Three-phase induction motor drive with frequency converter control.

p0001The invention relates to an AC drive with frequency, wherein the frequency converter comprises a pulse width modulator and a DC intermediate circuit asufweist whose voltage is generated by an uncontrolled rectifier on the net.

p0002For inverters whose DC voltage is generated by uncontrolled rectifiers from the network, as they are often used for cost reasons in particular for smaller AC drives, the DC link voltage is voltage-dependent, resulting in a direct function of the speed of the driven electric motor to power supply variation.

p0003Since this is disadvantageous for many applications, the present invention provides an object to prevent such fluctuations in speed with the simplest possible means in order to achieve high speed stability even in an otherwise unregulated AC drives of the type mentioned.

p0004The object referred to is in accordance with the invention in that a monitoring unit for continuous monitoring of the intermediate circuit voltage is provided which is connected to a control input of the pulse width modulator of the frequency converter and serves for a change in the DC link voltage inversely proportional change in the pulse width. Thus, the invention anticipates and makes use of the fact that in modern frequency converters of the above-mentioned type, the excitation voltage is generated for the three-phase motor by pulse width modulation, which opens up the very simple way, fluctuations in the intermediate circuit voltage, arising as noted by power fluctuations to use at an opposite change in the respective pulse width, so that the impact on the speed of the driven motor can be prevented by very simple means.

p0005According to an advantageous embodiment of the invention provides that the monitoring unit for continuous monitoring of the intermediate circuit voltage of a logic device such. B. formed a Mirko processor, which also serves as a pulse width modulator. Thus the extra effort in order to achieve a high speed stability is extremely low and enables easy, cost-effective and operationally reliable training.

p0006The invention is illustrated below with the aid of an exemplary embodiment schematically illustrated in the drawing in a diagram in detail.

p0007A three-phase motor l is connected via supply lines 2, 3, 4 with the center points of a three-phase bridge 5 having two power semiconductor 6 in each of its bridge arms. The three-phase bridge 5 is preceded by a DC intermediate circuit, whose intermediate circuit voltage is generated by an uncontrolled rectifier 7 directly from the AC mains. Parallel to the DC outputs of the rectifier 7, a filter capacitor 8 in order to reduce the ripple is turned on.

p0008The power semiconductor 6 of the three-phase bridge 5 are controlled by a microprocessor 9 on no further interest here kind, thus enabling the generation of the necessary for the operation of the AC motor l rotating field from the intermediate circuit voltage. It is only important the fact that this, as well as common in many modern frequency converters, pulse width modulation is applied, in which the supply voltage for the AC motor l is varied in accordance with the requirements of their pulse width. The corresponding Steueraus transition of the microprocessor 9 is indicated with l0.

p0009In consequence of the uncontrolled rectifier 7 fluctuations have in the mains voltage, which can easily reach a few percent, almost immediately in corresponding fluctuations in the intermediate circuit voltage, which in turn on the three-phase bridge 5 to an immediate influence of the three-phase motor supplied voltage and thus to an undesired fluctuation would result in its speed. To prevent this, the DC link voltage via lines ll, l2 and the voltage divider formed by the two resistors l3, l4 the microprocessor 9 is supplied, which also serves as a monitoring unit in order to control the current of the DC link voltage. The realized in the microprocessor 9 pulse width modulator is proportional signal supplied to a control input with the intermediate circuit voltage or the variation of the DC link voltage, thus a decrease According to the DC link voltage can be a sinking line voltage to equal offset by a corresponding increase in the respective pulse width very easily. Conversely, an increase according to the intermediate circuit voltage of an increase of the mains voltage are compensated by corresponding reduction of the pulse width, so that the supply voltage for the AC motor l kept constant and high speed stability can be achieved.

p0010Apart from the illustrated implementation of the monitoring unit for the intermediate circuit voltage and the pulse width modulator in a microprocessor, a purely hardware-configuration of these two institutions would of course also conceivable and would equally provide the advantage of an independent of voltage fluctuations in the grid constant supply of the AC motor l.