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A.C. motor control apparatus and method
| Details |
Inventors: Zankl, Frank; Kirkham, Edward E.; Stobbe, Richard E.; Schachte, John J.;
Assignee: Kearney & Trecker Corporation (West Allis, WI)
Primary Examiner: Rubinson; Gene Z.
Assistant Examiner:
Attorney, Agent or Firm: Hajewski; Cyril M.
In apparatus for controlling the speed and torque of a multiphase a.c. motor in accordance with digital control signals, a first regulation circuit regulates the frequency of the a.c. electrical signal applied to the motor and another regulation circuit regulates the voltage of the a.c. signal. Both regulation circuits are controlled by digital control signals in a series of discrete steps over a range of control values and the control signals are supplied periodically to the regulation circuits. The speed and torque are controlled in accordance with an algorithm relating speed and torque to the frequency and voltage of the source of electric power which is applied to the motor, calculated by computing apparatus on a periodic basis. In one embodiment the position of the magnetic flux of the stator is controlled directly. |
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DETAILED DESCRIPTION In one illustrative embodiment of the present invention there is provided a plurality of power transistors connected to the several phases of a multiphase induction motor, a variable frequency voltage source connected to said transistors, and a variable voltage d.
c.
source selectively connected to said motor through said transistors in accordance with said variable frequency.
Both the variable voltage source and the variable frequency source are controlled periodically by digital control signals.
In connecting the variable frequency source to the motor, the power transistors maintain a connection between the voltage source and the several phases of the motor for varying periods of time in order to obtain precise control.
The motor current is sensed, and an excessive current condition quickly temporarily disables the connection between the voltage source and the motor, and then permanently (until manually reset) disables such connection if the high current condition persists beyond a predetermined interval.
The digital control signals are produced by computing apparatus which periodically calculates the appropriate digital control signals by use of an empirically derived algorithm relating motor speed and servo following error to frequency and amplitude of the driving voltage.
Motor speed and servo following error are sampled during each successive 8.
3 milliseconds period, and the digital control signals are updated each period.
The digital control signal for the frequency of the driving voltage is chosen, for one mode of operation, such that the driving frequency is either 30 Hz.
or 60 Hz.
, depending on the instantaneous motor speed, and a variable frequency, up to 90 Hz.
, for a rapid traverse mode of operation.
A hysteresis quantity is recognized in the frequency selection, and the configuration of the motor is changed, under conditions of zero driving voltage, when a transition is made into, or out of, the rapid traverse mode.
The amplitude of the driving voltage is calculated, by applying a predetermined relation of voltage and following error for the chosen frequency, such that the voltage produces the torque necessary to maintain the servo following error at a predetermined level
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