DE3701208C2 - - Google Patents

Description

The invention relates to a control method for the Control the torque of one over Pulse inverter powered multi-phase electric motor according to the preamble of claim 1.

So far, there was where an electric motor to drive a Sewing machine or the like was used, the Operating speed is changed while the engine speed remains unchanged, the clutch force for transmission of the torque used to use the clutch Speed control. However, with this Procedure of maintenance of the clutch is rather tedious.

To overcome this difficulty, an inverter was created used the speed of an electric motor, in particular to control an AC motor. in the general became a Pulse amplitude modulation control method (PAM) and a Pulse width modulation control method (PWM) for control of the converter.  

A pulse width modulation control method for controlling the converter is described in Japanese Patent Application (OPI) No. 188377/1984. In this method, a triangular wave is compared with a sine wave, which can be controlled in terms of its maximum value and its frequency in order to form a control pulse for controlling the motor, and a nominal DC voltage, which is switched using the control pulse, is supplied to the motor. According to this method, by controlling the pulse width and the repeating period of the control pulse, the duration (or pulse width) and repetition frequency of the voltage supplied to the motor are controlled. If the duration of the voltage is long, this means that a high voltage is supplied to the motor, which controls the operating voltage of the motor. If the repetition frequency is high, this means that a voltage with high frequency is supplied to the motor, which controls the operating frequency of the motor. As shown in Fig. 1, until the operating frequency reaches a nominal value, a constant driving torque is achieved, the ratio of operating voltage v to operating frequency fv / f remaining unchanged, whereupon when the operating frequency exceeds the constant nominal output value, the drive with unchanged Operating voltage is carried out.

However, when driving the engine with the in this way pulse-controlled reversing stage during start-up the engine, d. H. during engine running at lower Speed, the voltage supplied to the motor, the voltage obtained by rectifying the AC input voltage with which it is excessively high. Consequently reaches the electromagnetic noise caused by Stray currents of the motor is caused considerable volume.  

If the converter is controlled by means of the pulse amplitude modulation control method, waveforms of the phase voltages supplied to the motor according to FIG. 2 result, which are referred to as "six-stage waveforms". The waveforms shown in Fig. 2 are thus square waves, including a number of low order harmonic components (the third harmonic, the fifth harmonic, etc.). Therefore, the engine rotates not only by means of the fundamental, but also by means of the low order harmonic components. Therefore, especially when the engine is running at low speed, torque ripple occurs, which is associated with vibration of the mechanical system.

Therefore, not only in the case where the converter according to the pulse amplitude modulation control method is controlled, but also in the case where it according to the Pulse width modulation control method is controlled generates unpleasant noise or vibrations. Becomes the motor is used to drive a sewing machine, so make this noise or vibration Operator nervous as the engine is on the bottom of the sewing machine table is housed.

From US-PS 34 03 318 is a conventional static Inverter system with frequency control of the Output voltage known as a function of the input voltage.

From JP-OS 59-188 377 is an inverter and Control circuit known, which the possibility of Offers pulse width modulation and a converter stage and contains a control device. This is just pulse width modulation performed.  

From the literature "Power Electronics - Basics and applications ", 1977, pp. 172-198 are inverters known in which the input voltage by a DC chopper is controlled, or the Output voltage controlled by pulse width control can be.

From "Frequency-controlled drives with cage armature motors", Brown Boveri Mitt. 9-71, pp. 407-415 Inverter circuits known for Frequency control of asynchronous machines are suitable. To the frequency of the supply voltage is changed for this purpose.

In "IEEE Transactions on Industry Applications", vol. JA-8, No. 5, Sept./Oct. 1972, pp. 584-592, is a circuit described for pulse width modulation, both the Frequency as well as the DC voltage of the pulses changed is so that the relationship between voltage and frequency remains constant.

The known from these publications Circuit structures are not suitable the previously eliminate the problems presented. It therefore join these circuit structures a torque ripple or Noise at low engine speeds.

From DE-OS 33 13 120 is a control device for an inverter known, which is a generic Executes tax procedure in which a DC link voltage is generated, which is proportional to Operating frequency is controlled, with a simultaneous Pulse width modulation takes place. This tax procedure However, requires a lot of circuitry because the voltage regulator very low voltages for the DC link voltage must be able to produce.

Based on a tax procedure of the aforementioned Art is the object of the invention, the known to further improve generic control methods, so that noise caused by torque ripple is reduced be, with a simpler circuit arrangement with a simple voltage regulator should be used.

The task is solved by a tax procedure of type mentioned at the beginning with the characteristics of the license plate of claim 1.

In the control method according to the invention DC voltage kept low until the operating frequency reaches a predetermined value, and after the Operating frequency exceeds the specified value the DC voltage increases with the operating frequency. During engine start-up, i. H. during the Engine operation at low speed, that will be Pulse amplitude modulation control method and the Pulse width modulation control methods in combination with Control of the engine used.

The invention has the advantage that the supplied Voltage is made low at the beginning, which makes Torque ripple and stray currents become small, so that the vibrations of the mechanical system on one Minimum be reduced.

