Method and apparatus for standstill recognition when restarting a power converter-fed AC motor without a tachometer generator

A method for recognizing a direction of rotation of a three-phase AC motor driven by an inverter, said method comprising the steps of:

• applying a DC current to a phase of the motor during a first time interval;
• measuring a time characteristic of a motor current during the first time interval;
• disabling the driving of the motor and allowing the motor to run in idle operation during a second time interval;
• short-circuiting the motor across its terminals during a third time interval;
• measuring a time characteristic of the motor current during the third time interval;
• recording a plurality of orthogonal current components based on the measurements in said first and third time intervals,
• wherein recorded reactive current components of said first and third time intervals are evaluated such that it is determined that the motor is rotating in a first direction when an instantaneous value of said reactive current component at a beginning of said first time interval has a negative operational sign and an instantaneous value of said reactive current component at a beginning of said third time interval has a positive operational sign, and it is determined that the motor is rotating in a second direction when said operational signs are inverted.

An apparatus for recognizing a standstill condition of a three-phase AC motor driven by an inverter, said apparatus comprising:

• a coordinate converter;
• a plurality of memory units;
• a plurality of double switches coupled between said coordinate converter and said plurality of memory units;
• an evaluation unit connected to said plurality of memory units;
• and
• a sequencing control coupled to said evaluation unit, said sequencing control being coupled to a control input of each of said plurality of double switches and to a control element and a trigger element of the power converter.