Member postings for Andrew Johnston

Superficially a worm mating with a helical gear, and involute tooth form. However, the width of helical gear is quite small so I suspect the teeth are simply straight cut at the nominal helix angle rather than being cut on a helical path.

Andrew

I believe that Tom said in another thread that he had a 3-phase supply available.

Andrew

Not sure losses are the main problem. At the generator the output voltage and current will be in phase. So the generator creates an amount of real energy X. The old style electricity meters (with the little rotating disc) only measure real energy consumed, ie, they take into account the power factor. So if the customer has a power factor less than unity he only pays for the real energy consumed, Y. The generator is paying to create X amount of energy but only charging for Y amount, where Y<X. Equals a hole in the bottom line. Modern electronic meters measure power factor as well as voltage and current. What is done with it is up to the electricity company. As SoD says I expect that for the ordinary consumer it makes little difference. Unless one has a garage full of machine tools with large motors.

Andrew

An inductive load isn't a load, strictly speaking, as it doesn't dissipate any power. Many years ago when I was designing VFDs for use as electric vehicle drives the company I worked for had the idea of using the VFD as a power factor corrector for industry. The mains connected to the VFD motor terminals and the battery was replaced by a capacitor bank. The software then controlled the capacitor bank to look like a varying capacitative load, cancelling out the normal inductive load of the factory and ensuring the power factor was unity. The unit didn't consume much power although the circulating currents in the capacitors could be large - hundreds of amps. Personally I though this was a better bet for making money than electric vehicles, although in the event neither worked out and the company went phut soon after I moved on.

An induction motor will be inductive on the output of a VFD, but it's isolated from the mains via the DC link. So what the VFD looks like from the mains is determined by the input of the VFD, not the load.

Andrew

Ok, but it's the same test as Maurice did and it tells us the same, ie, nothing about the problem in hand. Just because the LED being used is advertised as 12V it's not necessarily going to go phut at 12.01V. The whole point of doing the test with the actual unit is that it tells us what it will withstand. If it goes phut at 13.2V (+10%), say, then there's almost certainly a problem. If it survives to 20V then probably not. Either way we have a fact that helps solve the problem.

Andrew

Can't immediately see where? The test needs to be done with the actual device being used, not something out of the spares box. A generic LED and resistor should behave exactly as Mauruce details - it's a crude current source. But we need to know if there is anything odd about the actual device.

Andrew

Agreed, although you won't get an accurate or stable 12V limit. Since we don't know what the problem is there's no way we can say the circuit has a good chance of fixing it. It's the multiple monkey approach to problem solving.

Yes, the OCP71 was an OC71 with the paint scraped off. Until the manufacturer got wise and started sealing the OC71s with an opaque resin. The original clear resin was reserved for the OCP71s, at a premium price of course.

Andrew

If that's the case then I agree that the Zener might do something, albeit not necessarily that useful. The problem with Zeners is that they're not overly precise, the knee is quite soft at low breakdown voltages and they have a temperature coefficient.

An interesting test which might help solve the problem is to put an example of the LED on a bench power supply and see what happens as the voltage is increased. That should tell us if there is a maximum input voltage before the LED goes phut.

Ignore this bit if you're not an electronics nerd!

Most Zener diodes are not in fact Zener diodes at all. The Zener effect (quantum tunnelling made possible by very high electric fields in the p-n junction) is only dominant below about 5.6V. Above that avalanche breakdown dominates. The Zener effect has a marked negative tempco while avalanche breadown has a positive tempco. Look at the tempco of the breakdown voltages and you'll find a near zero value at about 5.6V. In old school electronics (before sooper-dooper IC voltages references were available) a 5.6V Zener was often used as a reference. Or even better in conjunction with a forward biased silicon diode where the tempcos of the two components tended to cancel. That's why old school voltage references were often at 6.2V.

Andrew

Is it safe to assume that the LED in question only comes on when the sidelights are turned on? If yes all we can say is that the LED fails to come on when the sidelights are turned on. We can't say whether it failed when the car was started or when the sidelights were turned on, or somewhere between the two events.

Andrew

That's where I'd start. Identify the problem and then one can create a solution.

Andrew

Whatever you do; don't give up the day job!

The LED is the wrong way round. Even with the LED the correct way round the Zener won't do anything against positive spikes. If there's 12V across the Zener there will also be 12V across the LED and since current in a diode is an exponential function with voltage that's a lot of current. In practise the resistor or LED will most likely fail long before the Zener breaks down.

Against a negative spike the Zener will protect the LED as it acts as a normal diode with a forward voltage characteristic much lower that the reverse breakdown voltage of the LED.