While the engine is running at low speed the voltage supplied to the motor is kept low and has a sine waveform so that the Torque ripple and thereby the extent of Vibrations and / or noise in mechanical System, d. H. in the engine.  

The noise generated in the mechanical system don't just include those through the Torque ripple are generated, but also electromagnetic noise caused by Motor leakage currents as a result of the rise and fall of the Control impulse are caused. However electromagnetic noise also made small, since the voltage supplied when the engine is started is small is made.

The drawings show:

Fig. 1 is a graph showing the operating line of an inverter,

Fig. 2 is a waveform representation of the six-step waveform of the inverter is present in the pulse amplitude modulation control,

Fig. 3 is a diagram for describing an embodiment of a control method of the invention for a frequency converter,

Fig. 4 to 6 waveform diagrams indicating the signals at various circuit points in the circuit of Fig. 3,

Fig. 7 is a graphical representation of the operating characteristics of an embodiment of the method according to the invention, and

Fig. 8 is a graphical representation to explain a switching characteristic.

To explain an embodiment of the invention, reference is made to the circuit of FIG. 3.

In Fig. 3, reference numeral 1 denotes a three-phase voltage network, 2 a- 2 n represent a diode rectifier, 3 a- 3 g are transistors, 4 is a coil, 5 a, 5 b are capacitors, 6 a, 6 b are resistors and 7 is an asynchronous motor. Furthermore, the reference numeral 11 denotes a Schmitt trigger, 12 denotes a differential amplifier, 13 a- 13 c reversing stages (non-gate), 14 a- 14 c comparators, 15 a triangular voltage generator, 16 a- 16 c multipliers, 17 a three-phase sine wave generator , 18 a voltage-frequency converter, and 20 a control circuit. The transistors 3 b- 3 g and the diodes 2 h- 2 n represent a converter.

The control circuit 20 provides a DC voltage value Vdc for controlling the output voltage of the transistor 3 a at its terminal 20 a, an AC voltage command value for controlling the pulse widths of the comparators 14 a- 14 c output control pulses, at its terminal 20 b, and a signal F to Control of the motor frequency. In particular, the control circuit is able to output a voltage setpoint and a speed setpoint for the asynchronous motor 7 ; that is, the control circuit performs constant control of the voltage / frequency ratio and acceleration and deceleration control. The control circuit 20 consists of microcomputers. The voltage-to-frequency converter 18 is also referred to as a "V / F converter". The three-phase sine wave generator 17 is capable of generating three-phase sine waves with a frequency corresponding to the output frequency of the V / F converter 18 .

The built up in the manner described above Circuit works as follows:

The signal Vdc supplied to the terminal 20 a of the control circuit 20 is fed via the differential amplifier and the Schmitt trigger 11 to the transistor 3 a, which works as a chopper, so that the capacitor 5 b is charged via the coil 4 . The voltage of the capacitor charged in this way is supplied to the differential amplifier 12 . Thus, the terminal voltage Vc of the capacitor 5 b by the signal Vdc is determined, which is supplied to the terminal 20 by the control circuit.

The signal F for controlling the motor frequency, which is supplied to the terminal 20 c from the control circuit 20 , is fed via the voltage-frequency converter 18 to the three-phase sine wave generator 17 , so that the latter 17 outputs three-phase signals for control, the frequency of which Signal F corresponds. The components used to control the three-phase signals are multiplied by the output voltage V b at terminal 20 in each of the multipliers 16 present 16 a- c. Thus, the amplitudes of the three-phase control signals output by the multipliers are controlled by the output voltage V.

The three-phase control signals supplied by the multipliers 16 a- 16 c are each fed to the comparators 14 a- 14 c, to which the triangular voltage output by the delta voltage generator 15 is fed. In this case, according to section a of Fig. 4, the pulse width is determined in relation to the amplitude of the three-phase control signal, while the maximum value of the three-phase control signal is smaller than that of the delta voltage, and that pulse is output as a control pulse which in the Section b of Fig. 4 is shown. The average value of the control pulse for each period changes according to section b of FIG. 4 with the same period as the three-phase control signal of section a of FIG. 4. Thus, the amplitude and frequency of the fundamental wave of the control pulse according to section b of FIG Fig. 4 is the same as that of the three-phase control signal, excluding low order harmonics. The pulse width and the repeating period change with the amplitude and frequency of the three-phase control signal.

If the amplitude of the three-phase control signal is increased and its maximum value exceeds the maximum value of the triangular wave, as is shown in section a of FIG. 5, a control pulse according to section b of FIG. 5 is emitted. This control pulse is not proportional to the change in amplitude of the three-phase control signal, and the pulse waveform of section b of Figure 5 contains low order harmonics; however, the amplitude of the fundamental waves of the three-phase control signal can be made larger than the fundamental wave amplitude according to section b of FIG. 4. If the amplitude of the three-phase control signal is made larger, the control pulse, as shown in FIG. 6, comprises only the fundamental frequency of the three-phase control signal.

