesn’t he design motors for fun?

To summarize this section for now – here’s a pic of the circular connector showing the pinouts I discovered.

That ought to fix all the buyer questions on the Surplus Center website, and hopefully make this thing a little more useful if you have a #80 chain hanging around that needs something to climb up it slowly.

But the story doesn’t end there. Oh, it’s just beginning. There was still 30 or so pounds of gear that I haven’t even opened up yet.

I’ve already found that my assumptions about the input stage were wrong. What ELSE don’t I know about this? Let’s remove the 25 miles of safety wire that hold the back gearbox cover plates on.

I actually decided to work forwards from the motor and start on removing the worm gear stage first, since I got very curious about what kind of worm gear it was which they’d still warrant a load-holding system on the motor in the form of the spring-loaded brake. The six inner screws release a cover plate for the bearing of the worm wheel axle. The outer six safety-wired screws hold the entire endcap on; it’s a mild press fit, so be prepared with a sharp paint scraper or knife edge to do the initial release.

The worm gear system is now exposed, but it’s not yet removable since it doesn’t have enough slop in it to wiggle past the wheel past the worm.

The other side of the worm wheel axle is just the large back-side cover. Time to take all those screws out and start paint-scrapering to release the press/grease fit!

First off: Holy crap, that’s a lot of peanut butter. Second: Holy crap, that’s another entire intermediate stage I wasn’t expecting! Third: Holy crap, those gears have stub-form teeth! I’ve only read about that in engineering textbooks of my bunny days – never seen stub gears in real life before.

Once this back cover pulls out, by the way, the worm wheel just falls out the endcap side (bottom in this photo).

The worm wheel is kept in place by the compression provided by the spanner nut, but power is taken through an involute-splined hub. Notice the small gear stage next to it. This is a mere 1.25:1. The gear at the bottom of the worm wheel axle is 12 tooth and mates with a 16/12 tooth cluster gear which does the final mate with the large output gear. I must wonder what type of packaging concern or ratio fine-tuning warranted this intermediate stage when you theoretically could have tuned the worm gear or output gear one way or the other slightly.

The worm stage is a double-enveloping type, also called a globoid worm design, for maximum strength and contact area. This thing has just been an engineering textbook in a peanut-butter filled box so far; it was actually quite pleasant to see again, given my recent forays into electronics and shitty vans.

I sprinkled the final drive out of the casing, only to discover….. more peanut butter. Go figure.

What was also cool to see: Inch-sized big-bore thin section bearings. These days, the dominant bearing is the metric single-row series (6002, 6200, 6801, etc.), but there is not a breath of metric on any of this. The bearings all were crunchy – unsure if it’s due to aged and dried lubrication or just specified to be a lower uniformity/finish grade, but I’m going to yank them all off and keep them.

After I cleaned the peanut butter off for the most part, here’s the final drive. The output shaft isn’t actually retained by anything but the back plate – once it comes off, the shaft will fall out. It also isn’t carried on its own bearing, depending on the distal bearing found in the aluminum mounting assembly with the swivels that I removed first. So the green gearbox wasn’t ever supposed to live on its own either.

There was still a worm gear stuck inside and the 3? stages of initial spur gearing I haven’t discovered yet, so back to the other end we go! The same procedure is used on the motor side: cut off all the safety wire and just start unscrewing. This side has some dowel pin alignment mates which are more press-fit than the rest.

Aaaaaaaand more peanut butter. By this point, I’ve almost half-filled a 44-gallon rolling trash bin with towels full of peanut butter stains.

The setup didn’t make any sense to me, as I was clearly spinning the motor pinion’s mating gear by hand but seeing two different rotation speeds. In fact, the hex shaft which couples through motor into the (missing) manual crank handle is geared up from the handle itself, but at a different ratio to the motor which is geared down in two stages. They put a complete independent gear path in here just so you can spin it by hand – I’d like to think it was informed by how much torque it was comfortable to keep applying versus how quickly your arms get tired from cranking.

But it was probably because steel was basically free back then.

After another container truck left for the JIF factory, we expose the final boss of this gearbox: the screw holding the input gear on.

Actually, no. It was in fact easy to remove that screw, but hard to remove the two gears in front of the worm input gear. They’re just press-fits with bearings ,which I had to “three phase screwdriver” pry the upper right (motor input) gear out, and slide hammer the hex shaft gear out (attempt 1 with a vise grip is shown…. attempt 2 is hooking my slide hammer onto this vise grip) trashing the tiny inch sized angular-contact bearings in the process.


