Do it because sure has Egg's is egg's they'll be out of stock when you want them.! . . PUSH THE BUTTON!!Quote:
Originally Posted by Rogue
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Do it because sure has Egg's is egg's they'll be out of stock when you want them.! . . PUSH THE BUTTON!!Quote:
Originally Posted by Rogue
Don't tempt me! I really, really shouldn't. If the drivers are out of stock then I can always move the ballscrew purchase forward by a month.Quote:
Originally Posted by JAZZCNC
House prices are rising, everything else will follow?
That is very possible but if an unexpected expense crops up and I've already cut into my "emergency petrol money", then I can't travel. If I can't travel, I can't work and can't invoice clients. That makes my bank manager sad and my wife furious :whistle:Quote:
Originally Posted by Swarfing
That's easy sorted.?? . . . .change both.!!Quote:
Originally Posted by Rogue
Now on another serious note.? if you haven't bought the screws yet then you don't want to be buying the electrics. Only buy the electrics when you need to fit them has just having them to spin on the bench is wasting the warranty and you won't learn anything from it.!!
Rouge the only thing i can say to that is what is more important you? On another site i saws omething that i liked, just came out on the market. I decided then and there to buy it. 6 hours later they doubled the price? and that was from China, go figure?
I know what you mean about work though it has to come first so it sounds like you have answered your own question and it will have to stay on hold. Hope you get there soon, bank managers are a PITA to please.
Wise words indeed, though (i) the steppers are already outside of a year old and (ii) in any event I've got 6 years to bring a claim for breach of SoGA; warranties do not do what most people think they do. Entirely academic as I'd not pursue that route for a hundred quid's worth of motors, and probably not even for three hundred quid's worth of drivers. Point taken, though, hassle is worth avoiding where possible.Quote:
Originally Posted by JAZZCNC
In fact, taking that on board, I might shift my plans around a bit. I'll still finish up the PSU first because it's nearly finished anyway!
If you have already bought the toroidal I say go for it, something else could crock the project and fixing the PSU would only waste money.Quote:
Originally Posted by Rogue
.
The problem with transformers is that when you rectify them to DC the voltage drops when you draw power. You can stuff capacitors in until the cows come home but if you don't rectify all 3 phases, the volts will sag.
.
Similarly the volts will soar if you get a mains surge so you really need to protect your drivers if they don't protect themselves. Fortunately protection is cheap and easy if you use a crowbar circuit and wire a diac to blow a fuse on the DC side if you go over volts. A crowbar circuit is cheap and simple. Leave yourself a note by the fuse because it is very easy to forget it is there even after it just saved you hundreds of pounds :rugby:
Quick question in relation to a soft start circuit on the 70V system.
In all the various circuits I've seen, there is some form of resistance inline before the transformer which, after a very short delay, is then bypassed. Many circuits also then isolate the resistors, though there seems to be various thoughts on whether this is really necessary.
My question relates to using a DPDT relay for this: when the relay activates, the first pole (NO) is closed, creating the bypass. The second pole (NC) which leads to the resistors is opened, taking them out of the circuit. In this arrangement, there is presumably the possibility that the second pole could open before the first pole has closed, so there is no power flowing at all for a very brief time. If this is correct, is this likely to have a significant impact given the timescales involved? Is any switching delay likely to be long enough to negate the effect of the soft start circuit?
If not practical then fair enough, but it would be neater to use one relay to do both functions!
If the rating of your transformer is less than around 600VA, I wouldn't bother with any soft start circuit.
Thermistor is an easy option, as it's resistance decreases with temperature so no real need to take it out of the circuit.
You can use a relay to short circuit the fixed resistor. Essentially you want to switch the power on, wait about a second, then switch the relay on. You've reminded me of a circuit I came up with when I was about 10 - put a largeish capacitor in parallel with the relay coil and charge it via a resistor, then wire the relay output to break the capacitor charging current and you end up with a simple oscillator! In your case you would just use the capacitor and resistor to make a 1 second delay, so calculate it such that it takes 1 second for the capacitor voltage to reach the threshold voltage of the relay coil. Remember to include the resistance of the relay coil in parrallel with the capacitor in your calculation. You can power the relay coil from the 70V output via a potential divider. It's a bit of a crude way to do it, but easy to understand. Bonus points for using a solid state relay.
It's 625VA. It might not be very useful but it's more useful than detrimental, I suppose.Quote:
Originally Posted by Jonathan
I understood the problems with thermistors to be the time needed to cool down again if they are not taken out of the circuit - if the PSU is shut off but restarted within quite a short time, the thermistor doesn't offer enough resistance to work as an effective soft start.
