随着超声波电机运行时间的增加，其谐振频率会下降，进而使电机速度降低。

As the increase of running time, the resonant frequency of ultrasonic motor will decrease, which then leads to the decrease of speed.

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电机运转过程中，如出现堵转现象，随着电机转速的降低，起动器会自动投入运行，达到增加电机转矩，减少起动电流的目的。

During the process of running, if it appears locked-rotor, the starter will automatically run for the reduction of motor speed to reach the goal of increasing the torque and reducing current of motor.

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