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.

The object of the invention is to achieve a new method for regulating the speed of rotation of an induction machine by suitably compensating in a control loop the deviation caused by the slip between the supplying frequency and the actual and reference values of the speed of rotation.

The present invention makes it possible to achieve an advantageous and reliable system for controlling the speed of rotation of an induction machine. The motor speed can be determined e.g. using a simple inductive detector without a separate tachometer or optic pulse sensor mounted on the motor shaft. Especially in cases where the axial length of the motor is limited, this provides a considerable advantage. In the system of the invention, the slip is compensated digitally via speed feedback, resulting in fast and accurate operation. The feedback is implemented using a few cheap integrated circuits.

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. 3 presents the pulse shapes of the signals, and

FIGS. 4a and 4b illustrate the arrangements for the measurement of the speed of rotation of the motor.

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.

The slip compensator 15 consists of a summing element 16, a voltage/frequency converter ]7, a phase comparator 18 and an integrator 9. The frequency signal n.sub.m obtained from the pulse sensor 12 is passed via conductor 14 to one of the inputs of the phase comparator 18. The synchronous speed n.sub.s is obtained as a voltage signal from the frequency converter 2 and passed to the summing element 16, which subtracts from its value the signal ds given by the phase comparator 18. The output signal of the summing element 16 is passed to the voltage/frequency converter 17, which forms a frequency signal n.sub.s ' corresponding to the reference speed f.sub.R of the motor. The phase comparator 18 forms a phase difference signal ds on the basis of the phase difference between its input signals n.sub.m and n.sub.s ' in the manner illustrated by FIG. 3. The phase difference signal ds is conveyed to the summing element 16 and to the integrator 9, which generates from the phase difference signal ds a correction signal C, which is applied to the correcting reference input 11 of the frequency converter 2. The summing element 16, the voltage-controlled oscillator 17 and the phase comparator 18 constitute the inner loop of the control system, which is structured as a so-called phase-locked loop.

The speed control system of the invention works as follows. The signal n.sub.s obtained from the frequency converter 2 is proportional to the output frequency of the frequency converter and to the synchronous speed of the motor. The voltage-controlled oscillator 17 produces a frequency signal n.sub.s ' proportional to the reference speed of the motor. The pulse sensor 12 produces a pulse train n.sub.m of a frequency proportional to the speed of rotation of the motor 1. The phase comparator 18 compares the pulse trains of signals n.sub.s ' and n.sub.m . As a result of this comparation, the phase difference signal ds increases if the pulses received from the motor are retarded. The phase difference signal ds is subtracted from signal n.sub.s, causing the output frequency n.sub.s ' of the oscillator 17 to fall and the oscillator to be synchronized with the frequency n.sub.m obtained from the pulse sensor. The mean value of the phase difference signal causing the synchronization is proportional to the difference between the synchronous speed and the speed of rotation of the motor, i.e. to the slip. The integrator 9 generates the correction signal C by filtering the phase difference signal ds. When the correction signal C is added to the original frequency reference f.sub.R, a closed speed control circuit is formed which determines the slip of the motor and corrects its supply frequency correspondingly. Thus, the static state speed of the motor is not dependent on the load but is adjusted to a value corresponding to the reference frequency f.sub.R as long as the motor is able to generate a torque corresponding to the load.

FIG. 2 presents a circuit designed to perform the function of the slip compensator shown in FIG. 1. The voltage signal n.sub.s (0 . . . 10 V) proportional to the motor supply frequency f.sub.s is taken through amplifiers 21 and 22 to the voltage-controlled oscillator 23. Via conductor 24, a phase difference signal corresponding to the slip is applied to the summing point at amplifier 22, which subtracts it from the signal n.sub.s. The oscillator 23 produces at its output a frequency signal n.sub.s ' proportional to the reference speed, which is passed to the phase comparator 25. The signal n.sub.m obtained from the pulse sensor connected to the motor is applied to the other input of the phase comparator. The output 26 of the phase comparator 25 is passed to an integrator 27 acting as a filter, whose output gives the correction signal C via amplifier 28. The signal ZS keeps the integrator output at zero when the machine has been stopped. The phase comparator 25 controls the input of the integrator 27 on the basis of the phase difference between the frequency signals n.sub. s ' and n.sub.m so that the integrator output increases when the pulse of signal n.sub.m lags behind the pulse of signal n.sub.s '.

FIG. 3 represents the input and output pulses of the phase comparator. The pulse sensor produces e.g. 24 pulses per revolution, in which case a motor speed of 1800 r/min corresponds to a pulse frequency of 720 Hz. The phase comparator is implemented using a CMOS circuit with a three-state output. In the area indicated by arrow A, signal n.sub.s ' causes the differential signal ds to go into the state +1, and signal n.sub.m causes ds to return into the 0-state. As the phase difference increases, the pulse duration of ds also increases. Similarly, if the motor speed tends to increase, the pulse of signal nm causes ds to change into the -1-state and the pulse of signal ns' back into the 0-state as is illustrated by the area indicated by arrow B.

FIG. 4a shows how the pulse sensor is mounted on the motor. The inductive sensor 41 is attached with a fixing element 42 to the body of a brake installed at the end of the motor. The brake wheel 43 is provided with a toothing 44 and the sensor is fitted in the immediate vicinity of the teeth. FIG. 4b presents a section through FIG. 4a along line A-A.

The invention has been described above by the aid of one of its preferred embodiments. However, the presentation is not to be regarded as limiting the sphere of protection of the invention, but the embodiments of the invention may vary within the limits defined by the following claims.