JPS5910157B2 - Slip synthesis method for induction motor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an all-base synthesis method for controlling an induction motor powered by a frequency converter.

Conventionally, when an induction motor is driven by a frequency conversion device that can variably control the output voltage and frequency, there has been an operation method based on base frequency control.

In order to perform this basic frequency control, it is necessary to control the frequency of the frequency converter using a value obtained by adding the rotational frequency detected by a rotation detector attached to the rotating shaft of the induction motor and the basic frequency and the synthesized value. Conventionally, analog synthesis and pulse synthesis are typical methods used for frequency synthesis.

A block diagram of a circuit for the former analog synthesis method is shown in FIG. A rotation speed generator 1 is used as the rotation detector, and an analog signal proportional to the rotation speed of this output and an analog signal from the Bae frequency setter 2 are added by an analog adder 3, and a signal proportional to this added signal is added. The voltage-to-frequency converter 4 converts the pulse train into a pulse train of the same frequency. The drawback of this method is that it cannot achieve the precision of frequency synthesis.
Generally, when an induction motor is subjected to Bae frequency control, the appropriate Bae frequency is about 1H2, and the rotation frequency is 50H.
When controlling the frequency synthesis accuracy to about 5% in the case of about 2, the accuracy of each of the rotational speed generator 1, analog adder 3, and voltage-frequency converter 4 needs to be 0.1% or less. be. In cases where control is performed by changing the base frequency to 1H2 or less depending on the load condition of the induction motor, even higher precision is required and this becomes difficult to realize. Therefore, we devised a pulse synthesis method whose block diagram is shown in FIG. The rotation detector uses a rotation pulse generator 5 that outputs a pulse train with a frequency proportional to the number of rotations, and converts this pulse train output and an analog signal from the base frequency setting device 6 into a pulse train using a voltage-to-frequency converter. The converted output is input to the pulse synthesis circuit 8. The pulse synthesis circuit 8 is this 2
One. If two pulses are input at the same time, one of the pulses is delayed to perform pulse synthesis. When the subbe frequency set value of the subbe frequency setter 6 is negative, the voltage-frequency converter T generates a frequency of the absolute value of the subbe frequency set value as a pulse train,
A signal indicating that the base frequency is negative is sent to the pulse synthesis circuit 8. The pulse synthesis circuit 8 receives this signal, and each time a pulse proportional to the slip frequency from the voltage-frequency converter 7 is inputted, the pulse synthesis circuit 8 erases one pulse at a time from the pulse train proportional to the rotation speed, and calculates the difference between the two pulse trains. Synthesize a pulse train with a frequency of . Although the output pulse train does not have a uniform period compared to the analog synthesis method, the pulse synthesis circuit By frequency-dividing the output of 8 and using it for control, periodic fluctuations can be made to a level that causes no problem in control. As described above, the pulse synthesis method does not have any major problems in terms of control, but the circuit structure requires a circuit to avoid simultaneous input of pulses, making it complicated. In addition, when performing continuous four-quadrant operation of an induction motor (for example, operation in which the induction motor is operated in four quadrants (for example, decelerated with a negative slip frequency from a forward rotation state, and then accelerated in the reverse direction after reaching zero speed with a positive slip frequency), the rotation frequency is Veri frequency may be larger,
It is difficult to synthesize continuous slip frequencies, and this requires a more complicated circuit configuration. That is, when an induction motor is operated in the reverse direction after passing through normal rotation, deceleration, and reverse operation, the composite angular frequency ω0 of the rotational angular frequency ω and the slip angular frequency ωS becomes ωo=ω−ωs, and this relationship can be expressed as Keep the induction motor slowed down.

Therefore, ωo and ω will gradually decrease, but if slip synthesis in this case is performed, for example, by the method shown in FIG. Since it is thinned out by the output pulse, the difference becomes zero when the absolute values of the rotational angular frequency ω and the slip angular frequency ωs become equal, and the rotational angular frequency ω decreases from this point, and the rotational angular frequency ω decreases. It is extremely difficult to synthesize slip in the period until it becomes zero.

