Typically you want the drive to match the peak amps of the servo motor, like a nema 42 motor that is about 10 amps continious, 40 amps peak, you want a 40 amp drive. If you use a 20 amp drive, sure it will make the motor take off fast, but when the servo motor slams on the brakes, (stops) the drive has to be able to take the amperage the servo motor shoots back at it for stopping. Thats when a too small of drive will blow! Your best bet is to get the motor specks, and give those to to the drive manufacture you are concedering buying from, and let them tell you what drive to buy. Also, if the drive is way to big for the motor, you will run into other problems like tuning the drive.
Maybe, before you buy anything, contact a drive or motor manufacture, and tell them your application, and the speed you want, and let them suggest what would work best. I know Servo Dynamics in California would do that, their stuff is good, but expensive, and they would hook you up with the drives and motors and cables. I think most any manufacture would help you.
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