There's 24V available for such electrickery, though I suppose the more self-contained the system the better.Quote:
Originally Posted by Jonathan
Alternatively, use a zero crossing point SSR to power on the transformer, which greatly reduces any switch on surge from the transformer itself. And if you really find a series resistor necessary, just use a resistor bypass relay powered from the DC side. By the time the DC is high enough to power the relay, any surge from bypassing the resistor should be pretty minimal.
I'm reading everywhere that zero crossing is the worst thing with regard to inrush.
You would be fine with 625VA and no soft start circuit, so if you restart the PSU with the thermistor still hot the current is still going to be less than without anything, so either way it's not a problem.Quote:
Originally Posted by Rogue
Reference?Quote:
Originally Posted by EddyCurrent
I used that strategy recently to charge a 0.06F capacitor bank from the mains via voltage doubler ... nice and simple. Pity what it was powering blew up.Quote:
Originally Posted by m_c
Not when starting something like a transformer. A mains powered transformer essentially has a zero current point 100 times a second, so if you can switch on at exactly that point, you won't see any surge on the input line, unlike if you were to switch it on when mains volatage is at it's peak you'll see a major surge as full peak voltage is essentially shorted out until the magnetic field builds up.Quote:
Originally Posted by EddyCurrent
The issue with surges comes when you add things like capacitors, as these need time to charge. For smaller transformers, the transformer itself will usually limit current enough to avoid any major surge issues, however with larger transformers they can allow that much current to pass that you need to limit it until things get charged up.
There is a lot to be said for the KISS approach.Quote:
Originally Posted by Jonathan
No point complicating things anymore than what you really have to!
I've known about zero switching for many years and was surprised myself.Quote:
Originally Posted by Jonathan
here's 2 examples
Inrush current - Wikipedia, the free encyclopedia
Time-delay relay reduces inrush current | EDN
What is the sort of level where soft start becomes important? I kept hitting the figure 500VA when reading, which is why I was looking at including it. Are there other factors that might impact on the choice, ie running it off a standard domestic wall socket, etc?Quote:
Originally Posted by Jonathan
That is true for a pure resistance, however the issue is the transformer in this case is better modeled as an inductor. I followed Eddy's links but unfortunately they only give 'hand waving' explanations, so instead I've just calculated it and it transpires that if you model the transformer as such, using Faraday's law to find the voltage across the inductor as a function of magnetising flux, then solve this differential equation to find the magnetising flux as a function of applied voltage and switching angle, the result is sinosoidal with a DC-offset. This DC offset depends on the initial flux (i.e. residual flux) and the cosine of the switching angle, so clearly if the angle is pi/2 (i.e. a voltage peak), the cosine term is zero and you get the lowest inrush current. To make matters worse, the relationship between flux and current will be non linear since in normal operation the core operates adjacent to saturation, so when switched on the core is operating well into the saturation region. This means that although the flux implied by Faraday's law is only up to twice the rated value, the current is many times higher. Unfortunately things change a bit when you have capacitors connected to the output via a rectifier, as they essentially present a short circuit to the secondary.Quote:
Originally Posted by m_c
To be honest my 600VA figure was just a rough estimate based on experience. Yes, you could consider the current capacity of the mains circuit you are connecting it to - e.g. if it's got a 30A RCD with no significant load connected then you're much less likely to have a problem, as the surge current trip is much greater than 30A...Quote:
Originally Posted by Rogue
I'll back up this based off experience because I often use 500 & 600Va transformers without any trouble with inrush trips. If there ever is I just use C or D rated MCB and I'm sorted.Quote:
Originally Posted by Jonathan
Following from Jonathan's post #99
Transformer Inrush - Open Electrical
Good article, but they could have expressed the final equation more clearly to demonstrate their point. Also, it's a bit pointless to solve the differential equation using Laplace transforms when you can trivially separate the variables. This is how I did it:Quote:
Originally Posted by EddyCurrent
From Faraday's law:
.
Seperate variables:
.
Use sin(A+B) identity to make it easy to integrate:
.
Integrate:
.
Use cos(A+B) identity to simplify:
. (1)
When the transformer is switched on, we have the initial condition relating to the residual flux:
.
So substitute this in to (1) to find constant, k:
.
So the solution is:
.
So now it's easier to see that the . term is a constant, which introduces a DC offset that disappears when the switching angle is . ... i.e at the peak voltage.
I used to like Laplace transforms but that was 40 years ago, I've never needed to use them since :hysterical:
I much prefer the Lapdance performs. . Makes me constant, which introduces a DC offset that disappears when the switching angle is ...at the peak. .:whistle:Quote:
Originally Posted by EddyCurrent