| 摘要 |
A circuit for controlling a d.c. motor (10) is provided with a current-limiting arrangement (27), designed as a time-delay amplifier, responding at a certain current value by interrupting the supply of current to the motor (10), and then after a certain time, determined e.g. by internal timing elements (46, 47, 49, 54), switching the current supply to the motor (10) on again. The switching operations occurring in such a motor, i.e., the pulse-duration/pulse-gap ratio and the pulse repetition frequency, are a function of the back-EMF that is induced in the motor (10) as a result of its rotation. This situation is exploited for the following purpose: to make the threshold value of the current-limiting arrangement high when the pulse-duration/pulse-gap ratio is high, namely e.g. when the application of current to the motor is practically continuous; and, in contrast thereto, to reduce this threshold value to, for example, one half when the pulse-duration/pulse-gap ratio is low, such as is characteristic when the motor is at a standstill and during start-up. In this way the motor, if blocked against rotation, draws power of a value substantially lower than at rated operation and, for this reason, cannot become overheated even if it is blocked against rotation, and during start-up no excessive values of start-up current can develop.
|
