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  #1  
Old Sun 23 May 2010, 01:24
mtgstuber
Just call me: Michael
 
Newman Lake, WA
United States of America
MW SP-300-48 for (4) PK296A1As?

So I'm finally getting back to my ShopBot PRT upgrade project. I have the three stock Oriental Motors PK296A1A-SG3.6s (X,X2,Y) and one stock PK296-A1A-SG7.2 (Z). Looking at the wiring, ShopBot hooked it up BiPolar. For redoing the control box (I've got my G203Vs and PMDX-122), I plan on switching to a unipolar configuration. Here's my question: Should I consider a new power supply?

I have the MeanWell SP-300-48 that came with the PRT. It's a 300 VA supply that will put out 6.25 A @ 48v. (http://www.meanwell.com/search/sp-300/sp-300-spec.pdf) The Oriental Motors spec sheet for the motor indicates 7.7 mh. Following the SQRT * 32 formula, I get 88 volts. If I multiple that by 2/3, I get 66 volts. This is also equal to the 20x rated voltage (3.3v) that the G203V documentation recommends as a max voltage.

It looks like the existing power supply will do everything I need BUT, my chief goals for this upgrades are improving the quality and the speed of the cut. In all the reading I've done, it would appear that moving to Unipolar will help the torque, and, provided I don't overheat the motors, I will get better performance with higher voltages. Is it worth it to build or buy a 60 volt power supply? How close to 66 volts should I bother going? Or will I never notice.

Thoughts?
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  #2  
Old Sun 23 May 2010, 06:30
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
The PK296A1A-SGxx motors draw 1-Amp when wired bipolar series and 1.5-Amp when wired half-coil. The Geckodrive G203v can handle up to 80 VDC. As you stated, at 7.7mH, the maximum voltage for the motors would be 88VDC, so the Geckdrive G203v is the limiting factor.

I would use a power supply rated up to 80VDC, depending on how stable your electricity is. In my shop, it ranges from 105VAC in the summer to 125VAC when everyone but me is off grid, so I limit my transformer to a 50VAC, which rectifies to 70VDC AT 115VAC input.


You would multiply the Amps by 66% to see how much amperage the power supply should produce, so 1.5A X 4 motors = 6A and 6A X 66% = about 4A.

Switching type power supplies, like the meanwell will work with stepper motors, but they are designed to hold the voltage constant with a constant current draw. Stepper motors do not have a constant current draw and they do not require constant voltage. A standard linear (non-switching) power supply works best and is usually less expensive. If you decide to use the switching type power supply, be sure to use filtering capacitors on the output, just like you would with a linear power supply. A 4,000uF capacitor rated at least 120V would meet your needs with those motors.

The 48VDC power supply will work to get you started. You MIGHT get faster speeds if you use a 70VDC or 80VDC power supply, but that depends on the load that the motors have to move. In either case, add the filter capacitor so that the motors aren't starved for power when the switcher is trying to regulate things and you'll be running.

(I often use two 24VDC switching power supplies connected in series without an output capacitor when I'm testing motors that have a maximum voltage of 50VDC. I can tell that I need to add an output capacitor for optimum results, but a test bench is not the same as a production machine.)
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  #3  
Old Sun 23 May 2010, 08:39
mtgstuber
Just call me: Michael
 
Newman Lake, WA
United States of America
Thank you Mike.

Should I worry at all the comments in the G203V installation notes: "In all cases the power supply voltage should be no less than 4 times or no more than 20 times the motor's rated voltage"? The rated voltage on these motors is 3.3 volts, which would indicate a 66 volts maximum.
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  #4  
Old Sun 23 May 2010, 13:55
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
Mariss, who wrote the caution about 4X to 20X the motor's rated voltage, also wrote the formula: 32 X SQRT( Inductange ) = MAXIMUM voltage.

My testing with various motors shows that using his formula gives the maximum possible speed without exceeding the motor's rated temperature. Personally, I prefer to reduce the maximum voltage by 15% to 25% so that the motor stays at 65 C or lower, but every motor that I have is rated at 80C or higher, so my caution is costing me a little performance.

In the case of the PK296A1A-SGxx motors, Oriental Motor has reduced the motor's rated current by about 1/3rd so that the gearbox is not worked beyond it's capability. That reduction in current automatically reduces heat in the motor. Remember that Watts = Voltage X Current in a resistive load, so reducing the Current automatically reduces the heat (Watts).

I have never run a PK296A1A motor on my test bench. I have the PK296A2A motors. They run fine at maximum voltage.
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