| 摘要 |
PURPOSE:To completely prevent the breakage of a frequency converter due to the backward-flow current generated when the motor is operated at a reduced speed by a method wherein a current limiter, having a preset current controlling value, a gate controlling circuit and the like are provided. CONSTITUTION:When an induction motor 5 is operated at an accelerated speed, the degree of increase of the output signal D on an adjustable speed limiting circuit 7 is reduced by an overcurrent Q1 on condition that a preset current controlling value has been exceeded on the motor which will be detected by a current transformer. On the other hand, when the motor is operated at a reduced speed, the backward-flow current of the motor 5 in excess of the preset current controlling value generated by the decceleration, in other words, during the period while the output signal Q2 is maintained, the degree of decrease of the output signal D of the adjustable speed limiting circuit 7 of the overcurrent Q1 is controlled by open-circuiting a gate 21 in such manner that the degree of reduction is maintained in the state which is based on the expected set point. Through these procedures, the decrease of the positive feedback of the output frequency and the output voltage on the frequency converter, generated by the backward-flow current of the motor caused by a powerful inertia of the motor load at the time of decceleration, can be prevented. |
