| 摘要 |
<p>An electric induction motor (10) is connected by a switch means (30) to a source (A, B, c) of three-phase alternating electricity. The speed of the motor is determined by detecting when the source voltage has made a zero crossing and thereafter sensing the polarity of the back emf voltage induced in each winding (11, 12, 13) of the motor when the switch means is non-conductive. After several zero crossings a set of polarity samples has been produced from each winding. A motor controller (20) determines the motor speed by detecting the occurrence of a predefined relationship among samples in each set. For example, the speed is in the range of fifty to about thirty-eight percent of full synchronous speed when three consecutive samples from one winding all have the same polarity. The occurrence of the predefined relationship among back emf polarity samples can be used to control the braking of the motor. <IMAGE></p> |
