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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: 3 phase supply (again)
14/01/2022 16:54:13
Posted by Andrew Johnston on 14/01/2022 10:34:06:

A funny thing happened just now, I've had to sit down to recover. I agreed with the whole of a post by SoD. smile

There is only a current surge when starting an induction motor direct offline because full voltage is applied with the motor not turning. A VFD should control the applied voltage to keep the current within the set parameters. There should be no need to select a VFD of four times the rating. A little over the motor rating may be helpful if using a speed/over-current tradeoff to overcome short terms overloads when running.

My understanding of digital phase converters is the same as SoD. They create two phases from the incoming supply and create a third phase using a half bridge and PWM drive, which is then filtered to provide a sinewave output.

I've got a couple of VFDs on my CNC mill, but for the rest of the workshop I solved the problem once and for all by installing a 3-phase supply. This is particularly useful as some of my machines have two speed motors, and star-delta switching in one case.


It's unusual but I have to disagree with you a bit Andrew,
VFDs are intended to be permanently connected to a motor and control start-up currents when they are commanded to start the motor. Even if set to maximum speed at turn-on there is some ramp up. Some also increase the current limit during ramp up at maximum rate. If you turn the VFD on and then switch a load onto it the only protection is the over-current. This is a fault condition and exceeding it will trip the drive off-line. Thus it has to be set high and may also be part of the 4x oversize recommendation. Of course setting the current limit high means the motor is no longer protected against over-load.

Starting a smaller motor first has also been mentioned on this thread and even running an un-loaded motor permanently connected is suggested by some sources. Not very efficient.
I'm not making this up I've had a serious look at and experimented with using VFDs as 3 phase converters. Just the amount of filtering required to tame the emissions when running off load was a show stopper.

Robert G8RPI.

13/01/2022 22:08:39
Posted by Clive Foster on 13/01/2022 12:28:31:


Apart from Phase Perfect and similar devices which are built as industrial 3 phase utility replacement devices with corresponding costs, all these boxes are based on VFD internals so stepping up to 60 hz should be no problem. Certainly the (expensive) ones from Drives Direct will do the deed.

Have to accept that 220 to 380 /420 volt boxes inevitably loose voltage and overload current headroom when compared to normal same input and output voltage devices. But the 50 to 60 hz change will have no noticeable effect.


Edited By Clive Foster on 13/01/2022 13:12:27

Hi Clive,

I don't need to ring them. The simple fact that they do not publish a datasheet, never mind an instruction manual on their website means I would not even bother.
Almost certainly the units pictured do not comply with UKCA electrical safety and EMC (interference) regulations. If they have modified them then they will have to re-qualify them.
I'm not too sure about drives direct either.

Robert G8RPI.

13/01/2022 21:44:10

You are missing some things. While using a VFD as a phase converter may work, it is far fom ideal. Apart from the start-up load issues switching loads on a VFD can generate significant voltage transients. These can damage the drive just over-sizing the drive is not good enough to prevent this. VFD's are also not intended to have long leads as would be required if wiring to several machines. This can daamage the drive and will also radiate excessive interference. Almost all VFDs have limitations on powered connection and disconnection of loads and on the length of leads on the VFD output.. These should be observed. Use one VFD per mchine.


VFDs are popular because they allow control of the speed, torque, ramp speed (accel and decel) etc but cost litttle more than a phase converter
VFDs do however require one drive per machine.
Rotary phase converters are completely out of date and very inefficent. Not a minor consideration with the rising cost of electricity.

Robert G8RPI.

Thread: Packing
11/01/2022 12:54:57

I would not use chipboard, old or new. It can have a lot of abrasive material in it. Kitchen worktop routers etc use carbide cutters for good reason.


Thread: LP gas tank construction
10/01/2022 21:35:07

Another consideraton is low temperature. The liquid gas will cool was it evaporates. Many materials, including many steels become brittle at low temperatures.

Thread: Three phase reversing
10/01/2022 20:34:40

DO NOT reverse the the motor switch with the spindle moving.
Doing so is likely to cause mechanical and electrical damage.


Thread: Highway Code
09/01/2022 13:52:57

Another rule often overlooked or not known about is the speed limit for vans, pick-ups and minibuses on a dual carriageway in the UK.

Anyone who thinks it is 70MPH needs a refresher. The speed limit for commercial vehicles on a dual carriageway is 60MPH. That's not just vehicles over 7500kg. There are exceptions. The main one is for "Car derived vans" but it has to say this as he body type on the registration document of the vehicle. There are also exemptions for Dual-purpose vehicles. Dual-purpose vehicles include 4WD and "crew-cab" types with two rows of seats. However the max GVW is 2005kg. Again it depends on what it says on the registration document. This makes pick-up trucks difficut to judge. A 2WD 2 or 3 seater is limited to 60 MPH otheres may not be. The curent boom in pick-up trucks is mainly due to their low taxation as a company vehicle.
The typical white "transit" is limited to 60 MPH on a DC and 70 MPH on a motorway. Over 7500kg is 60 MPH on both.

