| 摘要 |
PROBLEM TO BE SOLVED: To prevent thermal breakdown of a switching element by varying the generating time ratio between two switch patterns where the line output goes zero when the modulation rate is low, thereby regulating loss of the switching element in a three-phase two level inverter. SOLUTION: Output from a comparator 3, i.e., a PWM waveform, is inputted to a modulation rate detecting means which detects a modulation rate from the on/off ratio of the PWM waveform. The modulation rate is inputted to the minus terminal of a modulation rate comparing means which has the plus terminal receiving a reference modulation rate. No output is produced from the modulation rate comparing means, when the modulation rate is higher than the reference level and an output is produced, when the modulation rate drops below the reference level. Consequently, a time ratio regulating means functions to shift the command voltage of a command voltage generator 2, so that allotment of loss is varied between switch elements on the positive and negative bus sides thus controlling loss of the switch element and protect it against thermal breakdowns. |
