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Member postings for Robert Atkinson 2

Here is a list of all the postings Robert Atkinson 2 has made in our forums. Click on a thread name to jump to the thread.

Thread: A good toolpost drill design
20/01/2020 07:25:40

My plan for this is to take the motor and gearbox out of an old cordless drill. I've kept an old Bosch that the NiCad batteries died years ago fir just this purpose. Some need the case to hold parts together but this can be solved at various levels of neatness (hacking the handle off to machining a shiny new housing. It will run off a 12V supply with speed control modified to a rotary knob.

Robert G8RPI.

Thread: PSU for anodising.
15/01/2020 15:52:31

I'm going to reveal a hidden gem here:

Get one of the Low voltage AC/DC power supples intended for use in schools like this one

https://www.ebay.co.uk/itm/POWER-SUPPLY-EJ0032-IRWIN-L-V-Variable-AC-8A-DC-5A/254479418781

or 8A one

https://www.ebay.co.uk/itm/POWER-SUPPLY-EJ32-IRWIN-L-V-Variable-8-Amp-AC-DC-volts/124032526494?

Search Irwin power supply. The also made one with stitched positions fron 2 to 12V

Griffin & George also made them.

Some are over-priced eg https://www.ebay.co.uk/itm/Variable-Power-Supply-Griffin-and-George-ltd-London/202874378189?

Some are absolute bargains like this https://www.ebay.co.uk/itm/GRIFFIN-AND-GEORGE-GRIFFIN-LT-VARIABLE-POWER-SUPPLY-/113986088771?

(No I didn't get that one but have an identical one).
These are unregulated and are much more suited to to plating, and running small motors than modern solid state digital units.

Don't forget that if you want to have sulfuric acid stronger than 15% you need an Explosives and Poisons Precursors licence in the UK

https://www.gov.uk/government/publications/licensing-for-home-users-of-explosives-precursors/licensing-for-home-users-of-poisons-and-explosive-precursors

Robert G8RPI.

Edited By Robert Atkinson 2 on 15/01/2020 15:58:10

Thread: VFD Question
14/01/2020 01:01:54

In reality you need to know 3 electical specifications for a 3 phase AC induction motor

1. Voltage to Frequency Ratio (V/Hz)
2. Maximum Frequency
3. Maximum current

For a typical UK 1.5 hp motor these could be:

8 V/Hz, 60Hz and 2.5A

This motor is suitable for direct connection directly to the UK mains (50Hz 230/400V) as 8x50 = 400V.
If used on a VFD with voltage boosting it will run 20% faster at 60Hz and 480V with same max current (torque) so will provide 1.8 hp
Running at reduced speed reduces the power linearly to zero. At zero Hz (DC) you can apply zero volts. Note that you still get torque at 0V but its resisting turning, not providing it. This is how dynaimic braking and DC injection works. Try spinning a disconnected motor by hand and then try with the connections shorte together.

Note that the same physical size motor provides more power at higher frequency. This is why aircraft use 400Hz AC supplies, 8 times the power for the same weight of motor. The V/Hz rule applies to transformers too.
Aircraft voltage is 115/200V so V/Hz for motors is 0.5. This means you can run an aircraft motor on 50Hz but you have to reduce the voltage to 25V so there is little point.

Robert G8RPI

13/01/2020 23:07:59
Posted by Stuart Bridger on 13/01/2020 17:18:03:

A bit more research on VFD operation (specifically on the Emerson Commander that I use) shows that the the VFD will modulate output voltage progressively up to the "base frequency" set. Above the base frequency full supply voltage is applied and just the freqeuncy is varied This is where the 29Hz comes in. Setting the base frequency to 29Hz ensures that this voltage/frequency curve is optimised for 230V operation. Agreeing with Robert's post above

Edited By Stuart Bridger on 13/01/2020 17:19:52

No it's NOT.