That is, from the time when the absolute values of the rotational angular frequency ω and the slip angular frequency ωs become equal, in principle, the output pulses of the voltage-frequency converter 7 can be thinned out by the output pulses of the rotational pulse generator 5. However, since the output pulses of the rotational pulse generator 5 and the output pulses of the voltage-to-frequency converter 7 are not always input to the pulse synthesis circuit under constant conditions, the rotational pulse generation Since the interval between the output pulses of the generator 5 becomes gradually longer, the point in time when the output pulses of the voltage-to-frequency converter 7 and the output pulses of the rotary pulse generator 5 become equal within a certain sampling period can be quickly and accurately detected. This is extremely difficult. In addition, semiconductors such as microprocessors have recently become available at relatively low prices, and control circuits that used to be constructed from hard-wired logic circuits made up of individual logic integrated circuits are now replaced by soft-wired logic circuits. There has been a shift toward processing signals by treating them as digital values using software-based programs.

This allows different types of control to be handled by changing the program with the same hardware configuration, and mass production can be expected to significantly reduce costs. When trying to apply a microprocessor in this way, the pulse synthesis method is not suitable because the pulse frequency from the rotating pulse generator becomes very high. For example, if the number of pulses per rotation of the rotary pulse generator is 1000 pulses, the number of rotations of the induction motor is 3000.
The rotation per minute is 50 KHz, and the interval between pulses is 20 μs, and with the processing speed of current microprocessors, only the slip frequency can be controlled, and other controls such as ignition control of switching elements of frequency converters, etc. cannot be processed by the same microprocessor. The present invention was made in view of the above circumstances, and generally, frequency control of a frequency converter is performed by inputting a pulse with a frequency proportional to the output frequency to a counter circuit, etc., converting it into a value proportional to the angle, and igniting the switching element. Focusing on the point that control is being performed, slip frequency synthesis is performed entirely in angles, and is suitable for digital ratio control, especially microprocessor control, and also allows easy compensation of slip frequency for changes in induction motor constants. The purpose is to provide a method. Below, prior to explaining one embodiment of the present invention,
The principle of this invention will be explained.

FIG. 3 is an example of a circuit that generates a gate signal for a switching element of a commonly used frequency converter from a pulse signal synthesized by slip frequency. The pulse signal subjected to the slip frequency synthesis is frequency-divided by the counter circuit 9 to a frequency necessary for controlling the switching elements of the frequency converter. This frequency-divided pulse signal is further input to a counter circuit 10, and according to the count value of the counter circuit 10, a gate signal for each switching element is generated by a gate signal logic circuit 11 and sent to each switching element.
Thus, it can be seen that in order to control the frequency converter, pulses of the frequency synthesized by the slip frequency are not required, but a value proportional to the angle, which is a value obtained by counting these pulses by a counter or the like. Taking this into consideration, the principle of the present invention performs the slip synthesis necessary for controlling the slip frequency of an induction motor by using angles instead of synthesizing by frequency. In other words, conventionally, if the rotational angular frequency is ω, the slip angular frequency is ωS1, and the angular frequency resulting from slip synthesis is ωo, slip synthesis is performed according to equation (1): ωo=ω+ω8
・・・・・・・・・ (1) Time integration of both sides yields equation (2) θo=θ+θ8 ・・・・・・
(2) θ is the time integral value of the rotation frequency ω, which is the rotation angle, and θs is the time integral value of the slip angular frequency, which will be called the slip angle.

θo is an integral value of the angular frequency ωO obtained by synthesizing the slip frequency and will be referred to as an output angle. The output angle θo corresponds to the value counted by the counter circuit 10 in FIG. Therefore, it can be easily calculated by giving the rotation angle θ and the slip angle θs of the induction motor.

