Method for field-oriented control of a frequency converter for a three-phase motor

Abstract

A method for field-oriented control of a frequency converter for a three-phase motor includes the setting of a new position of the rotary field in the electric motor being performed by voltage pulses for the stator coils. An amplitude and the angle of the rotary field vector are specified by the duration of the voltage pulses for the respective coils and by their temporal offset. The duration and the offset of voltage pulses for the stator coils are the result of the calculation of manipulated variables in a digitally controlled process in a coordinate system fixed in respect of the rotor, depending on the prevailing angular rotation (theta) and the prevailing speed of rotation (omega) as well as on the prevailing current values, a predetermined torque and a predetermined speed of rotation.

Claims

1. A method for field-oriented control of a frequency converter for a three-phase motor, which comprises the steps of: performing a setting of a new position of a rotary field in the three-phase motor by means of voltage pulses for stator coils, wherein an amplitude and an angle of a rotary field vector are specified by a duration of the voltage pulses for respective coils and by a temporal offset, wherein the duration and an offset of the voltage pulses for the stator coils are a result of a calculation of manipulated variables in a digitally controlled process in a coordinate system fixed in respect of a rotor, depending on a prevailing angular rotation and a prevailing speed of rotation as well as on a prevailing current values, a predetermined torque and a predetermined speed of rotation, and the manipulated variables calculated in the coordinate system that is fixed in respect of the rotor are converted through a reverse transformation into manipulated variables in the coordinate system fixed in respect of a stator; determining times for edges of the voltage pulses from the manipulated variables in the coordinate system fixed in respect of the stator; and dividing the calculation of the manipulated variables in the coordinate system fixed in respect of the stator for a second edge of a voltage pulse into two partial calculations with an angle of rotation for a first edge and with an angle of rotation from the first edge to the second edge, wherein a first partial calculation of manipulated variables for the first edge is also used for the second edge, and a second partial calculation of the manipulated variables is used for a predetermined number of coordinate rotations.

2. The method according to claim 1, wherein, in the second partial calculation, a reverse transformation matrix approximates a cosine of a partial angle of rotation by 1 and a sine of the partial angle of rotation by the partial angle of rotation itself.

3. The method according to claim 1, which further comprises: using a value of a time for the first edge of the voltage pulse for calculating a time for the second edge of the voltage pulse; and performing the reverse transformation into the coordinate system fixed in respect of the stator by means of an angle of rotation that is extrapolated from the angle of rotation for the first edge of the voltage pulse and the prevailing speed of rotation.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a schematic illustration of a frequency converter with field-oriented control for a three-phase motor with a permanent magnet motor according to the prior art; and

(2) FIG. 2 is an illustration showing the conversion from coordinates that are fixed in respect of the a rotor into coordinates that are fixed with respect to the stator using a calculation of two partial rotations according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) Referring now to the figures of the drawings in detail and first, particularly to FIG. 1 thereof, there is shown a synchronous three-phase motor with a permanent magnet rotor PMSM is driven in the known manner using three pulse-modulated voltages UR, US, UT. The pulse-modulated voltages UR, US, UT are generated by a modulation unit 6 which, for example, carries out a vector modulation.

(4) The currents IR, IS, IT which flow through the stator coils of the three-phase motor as a result of the pulse-modulated voltages UR, US, UT are detected, for example by use of shunt resistors, and are supplied to a transformation unit 7 which calculates from these the current pointers lalpha, Ibeta representing the field position in the motor in a complex coordinate system fixed in respect of the stator.

(5) The current pointers Ialpha, Ibeta of the coordinate system fixed in respect of the stator are converted in a conversion unit 8 into the current pointers ID, IQ in a coordinate system fixed in respect of the rotor. This requires the angle of the rotor position theta, which is determined in a rotor position estimator 9 from the pointers lalpha, Ibeta of the coordinate system fixed in respect of the stator and the manipulated variables Ualpha, Ubeta in the coordinate system fixed in respect of the stator.

(6) The current pointers ID and IQ of the coordinate system fixed in respect of the rotor are supplied to a current controller ID 2 or to a current controller IQ 3, wherein the current controller ID 2 controls the field-forming or field-weakening current ID and, in the example illustrated, is supplied with a setpoint variable of 0, since neither field reinforcement nor field attenuation are wanted. The current controller IQ 3 is supplied with a setpoint variable for the torque-developing current IQ. This setpoint variable is made available by a speed controller 1, to which on the one hand a setpoint rotation speed omega_set and on the other hand a prevailing rotation speed omega determined by the rotor position estimator 9 are supplied.

(7) The output signals of the current controller ID 2 and of the current controller IQ 3 are each added to an output signal of a decoupling or pre-control unit 4, to which both the current pointers ID, IQ of the coordinate system fixed in respect of the rotor, as well as the prevailing speed of rotation omega are supplied. The voltage pointers UD, UQ of the coordinate system fixed in respect of the rotor obtained by the additions are passed to a reverse transformation unit 5 which, using the rotor position angle theta from the rotor position estimator 9, transforms the voltage pointers UD, UQ of the coordinate system fixed in respect of the rotor into the voltage pointers Ualpha, Ubeta of the coordinate system fixed in respect of the stator. These voltage pointers Ualpha, Ubeta of the coordinate system fixed in respect of the stator are passed to the modulation unit 6.

(8) The reverse transformation from the coordinate system fixed in respect of the rotor into the coordinate system fixed in respect of the stator is highly computationally intensive, but should however be carried out at high speeds of rotation for both edges of a pulse-modulated drive signal for a three-phase motor, since the rotor can rotate onwards significantly during the period of the drive signal.

(9) The transformation matrix for calculating the voltage pointers U.sub., U.sub. of the coordinate system fixed in respect of the stator from the voltage pointers U.sub.D, U.sub.Q of the coordinate system fixed in respect of the rotor is illustrated in FIG. 2 in the upper line in the formula on the left.b The calculation of the sine and cosine values for the new rotor position angles .sub.0+ of the matrix elements is very computationally intensive, and requires a microcontroller that is capable of higher performance than would be commercially appropriate for the majority of applications.

(10) In the manner according to the invention, therefore, the transformation is divided through a trigonometrical transformation and matrix decomposition into two partial transformations. The first partial transformation calculates the voltage pointers U.sub., U.sub. fixed in respect of the stator at the time of the first edge of the drive pulse for the three-phase motor from the voltage pointers U.sub.D, U.sub.Q fixed in respect of the rotor, and can consequently be used for both edges, which saves computing time.

(11) The second partial transformation into the right-hand part of the first line of FIG. 2 calculates the partial rotation through a small angle, and is, in the manner according to the invention, only carried out approximately, in that the sine and cosine functions are replaced by a number of terms of a Taylor series for the sine and cosine functions sufficient for the accuracy requirements. In addition, the calculation is used for a number of partial rotations sufficient for the accuracy requirements, and consequently only recalculated for example every three or four partial rotations. This too saves a great deal of computing time, but nevertheless yields sufficient accuracy without entirely foregoing calculation of the second edge.

(12) An approximation for very small angles is finally shown in the lower line of the equation of FIG. 2, where only the first term of a Taylor series is used, in that the cosine is approximated by 1 and the sine of an angle approximated by the angle itself.