Procedure and apparatus for compensating the slip of an induction machine

#The invention relates to a procedure and to an apparatus for compensating the slip of an induction machine.

#The speed of rotation of an induction machine deviates from the frequency of the supplying network in a known manner by a certain slip. When operated as a motor, the speed of rotation is somewhat lower than the supplying frequency divided by the number of pole pairs, i.e. the synchronous speed in motor operation and, correspondingly, the speed of rotation in generator operation is somewhat higher than the network frequency divided by the number of pole pairs. The magnitude of the slip depends on the load of the machine, which is why adjusting the speed of rotation often leads to a complicated control system, because the variable quantities cannot always be measured accurately. For this reason, it is difficult to maintain a constant speed of rotation of an induction machine, especially when the load varies.

#There are several previously known control systems for adjusting the speed of rotation of an asynchronous motor to a level corresponding to a reference value. These solutions have often led to the use of complicated and expensive regulating apparatuses which could seldom be realized in the case of small motors. Besides, the proposed apparatuses have required the measurement of the speed of rotation directly from the motor shaft or the measurement of the load current of the motor.

#In the following, the invention is described by the aid of the drawings, in which

#FIG. 1 presents a block diagram of the control system of the invention,

#FIG. 2 presents the circuit implementing the compensation,

#FIG. 1 illustrates the system of the invention for the compensation of the slip of an asynchronous motor. A three-phase asynchronous motor 1 is fed through conductors 3, 4 and 5 by a frequency converter 2, which converts the constant-frequency voltage of the supply network connected to its input terminals R, S and T into a motor supply voltage determined by the control, the frequency and voltage of which are adjustable. The frequency converter 2 consists e.g. of a PWM converter 6 and a controller 7 which generates a reference voltage u.sub.s and a reference frequency f.sub.s for it

#The structure and operation of the motor 1 and the frequency converter 2 controlling it are previously known systems in the art and their details are irrelevant to the implementation of the present invention.

#The speed reference f.sub.R for the motor 1 is connected to the reference input of the frequency converter 2 or it is generated within the frequency converter in accordance with external control. The signal C compensating the slip of the asynchronous motor is passed via conductor 10 to the control terminal 11 of the frequency converter so that it is summed with the speed reference f.sub.R ' producing a reference frequency f.sub.s. The system controlling the frequency converter determines, in a manner known in itself, the magnitude of the motor supply voltage on the basis of the frequency reference. The motor speed is measured by means of a pulse sensor 12, which outputs a frequency signal n.sub.m proportional to the speed of rotation of the motor. The pulse sensor 12 is preferably implemented by providing the motor with an inductive sensor responding to the brake wheel toothing, the cooling ribs of the rotor or e.g. to the fan blades. Thus, no separate tachometer or optic sensor is needed. The frequency reference f.sub.s of the frequency converter is divided by the number of pole pairs p by a divider 8, and the signal n.sub.s obtained from the divider, which is proportional to the synchronous speed of the motor, and the frequency signal n.sub.m obtained from the pulse sensor, which is proportional to the speed, are passed to the slip compensator 15 via conductors 13 and 14. The output C of the slip compensator 15 is applied to the input of the frequency converter 2 as described above. The frequency converter 2, the motor 1 and the speed detector 12 measuring the speed of rotation constitute the outer control loop of the system of the invention, controlling the speed of the motor.