The rotation angle θ may be determined by replacing the rotation detector with a rotation position detector, or by counting the pulses of a rotation pulse generator with a counter circuit. The slip angle θs may be given as an integral value of the slip angular frequency ωa, and a sampling calculation suitable for digitalization will be explained as one method for this integration. Slip angle θs
Since is the integral value of the slip angular frequency ωs, the relationship is expressed by equation (3). θ8−fω8dt・・・・・・・・・(3)
If this is determined by sampling calculation, the sampling time interval is △t, the sampling calculation value before △t of θs is θs(n-1), and the calculation value after △t is θs(n), then θs(n )3θs(n-1)+(t)s″6t゜゜゜(4), which can be calculated by repeating simple addition.

Furthermore, assuming that the required slip angular frequency ωs is fixed in a certain load state, a change in the secondary time constant T2 of the induction motor causes a proportional change in the characteristics of the induction motor, and the optimal slip frequency is determined by the secondary time constant T2. must be inversely proportional.
This secondary time constant T2 is the ratio L2/R2 of the secondary inductance L2 and secondary resistance R2 of the induction motor, and the secondary inductance L2 changes due to magnetic saturation, and the secondary resistance R2 changes due to temperature during operation. Conceivable. For example, suppose that only the secondary resistance R2 changes with temperature, the secondary resistance at a certain reference temperature is R2O1, the slip angular frequency required under load at this temperature is ωSO, and the secondary resistance at any temperature is R2O1. , the required slip angular frequency under the same load condition at that temperature is ωs.

Therefore, equation (4) becomes equation (6)R Here, if the sampling time interval Δt is Δt=Jc (C is a ratio constant), that is, the time inversely proportional to the secondary resistance value R2, θs(n θs( n-1) + COsO゜゜゜゛゜゜(7
), and there is no need to perform the calculation of R2.

This means that even if the secondary resistance value R2 changes, by varying the sampling time interval Δt, there is no need for calculations such as multiplication, and it is sufficient to repeat the same addition. For example, for easy understanding, the secondary resistance value R2
FIG. 4 shows the sampling calculation when the value doubles from a certain value due to temperature rise.

Figure 4A shows the case where the secondary resistance value is R2, and Figure 4B shows the case where the secondary resistance value is 2R2.
In both cases, although only C(l)SO is calculated for each sampling period, △t
changes in accordance with R2, indicating that the desired slip angle θs can be obtained.

Although the case where the secondary resistance value changes has been described, the case where the secondary inductance value changes can be dealt with in the same way.

Furthermore, it can be applied not only to changes in the constants of an induction motor during operation, but also to controlling different induction motors, and there is no need to change the sampling calculation to change the induction motor.
FIG. 5 is a block diagram showing a specific example of the process, and FIG.
The figure is a waveform diagram showing in analog form the waveforms of various parts in FIG. 5 when the digital value ωs proportional to the slip angular frequency is changed from positive to negative at time t.

In FIG. 5, a digital value ωs proportional to the slip angle frequency given from the outside is added to the previous slip angle θs held in the latch circuit 14 by an adder 16, and the result is added to the delay circuit 1.
5 and is input to the latch circuit 14. The signal corresponding to this new slip angle θs is sent to the sampling clock circuit 1.
The data is held in the latch circuit 14 by the clock pulses outputted from the circuit 7 at every sampling clock period Δt.
At this time, the delay circuit 15 detects that the output of the latch circuit 14 is the new θ.
s, and accordingly, the input of the latch circuit 14 simultaneously changes to the new θs and a value ωs proportional to the slip angular frequency.
changes to the added value, and the latch circuit 14 receives the new value θs.
This is to prevent it from becoming impossible to hold. Generally, the latch circuit 14 and the adder circuit 16 have a delay time, so
In particular, the delay circuit 15 is not required in many cases. The slip angle signal θs, which is the output of the latch circuit 14 and changes every sampling clock period Δt, is added to the rotation angle signal θ from the rotational position detector 12 in the adder 13, and the output angle is the slip-synthesized angle signal. The signal becomes θo. Angular signal θS1
θ, θo are values corresponding to 0 and 2π and underflow,
It is a value between O and 2π that overflows. The sampling clock circuit 11 includes a rotor temperature detector of the induction motor, etc., as necessary, and variably controls the clock frequency. As an example, a block diagram composed of a hardwired logic circuit was shown and explained.
In order to perform only this slip synthesis using a software program using a microprocessor, in addition to the microprocessor and the memory that stores the program, there is an input circuit that inputs the rotation angle signal θ, and an input circuit that inputs the slip angular frequency ωS. An input circuit for outputting the output angle signal θo, an output circuit for outputting the output angle signal θo, and a sampling clock circuit are required.