F :(


The input pre-duction has been freed!

….and there are more screws. And more safety wire. 2 health bars? Nope, this thing’s had like 20 extra lives by now.

After that gets undone, the worm gear pops right out! Hurray!

Here’s the final gear count: Seven, each intricately engineered and machined, with polished teeth and edges. Go watch some worm gear machining videos and then talk to me about how sweet this thing is. I’m not sure how I can even use any of these gears outside of the box they came in, because I sure as hell am never going to make the worm gear fit correctly in anything again. The temptation of bringing Cold Arbor back, though, went up several-fold after doing this surgery.

So what’s the ratio from the motor to the big 7-tooth sprocket? The input stages are made of two stages of 54:14 (14 tooth pinion on the motor, a 54/14 cluster gear, and 54 tooth worm shaft input gear). The worm stage is 30:1. Then you have the 1.25″1 intermediate stage (12 teeth on worm shaft onto a 16/12 cluster) and finally the 36-tooth output gear. That’s a total of 1673:1 and some spare change.

I’m inclined to say that this device is really best kept together, maybe with your own assembly in place of the swivel mount, and using the motor as-is also; maybe doing some work to adapt the input shaft to your own motor.

There’s so much excess it’s a joy to behold: In its day, steel was free, labor was cheap, and China wasn’t a real place.

alright, so what the fuck are you building that needs this precise kind of motor, the more American the better?

All will be revealed in due time, but it’s exactly what you’d expect. I’d like to collect some magnetization data on the motor soon, so I’ll report back in with its operating characteristics at several voltages and speeds. I’ll likely end up purchasing another one GREAT, MORE PEANUT BUTTER and making a face-to-face dynamometer to collect said data (maybe not full dyno curves, but at least the bemf-versus-speed info) since the best way to characterize a motor is to drive it with itself, in an electrically masturbatory loop of power.  I’m thinking of how I’d incorporate the rest of the gear stages, but it seems unlikely with the ratios I can arrange them in. All I can really say since I don’t have a definite timeline due to my startup-baby, is that it will bring some interesting new-old tactics into a game with an established meta; it won’t win anything, but will be glorious while doing so!

A Different Kind of Chinese Motor Controller?! Adding Dynamic Braking to your Inexpensive Chinese VFD

Here at Big Chuck’s Robot Warehouse, we love our Chinese motor controllers. I some times think that at this point in life, I’ve become a kind of Chinese motor controller evolutionary biologist…. or at least like the Identifying Wood guy of underpowered gate drive amplifiers, I hope. Taking apart and examining motor controllers, which I’ve written up many times on this site in “Beyond Unboxing”,  is a large part of how I came to understand them, at least to the degree that Man can comprehend the transcendent nature of motor controllers.

Navigating the Pacific Rim of Chinese mass-market industrial products is not for the feint of heart – often times, products are sold over-rated and inaccurately advertised, and much of the knowledge base of using these products exists on hobbyist forums and message boards/email threads. This means anyone else outside of a circle of knowledge who tries to buy something and use it is often frustrated due to the lack of official documentation… and to find any good documentation often requires sifting through a forum thread or (heaven forbid) Github repository. That’s what I try to remedy whenever I cross paths with it, with some detailed writeup and explanation of what’s going on. Because at least that appears on a search engine result in a comprehensible fashion!

Today, we *gets out David Attenborough voice chipset* will be getting a closer look at a different species of Chinese motor controller. Rarely seen in the North American continent compared to its smaller, domesticated brethren, it is the majestic Giant Chinese VFD.

This one’s an adolescent male, with a 9/2016 date code. You can tell from his unadorned, angular ABS plastic case, compared to the more ornate and filleted females. He’s just begun to venture into the wild alone to expand his territory.

He stalks his prey, an aging Bridgeport J-head, from the safety of his preferred observation grounds, a nearby wall:

Okay, that’s enough, David. Also, lyrebirds are cool.

So why do I need a VFD? The shop has easily-obtainable 208V single-phase power which we had installed, as seen by the new junction box behind the mill. 208V is just missing the 3rd phase to become 3-phase, but that doesn’t exist in the vicinity and wasn’t going to be cheap to run. Hell, even if I had 3 phase, I’d still be getting a VFD anyway to have the additional running envelope and ability to change to arbitrary speeds. You mean keeping the Bridgeport in low gear and revving the motor to 13,000 RPM isn’t a good idea?

