Typical dynamic responses are also shown. The motor is initially at standstill and at no load when a step command in speed is applied; when steady-state conditions are reached, a reversal of speed is commanded followed by a step load application. The system is highly nonlinear due to the introduction of saturation needed to limit both the current delivered and the voltage applied to the motor. The system is in the saturation mode when the errors are large; as a consequence, the controller functions as a constant current source, that is torque, resulting in the ramping of the speed since the load in this example is a. | - The tests described thus far must be modified for a bldc motor. A bldc motor must be tested with its servo amplifier. The step input will be a step in dc voltage to the servo amplifier input. The voltage step should be large enough to cause the motor to reach rated speed. With bldc motors it is not possible to measure the high frequency phase current in a WYE connected motor. Thus some other procedure must be used to measure the current or torque and velocity. Commercial bldc servo amplifiers have two dc output test points. One output is a dc voltage proportional to velocity with a given calibration. The voltage can be directly connected to a strip chart as described previously to measure the motor acceleration, A second test output provides a dc voltage proportional to torque or a percentage of rated torque. This calibrated voltage can also be recorded with a stripchart recorder to observe its maximum value for a step voltage input to the servo amplifier. The inertia can thus be calculated as done previously.
