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Member postings for Bob Worsley

Here is a list of all the postings Bob Worsley has made in our forums. Click on a thread name to jump to the thread.

Thread: Vfd sizing
09/08/2020 17:04:39

Let us have some basics.

All motors are rated on their output power, nothing else.

A VFD is rated on its output power, plus about 2000 other not important factors.

An induction motor is an amazingly complex electromagnetic machine. Pick up any book on motors from any charity shop to convince yourself. An induction motor is essentially a transformer where power is transferred from primary, stator, to secondary, rotor. With the huge air gap between the power factor of the motor just rotating, nothing whatever attached to the spindle, will be about 0.2. At full load the power factor of a 2.2kW motor will be 0.85. If you measure the current, note current, taken at these two points it will be near identical, about 5%, possibly 10%. Read up about circle diagrams.

To determine the current taken by an induction motor you take the rating plate Watts divided by voltage equals current, then divide by 0.8 for efficiency and again by 0.8 for power factor. This is pretty worst case. Note that efficiency and power factor improve as the motor gets larger. If you have a large machine with multiple kW motors then divide by 0.85.

This leads onto a little known fact about induction motors, they are very fussy about their voltage. The power output, that is torque, is proportional to the voltage squared. So if your mains is 216 volts, and your motor is old and rated at 240V then this is 0.9 down, so squared is 81% power!

Conduction angle. Oh, this has hurt many people! The VFD has a bridge rectifier stuck across the mains with the electrolytic caps after, the idea being to charge the caps. But the caps are also being discharged by the motor as it is driven. Now, the maximum voltage the caps can be charged to is the RMS line voltage times 1.41, root 2, the difference between the RMS and peak voltages. The conduction angle is the actual part of the sinewave that the mains is more positive than the cap, so current flows through the rectifier to charge the cap. If the cap is at 300V then it can be seen that this charging period is not 180 degrees, more like 40 degrees. The problem here is that all the power needed to run the motor, 370W, has to be passed through the rectifier in these 40 degrees. So whilst the motor is happily delivering 370W with a current of 1.1/2A for 180 degrees, the poor rectifier has to pass that power in a quarter of the time, so 6A nominally flows. We then have the problem that the current is actually peaky, more like 10A peak for very few degrees. This is why they are so infernally noisy. Need very very fast switching diodes to do it with minimum loaa and noise. Now factor in the usual case where the workshop is run from some lighting flex and it is obvious that current drop due both to the wire resistance and impedance start to become important.

Just to keep you all happy there were some posts about single phase in to 400V three phase out VFDs. Now these really are pushing the limits because they use a voltage doubling circuit to generate the 400V or so for the caps. So what is in the single phase VFD a full wave bride, in these it is half wave. One half wave charges the cap to 240V, the next half wave pushes the 240V on the cap to 450V. Not at all nice. Now you see what three phase is used, and wired to create 6 phases.

For SoD, more load means more REAL amps, not inductive amps. Power factor REALLY does vary widely as the current taken shifts from inductive to real amps. I don't mind at all, I make as many mistakes as anyone else!

Foe AJ, same as SoD, at no load the motor is a large leaky inductor with non0existant [power factor.

I said buy a VFD same size as the motor.

Buying a larger VFD really does nothing to improve the conduction angle or charge current.

The bottom line is that the electronics, particularly the rectifiers and caps, are seriously stressed. One 4kW VFD I have actually has a programme option to measure the impedance of the caps.

Thread: Copper boiler plate flanging, or not?
09/08/2020 16:22:33

My point about the 5 thou is that it is important. Bend a leg on a chip with lots of legs and it will work on the conductive flux, for a while. Crossover engineering could be really good, can't think of an example at the moment of course!

My point about Simplex was simply that it used a thick throat plate, not flanged, which I found interesting. What is the too small factor of safety?

I do have a bucket load of Silfos, was used to solder coax cables, but I won't be using it.

For 30 years I have sold items all around the world, and they were required to be good, however defined, even though the buyer had not a clue about what was inside. I take the same attitude with model engineering, it has to be good. This discussion is much better than none.