I did an initial sweep of the space of available Chinese VFDs back in January. Did you expect me to pay actual money for a real, working and supported product? Come on now, you know I’d rather jump into a pool of sharks. Chinese knockoff sharks!

As you can see, they all look kind of alike, and based on my brief research on DIY CNC forums and groups, they’re basically all the same genericized design. This is similar to other Chinese industrial products, including my favorite e-bike and R/C brushless controllers.

I have a rule called the “Law of Chinese Packaging Inertia” – if the Chinese product visually appears the same as a counterpart, it very likely is the same, or has trivial differences for marketing reasons. There’s been no better proof of this law than hoverboards seg-things, but it’s existed substantially in the past in the form of cordless drill motors for robots, the aforementioned e-bike controllers, and the like.

On eBay, there are numerous US-based resellers of the same products:



So I picked one which was severely overrated nominally for the motor it was to be running – a 3 HP (4kW) rated one, thereafter sorting by distance nearest and free shipping. You Only Line-start Once.

I figured I might as well err on the side of caution ratings-wise, since my other Chinese product rule is known as the “Harbor Freight Derating Factor”: derate by half if you intend to use it, and by 2/3rds if you’re standing under it. Vantruck weighs 3 tons. Have you seen how thin the metal is on a 3-ton Harbor Freight jackstand?! Don’t give me none of that shit…

The real reason, though, was because I picked the size originally for eventually powering the lathe, which has a pretty beefy spindle motor. I decided to outfit the mill first because it was a bit safer of a proposition to try something unknown on – there’s less rotating mass to bring to a halt.

Alright, my life is settling down a little after Motorama and the insurance & mechanics nonsense. Let’s wire up the mill!


Actually, speaking of “have you ever”…. have you ever seen inside a 1HP Bridgeport J-head “pancake motor”? I have actually never looked inside one until now, somehow, and it really is an axial-flux motor! For some reason I always mentally wrote it off as a very stubby conventional motor, but this makes so much more sense. Have a look at these photos! I didn’t take apart the motor since I “get it”, but that was a good trivia day.


I had to remove the drum switch (for manually powering in forward or reverse) and then drill an access hole in the 1/4″ thick cast iron junction box for a cable grip. This was when I discovered the previous operators used a 3-conductor service cord on a 3 phase motor with no ground. The ground was an extra piece of hookup wire mashed into the cable grip, electrical taped around the machine, and eventually into the 4-prong twist-lock plug. Well, at least it was grounded.

Wiring was pretty easy after that, and the instruction booklet which came with it was very Technical Chinglish but easily decypherable (and comparable to other more English manuals for VFDs).

Has anyone seen THE USE OF MANUAL???

These things will allow you to change a lot of parameters about the motor, and you can set the V/F line to have 2 slopes for more torque in certain operating regimes, etc. They call this “arbitrary” V/F curves, but no, it’s not really that. It came with a bunch of parameters set assume 50hz mains, which I changed to 60hz. Other parameters control what inputs the drive unit listens to – I hooked up an external potentiometer and told it to use the potentiometer to control the speed, as the unit DESPITE BEING ADVERTISED WITH ONE IN THE PHOTO didn’t come with a knob on the control board! See the very first photo above.

I cut the faceplate open to try and see if there was one hiding in there or something. Nope, missing. This will become a trend.

Most of the parameters I ended up leaving stock until I had a better feel for the system, since I’d not set up a VFD before. These inexpensive units are generally open-loop VFDs – they don’t have a tachometer input, though there seems to be an option in the settings… I’ll have to look a little more in detail.  They just bang out a frequency, and you can set how fast it increases that frequency for acceleration; if you set it too fast, you fall off the optimal slip region for maximum torque and your motor actually takes much longer to spin up (Induction motors require the supplied field frequency to be just a little faster than its rotational speed for torque production).


I call this the DOUBLE DANGLE


Slowing down was the hard part. Nominally, this thing had “braking”, and included terminals for a dynamic braking resistor, subway train style. I added one found in the bowels of MITERS – a 120 ohm, 50 watt unit. A little undersized, but it’s not like I’m stopping this motor every 10 seconds for a tool change.

Despite having the options selected, I couldn’t get it to actually perform any braking. I could either 1. set the ramp-down time to nearly as long as the machine would take to coast down by itself, or 2. just use “coasting stop” mode which was exactly the same damn thing because it just lets go of the output.