The sampling clock is input to the microprocessor as an interrupt clock for starting the slip synthesis calculation program. When an interrupt occurs, the calculation program inputs the slip angle frequency from the input circuit and adds it to the previous calculation value θs of the slip angle stored in a register in the microprocessor. Furthermore, the rotation angle signal θ is inputted from the input circuit, and the newly calculated θs is added. This is outputted to the output circuit as an output angle signal θo. A series of calculations is completed with the above steps. Although it depends on the type of microprocessor, the above program processing of about 5 steps can be sufficiently processed in a time of 501 ts or less. If the output frequency of the frequency converter is about 50 Hz and the accuracy of the output angle θo is about 2 degrees, the sampling clock frequency should be about 1 KHz, that is, about 1 ms period, and the 50 Hz required for slip synthesis calculations is sufficient.
The remaining 950 μs can be used for other controls, such as switching element ignition control, calculations necessary for speed control, etc. In addition, the present invention has the advantage of being able to perform continuous slip synthesis necessary for four-quadrant operation of an induction motor without requiring any special treatment for the rotation direction or the positive or negative slip frequency.

Therefore, according to the slip synthesis method of the present invention, since an output is obtained as an angle signal, a speed control device for an induction motor that performs slip frequency control can be realized with a simple configuration.

Fig. r shows an embodiment in which a combination of a thyristor rectifier and a current source inverter is used as a frequency converter and the slip frequency is controlled to be constant. The frequency of the alternating current 18 is converted by a frequency conversion device 19. Frequency conversion device 19
is converted into DC by the thyristor rectifier R1, and then the reactor L1, thyristors S1 to S6, and diodes D1 to D
6. The frequency is converted into alternating current using a current source inverter device called a series diode type, which is composed of capacitors C1 to C6. A detailed description of the frequency conversion device 19 constructed using these known techniques will be omitted, but the amplitude of the output current is controlled by the thyristor rectifier R1, and the frequency is controlled by sequentially firing the thyristors S1 to S6 of the current source inverter device. It has the characteristic that it can be controlled by The output of a rotation speed generator 22 attached to the shaft of the induction motor 20 and the output of a speed setter 32 are input to a speed control circuit 30. A signal corresponding to the output speed deviation of the speed control circuit 30 is separated by the absolute value circuit 28 into an absolute value signal ABS and a signal SIGN indicating the sign. The absolute value signal ABS is input as a current reference signal to the phase control circuit 24 together with the current detection value of the current detector 23, and the thyristor rectifier R1
The firing phase of each thyristor is controlled to control the current. On the other hand, the slip frequency set value of the slip frequency setter 31 is input to the complement circuit 29, and depending on the signal SIGN indicating the sign of the absolute value circuit 28, a signal proportional to the positive slip frequency as it is or a negative slip obtained by taking the complement. The signal is input as a frequency-proportional signal to the slip synthesis circuit 27 of the present invention together with the rotational position signal from the rotational position detector 21. The slip-synthesized output angle signal θo is input to a read-only memory, and the main thyristors S1 to S1 of the current source inverter corresponding to the output angle signal θo set in advance in this memory are input.
The ignition signal of S6 is output. This ignition signal is amplified and isolated by a gate amplifier 25 and is applied to the gate of each main thyristor. The relationship between the firing signal of each thyristor with respect to this output angle signal θo is shown in FIG. 8, and the current waveform of each phase of the induction motor 20 with respect to the temporal change of the output angle signal θo when controlling to such a relationship is shown in FIG. As shown in the figure. FIG. 9 shows that after time t, the frequency applied to the induction motor 20 is lower than before time t. With this configuration, the output angle signal increases or decreases depending on the polarity of the rotational position detector and the slip frequency, and the firing phase sequence of the main thyristor is switched, allowing continuous control from forward rotation to reverse rotation and from reverse rotation to forward rotation. can be done easily. This is an electric motor speed control device that takes advantage of the induction motor's robust structure and lack of brushes to provide easy maintenance and continuous reversible operation. As explained above, according to the present invention, slip synthesis is treated as a digital quantity, so it can be performed in a short calculation time, which is particularly suitable for microprocessor applications, and it is easy to change the constants of an induction motor or change the induction motor. I can handle it.

