Time for the run up of a hydraulic motor from standstill to a given rotation speed. Applicable when the valve is fully open and there is no external torque, the time constant To max is a characteristic for the motor control valve system. Based on ⅔ of the motor's rated speed, the run up time constant To for, say, a 50 kW axial piston motor would be around 0.2 s (the run up time constant To for an electric motor with the same power would be over 3 s).
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