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