I can't find the magazine with Keith Wilson Silfossing a huge boiler to check later issues.

Cost is obviously of importance, but I am currently selling off stuff I am no longer interested in, Meccano, valves, test equipment etc so I do have money to spend, more so no doubt than most people.

Thread: Silver soldering a Minnie traction engine boiler
09/08/2020 16:08:16

I did emphasise that it was MY OPINION.

You would think that Microsoft would be a professional outfit, but look at the mistakes they have made over the years!

Easy enough, now, to curve a piece of 90 degree copper to solder over the top, inside or out.

I have been trying to find out what the tearing strength of copper is. If the joint started to go, say a 3mm gap started weeping, then at what boiler pressure does it tear and leak or explode depending on the speed it tears. Way outside my area of expertise, which is why a read books.

Apologies to Mark if this is causing sleepless nights.

Thread: Vfd sizing
08/08/2020 13:09:37

The VFD is designed to drive a certain size motor, so buy that size motor VFD.

A motor will take pretty much the same current irrespective of its load, the power factor alters to produce the output power.

I was reading Oxford Welders web site, and they have some interesting comments on inverter electronics. I have to say that it fits with my experience and what I have heard, they will die sooner or later in any case. The electronics is seriously loaded, and buying a larger VFD doesn't alter the fact that the recitifiers and smoothing caps are really being driven hard. The conduction angle off the mains is only a few 10's of degrees, and really won't change in a larger VFD.

Thread: Copper boiler plate flanging, or not?
08/08/2020 13:02:34

Back, just replied to another post about a Minnie boiler, the throat plate to boiler joint doesn't have a flange, just a simple 1.5mm thick solder joint. In MY, emphasise MY, book that is simply unacceptable.

Duncan's comment about getting hung up on the 5 thou gap is simply wrong. I am an electronics engineer, and have had to cope with surface mount components, and a 5 thou gap simply means the item fails later on. Tin solder is awful stuff, I have to hand solder, fine if you have got the gear to flow solder the 98% tin 2% silver solder. But even that fails after a few years. I have been seriously burnt by lead free solder, I won't use it.

There is lots of comment about expansion, yes, but why is a solid tube plate different to a flanged one? The solder is easily able to cope with the stresses created, as is the copper sheet, not a problem. It all comes back to not really knowing just how much solder is in the joint. There will also be a slight reduction in water space, but just shift the plate to keep the 1/4" or whatever gap between plates, firebox isn't going to notice.

I will buy a little boiler and make it.

As said there have been thousands of boilers made, but where are they? Going back to the 40's and models were made everywhere, but do you see them? The exhibitions all seem to have new models. Beginnings to wonder if all the thousands have actually been turned into static models, or simply scrapped. Seems like making boilers from steel, up to 2" scale, just having to scrap the thing after 100 steamings because you have no idea of its condition.

Boiler bangs are not a problem, keep it that way!

Thread: Silver soldering a Minnie traction engine boiler
08/08/2020 12:41:41

I hate to criticize something that I have never done, but working my way towards.

I actually came across this thread trying to find one I started about boiler soldering.

The boiler joint between throat plate and barrel is just a line contact, 1.6mm or so? I really don't think this is good enough, particularly on a traction engine with all the driving stresses pulling that joint apart.

I have been working my way through a huge pile of MEs I have just been given. I see that Martin Evans on the Jubilee boiler does talk about using a thick throat plate without flanges, 5mm or 3/16" in that case. There are also a couple of articles by Martin Evans about boilers and strength in the early 60's. Issues 3107 and 3119

These are, of course, my opinions, and without ever having soldered up a boiler so with no practical experience. But, I am not going to start soldering any boiler until I know why you get weeps, I think poor fitting between flange and shell so solder can't flow.

Thread: Harrison M300 single phase from new
15/07/2020 16:43:22

Really not a good idea to run a three phase motor with a static converter in delta. Endless comments all over the web but principally because of circulating currents in the motor, excess heat.

Thread: I'm thinking of selling up, whats it worth?
15/07/2020 16:35:02

Not said where you are in the country?

