Apparatus to reduce no load current at low frequencies

A method to be used with a motor controller which adjusts motor operation at low frequencies and under no load conditions by altering PWM command signals as a function of a torque current i.sub.qe wherein current i.sub.qe is a real component of an apparent current and forming a phase angle therewith, the motor having a rated frequency and low frequencies being between zero hertz and a maximum percent of the rated frequency where the maximum percent is 50% or less, the controller including a sensor which is incapable of precisely determining the phase angle at low frequencies, the method for compensating for phase angle errors, the controller providing an applied stator voltage frequency signal, the method comprising the steps of:

• integrating the applied frequency signal to provide a perceived phase angle;
• generating a phase correction angle as a function of the applied frequency signal;
• providing stationary current feedback signals i.sub.qs, i.sub.ds, respectively;
• and
• mathematically combining the perceived angle, the correction angle and the current signal i.sub.qs and current signal i.sub.ds to provide a torque current i.sub.qe value.

An apparatus to be used with a motor controller which adjusts motor operation at low frequencies and under no load conditions by altering PWM command signals as a function of a torque current i.sub.qe wherein current i.sub.qe is a real component of an apparent current and forming a phase angle therewith, the motor having a rated frequency and low frequencies being between 0 hertz and a maximum percent of the rated frequency where the maximum percent is 50% or less, the controller including a sensor which is incapable of precisely determining the phase angle at frequencies less than about half of the motor rated frequency, the apparatus for compensating for phase angle errors, the controller providing an applied voltage frequency, a stationary current signal i.sub.qs and a stationary current signal i.sub.ds, the apparatus comprising:

• an integrator receiving the applied frequency and integrating the applied frequency to provide a phase perceived phase angle;
• an error module including a look-up table which correlates applied frequencies with correction angles for the motor throughout a range of low frequencies, each correction angle indicating the error in the phase angle, the error module receiving the applied frequency and generating a correction angle by correlating the applied frequency and a corresponding correction angle;
• a first summer which adds the perceived angle and the correction angle providing a corrected angle;
• a sine calculator which receive the corrected angle and generates the sine of the corrected angle;
• a cosine calculator which receives the corrected angle and generates the cosine of the corrected angle;
• a first multiplier which multiplies the sine of the corrected angle and the stationary current signal i.sub.qs providing a first component;
• a second multiplier which multiplies the cosine of the corrected angle and the stationary current signal i.sub.ds providing a second component;
• and
• a second summer which subtracts the first component from the second component to provide the torque current i.sub.qe.