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  1. #1
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    Power supplies

    I'm about to start putting together my controller box in a 2u 19" rack mount.
    After stripping the old computer out of the box (dual Intel P3 450mhz with integrated scsi/video. Anyone interested for cheap?) I had a thought about power supplies. I already am leaving the 300watt supply the computer was using installed in the rack and had thought to purchase a 48v power supply with aprox 12 to 14amp capacity (4 steppers at 3amp each). After looking on ebay however, 2 7.3amp 48volt power supplies can be had for really cheap. I can either wire them in parallel or take apart the gecko G540 and wire 2 stepper drivers to each power supply. I am thinking the parallel wire method might be the best option thereby not voiding the warranty. Any thoughts on this? or should I just byte the bullet and purchase a power supply with sufficient capacity? 2 48v power supplies for around 30 each plus 25 for shipping compared to around $140 and I don't know if it'll fit in the case. A normal computer power supply just squeaks in.

  2. #2
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    I am not an expert, but just a newbie, going through this with my setup. With that disclaimer, you have 4 drives at 3 amps each, you only need an 8 amp PS. Add you drive amps together and multiple by .67 to get your PS amp requirements.

    I saw some 48v 10a supplies in the $50 range

  3. #3
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    Scott, I'm curious where you are getting those numbers from. Are you taking the "4 drives at 3 amps each" from the G540 specs? If so, keep in mind that cd may not be driving his motors at the G540's full capacity. E.g. he might have a 1 amp/phase motor and so set the G540 to a lower than maximum rating. Having said that, it's a safe bet if you give the board all the power it can ever need, in case you upgrade motors in the future.

    Also, manufacturers can rate their motors under different systems. Most of the old round unipolar motors you pull out of old equipment have label ratings for 2 phases on. So 1.5A means both phases always running at 1.5A. But newer motors (like most square ones) are rated per phase, so 1.5A means 1.5 amps maximum with one coil running.

    So, as I understand it, you need 3 amps * 0.67 (if using a chopper type driver) to run that motor.

    More information at:
    techref.massmind.org/techref/io/stepper/power.htm
    James hosts the single best wiki page about steppers for CNC hobbyists on the net:
    http://www.piclist.com/techref/io/steppers.htm Disagree? Tell him what's missing! ,o)

  4. #4
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    I have 4 - 3amp motors (square kind) I purchased from a eBay seller in China. long_motor I believe it was. Anyways, I had wondered where the 0.67 came from and if I had to guess, it's the reciprocal of the value used to change from AC to DC. since we are actually going from DC to AC.

  5. #5
    Community Moderator ger21's Avatar
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    Quote Originally Posted by James Newton View Post
    So 1.5A means both phases always running at 1.5A. But newer motors (like most square ones) are rated per phase, so 1.5A means 1.5 amps maximum with one coil running.

    So, as I understand it, you need 3 amps * 0.67 (if using a chopper type driver) to run that motor.
    Not according to Gecko:

    That is to say, a 6 amp per phase motor will require a 4 amp power supply when wired in parallel and a 2 amp power supply when wired in series.
    Gerry

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  6. #6
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    Well, I should just keep my finger quiet. I'm new and didn't realize the older motors are rated different.

    Plus I was just assuming parallel wiring (how's that saying go again .

    But the comment was intended, (because of the gyrations he was going through to get an amp rating on his power supply that covered the sum of all his motors amps), to indicate that you don't just add the amperages together.

    Correct me if I'm wrong.

    And maybe he was intending to get the biggest PS for future growth.

    I think I'll just stick with my first line of thought here it suits me better.

  7. #7
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    And James I was going from his post of 4 stepper at 3 amps each to get 12 amps. He also stated that he was looking for a 12 -14 amp PS. I figured he was thinking that you just add the amps together to get 12 amps and was looking to get the maximum PS for the steppers.

    However, I see that I type DRIVER, when I meant MOTOR. Maybe I should have said, and see if this covers it better:

    You should take the smaller amperage rating of each driver-motor(2 phase rating) combination and add them together. Then multiple the total by .67 to get the PS amp rating.

    So if you had a driver rated at 2 amps max and had a motor rated:

    Check the motors amp rating in the appropriate Wiring configuration (for example, the omega 264/187 oz-in motor is rated as follows):

    Series at 1.41 amps. < driver use 1.41 to calculate
    Parallel at 2.83 amps >driver Use drivers 2 amps to calculate
    Unipolar at 2.00 amps. = driver Use either 2 amps to calculate

    Use this for each driver-motor combination and add them together, multiple by .67 for your PS amperage. My understanding with the .67 was because you'll never be running all axis at full speed at once except for a very short period of time.

    Now this is the max you'll need, if your application doesn't need a lot of power, then you wouldn't need the max.

    Is this a more accurate explanation?

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