Member postings for Robert Atkinson 2

Hi Bob,

I think you have mis-read. I said "3 phase excitiion alternators" i.e. the excitation alternator is a 3 phase machine. It is a alternator not a transformer with a moving coil.

The excittion alternator has a single winding DC powered stationary field coil on the frame and a 3 winding rotating output coil assembly on the main shaft of the total machine. The 3 phase output of the windings is rectified by six diodes, also mounted on the shaft. and the resulting DC is connected to the rotating, single winding field coil of the main alternator.
Typically the excitation alternator has more poles than the main alternator so the ripple frequency of the rectified DC suppled to the main fiel coil is higher than the output frequency.
By varying the current in the stationary excittaion alternator coil the AC output of the main alterator is controlled.

Hope this is clearer.

Edit, output ripple is important in a car system. Apart from modern electronics, AC ripple is damaging to lead acid bateries.

Robert.

Edited By Robert Atkinson 2 on 11/07/2023 10:25:54

Edited By Robert Atkinson 2 on 11/07/2023 10:26:32

Martin,
Yes the aviation brushless alternator use 3 phase excitation alternators with rectifiers mounted on the shaft.
Engines with electronic control often have small unregulated permanent magnetalternators to provide power to the engine computers if all other power fails.

The world of aviation power generation is undergoing a lot of change. Many aircraft now use wide frequency range alternators (typically still called generators in the industry) rather than the tradition 400Hz tharequired heavy constant speed drives. There are some that use higher frequencies and a AC-AC converter to generate 400Hz. The converters use the same technolgy as our VFDs.
There is also a move to more DC power. The standard is currently 270 V but ther are moves to higer voltages. Semiconductor based devices are replacing heavy iron transformers and where you do need a motor a local inverter taliored to the task gives all the advantages of a VFD. In particular starting load control and for applications like pumps pressure or flow control without wasteful regulators and bypasses. It's also lighter and safer than hydraulics or bleed air.

Robert.

I've not seen the original article, but if this is an alternator for a steam engine you could make it linear and get rid of the crank and flywheel.....

Robert

Edited By Robert Atkinson 2 on 11/07/2023 07:50:12

No, that is for AC.

Indeed,
I was replying to Duncan with an example. I should have referenced you though.

Robert.

Not always. See: https://www.youtube.com/watch?v=DbAKDJGykgY

Robert.

There was no need for a un-classed submersible to be used for tourism othe than cost. IT ppears from published information tht Oceangate were actively subverting the regulations regarding passengers and vessel classification.

I can say from direct knowlege that the failure of the Bloodhound project was not about safey. It was about the huge amount of money spent and sponsors / public inerest in somone breaking their own record.
ThrustSSC was run on shoesring compared to Bloodhound and proved it could be done. Personally I did not approve of the orginal Bloodhound project approch were sponsorship seem to take precedence over sound engineering decisions. An example was the oxidiizer pump prim mover selection. Apart from the validity of a petrol ICE they changed the engne OEM / sponser twice.

The New Yorker article has a link to this article from 2018:
https://news.wsu.edu/news/2018/02/21/a-new-generation-of-titanic-exploration/
It says the control system ws designed and built by gradute engineers and interns. Comments like

“We’ve created a whole ‘Internet of Things’ network on the sub, so you control functions like the lights by just sending a message over Ethernet,” Nelson said. “I’m super happy with that.”

seem to indicate little understanding of safety of life conrol systems. There is no internet on the submersible so why use that technology (IOT). I guess because it was the next great thing and "cool". I'm not saying the control system contributed to the incident, but it shows a lack of understanding of basic safety principles. Using graduates and interns is good, but they need supervision by experienced engineers when carrying out critical tasks.

Edited By Robert Atkinson 2 on 10/07/2023 10:15:23

Note that the Orbital Science Tristar, contrary to what is implied on a lot of websites and other media, was engineered in the UK. Marshalls of Cambridge designed and installed the modification. This included large changes to the pressure hull. This was a lot more complex than the Virgin Orbit modification that used the existing "spare" engine mount on the 747 Wing.

Robert.

A 747 does not go high enough. A 747 will get to around 43,000 ft while the Eve mothership goes to at least 50,000ft.

Edit: To clarify further. The NASA 747 was just for transport between site. The Eve is basically the first stage of the launch system

Robert.

Edited By Robert Atkinson 2 on 09/07/2023 17:05:28

Interesting video. Lots of other video and images used with no apparent referencing. I found the lmages of them apparently bonding a large metallic flange to the outside of a composite tube. This does not seem appropriate for the Titan pressure hull. However a short video on the OceanGate website (before it was shut down) appeared to show composite tape being wound on a mandrel complete with flanges. On the face of it starting with a metallic tube with integral flanges and wrapping it seems a better approach. The load paths in the flange area must be quite complex to say the least.
The fact that the acrylic window plug was missing from the recovered end bell is interesting. If it failed it seems unlikely to me that the rest of the hull would have failed. The window was a tapered plug so probably did not need or have much holding it in place. It is possible a shock wave (like water hammer) inside the end bell caused by the collapse of the main body tube disloged it.