Robert G8RPI.

05/01/2022 21:06:45

Sorry, can't agree on the cycle lanes. Around here (Cambridgeshire) they have laid new ones with flush LED lighting the lot and some cyclists still insist on riding on the road. One in particular is on a unlit country road and they won't use the LED lit cycle path. Then there are the bus drives who won't use the bus lanes. I've had one pull out of recessed bus stop, come across the bus lane and cut me up in the traffic lane. He then proceeded to do about 20 in a 30 zone. And n there weren't any ikes in the bus lane. There is another one where there is a traffic light cotrolled junction where buses go straight on. They have bus lane with it's own light that changes in their favour automatically. This stops the straight on main traffic so they can join. They use it when it's busy, but if its quiet and the main light is green they pull out at the lst minute into the main lane. Thay have already tripped the bus lane sensor so the main traffic lights recycle for no reason. If you are waiting to yurn right you have to wait a full cycle of the lights. that can be 2 minutes or more. I've been held at the line for three cycles due to two used doing this.
I can understand a bit it if drivers won't let them join when the bus lane ends, but it works both ways.
I also can't understand the number of cyclists and car drivers (ex cyclists?) who drive through red lights in Cambridge. At the same junction I mentioned above I had a cyclist ride into the back of my car when I stopped at a red light (two cars in front went through on amber) going straight on. When I asked the cyclist if he was OK he said he had expected me to jump the light! He could of course have used the bus lane, it has a permanentgreen for cyclists to go straght on. Again the same junction I stopped on red when turning right (again two cars in front went through on amber). A pick-up truck behind me actually stopped, then pulled left into the straight on lane, passed and turned right in front of me. He managed rude guesture at the same time.
No I'm not perfect driver, but I try to obey the rules.

Roert G8RPI.

Thread: Paranoid about Android
02/01/2022 22:23:53

Much as they would like you, and ask you repeatedly in several ways, you do NOT have to share or Sync your contacts with your Google account. You also don't have to log into your Google account to access the Play store. Just continue without agreeing. I just did all this with a new device I got for Christmas.

Thread: VFD - which is best please ?
02/01/2022 20:46:42

My concern was an installation where not even the filters required for industrial levels were fitted. Not a compliant factory installation.

Robert G8RPI.

02/01/2022 19:04:04

Steviegtr said:
" As for the home hobbyist needing to buy filters for the input i would say no. Not required. As an example I have a Dab radio sat on a shelf directly above my inverter, which i always have switched on. Never bats an eyelid "

Why did you say this? Are you just trolling?

EMC is not just about you. It's mostly about affecting others. How do you know that you are not interfering with aircraft navigation? Emergency communications? For example can you be sure that if an ambulance parked outside your VFD would not stop their datalink system or upset their ECG machine or other equipment?
You CAN'T.
As a minimum for your installation you might have done a quick sweep with a spectrum analyser but I doubt it. VFDs are typically used in an industrial environment and any internal filtering or external filtering recomendations are based on the allowable emissions for industrial. Hobby lathes are used in domestic environments. The allowable emissions for this are typically 1/10 of industrial (10dB lower. Exact levels depend on standard used, frequency etc). So to acheive the legally mandated emissions for home use methods such as additional screening and filters wil be required above those stated in the manual.

Suggesting even the minimum filtering is not needed is just irresponsible.

Robert G8RPI.

01/01/2022 15:43:25

I will not be posting pictures of my workshop as it is far from pristine and I would not want people to think what I do in my personal workshop is an examples of good practice. I have enough training and experience to assess the risk to me and put appropriate mitigation in place. My acceptable risk to myself is higher than I would expose others to either directly or by giving advice.
I will post pictures of my ML2 and it's new drive system when i get round to doing some more work on it. I've never been a "crop duster" but have designed and built systems, including motorised flight controls, for a British turboprop agricultural aircraft.

Happy new year and stay safe,

Robert G8RPI.

31/12/2021 22:11:53

I watched the whole video. The bit of old shelf or whaterver it is acting as a "swarf guard" does not protect the VFD. Indeed it appears to be putting additional loads on the VFD wiring.

As I've said before, you can chose to do whater you wish in your workshop but please don't recommend unsafe practices to others who may not have enough understanding of the risks involved. If, as you imply, you have done this sort of work as part of your employment you should know better.

Robert G8RPI.

31/12/2021 19:05:59

David J and Mike P. Thank you for the posts. Saved me saying it.
I hate it when people make unsupportable or unsafe statements and then respond to being "called" on it by applying impractical conditions.
I don't really care what people do in their own homes. I do care when they give unsafe advice to others or worse, challenge the safe advice given to others.

There is another side to all this. At some point there will be an accident and the "ambulance chasers" or an insurance company will come after either the supplier of substandard kit or someone who gave poor advice. We all have moral and legal responsibility for athe advice we give. Additionally I have professional responsibility.
Far fetched? Maybe, but lawers and insurance companies are always looking for new ways to make or save money.

