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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: VFD on a Boxford lathe
25/05/2019 05:05:18

To add to Andrews excellent response, this means that best use of a VFD is to run the motor faster than the "50Hz" spee but at reduced power and current for small parts and keep the belt ratio high so you maintain power at low speed for larger workpieces. Obviously you need to make sure you do not overspeed the bearings or rotor particuarly if its an older motor.

Robert G8RPI.

Thread: Childhood diseases
23/05/2019 12:48:59

Well it’s clear that the Americans who won’t vaccinate their children have caused a lot of trouble. I’m of the view that a bit of exposure promotes the immune system. That’s how vaccines work after all.

Robert G8RPI.

Thread: Mini-Lathe setup for an absolute beginner?
21/05/2019 18:47:02

Hi,

If using in the same room as electronics beware of getting swarf into it. Little bits can fly quite far. As long as top ventillation holes are under a shelf or similar it should be fairly safe though.

Robert G8RPI.

Thread: Larger VFD/Motors
20/05/2019 21:31:18

Andrew has explained this perfectly. I do note that SoD's calculations look bit suspect. For starters it's normal to use peak to peak rather than peak. More importantly, the root two factor only applies to pure sinewaves.
The 13 amp fuse will not be stressed at all by 18A at a couple of milliseconds. The main effect is thermal and this will be averaged by the mass of the wire and the sand packed around it. In fact a 13A BS1362 fuse will not blow when passing 20A (RMS or DC) for long periods of time (hours). It will take 100A for 10ms without blowing.

So a 13A plug fuse will not blow under this load and a 5A one would probably be a better choice as it will actually blow under sustained load. You might get failures if you try to start the motor under load though. An MCB needs to be higher current rating or/and B orC type as they operate on magnetic attraction so are much more sensitive to pulse currents.

Edit: found a good explanation of 13A fuse rating https://www.pat-testing-training.net/articles/fuse-operation-characteristics.php

Robert G8RPI.

Edited By Robert Atkinson 2 on 20/05/2019 21:31:45

Edited By Robert Atkinson 2 on 20/05/2019 21:32:58

Thread: What Did You Do Today 2019
19/05/2019 19:19:31

I went to the Dunstable Downs amateur radio boot sale. The only thing I was looking out for was a UHF signal generator. Typically I didn’t find a signal generator but did come away with an arbor press (BNL No.1 MP1), a Sorba HV5 5” rotary table and a 12” APE microball micrometer height gauge. The press seems better made with a round rack rather than the square common on cheaper modern ones. I’d not come across the microball height gauge before. It seems quite nice and the size suits my small surface plate. These were all items on my shopping list for the workshop so at £60 I’m pleased even though I didn’t get a sig gen (I did get some other electronics wink)

Robert G8RPI.

Thread: Adhesive Storage?
18/05/2019 09:00:37

Virtually all non water based adhesives will last longer if stored in a fridge or freezer. Locktite and PVA should not be frozen. datasheets available online often give advice or at least a minimum temperature. Aero space sealents that are normally two part are also supplied pre-mixed and frozen. Annealed 2017 and 2024 alloy rivets stay soft longer if stored in a fridge too.

Thread: Electronic switch forward/reverse/stop help
17/05/2019 21:01:30

Hi,

The new switch is not suitable as it does not have a no volt release coil. This is a safety feature and it is likely that the emergency stop will not operate without it.

Robert G8RPI.

Edited By Robert Atkinson 2 on 17/05/2019 21:05:24

Thread: Parkson M1250 Beast
15/05/2019 12:51:33

Where are you located? maybe there is someone nearby with electrical / control knowledge who could help.

Robert G8RPI.

Thread: North West 200 coverage
13/05/2019 22:11:48

That brings back memories. 40 years ago I was a fairly newly licenced radio ham and a member of RAYNET covering the races providing communications for the Red Cross and St Johns Ambulance. I was at the last first aid post on Quarry Hill when Tom Herron crashed. 1979 was a bad year, three riders died. I left NI a couple of years later.