Andrew

I've read this thread through twice and I can't see any evidence that points to the problem being ignition pick up. The di/dt and dv/dt of an ignition system can be quite high, but the energy is low. The only time I've had a problem with ignition pick up was when fitting thermocouples to a racing (F1) car. In fact they were a darn sight more reliable at measuring rpm than the "proper" rpm circuit.

The LED circuit is fairly low impedance so it is unlikely that enough current will be induced to cause the problem. There's an art to solving these sort of problems and the first step is to gather data without making any guesses at what is causing the problem. That's not as easy as it sounds. If one thinks one knows what the problem is then data collection is often biased towards verifying the expected cause. There are some simple questions we can ask:

If I've understood correctly the LED fails when the car is started? Is this repeatable, ie, every time the car is started a new LED fails.

Can the LED be powered without the car actually being started, and if so does it work?

If I've got it correct some other LEDs flicker when the car is started/running which is suspected to be due to variations on the DC supply. It would be useful to quantify those variations, although I'd agree with Tim that a 'scope is the only real way of doing so. An analogue meter will give some indications but will tell one very little about the nature of any variations.

One more general question; does the wiring for the failed LED run significantly closer to the ignition system than any of the other units fitted?

As an aside you've only got to look at the startups that have come and gone over the years, along with investors money, having had the wonderful idea of wireless charging ones 'phone/tablet/camera only to find that it's pretty darn tricky to reliably transfer significant power between randomly placed coils.

Andrew

Got it in one. Most, but not all, VFDs have a display that can be programmed. I've got two VFDs on my CNC mill, one displays frequency and the other rpm.

Personally if I was in your position I'd bin the VFD and use the 3-phase supply. It makes life a lot simpler, and gives you full power over the speed range without having to think about it. I assume that the lathe is a geared head? So It's fairly quick to change speed. Cutting speeds are not written in stone and are not critical. If you're somewhere in the ball park that's fine. To some extent the speed range needed will depend on tooling. If you use HSS tools then the low range of speeds will be fine. For insert tooling it might be a little low at the top end. Having said that my lathe goes to 2500rpm but I mostly use 800rpm and lower. For small work I might go to 1200rpm or rarely 1700rpm. I've never used the 2500rpm range in anger. If nothing else it's very noisy, which is a problem in a quiet village location.

Andrew

Simple arithmetic. You know the speed of the original motor and the speed of the spindle at each gear setting. So the gear ratio for each setting can be calculated. For any given setting of the VFD you know the output frequency. Knowing that, and the number of poles on the motor, you get the motor speed for each frequency, less around 5% for slip. The motor speed plus the gear selected will give you the spindle speed.

Andrew

They're the speeds for 50Hz and 60Hz. And the two currents are for 230V delta and 400V star respectively.

Andrew

Probably because the OP has 3-phase available and if he wants to run his industrial machine to capacity he'll be better off using the motor as designed rarther than an inverter. Power is more important than infinitely variable speed.

Oh, and welcome to the forum Tom.

If the lathe has a 2-speed motor I assume there must be an electrical switch somewhere on the controls for low and high ranges? Both of my machines that have 2-speed motors have these switches, two in the case of the horizontal mill.

Andrew

Which is no surprise as that's what one needs to generate 400VAC phase to phase 3-phase if one wants the "neutral" point to stay fixed. If the "neutral" can vary then one can get away with 565VDC.

Except for very low power VFDs I would think that a front end PFC is a simpler way of producing higher DC link voltages. The circuit is more compact and doesn't need two large flying capacitors and associated switches which may need isolated gate drives. A PFC has the additional bonus that it reduces the filtering required on the mains input to the VFD in order to meet harmonic limits.

Andrew

Standard 3-phase supply.

Andrew

There are two types of spiral tap, spiral point (aka gun tap) and spiral flute. They're for different applications. A spiral point tap pushes the swarf ahead, and is intended for thru holes. A spiral flute tap pushes the swarf back and out of the hole, and is intended for blind holes. Of course a spiral flute tap will work just as well on a thru hole, or by hand. Where possible I buy spiral flute taps - only one tap needed per thread size.

Generally I'm with DavidG. I prefer to use tapping heads; better torque and depth control, and much faster.

Andrew

I measured the current on one phase of my lathe motor this evening. First measurement was with the lathe on, but motor stationary. Current reading was 0.98A. That's partly the lathe control gear and possibly other things in the workshop. Next measurement was with the motor running but no load. Current reading was 1.90A. Lastly I took a reasonable cut in steel (0.1" DOC, 8 thou/rev feed and 540rpm) but nowhere near full power. As the cut came on the current increased to 2.84A. Funny sort of 5 to 10%.

Andrew