The transistors 3 b 3 are switched by the A of the comparators 14 14 c control pulses emitted conductive and locking g. A drive current is supplied from the capacitor 5b via the transistor rendered conductive to the induction motor 7 to enable these to rotate. In this case, if the output voltage V is controlled in the manner described above, the maximum value of the three-phase signal can be controlled as shown in FIGS. 4 to 6. If the signal F is controlled, the frequency of the three-phase control signal can be controlled. These control data are contained in the control pulses which are output by the comparators 14 a, 14 b, 14 c and the control pulses control the current supplied to the asynchronous motor 7 . Therefore, the power supplied to the asynchronous motor 7 , based on the operating voltage and operating frequency, is controlled by the voltage V and the signal F.

In general, in the case where the engine changes with the Operating voltage and operating frequency is operated, a drive with constant torque performed, the ratio of operating voltage v is constant at the operating frequency f, until the operating frequency reaches the nominal value f₃, and after the operating frequency the nominal value f₃ has exceeded, a drive with constant Output signal carried out, the operating voltage remains unchanged.

When driving with constant torque, the output voltage V and the signal F are controlled. If the level of the output voltage V is increased, the from the comparators 14 a- 14 c control output pulses shown in FIGS. 5 and 6, the highest voltage or there arise wide individual pulses reach the controller of FIG. 6 made when the operating frequency of the Nominal frequency exceeds f₃, and harmonics of low order and thus torque ripple and possibly vibrations occur. The torque ripple increases with falling operating frequency f. Therefore, it is desirable that no wide single pulses be generated as long as the operating speed is low.

To this end, FIG according. 7, to the operating frequency f₂ reached, is kept constant the capacitor 5 b the voltage supplied by the signal Vdc, which is supplied to the terminal 20 a of the control range at a minimum value VCDs. Under this condition, by controlling the output voltage V and the signal F, the drive is controlled so that the ratio v / f, as in the area A of FIG. 1, is maintained unchanged. Thus, until the operating frequency f 1 is reached, the voltage supplied to the motor is kept constant and an operation according to FIG. 4 is carried out so that the torque ripple is low.

If the operating frequency is between f 1 and f 2 according to FIG. 7, the operation according to FIG. 5 is carried out. Therefore the torque ripple increases.

Exceeds the operating frequency f₂, the operation shown in Fig. 6 is carried out, and therefore the pulse width can not be controlled with the output voltage V. Therefore, the output voltage V is controlled in place of the DC voltage Vdc to gradually increase the capacitor 5 b supplied voltage and thus to control the operating voltage; ie there is a drive with constant torque, so that the ratio of operating voltage v to operating frequency fv / f is kept unchanged until the operating frequency f₃ is reached.

Has reached the operating frequency f₃, a Drive performed with constant output signal, whereby the operating voltage is set to the nominal voltage Vc is.

After reaching the operating frequency f₂, the operation is performed with the waveform shown in Fig. 6 and the low-order torque ripple is obtained. However, in general, at an operating frequency of 30 Hz or more, the torque ripple hardly affects the mechanical system, and vibration and noise are neglected.

As is apparent from the preceding description, in the method according to the invention, if the operating frequency is f₂ or lower according to FIG. 7, the pulse amplitude modulation control and the pulse width modulation control are used in combination and the pulse width modulation control is used on the converter while the motor supplied voltage is kept low, whereupon when the operating frequency exceeds f₂, the voltage supplied to the motor is increased.

The pulse broadening starts at f₁ and is at f₂ to a full six step voltage waveform; is f₁ fixed, the frequency is f₂ also determined from which the control with constant ratio v / f is carried out. The frequencies f₁, f₂ will selected such that no resonance in the mechanical System is generated.

The switching frequency of the switching on and off of chopper 3 a depends on signal Vdc. As can be seen from Fig. 8, the switching frequency n is high until the operating frequency f 1 is reached and is abruptly decreased after f 1 until the operating frequency f 2 is reached, and after f 2 a square wave voltage form is obtained so that the switching frequency n is equal to Operating frequency.

In the previously described embodiment uses a chopper transistor; however, this can replaced by a thyristor circuit will.

Claims (5)

1. Control method for controlling the torque of a multi-phase electric motor (IM) fed via a pulse inverter, the pulse inverter being supplied with a DC voltage that increases with the motor frequency in at least one frequency range, characterized in that

• a) in a first, lower frequency range (to f₁) of the electric motor, the DC voltage (V _DCS ) supplied to the pulse-controlled inverter is kept constant at a low voltage value and the pulse width of the voltage pulses supplied to the electric motor (IM) increases with increasing motor frequency; and
• b) in a second, upper frequency range (<f₂) the DC voltage supplied to the pulse inverter is increased with increasing motor frequency, the pulse width of the voltage pulses being at a maximum.

2. The method according to claim 1, characterized records that the DC voltage by a switching regulator upstream of the inverter is set.   3. The method according to claim 2, characterized records that the switching frequency of the Switching regulator is matched to the motor frequency. 4. The method according to any one of the preceding claims, characterized, that the switching regulator with a high switching frequency works. 5. The method according to claim 1, characterized records that the electric motor to drive a sewing machine is used.