The VOLTAGE is reduced when the FREQUENCY is BELOW the rated frequency of the motor.
The VFD instructions you are reding assumes the motor you are using has the same rated voltage as the VFD output. Some VFDs can increase the voltage as the frequency rises to maintain torque and increase power but this depends on the insulation of the motor being able to withstand the higher voltage. Motors with increased insulaton ae available for this purpose. Note that the motor will not run any hotter with higher voltage because the main heating effect is I squared R and the current and resistance of the windings has not changed (yes there are increased bearing, windage and AC losses like skin efect but these are insignificant for the levels we are talking about).

I'll say it again it is FREQUENCY (speed) and CURRENT (torque) that determine the motor power, the voltage rating just sets the current at one frequency.

Robert G8RPI.

Thread: Homemade Lathe
13/01/2020 17:18:51

It's a bit concerning that the "bed" did not sit flat on this bench after he welded the two U sections together.........

Thread: VFD Question
13/01/2020 17:04:05

I've been travelling so have off-line.

Please READ ALL THE THREAD BEFORE POSTING.

I've already explained the situation on running a 400V motor (does not matter if it's star or delta just that it cannot be wired for lower voltage). Running it at 29Hz (with no voltage reduction, maximum VFD can give) Gves more torque at lower speed. Runnng at 50Hz (same voltage -maximum) gives less torque and higher speed. THE POWER IS THE SAME.
The voltage rating of a motor is only to get enough current to provide the required torque at full speed. Star-Delta starters provide lower inital current and thus torque by putting less than rated voltage (0.577 times) across each winding during starting. This can be done to reduce electrical loading so supply requirements can be less OR to reduce initial torque to prevent mechanical stress.


The VFDs with higher output voltage than input either use a simple two capacitor full wave voltage doubling rectifier. This is only suited to smaller units as it has poor power factor, but it's cheap. larger and better units use a boost switchmode regulator that can also provide power factor correction with a few extra components. While a true Cockroft-Walton voltage multiplier uses the same basic technique of multiple capacitors charged in parallel and discharged in series as the full wave doubler it is defined by many more capacitors and thus higher multiplication.

Robert G8RPI.

Thread: Shimming Myford ML7 spindle
12/01/2020 07:26:41

You can't tell f the bearings are fitting properly unless you get some blue on them. It sounds like the bearings were replaced but not fitted and adjusted properly. They maybe be fine but you can't be sure until you check them.

Robert G8RPI.

Thread: The cultural status of engineers in the UK
11/01/2020 15:23:25

Well I don't have a degree but I am an engineer. I qualified as a licenced aircraft engineer nearly 40 years ago. This is one of the few engineering jobs that actually need formal licencing. More recently I became a Chartered Engineer (and a Fellow of the Royal Aeronautical Society). So I don't agree that a degree defines an engineer.
As mentioned the image of engineers on the media does not help. Even on those programs showing engineering tend to concentrate on hitting things with hammers and things going wrong. This is becuse the media is looking to be entertaining, but the judge is the producer / director. I've been behind the scenes and on some engineering programmes and what ended up on the screen bore little resemblance to what actually went wrong.

SWMBO has degrees and a doctorate but does not earn anywhere near as much as I do smiley

Thread: Electric motor ratings
11/01/2020 09:06:40

I never said that k made sense just that it is the standard. That said there are actually grounds for using m as the thousanths multiplier that long predate SI and the Kelvin. The terms mil (1/1000" and milli have been used for centuries.

At least with k & K it is pretty obvious which is which. With m & M it may not be clear especially in electronics. A single design may use milliohms and Megohms, millihertz and Megahertz.

Robert G8RPI.

Thread: Co2 emissions.. Steam or diesel best?
10/01/2020 20:37:11
Posted by not done it yet on 10/01/2020 19:03:50:

There is not really any advantage of coal in comparison with electric.

Near 50% of electricity is currently (on average) generated by low carbon technology (nuclear, wind, solar, biomas, hydro, imports)

Most of the fossil fuel generation is with closed circuit gas turbines and these are greater in efficiency (over coal burning) by something close to 50%. It also produces only about half the carbon dioxide emissions, compared to coal generation (per unit produced).

Perhaps you should study the likes of:

**LINK**

or others of similar ilk. Plenty of information to be gleaned (if one is understanding the different statistics of electricity generation).

