Member postings for Robert Atkinson 2

Hi Richard,

The item pictured with 3 contacts separated by grey (asbestos?) insulation panels is the contactor. The contacts don't look great. If you can hear this clunking with the start lever operation the PC40 is powering it.

Do you have a multimeter / test meter? You really need to get into some fault finding.

First thing would be to check the contactor contact resistance you can do this with it unpowered by manually closing it , pushing down on the brown (Tufnol) insulator that carries the moving contacts and then checking the resistance across each contact.

Robert G8RPI.

I seem to have upset some people with my comment about not buying from far eastern companies.

I did not say ebay or Amazon were far eastern or not to buy from them. I said "I would not recommend buying ANY mains powered device from any far eastern supplier on ebay, amazon bangood etc."
If you do, you are the importer and responsible for complying with relevant regulations. The VFD on ebay linked to by Brian H clearly does not meet CE requirements just from the photographs. It does not matter that you are not selling the unit or using it for business, you have to comply with the regulations. If injury or property damage results from the use of the item you could potentially find your insurance invalidated. I admit that this is highly unlikely to happen but it could. Apart from the legality issue, a large number of mains powered items sold by these sellers do not meet safety standards and many are downright dangerous.

I'm a professional electronics engineer and stand by my recommendation.

Robert G8RPI.

The lawnmower is the result of a stupid requirement in many ISO9000 quality manuals that all suppliers and subcontractors are ISO9000 approved. Many "advisors" who "helped" people get accreditation when ISO9000 first came popular used pre-made manual templates with "find and replace" for company name, business type etc. By requiring suppliers and subcontractors to be approved they made more business for themselves. First thing I changed as QM for a ISO 9000 company was to change this to "all suppliers and subcontractors shall be formally approved by the company" with a process that graded the criticality of supplier and level of review accordingly. If they had ISO9000 AND appropriate scope then we didn't have to audit them ourselves.
Got one at the moment with AS9100 (aerospace) approval where their scope is "design and make our widgets". As there is no recognised standard or specification for their widgits it is meaningless but as long as they follow their process they are compliant.

Robert G8RPI.

I would not recommend buying ANY mains powered device from any far eastern supplier on ebay, amazon bangood etc. I would definitely not buy a VFD from one.

You can have little or no confidence that it meets applicable safety requirements (Low Voltage Directive EMC Directive etc) or that it even meets it's specification. Buy a branded unit from a UK (or possibly EU) supplier. If nothing else you will have some one to complain to. If they don't have a proper UK address don't bother.

Robert G8RPI

Sounds about right size but even so I'd reccommend attaching it to the wall as security against it toppling, Either directly or by a strap or chain. Or bolt the feet to the floor.

Robert G8RPI.

I think we have gone off on a tangent...

I think Mick's issue is that the sight glass does on go LOW enough so there is still some water in the boiler when the glass is empty. This is of course the safe situation. If I'm correct then yes adding a float that sits on top of the water will give you some extra range equal to the height of the float above water. Some vintage aircraft used this technique, even going so far as to have the float completely out of sight with just an attached index wire showing in the tube. This also works for an overhead tank with the tube full of fuel and the wire sticking out the bottom of the tank and the float on fuel surface above.

The problem is that you have introduced a significant failure mode. If the float sticks you will run the boiler dry. Probably best to just put up with the reduced running time.

Robert G8RPI.

Did I religiously check the expiry dates on "consumables" when I was quality manager for an aerospace company - of course I did.

Did I then take it home a few days before it expired - well if I'd waited until it expired the company woyld have to pay to dispose of it as chemical waste

Edited By Robert Atkinson 2 on 12/11/2019 18:21:40

Those who say they cannot see how disconnecting a running motor will damage a VFD do not understand high power electronics. When the current though a coil (inductor) changes there is a voltage developed that is proportional to the rate of change. For disconnecting a motor, that large motor inductance can cause enough voltage to case arcing at the disconnect contacts (and possibly feed back into the drive causing issues) but more importantly for drive damage the inductance of wiring and even transistor internal connections on the drive side can cause voltage spikes high enough to cause damage. Yes you can design to account for these but don't bet on a cheap no name inverter having such protection. This does not mean every drive will fail the first time you disconnect it the magnitude of the spike and ability to cause damage depends on (among other things) the mains voltage, load, speed, temperature and most importantly the exact timing of the disconnect in relation to the load current.

I note that the drives direct advert that was linked to says they are "special" in respect of switching loads and also that they need to be rated at FiIVE to EIGHT TIMES the actual load. So for a 1hp motor you need 5 to 8 HP inverter with presumably a suitable input supply. Most electronics will take mose abuse if you only run them at 20% of their design rating.

Robert G8RPI.

For cutting I'd suggest a diamond core drill rather than a hole saw. Difficulty is holding it in position. Make sure the drill has a clutch, if the bit snatches it will break your arm without.

As an alternative could you cut a slot in a toothed pulley, slip it over and clamp on and then use a belt to turn the pulley? To be fancy a slot half way through on one side could engage with one side of the valve handle. maybe even a shaft and drve pulley across the top of the "wall"

Robert G8RPI.

