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!

Reassembling a Bridgeport J-head with Uncle Charles! And More About Hooking Up Your Annoyingly Chinese VFD

You know what? I’m tired of having sweet-ass machinery sitting around not hooked up. Last time in “Charles takes forever to set up his own shop because he’s sick of setting up shops”, I did some battle with a generic Chinese VFD and completed what the damn factory couldn’t be buggered to by adding the dynamic braking components.

Though Bridget ( <3 ) ran since then, there were some issues. The spindle brake was so worn it was difficult to change tools, and the head made the “Bridgeport Clack” from the high/low speed dog clutch being worn. The motor’s V-belt was also severely worn. I wanted to tear it down for a rebuild of sorts, so I spent a little while watching “How to rebuild a Bridgeport head” videos. I decided that all of these videos sucked, and that I was really only interested in repairing the brake and replacing the timing belt and V-belts.

So here is my documented take on how to take apart a Bridgeport 1J head. In it, I discover that it wasn’t as terrifying as I had thought originally, and that old-school American engineers might commit some abominations but damn they’re good abominations. I guess this is kind of a Beyond Unboxing, too.

Step 1: Dismount the motor, which is retained by two studs, one with a set of two jam-nuts to let it move a little for belt tensioning, and another that’s the ball handle (you unscrew the ball handle and then untighten what it’s attached to). Then, crank the head about the Y axis (roll) 90 degrees.

Six socket head cap screws live underneath the belt cover casting and retain it to the steel back-gear housing. You can take all these off; pins retain the belt cover afterwards, and it needs to be yanked off. Don’t worry, it’s not heavy. But there’s one catch:

The back-gear timing belt pulleys both have flanges. To remove the belt cover means taking off one of the pulleys with it, and that means removing the belt with it. You have to remove the four slotted head screws that keep the pulley flange on. Once it’s gone, the belt slides off with everything, like this:

This setup is quite the abomination. The timing belt has no tensioner – it relies on good will and good spacing. Mine was getting a little loose from the years. While I haven’t run the machine hard in back-gear range to see if the belt skips, I ordered a new belt anyway since it’s a “Might as well” item. The belts, and other rebuild components which will be seen, came from H &W Machinery Repair.

While the cover was off, I cleaned off the thick layer of congealed rubber dust and spindle oil. I didn’t break into the back gear cavity, however – if you do, remove the nut on the big pulley and use a gear puller or Three-Phase Prybar to pop it off, then undo the remaining screws. Some times the gear cavity is filled with grunge; if your machine had multiple owners, chances are it has both grease and oil in it.

I loosened the cover and a lot of remnant oil started pouring out, so I’ll likely keep it together but drown it through the front oil port later.

The second step pulley and back gear timing pulley live with the belt cover and has a large bearing carrier assembly under it. To undo this, I need to remove the shifter mechanism.

The pins that ride in the shifter groove also help retain it completely. Problem: One of them was completely stripped and wobbly. Due to the pressure exerted by loading springs underneath the pulley, I couldn’t get the pin to bite on its remaining threads and back out. So I drilled straight down the center and threaded the hole for a #4-40 screw that I could then grab with pliers and pull on:

The stock machine has slotted head pins; H&W sells a replacement with a hex wrench drive. Here’s the victim screw driven in…

And a few tugs later, the shifter ring is freed.

The pulley then flies off the other side, since there are loading springs underneath it.

And here we have the brake assembly. The brake is simply a phenolic drum brake setup that crams against the interior of the pulley. Nothing sophisticated at all!

To remove the brake, you have to remove the 3 slotted-head shoulder screws holding it down. However, to do that, beforehand you have to undo the three hex nuts on the top side (underside in these photos) – they prevent the shoulder screws from loosening.  After that, the brake can be wiggled off gently. It will snap closed, due to its own return springs, so watch your fingertips .

The small tongue on the upper right of the bearing bore is the cam that toggles the brake shoes.

Many times, when a Bridgeport spindle brake is worn, it means two things – one, that the brake shoes are worn down, but what I found is that the cam had also dug a little trench into the brake shoes where it makes contact. So this has reduced the effective travel length and caused the brake shoe to lose engagement. In fact, it seems like the harder you wail on the brake lever, the quicker you induce this 2nd failure mode.

Also, Brigeport brake shoes are expensive. Speciality exotic part, sure, but I can do all 4 brakes on Mikuvan for less money using nice ceramic pads too! So I wasn’t going to replace these, but simply make the cam bigger.

