Member postings for Andrew Johnston

Are you still using the late lamented Wavedriver unit? Or did you ditch it in favour of a modern VFD?

Regards,

Andrew

Tin/lead solder is still available from professional electronics distributors. I don't think there's a ban on its use per se; what is banned is use in equipment put on sale within the EU after a given date. As always there are exceptions, including large scale fixed industrial equipment, exclusively military equipment and some medical devices.

To be pedantic the exemptions for medical devices were removed in 2011, but there are moratoriums in place for 3 or 5 years depending upon the precise use of the equipment. You wouldn't want a duff joint in your pacemaker!

Regards,

Andrew

Neil: Dooooh, now I think I get it, you have to be patient with us country yokels. I couldn't really imagine that the post was anything but humorous, sorry for my lack of understanding.

Regards,

Andrew

You definitely don't want a contactor between the VFD and the motor. That will probably result in a damaged VFD.

Most VFDs should have a 'stop' function, which can be wired as an emergency stop. Depending on the design of the VFD there are several things that can be done once the 'stop' button is activated.

1: The VFD simply stops driving the motor, and the motor coasts to a stop exactly the same as a motor where a contactor is opened.

2: A DC current can be injected into the motor windings to oppose the motor rotation and provide a braking force; known as DC injection braking.

3: Resistors can be used to dissipate the energy in the motor windings and thus stop the motor more quickly than by coasting.

The VFD on my CNC mill uses a braking resistor to stop the spindle within a couple of turns, so that the spindle can be reversed quickly when using compression/tension tapping heads.

In the manual for my Britan repitition lathe it is suggested that one way to stop the motor quickly is to reverse the direction momentarily, thus reversing the currents in the windings. I guess that's a kind of manual method of current injection.

Regards,

Andrew

I used to be a student member of the IEE, but didn't bother to get chartered status. If nothing else it p*ssed me off that the IEE wouldn't count post-grad research as a 'proper' job placement. I've never been asked in interviews whether I was a member of the IEE or had chartered status, so it doesn't seem to have done my career, such as it is, any harm.

Regards,

Andrew

Otley: That's a neat explanation, thanks. I believe that the technique is also called wobble broaching, which may help visualise the action of the cutting tool.

Andrew

There's nothing to say that the 6BA grub screws have a standard size hex hole, depending upon their provenance.

Regards,

Andrew

Thanks for all the comments.

Alan: Interesting link; could have done with some of those when my girlfriend got over-excited and knocked a glass of red wine over the carpet.

Norman: The kitchen is even worse than it was, as I now have all this year's seeds spread out ready for sowing; if the weather ever improves. But, to compensate, the workshop is a lot tidier now with new shelves, new racking system, junk binned and milling cutters all in neat rows.

Neil: I'm afraid that I simply don't understand your post, is it humorous or are you telling me to sling my hook? With regards to vessels, a full kettle whistles when it boils, but an empty one doesn't.

Regards,

Andrew

Michael: Thanks for the link to the electricity supply guidance document. I'm pleased to say that in general I meet the requirements, as the biggest three phase motor I run DOL is 5hp. My TIG welder also comes in under the single phase limits. However, the less said about the air compressor the better.

Reading between the lines I assume that the document is aimed primarily at commercial users? I doubt many domestic users would understand the terms used, know how to measure the parameters mentioned or have the equipment for the measurements.

Regards,

Andrew

Well I never, I didn't know Dykes rings were still used. Paul Dykes was listed as an engineering fellow of college when I matriculated in 1980, but I don't think he was active in college by then. I certainly never met him.

I got rat a***d last night; but today I made a master tool for my CNC mill, and then set up and ran my first job using tool tables and automatic compensation for differing tool lengths. Then I set up the repetition lathe for the first operation on 500 threaded spacers which I've been asked to make. Plus oiling and fettling the hitch on my glider trailer so I can get the d*nm thing off the car, and getting the next set of seeds on the propagator on the kitchen windowsill.

Andrew

Oh dear, if I've got my calculations right this is my 1000th post on the ME forum. That raises a conundrum; here's a partial quote from a previous thread with the names of innocent, and possibly the guilty, removed:

The culprits are clearly the seasoned posters that have a score of plus 1000 posts. with the exception of ***** who is a good guy with over 2000 posts. Now they have turned their attention to ***** with their bullying tactics with racial discrimination thrown in as well!

***** has turned a blind eye on these pests who contaminate the threads with their nonsense.

*****, you remember my tussle with a 1000+ poster on another thread, don`t become one of them.

So it would appear that I'm now irrevocably accursed and can only contribute nonsense. In the words of the Clash:

Should I Stay or Should I Go

Regards,

Andrew

That can't be right? I suspect the mill would have gone up by £300, but the woman would have made a decision, spent the money allocated to the mill and the extra £300.

Andrew

Michael: I'm unable to find any specfic rules applying to domestic electricity consumers regarding the use of converters; can you elaborate?