Robert G8RPI.

Thread: A dynamo question: rotation direction?
12/05/2019 16:33:36

Mid Wales is a bit too far I'm afraid.
Any imbalance or residual magnetism will cause it to rotate. Not much torque though.

Robert G8RPI.

12/05/2019 15:11:02

Where are you located? I could come over - seriously.

I like a challenge. One motor problem I had was a machine with a geared squirrel cage induction motor, The gearbox output was running backwards after somene "fixed" it. After spending time studying the gear box I finally reallised they had flipped the coil and stator over (not the whole motor, think about it) wich caused the shaded pole to be on the other side and the motor to run in reverse

Robert G8RPI.

Thread: Collet blocks
12/05/2019 12:32:34

One option to get around keeping a credit card account is to use one of the pre-paid cards that are available from places like supermarkets, normally on display with gift cards. You do a pay a premium and may be left with a unusable balance that can expire but you don't give out your bank details. Most you can loose to fraud is the balance on the card. Also useful for recurring subscriptions that are hard to cancel like gym membership.

Robert G8RPI.

Thread: A dynamo question: rotation direction?
12/05/2019 12:19:12

Some info from contemporary literature

 

dynamo1.jpgdynamo3.jpg

dynamo2.jpg

Images in my Album.

Robert G8RPI.

Edited By Robert Atkinson 2 on 12/05/2019 12:21:23

12/05/2019 11:14:25

Reversing either the field OR the armature (brush) connections will reverse a "universal" motor which is what a Dynamo in motor mode is. Are you sure you actually reversed only the field? Does the dynamo produce plenty of torque when motoring? you might have only reversed on section of the field winding or wound / connected one or more section in the wrong direction. "Flashing the Field" produces a residual field of fixed polarity that makes the dynamo self starting in the correct polarity. If for instance there are two field coils and you would one in the wrong direction or connected one backwards the two electic fields would cancel out but the dynamo would still draw a large current and possibly turn on the residual permanent magnetic field. The torque would be much lower though. If a field s open circuit the ame would apply but current would be lower.

Direction of rotation does make a difference in two ways:
1/ Field distortion. The rotating and fixed fields interact changing the neutral position of the brushes and causing sparking at the commutator. Often the brushes are offset to compensate. Reversing the direction of rotation increases the offset and makes sparking worse. Compensating windings, or on old desigins moving brush holders, are also used on big machines to acheive the same effect over a range of loads.
2/ Cooling fan will run the wrong way reducing efficency.
These may not apply to a small Austin 7 dynamo of course.

Dynamos are still used as starter-generators on small turboprop and turboshaft engines on aircraft.

Robert G8RPI.

Thread: H2O2?
11/05/2019 11:41:01

Light, particuarly the shorter wavelengths, will speed the decomposition of hydrogen peroxide so it should ideally be stored in opaque or light filtering containers. The online sellers are being cheap. If kept in a dark place it makes no difference.

Robert G8RPI.

Thread: E.stop wiring
11/05/2019 11:33:41

That looks like the original nuclear reactor SCRAM (shutdown) system. Allegedly the first reactor CP-1 had a control rod hung from a rope and the plan was to cut the rope with an axe if the reaction ran away. They still call a emergency reactor shutdown a SCRAM but there is no longer a Safety Control Rod Axe Man

Robert G8RPI.

11/05/2019 10:00:01

Hi Mike,

I agree most industrial installations would have the E-Stop in the control line, but most industril machines will be 3 phase and higher current. If we were talking about using industrial quality contactors and switches from reputable manufacturers and suppliers I would be happier with just control line E-stops. However we are talking cheap all in one NVRs made in the far east and I have little faith in their reliability. There is virtually no cost or difficulty impact from putting one E-stop that is close to the NVR in the two phase power input before the NVR. Any additional E-stop(s) can be in the control (holding coil) circuit to simplify wiring and not have power on the wiring. My reasoning is if the NVR does not release when toy press the button you will go for the nearst E-stop which will work 100%. If a remote E-stop does not work you will again go back to the NVR and the primary E-stop. This may seem like overkill, but why not when it is very little cost in time or effort that may save a work piece, machine or bodily harm some time in the future.
One thing is not clear on my drawing, I say the input comes from a CB. Obviously ther shoyld be some kind of isolator between the power source and first E-stop. For single phase machines this would normally be a 13A plug. For larger machines either an industrial plug and socket ot a wall mounted isolator switch.

