Subtleties of servoamps

Properly sized power supplies eliminate erratic servomotor performance in motion systems. Many engineers are familiar with dc power supplies used in motion control systems. In fact, they're so familiar with these supplies that they tend to give them little consideration during an installation. Yet unexplained shutdowns, underspeed operation, and lengthy accelerations describe just a few of the problems encountered when there are mismatches between servoamplifiers and power supplies. It can take a long time to track and correct faults caused by the wrong power supply. Batteries and dc power supplies behave about the same when connected to ordinary resistive loads. That similarity ends when it comes to servoamplifiers. Servoamplifiers do not present a steady load to supplies. In fact, during regenerative braking, the servomotor itself becomes a generator delivering energy back to the supply. Supplies designed only to source power have difficulty coping with external power applied to their output leads. Servomotors that draw 5 A at full speed typically require 15 A or more when accelerating from standstill. Power supplies may not have enough reserve capacity for this extra demand. Without sufficient reserves, motors take longer to reach full speed, may cut-out during acceleration, or may fail to reach speed at all. Excessive regenerative braking energy may overheat reservoir capacitors, possibly to the point where the capacitor explodes. The good news is that exploded reservoir capacitors make diagnosis relatively straightforward. The bad news is that the same excess energy may only cause lingering capacitor death. Failure-diagnosis becomes far more difficult when the cause is hidden. Most engineers know that the