Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: adrian5 on June 28, 2014, 10:53:49 AM
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I have a very cheap 90V, 200W spindle, in order to power it i'm using multiple laptop adapters connected in series (of course none is grounded to AC).
The DC output is 96V. I've changed the cable today on the DC side, to a cable that has the ground wire. If the spindle is turned on, when i measure the DC voltage between ground wire and minus(-) wire or ground wire and plus(+) wire i'm getting crazy high voltage pulses 200-600V.
So my question is, am i getting all that noise due to my inadequate power source?
Should i connect the DC minus(-) to the ground wire? (The ground wire from spindle is already connected to the CNC and all other ground wires from stepper motors )
Should i connect the DC (-) from spindle to the minus(-) DC2 output that powers my gecko board and all that to ground CNC?
The system seem to work well the way it is right now, but i'm worrying about the expensive gecko board.
Before changing all the cables to cables with ground wires, even with debounce interval set to 2000, i used to get some unexpected "Emergency mode activation"
thanks
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Frightening!
Yes, you should ground one of the wires going to the spindle - I think! I am assuming the laptop power supplies are all floating. If they aren't, you'll soon find out when the smoke comes out.
Better, MUCH better, would be to buy a proper power supply !!!! Please!
Cheers
Roger
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The laptop power supplies are all OEM, Sony Dell and IBM, so the voltage cumming out of them is stable.
I'm sure the spindle power supply made of laptop adapters is good, except the fact it's not AC grounded because:
1. With the original spindle power supply the spindle was getting warm-hot to 55-65 Celsius within 10 minutes of work
2. With laptop adapters it is 40-45 Celsius after 30 minutes, doing the same job, and motor works smoothly, i can hear it's running much better on these laptop adapter rather than on that cheap power supply it came in by default
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Um - OK.
I suspect, from that info, that the PS you had been sold with the spindle, was a VERY crude switch-mode unit emitting huge amounts of RF. The RF would have been heating the motor. The laptop units will be made to a much highr standard simply because they have to be, to get FCC aproval. Oh well, hooray for Standards!
You could use the original PS IF you filtered the output well: a series choke and a good shunt capacitor (or two) should work a lot of good.
Regardless: earth one end of the overall PS!
Cheers
Roger
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Adrian,
You still need to treat these series supplies as a single unit, that is; you need to filter the DC output. Supplies in series is not a good method unless they are all exactly the same voltage and current rating. What would probably work to quiet this supply down is a motor run capacitor or even a motor start capacitor - of ample voltage of course. Or better yet an electrolytic of ample voltage and capacitance - (like 1000 uF or above) Stay above 100 V and I believe standard motor capacitors start at 180 V (for a 120v motor). Grainger usually has these on the shelf....but these don't go very high in capacitance working with AC motors...
Also be sure to put a bleeder resistor (~47K Ω - 5 watt) across the capacitor for safety's sake - you're working with lethal voltages there. And - filter at the output of the supply, not at the motor. If anything at the motor a high voltage ceramic capacitor (about .1 uF) across the leads will help too. That will help quash some of the bask EMF that your seeing.
Careful with those voltages!
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http://www.batteryspace.com/dc-motor-controller-electronic-speed-controller-kit-for-90v-dc-motor.aspx
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Um - OK.
I suspect, from that info, that the PS you had been sold with the spindle, was a VERY crude switch-mode unit emitting huge amounts of RF. The RF would have been heating the motor. The laptop units will be made to a much highr standard simply because they have to be, to get FCC aproval. Oh well, hooray for Standards!
You could use the original PS IF you filtered the output well: a series choke and a good shunt capacitor (or two) should work a lot of good.
Regardless: earth one end of the overall PS!
Cheers
Roger
What would be the correct value for choke? I already tried a 200V electrolytic capacitor without choke, and it got hot within 1 minute
thanks
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Adrian,
You still need to treat these series supplies as a single unit, that is; you need to filter the DC output. Supplies in series is not a good method unless they are all exactly the same voltage and current rating. What would probably work to quiet this supply down is a motor run capacitor or even a motor start capacitor - of ample voltage of course. Or better yet an electrolytic of ample voltage and capacitance - (like 1000 uF or above) Stay above 100 V and I believe standard motor capacitors start at 180 V (for a 120v motor). Grainger usually has these on the shelf....but these don't go very high in capacitance working with AC motors...
Also be sure to put a bleeder resistor (~47K Ω - 5 watt) across the capacitor for safety's sake - you're working with lethal voltages there. And - filter at the output of the supply, not at the motor. If anything at the motor a high voltage ceramic capacitor (about .1 uF) across the leads will help too. That will help quash some of the bask EMF that your seeing.
Careful with those voltages!