This slip synthesis method is particularly suitable when applying control by a microprocessor to the entire control device of an induction motor. This can significantly reduce the cost and improve the reliability of the control circuit.

[Brief explanation of drawings]

Figures 1 and 2 are block diagrams for the conventional slip frequency synthesis method, Figure 3 is a block diagram for controlling the firing of switching elements of a frequency converter from pulse signals synthesized by the slip frequency, and Figure 4. 5 is a diagram for explaining the method of deriving the slip angle by sampling calculation, FIG. 5 is a block diagram of an embodiment of the present invention, FIG. 6 is an operation waveform diagram of each part of FIG. 5, and FIG. A block diagram of an example of application of the invention suitable for speed control of an induction motor, FIG. 8 is a diagram showing the relationship between the output angle in FIG. 7 and the firing signal of the thyristor, and FIG. 9 is a diagram showing the relationship between the output angle in FIG. FIG. 2 is a waveform diagram showing the relationship between each phase current of an induction motor and a temporal change. 1, 22... Rotational speed generator, 2, 6...
...Slip frequency setter, 3...Analog adder, 4,7...Voltage-frequency converter, 5...
... Rotating pulse generator, 8 ... Pulse synthesis circuit, 9, 10 ... Counter circuit, 11 ...
...Gate signal logic circuit, 12, 21... Rotational position detector, 13, 16... Digital adder,
14...Latch circuit, 15...Delay circuit, 17...Sampling clock generation circuit,
18... AC power supply, 19... Frequency converter, 20... Induction motor, 23...
・Current detector, 24... Phase control circuit, 25.
...Gate amplifier, 26 ... Read-on memory, 27 ... Slip synthesis circuit, 28 ...
... Absolute value circuit, 29 ... Complement circuit, 30.
... Speed control circuit, 31 ... Slip frequency setting device, 32 ... Speed setting device.

Claims (1)

[Scope of Claims] 1. A device comprising means for obtaining a digital quantity proportional to the slip angle of the induction motor and a means for obtaining a digital quantity proportional to the rotation angle of the induction motor, and adding the digital quantities obtained by these means. A slip synthesis method for an induction motor, characterized by: 2. The induction motor according to claim 1, wherein the digital quantity proportional to the slip angle is a value obtained by sequentially adding digital quantities corresponding to the slip frequency of the induction motor at predetermined time intervals. Slip synthesis method. 3. The digital quantity proportional to the slip angle is a value obtained by sequentially adding digital quantities corresponding to the slip frequency of the induction motor at time intervals proportional to the secondary time constant of the induction motor. A slip synthesis method for an induction motor according to scope 1.