New Machine Build- Help designing PS for MK board driven machine

[vikingboy]

Hi All,

I have been reading these forums for some time and this is really my first post abut the machine I am building, I have got 3 Nema 34 motors driven by MK boards.
So far I have had the power supply together and tested the motors one at a time. but when I tried testing all 3 axis together I started to run into problems, blowing fuses then a couple of Mosfets etc, I think I have eventually tracked down most of the gremlins causing the trouble ( with allot of help from Kreutz and also this website ,THANKS)
Anyway, now I have ordered a new toroidal and some extra parts etc, I also have decided to add in the emergency stop as per Mariss design and I also thought it might be a good idea to also add in a soft start as well just in case.
I have modified Mariss original emergency diagram to include an extra pole on the start momentary switch which drives another relay switching in a load before the transformer to slow the current into the transformer and capacitors until the switch us released, my idea is that you would hold down the start button for a second or two until the current is able to slowly rise and the other relay has latched the power to the main transformer.
An other idea I would like to add but I have not yet worked out is a way to have an indicator neon or led to show when it may be time to release the button, also I am not sure of the value for the R2 soft start load, If anyone can help with these items and also if anyone can tell me if this is a crazy and totally stupid idea I would be grateful.

thanks

Andrew

[vikingboy]

Havnt got any replies yet , I am all ready to put this together and so do I take the lack of response to mean I probably wont blow myself up ? .. thing is though I really need some advice on the value of the resistor for the soft start, I am using a 250V ac supply into the transformer with a 40V secondary at 10A X2 making 60volts dc and with 2x10000 uF caps smoothing,, the motors are 5volts 4amps, and 2 x 3 volts 6.7amps.. could use some help..

thanks

rgds,

Andrew

[Al_The_Man]

I have not yet had to resort to this method, but if I did I would be tempted to use one resistor.
The inrush of the transformer itself should not cause any problems on switch on, the inrush normally lasts 1 cycle at most.
The problem is often that until the cap is charged it appears as a short circuit to the transformer.
The dump resistor could be switched in series with the cap on switch on for the momentary period, and shorted out for the normal on stage.
The 'hot' end could be connected over to common when switched off for capacitor dump purpose.
It may take a bit of creative switching.
Al.

[vikingboy]

Hi Al,

thanks for the reply, I guess I could with a bit of thought and fiddling around do as you suggest and use the dump resistor for both purpose , I may even be able to somehow use only the one relay, But I get the impression from your post that I may be trying to solve a problem that dosn't really exist ? do you think I am overdoing this and may be better off just using the dump resistor and emergency stop as is ?
The main problem I was having was blowing mosfets when I blew a fuse on the DC side of my PS, I spoke to Kruetz and he said to remove the DC side fuses, which I did, now I have only a fuse on the AC side of the transformer and another in the main IEC inlet socket. I also increased the size of my transformer from 300 to 800VA and upped the smoothing caps to 20000uF, so I hope I have now solved mist if my power supply probs.

One other thing I was considering is that I may need some more caps in my supply, at the moment I only have the 2x10000uF smoothing caps, and I have read in various places that I may also need a lower value cap across the supply out to the motors functioning as a "flywheel" to ensure the supply can "keep up" with the motors demand, I also see some other designs have ceramic caps across the diodes in the rectifier or a 47uF cap after the rectifier.
I have not done any electronics of this kind for some time so am really rusty and would appreciate if you could guide me on if I need any of these other enhancements to my very basic power supply ?

thanks

Andrew

[Al_The_Man]

I have often used large toroid transformers for supplies which tend to have large inrush and so far have not experienced any problems with nuisance fuse blowing.
The practice of fusing the DC is not a good idea.
The other smaller caps you mention are in some case a .01µf ceramic is connected across the diodes in order to eliminate high frequency pulses across the rectifier, the smaller one across the main caps is to de-couple the DC supply as the high value electrolytic's can have an inductive quality also.
Both of these additions are mostly used in high frequency applications such as radio and plasma supplies.
A sometimes overlooked effect of going very high in smoothing capacitor value is it can raise the VA requirement of the transformer when operated at maximum load.
Al.

[vikingboy]

Guess then I will try it out as is now, and then I can add complexity etc as needed once it is functioning, probably the smarter way to go when developing a new system anyway .. So long as I can be pretty sure that I have done enough to not damage any other components.

thanks

Andrew