Biomass is NOT low carbon, The reports say the Drax power station is now carbon neutral because it runs on biomass. Unfortunatly the biomass is wood pellets at least some of which is produced in america. The CO2 emitted in harvesting the wood, drying it or transporting it (ships burnig dirty bunker oil) is not counted. And biomass produces more CO2 per Watt than gas.

Robert G8RPI.

Thread: Special nuts: Any name to what I may need?
10/01/2020 20:28:31

JasonB beat me to it!

10/01/2020 20:27:37

How about a joint connector nut

www.screwfix.com/p/joint-connector-nuts-m6-x-17mm-50-pack/83521

www.screwfix.com/p/joint-connector-nuts-m6-x-12mm-50-pack/83501

Robert G8RPI.

Thread: VFD Question
10/01/2020 20:19:00

On the linked page, Inverter drive Supermarket, remember that like many suppliers they are providing information that supports the product(s) that they are selling. This particular page looks OK. However, note that the mechanical power available running a 400V Delta motor at 50Hz/1.73 (29Hz) is THE SAME as running it at 50Hz. The difference is the available torque. If you are also changing the machines gearing then it's 6 of one half a dozen of the other. If the gearing remains the same then it depends on the machine type. It may even be worth considering running both settings one for work that needs high speed (50Hz) and one for high torque for mills and lathes.

Robert G8RPI.

Thread: Electric motor ratings
10/01/2020 13:03:29

Simon and Andrew have pretty much nailed it except K is absolute temperature (Kelvin), the multiplier for thousands is k (lowercase).


Ahh, VAR's some early turbine aircraft actually had VAR meters so the flight engineer could ensure limits were not exceeded.

Robert G8RPI.

Thread: VFD Question
09/01/2020 19:03:22

I've come to the thread late but OMG what a confusion. Lot's of good info, some "simplified electrical" and some just wrong.

One key point that seems to be missing is that (assuming same speed) the output power of a motor is determined by the torque. The torque is directly proportional to the CURRENT in the windings. If the current required to provide the load torque is less than the available current the motor provides the required power at the output (note this could be an overload and damage the motor). If the available current is less tthan the requirement the motor will slow and stall. A much better way to consider motor rating is the maximum torque and speed not power.
The above applies to any motor, 3 phase, single phase, universal including DC. For permanent magnet motors the magnet field is an additional limting factor.
Note there is NO mention of voltage. Voltage does not affect the available power IF enough current can be drawn. Voltage applied to a motor sets two parameters - A/ the off-load current (and thus speed for a DC motor) and B/ the maximum available current and thus (indirectly) available power.

So what controls the current? Well according to mr Ohm's law, Voltage divided by resistance for DC. We are mostly talking about AC but if we simplify things its Voltage divided by impedance (AC & DC resistance).

For a given frequency (speed) the impedance of the motor windings is fixed. This sets the stalled current. So what set the running current? That is the apparent voltage across the windings divided by the impedance. This apparent voltage is the applied voltage less the "Back EMF". This is the voltage developed by the the current flowing in the motor and opposes the applied voltage. It decreases with the load on the motor. At no load it is equal to the supply voltage and no current is drawn (This is a gross simplification, there is always some load, bearings windage etc so some current. and ignores phase relationships beteen voltage and current in coils). As load increases the Back EMF decreases and the current increases. The voltage rating of the motor is that which limits the input current and power (and thus output power) to the rating of the motor (set by the amount and quality of the iron and copper in the motor). For AC motors this includes the frequency. Too much current will overheat the windings due to rsistive losses.
The impedance of the motor is directly proportional to the frequency applied (all else being the same) so if you halve the frequency the impedance will halve and the current will double. This is why when running at reduced frequency (speed) the VFD reduces the voltge proportionally to keep the currrent the same. The available power reduces not becase of the reduced electrical input but because of the reduced speed at the same torque. You cannot exceed the torque rating of a motor without overloadding it and causing damage. If you increase the frequency (speed) the VFD increases the voltage to compensate for the increased impedance. This means you can get MORE power from the motor assuming you don't exceed the mechanical ratings (mostly bearings and centrifugal forces) or the voltage rating of the electricl insulation. Some "inverter" rated motors allow for this in their design. So ideally yous hould run a motor as fast as possible. 60Hz is OK for most normal motors.

