A.C. motor control apparatus and method

Claim 2

Apparatus for controlling a induction motor in accordance with digital input signals comprising, in combination; an input terminal for receiving a digital signal respresentative of a voltage level to be applied to the motor, counter means connected with said input terminal and adapted to be periodically set to a predetermined condition in accordance with said digital signal, a source of an a.c. signal connected with said counter means for causing said counter means to step sequentially through its states in response to said signal at a rate proportional to the frequency of said signal, said counter means being adapted to produce an output pulse at a predetermined time following each said setting, in accordance with the quantity represented by said digital signal, a source of a.c. voltage, rectifying switch means connected to said a.c. voltage source and to said counter, and responsive to the output of said counter for selectively rectifying said a.c. voltage to produce a d.c. voltage corresponding to said digital signal, and means for selectively applying said d.c. voltage to said motor.

Claim 3

Apparatus according to claim 1, including a power switch, means for connecting said power switch between said rectifying switch means and one terminal of said motor, and means for selectively energizing said power switch to selectively connect said d.c. voltage to said motor to energize said motor for operation.

Claim 4

Apparatus according to claim 2, including a plurality of power switches, one for each phase of a multiphase induction motor, and means for sequentially energizing said power switches for sequentially connecting said d.c. voltage to the input terminals of said motor.

Claim 5

Apparatus according to claim 1, wherein said source of an a.c. signal comprises a three phase source, means connecting the input of said counter means with one phase of said three phase source, to cause said counter to step sequentially through its states in synchronization with said one phase, second counter means, means for connecting said second counter means with a second phase of said three phase source to cause said second counter means to step sequentially through its states in synchronization with said second phase, third counter means, means connecting said third counter with the third phase of said three phase source to cause said third counter means to step sequentially through its states in synchronization with said third phase, and means responsive to said digital signal for periodically setting said first, second and third counters to an identical predetermined condition, said a.c. voltage source comprising a three phase voltage source, said rectifying switch being connected to one phase of said voltage source, second and third rectifying switches connected to the other phases of said voltage source and to said second and third counters, and responsive to the outputs of said counters for selectively rectifying said three phase voltage to produce said d.c. voltage at a level corresponding with said digital signal.

Claim 6

Apparatus according to claim 4, wherein said rectifying switch is an SCR, and including three additional SCR's individually connected with said counter, means for individually connecting said additional SCR's to said three phases of a.c. voltage, and means for causing each pair of SCR's which are connected to the same phase to selectively rectify said phase of said a.c. voltage at times spaced by 180° of phase, whereby said six SCR's comprise a three phase full wave rectifier.

Claim 7

Apparatus for controlling an induction motor in accordance with digital input signals comprising, in combination; an input terminal for receiving a digital signal representative of a frequency at which a.c. power is to be supplied to the motor, counter means connected with said input terminal and adapted to be periodically set to a predetermined condition in accordance with said digital signal, an oscillator connected with said counter means for causing said counter means to step sequentially through its states in response to said oscillator at a rate proportional to the frequency of said oscillator, said counter means producing a series of pulses having a pulse repetition rate equal to the frequency of said oscillator divided by a factor determined by the condition to which said counter means is set by said digital signal, a plurality of power switches comprising at least one power switch for each phase of said motor, a source of d.c. voltage, and means connected with said source of d.c. voltage, said counter means and said power switches to sequentially and individually connect said power switches with said d.c. voltage, under control of the pulses from said counter means.

Claim 8

Apparatus for controlling an induction motor in accordance with digital input signals comprising, in combination; an input terminal for receiving a digital signal representative of a frequency at which a.c. power is to be supplied to the motor, counter means connected with said input terminal and adapted to be periodically set to a predetermined condition in accordance with said digital signal, an oscillator connected with said counter means for causing said counter means to step sequentially through its states in response to said oscillator at a rate proportional to the frequency of said oscillator, said counter means producing a series of pulses having a pulse repetition rate equal to the frequency of said oscillator divided by a factor determined by the condition to which said counter means is set by said digital signal, a plurality of power switches comprising at least one power switch for each phase of said motor, a source of d.c. voltage, and means connected with said source of d.c. voltage, said counter means and said power switches to sequentially and individually connect said power switches with said d.c. voltage, under control of the pulses from said counter means.