Avoid ebay like the plague. Try using a local industrial auction house, around here in Lincolnshire there is PP. The advantage is that they sell stuff, machine tools, all the time, they know them. But you take them to their place and leave them, they lot, sell and deal with the buyers. Never underestimate the shear time and hassle of selling things, packing, postage, waiting in for collections, drives you nuts.

Prices, a place like PP (no connection other than buy and sell for 15 years) always has people looking. So the prices gained should be ok. But don't forget that things like 40 int tooling is past its sell by date, no one buys it, likewise huge drills and milling cutters. Yours is more home use size, and people will be interested.

I am in a similar situation, but selling up electronics stuff after 50 years to go to model engineering. Some items will sell, some won't, but using an industrial auctioneer, not ebay, will get the best price. At the moment stuff seems to be flying abroad and silly prices. On the other hand I have scrapped lots of stuff that ought to be worth money, just too hard to sell individually.

Just my thoughts, take it or leave it.

Thread: Copper boiler plate flanging, or not?
09/07/2020 19:57:17

Just been given a large pile of MEs, and quickly scanning my way through them.

One from the 1980's I think had an article by Keith Wilson about how great silfos is to soldering up boilers.

Then I have just read Wardle's article in 1st Feb 1980 about a couple of silfos boilers that literally fell to pieces after about 1000 hours of steaming. I found that incredibly frightening.

The solder had been attacked by the sulpher in the coal causing it to turn to black mush. This is one of the reasons I hesitate about ME flanging plates, that the 5 thou gap might be 50 thou and I do have a bucket full of silfos (used to solder coax cables in the days before fibreoptics) and was tempted to use it. I still look at the reverse curve on a loco firebox and think, without rivets, how do you keep those pieces of copper at a 5 thou distance. Note, not more than 5 thou, or less than 5 thou, but at 5 thou. If bits of the gap are 5 thou then what percentage?

Yes, I worry, but my professional engineering was like that too, eliminate by design known possible failures.

09/07/2020 10:54:27

Thanks for the continued interest, or exasperation!

Oxy acetylene.

If you are skilled then, as people say, use it. If you are not skilled then steer well clear. After reading books and articles on the use of oxy then that is my conclusion. The problem is that the flame temperature is so hot, 3,300C, and the boiling point of copper is 2560C. So the chances of overheating rises to, what, 100%? Look at the tubeplates, the firebox one is hard to see with lots of very thin copper. Gas is 1900C so can't melt and the flame size spreads the heat so even oxidising is unlikely.

The snag with reading older books and articles is that things change. Silfos was recommended ears ago, now it isn't, with large warnings of sulphur effects in coal fired boilers if the temperature exceeds 200C.


What troubles me is providing the guarantee that the flanged joint is consistently the required 5 thou. If you are making some steelwork then you use bluing paste to make sure that the whole area that needs to be in contact is in fact in contact. This is easy on the tubeplate, but that can be machined anyway, it is the firebox tubeplate and the springiness of the joint. This is why it is suggested you tack the joint with rivets, but then how do you know if you still have the 5 thou. At this point you have filled the joint with flux and any gaps are now invisible.

Solder is strong, far stronger in tension and shear than copper. This means that with a 3mm plate then the solder only has to be 0.5mm for the same strength. So the 6 or 8mm flange has plenty of redundancy, the snag is you have no idea how much of that redundancy you are using. Is the flange width so wide simply because they are so poor and the maximum redundancy is needed? My suggestion using thick plates might bring some clearer idea of how much is being used. Many years ago I found Frank Price's book Right First Time in the library, and bought a copy, and that has been a guiding principle ever since in design.

As mentioned, using a sandwich of formers seems a very good idea to improve the quality, however defined, of the flanges.

The last jobs I had to do that were completely new to me were felling trees. Standing at the bottom of a 90' tree smartened up you risk assessment. But even more so did the cutting of wind throw and hung up branches, they look small, lots of them, just hack away with the saw. One tree about 70' high and only 5" DBH I felled, oblivious to the squirrel damage, invisible, half way up that caused to tree to fold in half as it came down. If I hadn't obeyed the rule, watch it, then it would have hit me. Likewise with hung up branches, you wouldn't believe how much weight a 2" branch can support without breaking, cut it and twang. You hide behind the stem and cut from there. Similar situations is repairing valve audio amplifiers, 600V HT is nasty. Don't really want to have a boiler experience.