Attempting to set the spindown time faster simply resulted in the unit shutting down outputs and displaying an overvoltage error. Yes, it would make sense – when the motor regenerates power into the controller, it needs to go somewhere. In EV controllers, it’s back into the battery. I’ve never heard of a ‘grid tie VFD” for controlling machines before, though conceivable it could track the mains voltage to try and dump current back into the building, but why would you do that…. Or, you burn it off in a braking resistor.

Without any of those sinks of power, the voltage on the DC power rails of the VFD will spike upwards uncontrollably. It looks like this one will shut off at 400V on the DC power bus. I investigated a little more with stopping from different speeds, and it’s definitely correlated to the energy contained in the rotor and how fast I try to slow it down. So, it thinks it’s doing braking, but nothing is happening.

Well, I could leave it in coast mode, but what fun is not going down without a fight with a poorly documented Chinese product to the death?!


Step 1: Crack it open. Here’s what the power stage looks like. All the familiar trappings of a motor controller are there! Immediately, I can see that one of the gate drive optocouplers is missing…. probably the one that tugs on the braking IGBT.For a rundown on the symptoms I described here, read that article. It’s nice.

With some more research (read: forum threads… literally, read forum threads, like this one and this one) I found hints that a lot of these Inexpensive Chinese VFDs ship without any of the braking components populated. Given that this thing came with no potentiometer either, I’m entirely unsurprised. What I don’t get is what market they expect to sell to; a lot of them are advertised for process pumps (e.g. water pumps, blowers, oil pumps and the like) which I presume is a thing that doesn’t really need braking and doesn’t need constantly variable speed control, but maybe just 2 or 3 speeds and an on/off.

That’s another thing about Chinesium I can appreciate, even if I find it frustrating. Everything is stripped down and rat rodded to the point of doing only 1 thing, but it will probably do that 1 thing very well.

Staring at the P+ and PR terminals for the braking resistor under a backlight shows that there’s nothing connected to PR. It looks like there should be a wire jump…

Probably to here. The missing IGBT is connected via a wire jump to something. It’s functioning (based on the pinout of most IGBTs of this package) as a common-emitter  switch, one leg tied to ground and the other leg pulling on something. That something is supposed to be the DC rail (P+) through the braking resistor (between P+ and PR). My board seems to be a newer revision than the ones found on those threads, as a lot of the parts which were 8pin through-hole parts are now SMT parts, and the layout is different. Either way, from my investigation, 2 parts are missing: Q23, the bremschopper, and PC11, the optocoupled driver which tells it what to do.

So, if I haven’t reiterated, I fucking hate digging through forum threads to find the answer to my question. All y’all need to learn to keep a website. Read on if you want to add dynamic braking to your Inexpensive Chinese VFD!

I figured the parts used for this extra drive circuit should just be the same as the rest, so I ordered a pack of the IGBTs used on the board – FGH60N60SMD. The optocoupler driver TLP701AF didn’t have an exact match in-stock at Digi-Key, so I went for a similar equipped part number, TLP701HF.  The -AF part seems to have tighter switching time tolerances. In a single switch configuration here, I figured it doesn’t matter.

By the way, fully optocoupled drive is something I really, really want for Brushless Rage… but it takes up a whole lot of space compared to some driver ICs :(

Mounting the IGBT onto the power stage required some creativity. I cut up a spare RageBridge silicone insulation tab for it, and mounted it on the heat sink plate where it should go. Then I bent the legs up to the point where they should fall right into the empty solder eyes on the board. I decided to do it this way since trying to solder the IGBT to the board first wouldn’t have guaranteed it being able to lie down flat on the heat sink.

On the board itself, I made the wire jump from Q23 to the PR terminal.

And finally, I reflowed PC11, the optocoupler, onto the board.

And you know what?! That was it!

Man, whoever made this just couldn’t be motivated to put the extra 3 parts on it, eh? Guys, we saved like 80 cents! Yay!

Granted, again, if 99% of your users just drive their hydroponic pot farms with it, they’ll never need the braking feature and you might as well leave those parts out. For everything else, there’s my fucking MasterCard. Ugh.

Here’s a test video showing the braking in action. I cycle through the viewable parameters when the motor is running so you can see the DC bus spike up before the resistor does its job.

“DCB” is an added braking option where after the frequency gets low enough, it will just short the leads of the motor together. This provides extra braking power for speeds that are too low to generate any voltage to push across the braking resistor.

So there you have it. That’s literally the only thing stopping these controllers from being more useful running machinery! Now that I have additional parts, I’m going to purchase another one and wire up the lathe too.