Robert G8RPI.

10/05/2019 19:53:47

This is my recommended circuit for a KDJ17

nvr-1.jpg

It is fine for anything up to a few kW. 0.5mm "twin red" lighting cable would suit the wiring to the secondary E-stop(s)

Dave, I think it is quite reasonable to assume that mot of the KDJ17 NVR's sold on ebay are for replacement purposes rather than for manufacturing. For 3 phase you do have to use contactors etc but for safety critical circuits on production equipment you have to use suitably safety rated components. https://uk.rs-online.com/web/c/automation-control-gear/machine-guarding-safety/safety-contactors/

Robert G8RPI.

Edited By Robert Atkinson 2 on 10/05/2019 19:54:04

Thread: Larger VFD/Motors
10/05/2019 18:36:24
Posted by SillyOldDuffer on 10/05/2019 14:12:01:
<SNIP>

Robert, have you never used an oscilloscope on a hefty valve radio transmitter? My scope probes are suitable. The Gnd is the signal ground, and unless there is an earth fault, there is no difference between earth and neutral. Popping the RCD, or damaging the instrument, or electrocuting the operator whilst using an oscilloscope to measure mains voltage is a new one on me!

But as explained, Plan A isn't how I'm going to do it. Andrew suggested a current transformer, £4, which I've ordered. No need for an expensive differential probe, and no direct connection to the mains. The disadvantage of the current transformer method is it might not have the frequency response needed to catch spikes and it can't give accurate current measurements without me doing some sort of calibration. Nonetheless, I'm hoping the results will be sufficiently indicative. Unless EMC wrecks the experiment; the waveform close to the lathe could be dominated by VFD switching transients.

The question I'm exploring is: 'What current waveform is drawn from the single-phase supply by a VFD/3-phase motor combination when they are first switched on.'

I'm interested in the current profile over the first few seconds. With my lathe, I'm expecting to see a largish current, about 20A peak lasting a few hundred milliseconds, after which the current steadies down to a much lower level. Thereafter I expect the average current to be entirely proportional to the load on the 1.5kW motor, couple of amps idling, rising to 6 or 7A working hard.

Part of the fun is checking theory with an experiment. Double fun when the experiment itself is challenging.

Dave

Hi Dave,
Yes I have looked at valve transmitters (2x4CX250) and similar with a 'scope. I use a Tektronix P6015 or HP 1137A EHT probe for that sort of work. For high energy e.g. connecting to mains, I use a Fluke ScopeMeter which has fully isolated inputs and properly rated, fully insulated probes. Using "normal" 'scope probes on mains is not immediately hazardous, but its certainly not safe. I've seen a scope lead go up in flames during this type of connection (no it wasn't me connecting it) The 'scope was destroyed by the current through the ground traces on the PCB. Connecting the 'scope common to neutral connects it to ground which will in almost all cases trip an RCD if fitted. The earth and neutral only need to be a few millivolts apart to drive the 30mA required to trip a typical RCD.

You may be surprised by the current waveform you see at the input to the VFD on load. If it is a non power factor corrected drive you will see narrow high amplitude spikes, not a clean sinewave current. These current spikes are "topping up" the charge on the DC capacitor.

10/05/2019 12:25:08

At CAT III 1000V and 80M input impedance that differential probe is effectively isolated and is ideal for the job in conjunction with a current clamp.

Croc clips are a bit dodgy though.

Robert G8RPI.

Edited By Robert Atkinson 2 on 10/05/2019 12:26:06

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