I await the TSBC report with interest. They normally take about a year.

Robert.

Why do people keep saying BASIC is interpreted? While many BASIC implementations do use interpretors compliers are available and some versions are solely compiled. And if interpreted languages are so bad why is Python so good? Pyton needs huge resources. OK silicon is cheap now but more still costs more and my be less reliable. Even micropython needs a processor with 256kB of program memory and 16kB of RAM. I've written functional code used in professional commercial products in BASIC (Pic Basic Pro) that ran on 256 words (12 bit) of program space and 25 bytes of RAM. Yes, that is 409 Bytes total memory.

Robert

You are comparing apples and oranges.
The cost you pay a private audiologist for "the hearing aids" has to include the costs of initial and on-going consultations and support including facilities. The NHS price is for just the hardware. As you say the NHS will have negotiated a large bulk order price, but that almost certainly does not include any consumer warranty cost from the manufacturer for the items.

Robert.

I should have said: My aids are re-chargable. Not easy to forget to charge. They just drop into the storage case / charger on the bedside table at night. A lot greener than zinc air cells. Cost of aids includes annual check-up. I was given a 1-2 month wait for a initial NHS appointment. It might be better elsewhere or if you are already in the system.

Robert.

No experience with NHS aids but recently used Boots (central Cambridge). Very impressed with the service. I have loss of high frequencies. Bought some higher end Phonax receiver in ear aids with bluetooth (SWMBO insisted) and must admit it is very handy having wireless TV, handsfree for phone etc.

If I'd gone with NHS I'd still be waiting for an appointment.....

Dave is correct, the mains "earth" is not a good radio earth. That is why the VFD and any filters screens should be mounted on a conductive surface with the cable screens connectd 360 degrees to this surface. A good, typical design uses a bare metal plate mounted at the back of a metal enclosure. The low impenace plate causes interference currents to flow through the smalleas possible area which limits radiation and generation of voltage.
The outer metal case helps with interference but is mainly for electrical safety and mechanical protection.

Robert.

EMC ferrites while they do increase inductance they are different to a conventional choke. A inductor (coil, choke) limits AC current flow and develops a voltage across it. A good inductor has very low loss. EMC ferrites are designed to be lossy at high frequencies. As a result they absorb energy converting the interference to heat (a tiny amount). A coil of wire with the same inductance would be less effective.
They do work. See post by Tony Pratt 1 near the bottom of this (rather contraversial in other areas of the thread) page: https://www.model-engineer.co.uk/forums/postings.asp?th=176304&p=4
A sleeve cured interference emitted by a DRO.

On EMC earths Emgee is correct. I would add that for TNC-S (PME) earthed supplies there is a particular hazard with connecting additional "earths" to the mains earth. In the case of a failure of the neutral conductor outside your premises the load current for all the houses affected by the fault can try to flow through your extra earth connection. This can result in damage or a fire. It's a particular problem for radio amateurs who tend to like large earths.. Don't do any creative earthing without consulting a competent electrician who has seen your installation.

While professionals will comply with the regulations, certainl for production designs you don't throw everything at the system. There will be a systematic approach to meet the emission and susceptability requirements. This may meen repeated iterations of measure, modify, measure... If you are producing thousands of an item you dont want to use a £3 filter if a £2 will do the job. The EMC components do not affect the noramal operation of the item which is why a lot of the cheap items leave them out.

Robert.

Edited By Robert Atkinson 2 on 05/07/2023 07:56:26

Edited By Robert Atkinson 2 on 05/07/2023 07:57:01

I concur with Emgee. keep VFD and DRO as separate as possible. or instance don't plug them int the same extension lead. Take each back to the socket on the wall.
Best to do one thing at a time. If you are still having issues with the fancy motor cable, filter and box on he VFD we can look at improving the immunity of the DRO.

Robert.

Just noticed Dave's question on ferrites. Yes, ferrite chokes on the VFD cable can help. They mostly help with high frequencies. There are different grade grades of ferrite for different applications and they can be a ring or sleeve. Unless the VFD manufacturer makes a specfic recommendation you have to use judgement for initial choice and then make measurements to check on effectiveness.
I sent Rockingedge a ferrite ring (donut) of a type that may help. The 3 motor wires are passed through this with it located at the VFD output before they go into the screened part of the cable.

Robert.

Hi,
On earthing, the mains input earth should go directly to a stud / bolt on the bare metal plate. The VFD and filter both need a wire to taht stud/bolt or another one next to it on the plate AND be bolted directly to the plate. The cable screens need to go to a 360 degree gland either where it enters the box or to a angle bracket bolted to the metal plate and grommets or plastic strain reliefs where they enter the box. For the "triple ground" wires In the motor cable I wound take all three to a bolt on the metal plate close to the VFD terminals. This could be one of the VFD mounting bolts. At the motor end take all three to the earth terminal in the motor.

On screen grounding, it must be connected to the plate at the VFD end. At the motor end it is a choice as SOD says. Personally I'd try ungrounded at the motor end first. If still having trouble try 360 degree grounding. There is no right or wrong way.

Robert.