06/07/2020 20:05:52

Short delay while I bought and read Alec Farmer's book as suggested. Yet another person who is incapable of creating an index!

Thanks for the comments on wire drawing.

I see that it uses oxy acetylene for all soldering work. The other boiler books are all very wary of this, and some ME articles positively say no. Basically the flame is too hot, and can/will result in the alloy metals being boiled out. Possibly of more importance with cadmium alloys.

To reply, not answer, comments. Copper is a very good conductor of heat, and I don't have the expertise to say that heating the water will result in an x deg C temperature drop along the copper. Many experiences with a soldering iron and far thinner wire than a 3mm sheet of copper suggests this speed of conductivity is high. So I really don't see that using a 6mm tube plate without flanges is in any serious way different to a 3mm flanged plate. What about around the firebox? Again don't see that it is important, especially since the foundation ring could be 1/2" square copper. Also the firehole ring is a piece of far stiffer metal than the surround, don't get fracture failure around them.

The shell will expand. But all the descriptions of boiler testing are that the whole boiler can, probably will, move slightly as the pressure comes on. The metal will move until enough is there to take the stress, the strain in the metal is minimised. With firing of the boiler hopefully this movement won't reoccur, but if it does then the copper is as soft as it can be, and will adjust. Again, don't see this as a problem.

Other comments are that the instructions to solder a boiler very frequently say that it is the firebox plate where the leaks are. Could be for a number of reasons. 1 - There is very little metal left after the tube holes are cut, so could be partially melted or other nasties. 2 - It is always a tricky shape, not like the smokebox plate, so can't just stick in lathe and machine the flange. 3 - Because it is hand work to get the plate to match the firebox then who is going to claim their filing is within 5 thou over a distance of 10" and a complicated shape. 4 - And how are you going to justify that claim.

This is why using a thicker piece of metal, no hammering and banging, has got to be an improvement.

Thread: How to contact another person
02/07/2020 18:43:29

But there is an email account, no phone number?

02/07/2020 18:39:54

Oops, thought all that had been done.

02/07/2020 17:11:37

Wanted some Fowler Lion drawings, a reply but saying they can't directly contact me, and now I can't directly contact them.

Thought that once logged in all the contact details were there to be used?

Thread: ISO Container for Workshop
30/06/2020 11:21:53

Try looking regularly on

in their auction. Often have the ex-military workshops, toilet blocks etc which have 2" foam insulation around them. Most are only 15' or so long, check. There are a few of the curved top type, but inside heght less than 6', don't buy unless you are shorter.

I bought one of these years ago, got logburner in it and really comfy.

I have used Worcestercontainers to move several of my lumpy bits, nice helpful people.

Thread: Copper boiler plate flanging, or not?
29/06/2020 12:21:38

Re-starting model engineering and need a boiler.

Got and read the various ME books such as Tubal Cane, Martin Evans etc. First gripe, haven't they heard of indexes? What is wire drawing?

It is clear from these that the clearance between copper surfaces in the boiler are of critical importance when silver soldering, something like a few thou.

Never made a boiler but done some limited, evening class, copper bashing and seems to me that the hammering needed for a 3mm copper plate to flange it results in anything other than a few thou close fitting joint.

One example I am working on is a part complete Clayton by Robin Dyer. Nice and simple with just two plates, top and bottom. But they are both, particularly the firebox end, with the tube holes very close to the flange with consequent pulling of the metal and hammer damage.

Why not just use a piece of 5mm or 6mm copper plate and just bore the holes, turn to exact size on the lathe, and solder? Ok, the copper cost will be higher but in the whole model that isn't really very much, so ignore cost.

Yes, copper is sticky and not the easiest metal to drill. But use a lubricated step drill, I find these are amazingly useful drills and solve endless problems, probably the greatest one being the end result is circular! Or could bore the holes on a mill.

