Apparatus and methods for controlling induction motors

• US 4,800,326 A
• Filed: 06/06/1986
• Issued: 01/24/1989
• Est. Priority Date: 09/26/1980
• Status: Expired due to Term

• Current Assignee: British Technology Group Ltd. (Boston Scientific Corporation)
• Original Assignee: National Research Development Corporation
• Inventors: Unsworth, Peter J.
• Primary Examiner(s): Smith, Jr., David
• Application Number:
  US06/873,454
• Time in Patent Office: 963 Days
• Field of Search: 318/729, 318/728, 318/798, 318/800, 318/805, 318/802, 318/430, 318/436
• US Class Current: 318/729
• CPC Class Codes: H02P 23/26 Power factor control [PFC]
• CPC Class Codes: Y02P 80/10 Efficient use of energy, e.g. using compressed air or pressurized fluid as energy carrier

Abstract

When induction motors are lightly loaded their power factors and efficiency are poor but in the present invention power factor is controlled regardless of load. An induction motor is connected by way of a triac to a supply. The voltage across the triac is monitored by a comparator for voltage steps which correspond to current turn-off and a signal is developed at the output of an amplifier which represents error from required phase lag. A further comparator and a trigger pulse generator trigger the triac in accordance with the error. An override circuit overrides the control system during starting. A number of further induction motors may be connected in parallel with the motor. Additional circuits deal with problems arising when a three-phase induction motor is connected by three wires only.

Claims

1. A power controller for an induction motor which is coupled to an alternating current supply operating at a predetermined frequency, comprising:

at least one SCR device, each said SCR device connected between a phase of said alternating current supply and the induction motor to be energized from the supply, one SCR device being provided for each phase of the supply, each said SCR device also including a trigger input for trigger signals for turning on said SCR device;

ramp generating means for generating a ramp waveform with a frequency double said predetermined frequency of said alternating current supply;

means for detecting zero crossings of a voltage waveform of one phase of said alternating current supply;

synchronizing means, connected to said ramp generating means, for synchronizing zero crossings of said ramp waveform with said zero crossings in the voltage waveform of said one phase of the alternating current supply;

detecting means, coupled to said phase of said alternating current supply, for detecting when a current in said one phase of the supply ceases;

sample and hold means, coupled to said ramp generating means and said detecting means, for sampling and holding a voltage of said ramp waveform from said ramp generating means each time said current cessation is detected, said sampled and held voltage being a monitoring signal, proportional to intervals between said zero crossing of said voltage waveform in said one phase of the supply, and a cessation of current in said one phase which preceeds current reversal therein;

a first comparator, connected to said sample and hold means, for comparing said monitoring signal with a reference signal to derive an error signal indicative of a difference between said error signal and said reference signal; and

control means, connected to said SCR devices and to said first comparator, for generating said trigger signals to said SCR devices responsive to said error signal, to change a time relationship between a waveform of the supply and the trigger signals in a direction to shorten a period of conduction of the switching means when said interval increases, and lengthen said period of conduction when said interval decreases.

2. A power controller according to claim 1, wherein said control means includes:

a second comparator, connected to said ramp generating means and to said first comparator, for comparing said ramp waveform from said ramp generating means with said error signal from said first comparator and producing an output signal that changes state based on whether said ramp waveform is greater than or less than said error signal; and

generator means, connected to said second comparator, for generating said trigger signals in response to said output signal of the second comparator,

whereby the said ramp waveform is used by said sample and hold means to derive said monitoring signal and is used by said generator means to generate said trigger signals.

3. A controller as in claim 2, wherein said detecting means comprises voltage monitor means, coupled across said SCR device, for detecting a voltage step across said SCR device when said SCR device ceases to conduct;

and

said first comparator comprises signal deriving means for providing a signal representative of a time interval between a zero crossing in the supply voltage and a time at which a next voltage step occurs across said SCR device.

4. A controller as in claim 2 further comprising:

reference signal generating means, coupled to said first comparator, for generating said reference signal; and

said control means comprises means for changing a level of said reference signal in order to change a time relationship of said error signal.

5. A power controller as in claim 2 further comprising means for inhibiting trigger signals after a first trigger signal has occurred, and until a next return waveform edge occurs synchronized with a zero crossing in a supply voltage.

6. A power controller according to claim 4 further comprising:

means for detecting a back e.m.f. of said motor;

means for determining if said back e.m.f. indicates a tendency of said motor to stall; and

means for changing said reference value in order to lengthen said interval when said motor is tending to stall.

