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Offline olf20

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Power Supply Question / Problem
« on: March 11, 2008, 07:58:45 PM »
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
Mach3, Atlas Knee Mill, 4th Axis, VcarvePro, ESS, Super PID.
Been Heating with corn since 1998
Re: Power Supply Question / Problem
« Reply #1 on: March 11, 2008, 11:02:04 PM »
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.

Offline Chip

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Re: Power Supply Question / Problem
« Reply #2 on: March 12, 2008, 01:05:20 AM »
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

 

Offline olf20

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Re: Power Supply Question / Problem
« Reply #3 on: March 12, 2008, 08:33:25 AM »
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
Mach3, Atlas Knee Mill, 4th Axis, VcarvePro, ESS, Super PID.
Been Heating with corn since 1998
Re: Power Supply Question / Problem
« Reply #4 on: March 12, 2008, 11:31:01 AM »
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
« Last Edit: March 12, 2008, 11:40:01 AM by cjmerlin »
Re: Power Supply Question / Problem
« Reply #5 on: March 13, 2008, 05:53:32 AM »
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

Offline olf20

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Re: Power Supply Question / Problem
« Reply #6 on: March 13, 2008, 08:45:44 AM »
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
Mach3, Atlas Knee Mill, 4th Axis, VcarvePro, ESS, Super PID.
Been Heating with corn since 1998
Re: Power Supply Question / Problem
« Reply #7 on: March 13, 2008, 09:39:10 AM »


That's the way! nice and simple. use a rather slow relay for best results.

Offline olf20

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Re: Power Supply Question / Problem
« Reply #8 on: March 16, 2008, 09:36:27 AM »
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
Mach3, Atlas Knee Mill, 4th Axis, VcarvePro, ESS, Super PID.
Been Heating with corn since 1998

Offline olf20

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Re: Power Supply Question / Problem
« Reply #9 on: March 17, 2008, 01:14:47 PM »
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
Mach3, Atlas Knee Mill, 4th Axis, VcarvePro, ESS, Super PID.
Been Heating with corn since 1998