Sunday, February 17, 2013

Power Supply Pt. 1: Planning stages

This planning/reading/thinking about buying stage took place between April and August 2012 So I'll start this entry by saying that this mill didn't have a power supply or any servo drives when I brought it home. I'll assume that most of you reading this who get a mill with intentions of retrofittting it won't have to go thru this sort of headache since you'll already have most of the power supply and drive components.

There are some excellent bits of info out there most notably a power supply basics paper on The long and short of it is that you need a significant amount of power to sling around a several hundred pound table + a heavy work holding fixture + a potentially heavy part in 2-3 axis + a spindle depth axis and (eventually) I hope to add a rotary axis too. For the sake of future expansion and the expensive heartbreak that comes from watching undersized components fail to perform or let the smoke out, I decided I want a good amount of overkill in the size of my power supply.

Many CNC machines use an "unregulated" power supply which basically means that its output can fluctuate based on the load placed on its output or the voltage fed to its input. For the size demanded of our power supply in voltage and kVA, a regulated supply would cost at least $500 and probably more, without much additional benefit since the drives I expect to use are capable of running over a wide range of voltages.

*if* and this is a big IF, I had a lot of money to throw at this machine I would probably have invested in a turn-key solution to these problems from a company like Kollmorgen, who can set you up with a power supply, drivers and motors all at once and reputation that it will all work once installed. I hope that by posting some of my concerns and how I came to a workable solution that this can be of help to other people chasing the same problems.

The first concern is going to be voltage. Since I originally planned on using Gecko servo drives I check their spec sheet and see that they top out at 80VDC with some (but not much) gimme space around that voltage. I'm told that the original servos on the Lagun operate near the same voltage so I want to use the maximum possible voltage to get the best reaction time and torque out of my servo motors, without exceeding the maximum of either the servo or drive.

Since I am going with a transformer-fed supply, my first concern is stepping down from a ~220VAC mains voltage down to 80VDC, but since there is going to be a transition from AC to DC, the desired AC voltage needs to be less than the DC voltage desired by a factor of 1.4, I.E. AC volts * 1.4 = DC volts because of how AC voltage is measured versus what we need for rectified DC volts. So if we need 80 VDC then we need to divide 80 by 1.4 to get approximately 57 volts.

Next question is going to be how big it needs to be, expressed in VA or kVA (kilo-Volt-Ampere), in a DC circuit for our example its basically the same as wattage (volts * amps). For this I read a few different papers about sizing power supplies and a lot of build logs but ultimately decided that I needed a 1500 VA or 1.5 kVA transformer to deliver the power that I expected to need and leave some room for expansion in the future.

Determining necessary VA seems harder than it needs to be because so many items are rated in "maximum capacity" and building a power supply large enough to handle the max capacity of 5 axis worth of Gecko G320 drives would be ludicrously expensive - it would have to be capable of delivering 100 Amps DC, requiring a mains input of ~40 Amps @ 220VAC for just the motion, never mind the spindle, coolant pump, lights, brake, logic step-down transformer or any other powered accessories. Nevermind the fact that such a transformer that can deliver 100% duty cycle would be as big or bigger than a decently sized welder, it would also cost a lot of money to buy, ship and consume a lot of power once plugged in!

Fortunately its unlikely that we'll be running all 5 (planning for the future) axis at full draw at the same time - the maximum draw will come from a rapid change in direction at speed and it would be unwise and unlikely to be accelerating all axis at maximum speed with maximum load at the exact same time.

With a linear unregulated supply like we're going to build around a toroidal transformer, we can briefly overload the transformer for a few seconds with no ill effects (from Antek's data sheet "In most of the cases, this transformer can be output 20% more power from its rating at 60Hz power source without any problem." In addition to this, when a servo decelerates it feeds some energy back into the capacitor bank that can be re-used, provided it doesn't decelerate so rapidly that it back feeds more power than the drives or capacitor bank can handle.

While you can calculate out the necessary wattage needs for your mill, and the power supply and servo vs. stepper papers on Gecko's site show you how to calculate that, I considered the calculation I drew from that as well as looking at a number of other similarly sized mill build logs on CNCzone and what was available at reasonable prices thru a few online distributors, Antek being a very popular and well recommended one. I decided that I wanted to buy an Antek AN-15458, rated 1500 VA @ 58 volts.

Next I needed to worry about rectifying all that current from AC into DC, and had been advised by a number of people that I should double up bridge rectifiers for redundancy and power handling on my servo voltage supply, so I knew I was going to need 4 for the 2x58V secondaries, plus two more for each of the 12 volt secondaries.

Last I was going to need some capacitors, and we calculate the minimum value by the formula (80,000 * I) / V = C, where I is amps, V is voltage and C is capacitance in uF. (80,000*25)/75 = ~26,666 uF, but more capacitance on your filter isn't necessarily a bad thing (within reason of course).

I should also mention that you should probably have some sort of plan to mount all this stuff, cords to get power to it, a switch to turn it on and off and a box to put it in too. Until next time…


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