The soldered joint needs to be in shear, from books, so the wide plate joint will be in shear. In fact exactly like the foundation ring. The actual area of solder will be the same for plate and ring, so no hazard there.

Inspecting the joint is equally impossible for flange or plate, no extra hazard there.

The next model wants to be a traction engine with a loco type boiler. Here again I can't see why the plates need to be flanged, a solid 6mm plate will be just as good. Yes, there will be a reduction in the water space around the firebox, but just make it slightly smaller to maintain the 3/8" or whatever gap. There is the throatplate, double flanged, but could be two 6mm plates brazed, not soldered, together with one for the boiler, one for the firebox. Again, cost but seems a cheap solution to hammering and bashing a poor piece of copper sheet.

Any one already doing this?


Thread: milling machine which one ?
08/06/2020 15:04:25

The motor rating plate is the mechanical power out. Multiply by 1.2 for power factor and another 1.2 for efficiency to get power in.

Thread: What *should* a Warco Super Major Milling Machine be able to accomplish?
22/05/2020 10:33:34

Why practice being silly?

This is a home workshop forum, and the machines as used in such an environment. A column mill is the mill drill and similar mills, which took years before they bothered to put keyways or other anti rotation devices on the column.

Look at your wonderful Bridgeport, just how far is it from the tip of the cutter, through the head, down the column, into the knee then up to the vice? And how many joints and gaps are there? It isn't rigid. Do the finger and DTI test, but you won't, nobody ever does. Then you have the Senior mills, nice solid column but the table dovetails are square, as long as they are wide. This means that you can do nothing to stop the table twisting on the dovetails. This is not rigid.

Each machine has a limit to what it can do, to push that limit buy a Huron, for spotting holes and thin aluminium buy a Bridgeport. Most work is lightweight, works fine, unfortunately as a model engineer the work is far more diverse.

21/05/2020 16:16:21

Rigidity is what you want, and a Bridgeport or any column mill doesn't provide it.

Put the tool in the chuck, bring it up to the work but not touching, then get a DTI between the work and the cutter. Using finger pressure only push the cutter to and from the work and be amazed at how many thou movement you get. That is the play.

If you get 10 thou then buy a file, even a couple of thou is far too much. A real mill will only move a couple of tenths, that is what you need to get cutting.

Look at a real industrial mill, and the amount of cast iron in it.

Thread: Digital Phase converter...
14/12/2019 17:00:04

Interesting, not come across a digital phase converter before, but then I have three phase in the house.

As has been mentioned just seem to be a standard VFD with a voltage booster on the input. This is both a tribute to the amazing electrolytic capacitors now available, and a problem.

To step 220V up to 440V needs a capacitor, it is charged to peak volts on the negative half or the sine wave, and then lifted up to charge the output capacitor on the positive peak of the sine wave. This is identical to the dual voltage switch mode power supplies that have been around for years. The snag comes because the output capacitor is only charged once per cycle, so 50 times per second. And that charge is just a rapid dump of charge from the input capacitor to the output capacitor. If the VFD is running 5hp, then that is 3730W, divided by 0.9 pf divided by 0.9 efficiency giving 4600W. So this 4600W for one cycle needs to be dumped in the output cap in about 5ms to run the motor for the 20ms cycle. Simply put if the motor takes 10A then this current is 40A.

Certainly works the caps hard.

You can emulate this with a standard 400V 3ph VFD, the few I have seen all have terminals on the back to add in extra smoothing caps. Just make your own voltage doubler and attach the output cap to these terminals. It is seriously lethal, so perhaps not.

An induction motor will take an 8x overload to start. Don't forget that all that is limiting the current is the synchronous impedance of the motor. This is why a 5kW generator won't start a 3hp compressor. To some extent it is relying on the inertia of the rotor to keep turning to get the voltage regulator to crank up the excitation current.

If the rotary converter is noisy then switch the motor off as soon as the lathe is running, not then needed provided you check that there is enough running capacitance to keep the phases reasonably at 120 degrees.

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