Edited By Robert Atkinson 2 on 09/11/2019 16:40:19

That application makes some sense, being a secondary interlock after the motor has stopped and VFD output is off. It's not putting an E-Stop in series with the motor. Probably more cost effictive than using a safety rated VFD.

Robert G8RPI.

I've never seen a switch between a VFD and motor on an industrial installation. Can you give an example?

As stated putting any switch between the VFD and motor risks damaging the drive. Apart from this most E-Stops are not adequately rated for this.

Havig a faulty system that starts when it should not is not a got reason for fitting a switch in the motor. The root cause should be identified and fixed.

Robert G8RPI.

Edited By Robert Atkinson 2 on 08/11/2019 12:45:39

The problem is that you say you have taken that first "Unsafe action" - removing the guard- in your workshop. You also seem to be advocating removing guards to others.
Assessing the suitability of guards should be part of the machine selection process. If it's not suitable get the vendor to fix it or look elsewhere.

Generally in industry it is insurance assessors that drive good behaviour regarding guards and the like. If they refuse cover you can't legally work (at least in the UK if you have employees or public access to your business) Note that even in the home workshop removing guards could have accident or life insurance implications if the worst happens and they find guards were removed. Not very likely but some insurance companies seem to be looking for any excuse to reduce a settlement or increase a premium.

Robert G8RPI.

OP has said they are cutting discs, not grinding. Still need to know what is being cut and speed expectations and why 400RPM. If it to reduce heat then faster cutter speed with slower feed might still be a better choice. The heat put into the work piece (opposed to at the cutter interface) is proportional to the rate of material removal, which includes depth of cut, cutter speed and feed rate. If work piece heating is the issue and it's a dry cut, look at it's mass and thermal capacity of the materiel and the maximum temperature you want. Basic physics (how long does it take to boil a litre of water?) will tell you the maximum time you can cut it for a given power input (torgue and speed) This will give an idea of the maximum motor power and feed rate. It will be conservative because of losses etc but will give an idea if the motor is too big for your limits. It the feed rate turns out to bee far too slow you need to rethink e.g. wet cutting.

Robert G8RPI.

Hmm,

You don't need to know the power of the motor, you need to know the torque required to turn the disk. This will depend on what you are cutting and how fast you want to cut it.

More information please.

Robert G8RPI.

I suggested just a replacement SSD as I got the impression from the original post that Doglas was not a big PC user. I addd the caveat "if you are not planning to buy any new software". It's easy for computer "nerds" to forgetf a machine is otherwise OK and powerful enough to run the existing software does not have to replaced. A faster disk and "tune-up" can work wonders.

Robert G8RPI

The wiring regulation (BS7671) are not applicable to this (or the law, you can in theory show alternative means of compliance, but good luck trying). The quoted text applies to electrical isolation, not machine E-Stops. The mention of machines is to allow the input to motor drive to NOT be isolated from the supply so it can stop the machine more quickly.

If using a VFD (inverter) with a separate E-Stop input (and you should be if the machine needs an E-Stop) that is what you should use for the E-Stop as it stops the machine in the shortest possible time (loose chucks are a separate hazard and should be dealt with accordingly). Use the Stop / Start inputs of the VFD for just that and connect a proper isolator switch (Rotary red and yellow with provision for a padlock in the OFF position) at the VFD supply input.

A No Volt Release (NVR) should not be required with a VFD because you should set it up not to power the motor on power up or when the E-Stop is released. (This is normally the default but can be bypassed when the drive is used under automatic control)

E-stops should not be used for routine stopping of the machine but should be tested regularly.

If you want to get legal, then there are a host of assessments to do and all safety components must be designed and approved for the application. Most Hobby machine tool safety provisions, though CE marked, would not pass a through assessment. The NVRs with flip-over E-Stops are a particular example.

Any E-stop is better than none and you should NEVER EVER use the mains plug as the only means to start or stop any machine.

Robert G8RPI.
(CEng and have done this stuff for a living on machines to CE / UL & C-Tick).

You should use the E-Stop function of the inverter if it has one. Almost all of them do. This will switch off the invertor output instantly and with most units provide at least some dynamic braking where inverter "shorts" the motor. On simple inveters this is limited due but some have positive braking either by dumping the energy or injecting DC into the motor.
There is one big problem with putting the E-Stop in the input supply - stored energy. The inverters convert the incoming AC to DC before inverting it back to AC. As part of the AC to DC conversion there is a filter capacitor that stores energy. Sometimes this is called a DC bus or link capacitor. If you just cut the supply the motor will continue to run on the energy stored in the capacitor. If the inverter is lightly loaded compared to it's maximum rating the machine can continue to run for a significant time on this stored energy. It will certainly run longer than if you use the E-Stop function.

Robert G8RPI.

Edited By Robert Atkinson 2 on 06/11/2019 20:43:32

It should be noted that while (in the UK at least) you can do what you like in your own home workshop, if you are in business or an employer (even casual) you have to comply with the regulations which includes guarding.
Guards may be inconvienent at times but not as musch as missing part of a finger or worse.