So;

1\ you do not need to limit the upper frequency when running a motor at less than its rated voltage. Running a higher voltage rated motor (e.g. delta) with at lower speed and maximum available voltage allows you to get more torque out of it so keep the voltage up.

2\ The ratio of line to line and line to neutral is always root 3 (1.732) for a 3 phase system (five phase is root 5) so 230V/440V is not possible.

Robert G8RPI.

Thread: Stuck Chuck
07/01/2020 19:08:25

I know is obvious, but you have tied turning it both ways?

Robert G8RPI

07/01/2020 12:40:12

Steve, how is the adaptor initially installed in the chuck? Is it screwed in from the front (jaw) side or the back side? Is the section in the chuck fully threaded? I could assume its not fully threaded and goes in from the front , but assumptions are not a good basis for sound engineering.....

Robert G8RPI.

Thread: Watch servicing
06/01/2020 12:42:34
Posted by Howard Lewis on 05/01/2020 17:09:21:

This highlights my concern that a lot of traditional skills are going to be extinct.

<SNIP>

Witness the following for "The Repair Shop" programme on UK TV.

Howard.

I watched a few episodes of "repair shop" but had to stop. I have a couple of problems with it. First they are not passing on knowledge in particular details of materials used. e.g. a ceramics expert doing a repair with clearly two different adhesives but not a clue as to what type they were or why. Another using a "special solvent" on a motor in a plastic toy again with no details or even a warning that many solvents would destroy the plastic. secondly they show lots of poor practices, bodges, misuse of tools etc.

Back on topic my everyday watch is a 20 year old Breitling Aerospace (the old, compact thin titanium design not the modern chunky "jewellery" bought with my small part of the legacy of a relative who was an amateur horologist. The minute hand fell off last month. No sign of a broken staff so just need putting back on as far as I could see. As it was due a battery it was put in for a service and estimate (factory repair). This came back when I was out of the country for work and included repair, service, new crystal and case polish at just under £900. I would have said no thanks and done it myself but SWMBO said OK. Guess that's my birthday present sorted.

Robert G8RPI.

Robert G8RPI.

Thread: Ebay being clogged up by certain sellers
06/01/2020 07:30:40
Posted by peak4 on 06/01/2020 00:26:36:

Old Mart, what was it you were searching for?

I do appreciate the problem as sometimes when I seek a particular item, one seller includes dozens or even hundreds of the same item.

i.e. I look for a generic item for my motorcycle and one particular product is listed dozens of times by the same seller as xxxxx-Honda, xxxxx-Honda CB250, xxxx-Honda Xl500 etc etc. It takes forever to scroll past to find the next vendor selling a different product.

Bill

Report them to ebay. They are not supposed to list multiple copies of the same item. There is a limit of 10. There is a "Report item" link on every listing. This comes under listing practices.

Robert G8RPI.

Thread: Watch servicing
05/01/2020 14:06:23

The bigger problem is companies don't want or can't actually fix things. They uust want to replace "modules" or sell you a new one. This means owners of those items that can't be replaced are forced to pay high prices. Recent examples

My car has a worn bearing causing noise in 6th gear. It's a known problem (M32 'box). bearing are in the end cover so easy to change I talked to my local garage nd they won't do it. They will only fit an exchange garbox. I'll go bck and negotiate them re-moving nd refitting the gearbox with me taking it away and doing the bearing myelf.

A friend asked me to look at a Singer sewing machine that the local repair shop said was not repairable. I quickly found that the flying hook drive gear had broken up, This is a polyurethene gear with a metal hub. New gear cost £3. The problem is it is press fitted to the shaft flush to the bottom of the hook so getting the old hub off is an issue, but I just split it with a cutting disk in a dremel and pressed the new one on.

As to labour costs, near me there are a Jaguar and Ford Dealerships on the same site. The Jaguar labour rate is twice that of the Ford but a Jaguar X type is basically the same car as a Ford Mondeo,,,,,,,,

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