I'll add the resistor and motor capacitor to see if i'm getting better results than with electrolytic capacitor
thanks
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What would be the correct value for choke? I already tried a 200V electrolytic capacitor without choke, and it got hot within 1 minute
Ah, well that is a good question! But not a simple one.
I think you need a choke, an inductance, rated for a bit more current than your motor draws, in one of the wires to the motor, plus a mains-rated film capacitor across the motor. The cap should be rated to something like 630 VAC, and will only be maybe 0.1 uF. Not expensive stuff. An alternative would be to buy an IEC filtered mains socket, but then you would need an IEC plug as well. Not a silly idea, mind. You would (have to) earth the earth pin on the socket.
The 200 V cap was simply the wrong thing to try. It sounds right, but it could not handle the RF spikes.
A caution: do not try using an 'ordinary' capacitor. It would probably fail fairly soon. By the sounds of things, this is a harsh environment!
Now, this is quite different from putting in a large filter capacitor. But try it and see how it goes. If you need to add a large electrolytic filter cap as well, you will probably need a rating of about 400 VDC at least.
I doubt you need a bleed resistor across the 0.1 uF capacitor: it's very small, but it won't hurt.
Cheers
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Since the supply(s) are already rectified within each series supply, you're not dealing with the spikes that occur from the bridge mains rectifiers - so a 200VDC electrolytic filter capacitor will be just fine at the 90 DC volts - you're not dealing with mains RMS voltages. Again - 1000uF or larger at 200VDC. (You didn't mention the capacitance of the electrolytic capacitor that you tried....) The only way a large electrolytic will heat is if it were inadvertently wired in series or backwards - that could cause it to explode as well. Watch the polarity. The large electrolytic capacitor is the only one that will need a bleeder resistor. All it does is discharge the capacitor when the power is shut down since a large capacitor will store its charge and will completely discharge into you if you happen to touch the hot lead. Many joules of energy is released instantaneously! A large capacitor charged to the voltages that you are dealing is lethal!
The filter capacitor is wired across the output - positive lead to ground lead - and if you use an inductor (choke) it is wired in series with the hot lead so must be sized to carry the motor current. A choke blocks AC on a line carrying DC (or very noisy DC supply line carrying back emf from the DC motor). A capacitor used as a filter attenuates AC signals on a DC line or charges to the DC voltage and releases current during dips and absorbs current during spikes - so smooths the DC... The small capacitor at the motor brushes to ground will help to eliminate brush noise on the supply line and help to quash spikes due to back EMF from the motor. You don't need any 600 volt capacitor - just size them above the voltages that you're dealing with. If you have eliminated the spikes then you need not deal with their voltages. You could scramble wind about three feet of #10 insulated copper wire on a broomstick form and it would be ample to attenuate the back emf or brush noise from your motor and carry the 90 volts at 200 watts that you are dealing with. Also try a .47uF (200v ceramic disc or metal film) to ground on the motor brushes.
Good luck and again - be careful. Line voltage is a killer.
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Since the supply(s) are already rectified within each series supply, you're not dealing with the spikes that occur from the bridge mains rectifiers - so a 200VDC electrolytic filter capacitor will be just fine at the 90 DC volts
All of that would be correct if the laptop supplies are simple linear jobs. But since there seems to be a of of RF around, I question that assumption. My experience has been that in order to get an input rating of 100 - 250 VAC, which most of them have these days, they use a switch-mode supply (SMPS). I have a tiny charger for a camera battery from eBay which cost me just $5.05, and it too has an SMPS in it. Dunno how they do it for the price.
Cheers
Roger
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But then look at what is being done with this supply (or supplies in this instance).... whether they are a group in series or better yet one switching power supply of ample power, the ultimate use is as if it is a linear supply of varying load. If we were charging a battery - OK; or a fixed load - OK. But he's attempting to run an electric motor with a group in series of switching supplies. They still will need to be filtered as if it was a linear supply and even very much more so in this instance. These type supplies are designed for specific loads and specific variables in load but keeping the load demand in mind as has apparently been accounted for, all we need to do for Adrian is help him quiet down the effects of running a DC motor on his group of switching power supplies..
I will respectfully bow out of this....
Sorry Adrian, just attempting to help you out. My experience with power supplies of all types goes all the way back to the thyratron tubes that Monarch used in their EEE lathes, which I serviced and maintained for years. Computer supplies are a shortcut in technology to satisfy a narrow line of demand and price. 'Simple' linear supplies are not so lost in the mist of current thinking - might be some lack of experience or respect.
Thanks,
Bill C.
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The bottom line in all of this is that power supplies are not the simple beasts they are so often made out to be. Without being able to look at Adrian's system with a CRO, it is very hard.
I do remember listening to a conference presentation on a 5 V 600 A PS once - it was for a rather large computer (and no, there are no typos there). The complexity needed in the design was ... iinteresting!
Cheers
Roger