Need some AC servo motor education

Discussion in 'CNC Machines and Conversions' started by jim_geib, Jan 29, 2010.

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  1. Jan 29, 2010 #1

    jim_geib

    jim_geib

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    I need some brushless AC Servo motor education.


    I pickup a brushless Ac servo motor and a Pacific Scientific controller this week. I did a quick web search before buying and they seem to match. My question is the motor is sped'ed at 750 Watts. I converted that to HP on Google converter and it came back as 1 HP. Is this right, this motor is about 4”
    sq. and maybe 6”-7” long and that includes the encoder. That seems to be a lot of HP in a small package.
     
  2. Jan 29, 2010 #2

    Kermit

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    It will use 1 HP of power from the wall socket, but will probably give just less than 1 HP of actual mechanical power. Perhaps as low as 3/4 HP depending on the power factor and motor efficency. Other than that you heard right. 750W approx. equals 1 electrical HP.

    Kermit
     
  3. Jan 29, 2010 #3
    746 W = 1 HP. The question is, where do you measure it? Most metric motor manufacturers measure the power consumed whereas traditional American practice is to measure it at the output.
     
  4. Jan 29, 2010 #4

    Bluechip

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    Jim

    Does seem rather small, is it 'continous rated' ?, some servo motors have a duty cycle. They should not be run at the full rated output power continously.

    Otherwise they overheat ... etc.

    Dave BC
     
  5. Jan 29, 2010 #5

    jim_geib

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    Kermit,

    Thank You
    This now opens a whole new project for me. My Smithy has a 1/3 HP DC motor now so even at 3/4 HP that's a plus. Also the control has step/Direction inputs which I think will work with Mach3. This just might give me positioning. This could be fun.

    I also got a 1500 watt Brushless ac servo from the same place for $15.00 the 750 Watt was only $10.00 and the controller was a bit more $99.00. This is still less than a VFD drive.


    Thank again
    Jim
     
  6. Jan 29, 2010 #6

    GailInNM

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    Jim,
    You may have to dig a little bit deeper into the motor specification sheet from the maker. Servo motors generally have a variety of ratings depending on the application and the nameplate may not have all of these listed.

    When used for driving axis loads, such as moving a milling table, peak and short term power loads are often quoted to give an indication of the power available to accelerate the table for rapid moves. The continuous power may differ considerably.

    When used as spindle drive motors, it is common to quote a long term power and a short term power. An example is vertical machining center spindles where there will be a short, 5 to 20 minute, power specification and a continuous power specification. Typical on small machining centers will be 10 HP continuous and 15 HP for 15 minutes. This allows an operation requiring a lot of metal removal with high HP requirements followed by small diameter cutter use for a longer period of time to do detail work. Even higher peak currents are allowed to take care of things like spindle reversal for rigid tapping.

    A relatively small lathe that I have has the spindle driven by a DC servo motor. It is rated at 2 HP continuous duty. The peak power rating is 67 amps at 180 volts.This would correspond to about 12KW input or 16 HP peak. I have it set up at 170 volts 50 amps peak or about 11 HP. This is used short term to accelerate the spindle and to make rapid minor speed adjustments such as occur during threading.

    To your specific motor, the size is not too far off for 1 HP intermittent duty and 3/4 HP continuous. It sort of depends on the application the maker designed it for and how they rated it.

    Sorry if this is clouding the issue even more for you.
    Gail in NM
     
  7. Jan 29, 2010 #7

    Noitoen

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    Standard motors are rated by the shaft power. The power that's consumed from the wall socket depends on the power factor and the efficiency.

    The physical size of a ac motor depends on the rated frequency. Here in Portugal, in my shop I've repaired a 30Kw motor that drives a 1,5 meter diameter stone cutting disc and the physical size and weight of a standard 3Kw motor. The difference was the frequency since this motor can only run connected to a frequency converter that turns our regular 400VAC 50 Hz into 400VAC 1000Hz. Since you need water to cool/lubricate the disk they use it to cool the motor. The winding looks like a stepper motor with many poles to cut down on the rpm but this is still so high that the bearings are made of ceramic.
     
  8. Jan 29, 2010 #8

    RonGinger

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    I also have a 750w AC servo and I will be installing it on a lathe to be controlled by Mach. Mach does support a step and direction spindle. I had this motor on another lathe, an Emco PC5 and it was overkill for that spindle. I hope to try some live tooling on this machine and use the spindle motor as an indexer.
     
  9. Jan 30, 2010 #9

    jim_geib

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    I wish THANK all who have replied. I will be doing a little homework before flipping the switch.

    Thank Again

    Jim Geib
    Mansfield, Ohio
     

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