| 摘要 |
PURPOSE:To reduce the cost by making the current capacity of a semiconductor smaller by lowering motor current immediately when the motor current exceeds a limit value by a method wherein the frequency is lowered while the voltage is kept fixed and, after a certain fixed period of time, the voltage is also lowered. CONSTITUTION:Input current to a motor 1 is detected by a current transformer 3 and inputted through an analog-to-digital converter 4 to a CPU5 so as to be compared with current limit value memorized in a ROM6 in advance. When the value of the input current exceeds the current limit value, the CPU5 outputs its frequency through an I/O8 to an inverter 2 under the condition that the voltage is kept fixed and simultaneously the time counting is started. After a certain period of time, a signal to lower the voltage is outputted through the I/O8 to the inverter 2. Consequently, because the current flowing across the motor can be lowered immediately, the need for giving margin in the current capacity of a semiconductor is eliminated, the use of a semiconductor with low current capacity can be realized and a low cost rotational frequency control device can be obtained.
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