Andrew

Probably a misprint; it should be 4.5625", then it'd be 4-9/16".

Andrew

Silver steel is normally centreless ground, so it'd be difficult to form a hexagon section. I've never come across hex silver steel. I've never seen hex tool steel either. I suspect you'll have to machine or grind your own hexagons.

Regards,

Andrew

Only in the sense that it includes the factor 'j'. It may lead to complex impedance, but still results in real currents flowing in the real world.

Andrew

Swarf Mostly: Many thanks for the excellent electronics exposition. It's always interesting to revisit older, but not necessarily obsolete, techniques.

Another way of looking at the choke/capacitor combination is as a filter. The rectifier output is a full wave rectified sine wave, with a fundamental (in the UK) of 100Hz. The choke and capacitor act as a low pass filter, removing the fundamental and harmonics while leaving the DC component. Herein lies a problem if using an LC filter in a VFD. The output of the LC filter will be the average of the input voltage, not near the peak of the incoming sine wave as for the diodes and capacitor. If we look at the Fourier series for a full wave rectified sine wave the DC component is 2A/pi, where A is the amplitude. So, assuming we start with a 240Vrms incoming sine wave, for a diode/capacitor we get a DC voltage of around 339V minus diode drops, and less some ripple voltage depending on the load current. For an LC filter that removes all AC components we get an output voltage of 216V. It would not be possible to generate a 240Vrms, between phases, 3 phase output from 216V.

If I remember correctly resevoir capacitors were often fairly large, let's say 470uF. The nominal cutoff frequency of a two pole LC filter is f=1/2pi*sqrt(LC) and the filter rolls off at 12dB/octave. So let's set the filter cut off at about 12.5Hz to give 36dB of attenuation at 100Hz. With 470uF that gives an inductance of 0.345H. That's quite a large value, which equals a large component, especially one which needs to carry quite a few amps. It is often forgotten that engineering is as much about money as science. Given that the VFD market is pretty competitive I doubt manufacturers would include large inductors and capacitors requiring manual assembly.

Incidentally I suspect that the 'bleed' resistor was there to critically damp the LC circuit at resonance.

I agree that many people shy away from inductors. Of all the passive components they are the ones least likely to approach the ideal. For power applications they nearly always needed to be custom designed. That has changed a bit with the proliferation of switch-mode power supply ICs but you still need to design your own for anything out of the ordinary.

I would agree with Russell in that I suspect that most VFDs incorporate an active PFC at the front end, if anything. The PFC inductor, and any front end AC filtering, would involve small components that could be auto assembled on a PCB.

I'd like to say that none of the above is intended to denigrate the post by Swarf, which I enjoyed, but just to highlight some issues that may preclude the use of LC filters in VFDs.

Regards,

Andrew

Martin: It's a case of terminology. In an AC system there may be current flowing in the load that is 90 degrees out of phase with the applied voltage. This is generally known as the reactive current as it usually arises from reactance in the load from inductors, capacitors, or a combination thereof. Since the reactive current is at 90 degrees to the voltage the actual power dissipated by the reactive current is zero; which is the point I think Russell is making.

However, from the point of view of the generating company the reactive current at the load has to be generated in phase with the voltage at the power station, and hence it takes real mechanical power to generate it, which obviously costs money. I think the older style UK electricity meters, with the dials that went whizzing round, only measured the real power used and did not account for any reactive current. Hence the electricity board was paying to generate a certain amount of power, but only charging the customer for a lower amount; which could be regarded as a bad business model.

Regards,

Andrew

Bazyle: I only have one meter, which is 3 phase. Before I had 3 phase installed I asked two questions. One, would I pay the normal domestic rate per kWh and two, would the 3 phase meter cope with the highly unbalanced load of the house on one phase and nothing on the other two, if I wasn't using the workshop. The answer to both questions was yes.

Murray: Thanks for the info on the North American supplies. While the UK is nominally 230V for single phase my voltage at home is normally above 240V, and it's not unusual to see it above 250V.

Regards,

Andrew

Michael: Thanks for your concern, but it does seem slightly gloomy. Your idea seems to be that once a problem, real or imagined, is identified we'll all be banned, or similar. If we take that idea to it's logical conclusion, given that life has so far proved to be 100% fatal for all concerned, everybody should be killed at birth, until life is extinct, and there is no danger of injury or death for anybody.

Bazyle: I have installed a 3 phase supply at home, running into a 3 phase distribution board in the garage and thence to the machine tools and a single phase garage ring main, plus a 32A single phase outlet. One phase is tapped off from the electricity board fuses via a normal consumer unit and supplies the rest of the house. As far as I'm aware I pay a normal domestic rate per kWh. I'm not at home to check, but I don't think my electricity bills even mention 3 phase, they just list the kWh at the two rates I pay per unit according to usage.

Regards,

Andrew