Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: olf20 on March 11, 2008, 07:58:45 PM
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Hi all, hope everyone is doing well!
I built my power supply from a Micro wave transformer, bridge rectifier two large filter caps.
It has worked very well. However twice now when I turn the power on it trips the circuit breaker
in the panel. Once it blew my breakout board + a Gecko drive. This last time no appearant damage.
The output voltage is 72VDC to the Gecko drives.
I'm just wondering if when it sets for a weekend or its cooler (50F) over the weekend, if the inrush
current is causing some sort of problem.
All the wiring is ok, as I double checked it many times.
Any idea's are welcome!
olf20
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Hi, I'm currently building a servo system and thinking about the same problems. It's not finished yet so I haven't tested it. If the problem is inrush current then perhaps connecting an NTC thermistor in series with the primary winding of the tranformer should work. The thermistor will have a high resistance when cold and as the current flowing through it heats it up the resistance lowers providing more current to the transformer. This provides a slow startup sort of thing. You will need to provide a switch (contactor) on a timer to short across the thermistor otherwise the thermistor will be very hot,burnout or cause more problems if left to it's self.
Another thing I was concerned about with my power supply was the voltage is near the 80v limit for Gecko servo drives. At 74 volts I got worried and searched CNC Zone for a few answers. I managed to download this circuit diagram for an over voltage protection circuit originally created by Gecko.
I have enclosed it here. I havn't built it so I cannot guarantee if it works.
Hope this is helpful.
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Hi, olf20
The breaker may be weak or undersized and spiking due to in-rush & out-rush when the breaker trip's.
After the Power-Supply start-up/stabilizes, That's after the brown-out is over, lights back to normal.
How about a switch or relay to turn on your Drives & the 5 volt supply power.
Converted a bridge-port size mill with servos over to Gecko's with 75 volt power supply wired the relay to the E-Stop Button Reset.
Also depending on the size of your Capacitors maybe you only need one for needed filtering, That would decrease the in-rush time.
Not an expert hear.
Hope this Helps, Chip
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Thanks for the replys!
cjmerlin, I think that I might try your idea. I read a series of threads on CNC forum and a couple of guys
discussed the same problem / ideas's.
Chip, I think the breaker is fine although its worth trying another one. It has never tripped in 8 years
until I put the CNC Mill on this circuit.
Thanks for the replys!!
olf20
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Hi, It just sounds like a sensitive breaker. When I first built the power supply I tested with no load and the second time I switched it on it tripped the breaker in the house. I hate the damn things but I suppose one day they'll save my life.
If you are going to try the thermistor idea then make sure it is a NTC type and will handle the current at it's lowest resistance. It will only need to be in circuit for a short time. I used a contactor to short across it which was connected to a circuit I made using a NE555 timer connected to a small relay to power the contactor coil. I cant remember the timing value, it was about 1/2 a second or less.
Cheers
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You may need a different breaker type instead of a different rating. Over here at least they are to be found in B, C and D flavours. So a B60 would protect a 60A (running current) circuit, and have close protection, tripping instantaneously at 5x the rated current for fault protection. So you'd use it for resistive loads like heaters, boilers etc. A C type breaker allows its magnetic trip to operate at 10x its rated current value for motors and fluorescent lamps, and a D type at 20x for induction heaters and transformers. The thermal response curves are the same, but the magnetic fault protection differs.
A microwave oven transformer - hefty kit - tramsformers often suck in 10 to 15x their maximum running current as inrush so do consider some current limiting measures. Even a series resistor shorted out by a delayed contactor will help, allowing the core to saturate then giving it full power. You will need a high power, low value resistor. incidentally, a lot of the high-voltage guys use microwave oven magnetron transformers to drive the primary tank circuits of Tesla coils and other insane devices... 4hv.org will tell all. Summary: bonkers ;D
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BluePinnacle, thanks for the reply. I done a google search and found this site that described how Micro Waves limit the current.
http://www.kronjaeger.com/hv/hv/src/mot/index.html
I went on ebay and found numerous wire wound resistors and ordered a 39 ohm / 5 watt resistor.
As soon as I get the resistor I'll post back.
By the way, I remember this setup in the micro wave when I tore it apart.
Thanks to all who replied. This is how we solve problems!
olf20
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(http://www.kronjaeger.com/hv/hv/src/mot/mot_switch.png)
That's the way! nice and simple. use a rather slow relay for best results.
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Hi all, hope everyone is doing well!
Well I got the resistor(s) and I still had the original relay for the
microwave. I will install tomarrow and hopefully it will solve the
problem. Can not afford to have the machine down, or the expense.
olf20
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Hi all, hope everyone is doing well!
I hooked up the relay and resistor today. Nothing happened, so that's good.
Time will tell if that solves my problem.
Thanks again to all who replied!1
olf20