Method and apparatus for RMS measurements in induction motor without sampling

An apparatus for determining the RMS current in induction motor windings, the RMS current being used for motor control, the apparatus to be used with a motor controller, the controller controlling the motor by providing an alternating current in each stator winding, the apparatus comprising:

• a detector sensing the analog waveform of one stator winding current and producing an analog signal representative of the analog waveform;
• a converter to convert the analog signal into a series of pulses, the frequency of said pulses varying with the amplitude of the analog signal;
• a frequency averager receiving the pulses from the converter and determining an average pulse frequency;
• a current averager receiving the average pulse frequency and determining an average line current;
• and
• an RMS current calculator receiving and using the average line current to determine RMS current in the stator winding.

A method for determining the RMS terminal voltage for at least one motor winding, the RMS terminal voltage being used for motor control, the method to be used with a motor controller, the controller controlling the motor by periodically triggering solid state switches which are connected in series to each stator winding, untriggered switches operating as open circuits and triggered switches operating as closed circuits, the controller also providing a power factor angle between current and voltage, a cycle period equal to the time necessary to complete one-half of a cycle of the alternating current, and a notch value being the duration of a non-conducting state in terms of degrees of a cycle of the alternating current, the method comprising the steps of:

• sensing a supply line voltage signal;
• determining RMS supply line voltage from the supply line voltage signal;
• sensing the voltage drop across said solid state switch;
• and
• calculating the RMS terminal voltage using the supply line RMS voltage, the voltage drop across the solid state switch, the power factor angle, and said notch.

An apparatus for determining the RMS terminal voltage for motor windings, RMS terminal voltage being used for motor control, the apparatus to be used with a motor controller, the controller controlling the motor by periodically triggering solid state switches which are connected in series to each stator winding, untriggered switches operating as open circuits and triggered switches operating as closed circuits, the controller also providing a power factor angle between current and voltage, a cycle period equal to the time necessary to complete one-half of a cycle of the alternating current, and a notch value being the duration of a non-conducting state in terms of degrees of a cycle of the alternating current, the apparatus comprising:

• a first detector for sensing a supply line voltage signal;
• a supply calculator which receives and uses the supply line voltage signal to determine the RMS supply line voltage;
• a second detector for determining the voltage drop across the solid state switch;
• and
• an RMS terminal voltage calculator receiving and using both the supply line RMS voltage, the voltage drop across the solid state switch to determine the RMS terminal voltage, the power factor angle, and said notch.

A method for determining the RMS terminal voltage for motor windings, the RMS terminal voltage being used for motor control, the method to be used with a motor controller, the controller controlling the motor by alternating an average stator winding by producing a non-conducting period in each stator winding, the controller also providing a cycle period and a proportionality constant relating an average pulse frequency to an average current, the method comprising the steps of:

• detecting the current waveform in a single stator winding;
• generating an analog current signal representative of the current waveform;
• rectifying the analog current signal to produce a rectified signal;
• converting the rectified signal into a series of pulses, the frequency of said pulses varying with the amplitude of the rectified signal;
• counting the number of pulses during a single cycle period to produce a pulse count;
• dividing the pulse count by the cycle period to produce an average pulse frequency;
• determining the average current through the stator winding from the average pulse frequency according to the expression:
• I.sub.ave =K.sub.f .multidot.f.sub.ave
• where I.sub.ave is the average current through the stator winding, f.sub.ave is the average pulse frequency, and K.sub.f is the proportionality constant relating the average current and the average pulse frequency;
• and
• determining the RMS current in the stator winding according to the expression:
• ##EQU20## where .gamma. is a duration of the a non-conducting period in the stator winding, I.sub.ave is the average stator winding current, and I.sub.rms is the RMS current.