Apparatus to detect loss of rotation of a sensorless-brushless motor in a PWM driven system.
Original Publication Date: 2002-Aug-21
Included in the Prior Art Database: 2003-Jun-21
Disclosed is an invention to detect the loss of rotation of a sensorless, brushless DC motor when being driven by a PWM type controller. A problem arises when PWM speed/torque control is used in sensorless motors, specifically that an external mechanical load or disturbance may pull the motor (a synchronous machine) out of sync. This is not easy to detect without the use of a speed sensor. To make matters worse, a sensorless controller can easily be "fooled" into thinking a motor is spinning due to the close electromagnetic coupling inherent in many motor designs. It is this "fooled" state that allows the motor drive to continue to PWM motor phases and see back-EMF. This invention detects this fault state without the use of sensors. In existing pure sensorless systems in which PWM drive is employed, overcurrent protection crudely and sometimes ineffectively detects this fault, or software may look for a speed to drift high or low. The use of overcurrent protection or speed drift is not adequate for mission critical or life support applications applications, as it is neither swift nor consistent in its ability to detect the fault. This invention claims the ability to detect this fault instantaneously. This problem is especially prevalent is inverted stator motors. This invention solves the problem of detecting rotation loss in the presence of PWM motor drive systems by using a set of amplifiers to subtract and scale PWM drive signals from phase voltage leaving only the induced Back-EMF of the motor, the magnitude of which can be analyzed to determine if the motor is rotating. Since Back-EMF is linearly proportional to speed, an added benefit is that the actual speed could be readily computed. Computing speed is not the purpose of this invention, determining rotation is. Fig. 1) Current State of the Art: Employs speed sensor(s)