Claim 9

Apparatus according to claim 6, wherein said last named means comprises a plurality of bistable devices, each of said bistable devices having one input connected to the output of said counter means and another input connected to an individual one of said bistable devices, whereby said bistable devices are caused to sequentially change their states by the pulses produced by said counter means such that a number of pulses from said counter means equal to twice the number of said bistable devices is required to cause each bistable device to resume its initial condition, each of said bistable devices being connected with one of said power switches whereby said power switches are individually energized in time sequence in accordance with the states of said bistable devices.

Claim 10

Apparatus according to claim 6, including a pair of said power switches for each phase of said motor, one of each pair of power switches being connected from a source of positive d.c. voltage to a terminal of said motor, and the other of each pair being connected from the same terminal of said motor to a source of negative d.c. voltage.

Claim 11

Apparatus according to claim 8, including three bistable devices and six power switches, each pair of said power switches being connected individually to two mutually exclusive outputs of said bistable devices.

Claim 12

Apparatus according to claim 7, including means connected to two of said bistable devices for selectively changing the order in which said three bistable devices change their states in response to pulses from said counter means.

Claim 13

Apparatus for controlling an induction motor in accordance with digital input signals comprising, in combination; a first input terminal for receiving a first digital signal representative of a voltage level at which a.c. power is to be applied to the motor, a first counter connected with said first input terminal and adapted to be periodically set to a predetermined condition, in accordance with said first digital signal, a source of an a.c. signal connected with said first counter for causing said first counter to step sequentially through its states in response to said signal at a rate proportional to the frequency of said signal, said first counter being adapted to produce an output pulse at a predetermined time following each said setting in accordance with the quantity represented by said first digital signal, a second input terminal for receiving a second digital signal representative of a frequency at which a.c. power is to be supplied to the motor, a second counter connected with said second input terminal and adapted to be set to a predetermined condition in accordance with said second digital signal, an oscillator connected with said second counter for causing said second counter to step sequentially through its states in response to said oscillator at a rate proportional to the frequency of said oscillator, said second counter producing a series of pulses having a pulse repetition rate equal to the frequency of said oscillator divided by a factor in response to the condition in which said second counter is set by said second digital signal, a source of a.c. voltage, an SCR connected to said a.c. voltage source and to said first counter, and responsive to the output of said first counter for selectively rectifying said a.c. voltage to produce a d.c. voltage corresponding to said first digital signal, a plurality of power transistors comprising at least one power transistor for each phase of said motor, and means connected with said SCR, said counter and said power transistors to sequentially connect said transistors with said SCR, under control of the output pulses from said second counter.

Claim 14

Apparatus for controlling an induction motor in accordance with digital input signals comprising, in combination; a first input terminal for receiving a first digital signal representative of a voltage level at which a.c. power is to be applied to the motor, a first counter connected with said first input terminal and adapted to be periodically set to a predetermined condition, in accordance with said first digital signal, a source of an a.c. signal connected with said first counter for causing said first counter to step sequentially through its states in response to said signal at a rate proportional to the frequency of said signal, said first counter being adapted to produce an output pulse at a predetermined time following each said setting in accordance with the quantity represented by said first digital signal, a second input terminal for receiving a second digital signal representative of a frequency at which a.c. power is to be supplied to the motor, a second counter connected with said second input terminal and adapted to be set to a predetermined condition in accordance with said second digital signal, an oscillator connected with said second counter for causing said second counter to step sequentially through its states in response to said oscillator at a rate proportional to the frequency of said oscillator, said second counter producing a series of pulses having a pulse repetition rate equal to the frequency of said oscillator divided by a factor in response to the condition in which said second counter is set by said second digital signal, a source of a.c. voltage, an SCR connected to said a.c. voltage source and to said first counter, and responsive to the output of said first counter for selectively rectifying said a.c. voltage to produce a d.c. voltage corresponding to said first digital signal, a plurality of power transistors comprising at least one power transistor for each phase of said motor, and means connected with said SCR, said counter and said power transistors to sequentially connect said transistors with said SCR, under control of the output pulses from said second counter.

Claim 15

Apparatus according to claim 1, including current-sensitive control means for selectively disabling said applying means in response to an excess current condition; said current-sensitive control means comprising first timing means for temporarily disabling said applying means in response to an excess current condition which persists for a first predetermined interval, and second timing means for disabling said applying means until manually reset, when said excess current condition presists for a second predetermined interval longer than said first predetermined interval.