Robert G8RPI.

31/12/2021 12:45:59

I was trying to avoid this thread but I have to respond.

Emgee. A VFD mounted without an englosure is unsafe / unacceptable for a number of reasons. Firstly as Ian McV says the IP rating is not suitable for a workshop environment. Secondly the cable connections generally do not have adequate protection from contact or adequate strain relief. Thirdly the installation manual will (on better drives at least) tell you it has to enclosed. The safety and approval of the drive depends on following the instalation requirements. Note that these drives are components not equipment. They have to be correctly incorporated with other item to make a finished system. The supply of them assumes that the equipment they are incoprorated into is designed and constructed by competent persons. Note that this does not include the average electrician. In theory the completed equipment should be inspected and tested for compliance with the machinery, electriacl and EMC safety directives. Few people do this even for commercial kits.

Steviegtr. I've said this before. The installation in the video you linked to is a safety horror show. Exposed VFD, multiple adaptors and extension leads including an obviously illegal 2 pin adaptor. This i the grey one in the red extension lead. All this exposed to swarf and coolant. And to make things worse you have to reach across the lathe to reach the controls. I see no E-Stop on the operator side of the lathe either.
do whatever you want in your own workshop as long as no one else uses it but please don't put it forward as a good example. It is not. A bit of Dunning-Kruger effect maybe?

As Andrew J says there is no correct answer to the original question. ANY VFD choce will be a compromise. I'm sure Andrew was going to say this in his next installment but the VFD should adjust it's output voltage with frequency to maintain the rated current and torque. Not setting this correctly is one cause of poor low speed performance. Of course even if you maintain the current and torque halving the speed halves the power.

I do not like the cheap far eastern drives sold on ebay and the like They are generally poor quality. Most of the ones I've looked at are clearly not UKCA (CE) compliant just by looking at photographs and reading manuals. Saying that they are "made by omron" or what ever is just nonsense. It's been discussed for mechanical items that thes low cost manufacturers will build to the cost / quality the customer wants. The same applies to electronics. The cheap ones leave out parts not essential to basic operation. This includes interference (EMC) limiting components and over current / voltage protection parts. Even the basic electronic components from reputable OEMs are variable cost / performance. Capacitors are a classic case. You my have a basic requirement of 100uF 350V but do you want 85 deg C or 105 deg C temperature rating? 1000 hour life or 10,000 hour? Al else being equal a 105 deg 10,000 h capacitor will last much longer. And this is without basic improvments like de-rating for voltage and current.
Also be wary of sellers who recommend using over-sized "cheap" drives. Ask yourself why the "XkW" drive isn't considered good for XkW. The rating of a drive should be for continuous commercial service at the top of it's rated temperature for years. Such a drive should last much longer in hobby service.

If you have to ask the question of what drive to buy you probably should be buying a preconfigured kit or paying a competent person to do the work for you.

This will not be a popular answer but it is factually correct.

Robert G8RPI.

PS must type faster several other comments came in while I was typing.

Edited By Robert Atkinson 2 on 31/12/2021 12:48:19

Thread: Suitable Metal for Electro-magnet Levers
29/12/2021 15:32:57

Use steel. If there is not a lot of force to pull it off put a thin nonmagetic (plastic aluminium etc) shim on the electromagnet to maintain a small gap. I'd expect maxium force in the contact positionfor you application so shim should not bee required.

Robert G8RPI.

29/12/2021 12:54:08

Do your "solenoids" have magnetic cores? I read the original post as they had. If they have fixed magnetic cores technically this makes them electromagnets, not a solenoids. A solenoid has no fixed magnetic core and the moving part is normally Iron. This works because there is nothing ferromagnetic for the core to stick to if it retains magnetism. The moving part attracted to an electromagnet, the armature, is normally made from a material thati s harder to magnetise. This is typicall steel.
I think this subtle distinction between soleniods and electromagnets has caused some confusion as to a suitable material for the armatures.

Robert G8RPI

28/12/2021 13:28:15

Some supporting data from Eclipse

" The armature plate may also be known as a keeper plate. An electromagnet or electro-permanent magnet always needs a ferromagnetic surface to clamp onto – the ferromagnetic surface required is generally a mild steel or ferromagnetic stainless steel. "

Robert G8RPI.

28/12/2021 13:09:46

Note that iron is more easily magetised than steel. It is also more easily de-magetised which is why it is used for transformers. As the holding electromagnets are DC they will not demagetise the armature (moving part). This will eventually result it it sticking even with the electromaget de-energised.

The armature should be STEEL not iron.

Robert G8RPI.

28/12/2021 11:12:50

Steel. Mild is OK but when doing this sort of application in the day job we used gauge plate. We had it electroplated but the environment was fairly agressive. Don't go too thin, the force will be reduced.
Iron will get magnetised.

Robert G8RPI.

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