Returning to the top side, the brake cam escapes if you untighten the set screw holding its handle pin in place. The pin slides out and the whole thing falls apart.  The cam and shaft assembly are on the upper right.

The fix? Make the cam bigger by welding repeatedly over it, building up more metal, then sanding and filing it down! This was after the rough-sanding stage. I filed a gentle round onto the engaging edges so it doesn’t cause further erosion of the phenolic laminate brake shoes.

Alright, we’re now on the reassembly path. The brake cam is going in back in…

Secured up top, along with installed brake shoes and re-tightened locking nuts.

I reassembled the shifter ring after cleaning the whole area and thoroughly greasing it. In Bridgeport maintenance, you’re supposed to oil the shifter ring daily in production use. I think I’m fine with putting in a few greasewads where it needs to be instead of having to clean up even more crusty oil grunge down the line.

The belt cover is remounted now.

Before final assembly, make sure to thread the timing belt and V-belt back onto the pulleys. Then as you line the belt cover on, wiggle the timing belt onto its large pulley.

When finished, you can then replace the small screws and pulley flange.

Putting this motor on was the precarious part, since it involved holding something pretty heavy and wiggling it from an awkward angle! I threaded the two jam nuts onto one side in order to hold it in place for….

Final head tilt. Here are the newly installed parts! And there we  have it. Shifts great, runs smoothly. Still makes The Bridgeport Clack, but further research showed me that is all in the quill spline drive and there is not really a way to R&R that short of replacement. I’m fine with it.

Moving onto controls! I can’t use this thing from a potentiometer dangling by its wires forever. You may, but I have standards.

I put a little money on eBay into some more machine style switches and buttons.

I had two buttons left over from a project long ago, so they were going to be used as the Run and Stop functions. The same potentiometers got transplanted into a panel mount which I screwed into the housings. Knobs were a matching pair (rare! legendary!) found at MITERS.  The two-position switch will control forward vs. reverse.

The wiring was concocted using disembodied Ethernet cord, which is one of my favorites for pirating cables from their intended purposes. The VFD’s Use of Manual™ just showed a bunch of normal looking switch symbols connected to the forward/reverse, start/stop/reset, etc. inputs.

This is where I discovered another great undocumented feature of Use Of Manuals. The diagram was a lie, but only enough to get you in trouble.

I had problems with it accepting my switch configuration. I found that the VFD didn’t want to read my stop button at all, and it accepted any flip of the direction switch as a “run” command. That is, I can toggle the forward-reverse switch for it to change directions, but it wouldn’t take my stop button input. I’d have to hit the STOP button on the control panel of the VFD. After that, I couldn’t start it by using the start button, but just changing the state of the direction switch would let me turn the knob and increase speed again. Well, all of my settings seemed to be correct for the job, so I was a little confused and figured there must be Undocumented Behavior. This was certainly inconvenient to use the damn thing intuitively, and I certainly wouldn’t let anyone else touch it in this condition.

It took a few friends with experience in industrial controls to point out what I was doing wrong.


That is a diagram for a normal industrial magnetic contactor, showing how Start and Stop buttons are typically wired. In these things, the STOP switch is always closed unless something causes it to open (either by accident or on purpose). The Start switch, on the other hand, briefly powers the contactor coil which pulls in not only the main contacts, but a little auxiliary contact that keeps the coil energized and hence the contactor latched. You can see how any number of interlocks (e-stop systems, overload detection, etc.) can work its way into the STOP circuit and turn the machine off when needed.

The VFD is technically designed to replace this setup, so it’s expecting the Stop button to be normally closed. Well, all my switches are N/O type (close when pressed). So the VFD was waking up in an unexpected mode, I guess, where it seems to default to treating any forward/reverse switch inputs as “Okay, start running”. Well this seems a little scary of a failure mode.

Anyways, the Use Of Manual shows all switches as N/O, so it definitely assumes you already know industrial control practices to use it. That’s another endearing characteristic of Chinesium… you better know exactly what you’re searching for, or else you might find it.

Well that’s quick fix. I didn’t order modular contacts with my switches, but luckily they’re manufactured modularly enough to use the same set of contacts, just internally turned upside-down, to become N/C. Now my control panel works as expected – the stop button puts the VFD into slow-down-and-brake, then start will ramp the motor back up to the previous speed it was at. In run mode, I can change speeds at will, including braking down to zero speed manually.

And here’s the test video.

Now that I understand this setup (or do I….), I can build the second control box accordingly. It’s also easy now to add an anti-face-eating emergency stop mushroom button anywhere in line!

The next machine to go online will be Bridget’s cute Japanese friend, Taki-chan!

how about no