US 4,361,792 A · Digital induction motor control system

Abstract

A digital self-calibrating power factor controller for an AC induction motor. During start-up a first number corresponding to the actual phase angle between motor voltage and current is determined and compared with a second number corresponding to a desired delay in energizing the motor. The second number is varied until both numbers are approximately equal. A third number corresponding to a desired phase angle is determined and stored and the system switches from start-up to run. The first number corresponding to the actual phase angle is now compared with the third number corresponding to the desired phase angle. The second number corresponding to a desired delay is now varied with motor load so as to keep the actual phase angle equal to the desired phase angle.

Claims33

Dependent claims

A digital induction motor control system for controlling an induction motor based on a calibrated phase angle comprising:

(a) a clock for producing a train of pulses;
(b) a phase angle counter connected to said clock for counting said pulses to produce a digital number corresponding to the actual phase angle between motor supply voltage and motor current zero crossings;
(c) variable delay counter means connected to said clock for delaying energization of said motor by counting said pulses, beginning with a first digital number, to produce a firing command when said count has reached a second digital number, said delay occurring following said motor current zero crossing;
(d) phase angle storage means for storing a digital number corresponding to a calibrated phase angle;
(e) digital comparator means connected to said phase angle counter and to said storage means for comparing the actual phase angle in said phase angle counter with the calibrated phase angle in said storage means and for altering at least one of the two numbers in said delay counter means in response to said comparison so as to vary the delay in energizing said motor to maintain the actual phase angle in a predetermined relationship to the calibrated phase angle irrespective of variations in motor load;
and
(f) a switchable device adapted to be electrically connected in series with a winding of said induction motor and to said delay counter means for energizing said induction motor in response to said firing command from said delay counter means.

4 more elements

A self-calibrating digital induction motor control system comprising:

(a) a clock for producing a train of pulses;
(b) a phase angle counter connected to said clock for counting said pulses to produce a digital number corresponding to the actual phase angle between motor supply voltage and motor current zero crossings;
(c) variable delay counter means connected to said clock for delaying energization of said motor by counting said pulses, beginning with a first digital number, to produce a firing command when said count has reached a second digital number, said delay occurring following said motor current zero crossing;
(d) phase angle storage means connected to said phase angle counter for storing a digital number corresponding to a calibrated phase angle;
(e) digital comparator means connected to said phase angle counter and:
(i) during start-up being connected to said delay counter means for comparing the actual phase angle in said phase angle counter with the difference between the two numbers in said delay counter means and for automatically altering at least one of said two numbers so as to vary the delay in energizing said motor until the number in said phase angle counter and the difference between the two numbers in said delay counter means differ by a predetermined amount to thereby derive said calibrated phase angle;
(ii) during run being connected to said phase angle storage means for comparing the actual phase angle in said phase angle counter with the calibrated phase angle in said phase angle storage means and for altering at least one of the two numbers in said delay counter means so as to vary the delay in energizing said motor to maintain said actual phase angle in a predetermined relationship to said calibrated phase angle irrespective of variations in motor load;
and
(f) a switchable device adapted to be electrically connected in series with a winding of said induction motor and to said delay counter means for energizing said induction motor in response to said firing command from said delay counter means.

6 more elements

A method for operating a self-calibrating digital induction motor control system comprising the steps of:

(a) generating a train of clock pulses;
(b) counting said clock pulses to produce a digital number corresponding to the actual phase angle between motor supply voltage and motor current zero crossings;
(c) counting said clock pulses to delay energization of such motor, beginning with a first digital number, to produce a firing command when the count has reached a second digital number, said delay occurring following said motor current zero crossing;
(d) digitally comparing said number corresponding to said actual phase angle with the difference between said first and second numbers;
(e) automatically altering at least one of said first and second numbers in response to said comparison to vary the delay in energizing said motor until said number corresponding to said actual phase angle and the difference between said two numbers are approximately equal;
(f) storing one of said approximately equal digital numbers as the calibrated phase angle;
(g) digitally comparing said number corresponding to said actual phase angle with said number corresponding to said calibrated phase angle;
(h) altering at least one of said first and second numbers in response to said comparison so as to vary the delay in energizing said motor to maintain said number corresponding to said actual phase angle in a predetermined relationship to said number corresponding to said calibrated phase angle irrespective of variations in motor load;
and
(i) energizing said motor in response to said firing command following said delay.

7 more elements

A method for operating a self-calibrating digital induction motor control system comprising the steps of:

(a) generating a train of clock pulses;
(b) counting said clock pulses to produce a digital number corresponding to the actual phase angle between motor supply voltage and motor current zero crossings;
(c) storing a digital number corresponding to an initial actual phase angle as .theta..sub.1 ;
(d) counting said clock pulses to delay energization of said motor, beginning with a first digital number, to produce a firing command when the count has reached a second digital number, said delay occurring following said motor current zero crossing;
(e) digitally comparing said number corresponding to said actual phase angle with the difference between said first and second numbers;
(f) automatically altering at least one of said first and second numbers in response to said comparison to vary the delay in energizing said motor until said number corresponding to said actual phase angle and the difference between said first and second numbers are approximately equal;
(g) storing one of said approximately equal digital numbers as .theta..sub.2 ;
(h) determining N.sub.1 =/.theta..sub.1 ;
(i) determining N.sub.2 =K N.sub.1, where K is a constant;
(j) determining .theta..sub.3 =N.sub.2 .theta..sub.2 ;
(k) storing .theta..sub.3 as the calibrated phase angle;
(l) digitally comparing said number corresponding to said actual phase angle with said calibrated phase angle;
(m) altering at least one of said first and second numbers in response to said comparison so as to vary the delay in energizing said motor to maintain said actual phase angle in a predetermined relationship to said calibrated phase angle;
and
(n) energizing said motor in response to said firing command following said delay.

12 more elements

Dependencies are inferred from the claim text.

Digital induction motor control system

Life of this patent

Digital induction motor control system

Life of this patent

Abstract

Claims33