7. A power controller as in claim 6, wherein said means for changing the reference value comprises a diode and a Zener diode connected to said SCR device, and a capacitor in series with said Zener diode.

11. A power controller according to claim 2, wherein said control means includes

direction-detection means for indicating a sequence in which the voltages of the supply phases become positive, and

secondary logic means for each phase, coupled to receive input signals from the direction-detection means and from said first comparator for providing primary trigger signals for said each phase, said secondary logic means of said each phase supplying secondary trigger signals to one of the switching means of the other phases in dependence upon its input signals.

12. A power controller according to claim 11 including means for suppressing at least use of the monitoring signals whenever current through a switching means ceases but next flows again in the same direction.

13. A power controller according to claim 11 further comprising:

a ramp generating means detecting means, synchronizing means, current detecting means, sample and hold means, and comparator for each phase, and the power controller includes means for suppressing at least use of the monitoring signals of a phase when the switching means of the preceding phase in the sequence in which the phase supply voltages go positive, is not conducting.

14. A power controller according to claim 13 wherein said means for suppressing use of the monitoring signals includes suppression logic means for each phase coupled to receive input signals from direction-detection means and, by way of delay means, from the voltage monitor means of the other two phases, the suppression logic means of each phase being connected to suppress the output signal of the voltage monitor means of that phase in dependence upon its input signals.

15. A power controller according to claim 11, further comprising:

two additional ramp generators and second and third comparators, one generator and one comparator for each phase, each ramp generator for each respective phase generating a ramp signal having a repetition frequency equal to twice the frequency of the supply and having return waveform edges which are synchronized with the zero crossings in the supply voltage of that phase, and each comparator for each respective phase, in operation, comparing the ramp signal from the ramp generator of that phase with a signal dependent on the monitoring signal to provide a primary trigger signal when the signals applied to that comparator bear a predetermined magnitude relationship to one another;

direction-detection means for indicating the sequence in which the voltages of the supply phases become positive, and

secondary logic means for each phase coupled to receive input signals from the direction-detection means and from said comparator which provides primary trigger signals for that phase, the secondary logic means of each phase supplying secondary trigger signals to one of the switching means fo the other phase in dependence upon its input signals.

8. A power controller for an induction motor which is coupled to an alternating current supply operating at a predetermined frequency, comprising:

at least one SCR device, each said SCR device connected between a phase of said alternating current supply and the induction motor to be energized from the supply, one SCR device being provided for each phase of the supply, each said SCR device also including a trigger input for trigger signals for turning on said SCR device;

ramp generating means for generating a ramp waveform with a frequency double said predetermined frequency of said alternating current supply;

means for detecting zero crossings of a voltage waveform of one phase of said alternating current supply;

synchronizing means, connected to said ramp generating means, for synchronizing zero crossings of said ramp waveform with said zero crossings in the voltage waveform of said one phase of the alternating current supply;

detecting means, coupled to said phase of said alternating current supply, for detecting when a current in said one phase of the supply ceases;

sample and hold means, coupled to said ramp generating means and said detetecting means, for sampling and holding a voltage of said ramp waveform from said ramp generating means each time said current cessation is detected, to derive a monitoring signal proportional to intervals between said zero crossing in the voltage waveform in said one phase of the supply, and a cessation of current in said one phase which preceeds current reversal therein;

a first comparator, connected to said sample and hold means, for comparing said monitoring signal with a reference signal to derive an error signal indicative of a difference between said error signal and said reference signal;

a second comparator, connected to said ramp generating means and said first comparator, for comparing said ramp waveform from said ramp generating means with said error signal from said first comparator and producing an output signal indicative of which is greater; and

generator means, connected to said second comparator, for generating said trigger signals in response to said output signal of the second comparator,

whereby the said ramp waveform is used by said sample and hold means to derive said monitoring signal and is used by said generator means to generate said trigger signals.

9. A method of operating a power controller for an induction motor which is coupled to an alternating current supply, comprising the steps of:

connecting one SCR device, which has a trigger input for trigger signals to turn on said SCR device, between each phase of said alternating current supply and the induction motor to be energized from the supply;

generating a ramp waveform with a frequency double a frequency of said alternating current supply;

detecting zero crossings of a voltage waveform of one phase of the alternating current supply;

synchronizing said ramp waveform to have zero crossings that are synchronized with said zero crossings in the voltage waveform of said one phase of the alternating current supply;

detecting when a current in said phase of the supply ceases;

sampling and holding a voltage of said ramp waveform each time said current cessation is detected, to derive a monitoring signal proportional to intervals between the zero in the voltage waveform in said one phase of the supply and a cessation of current in said phase which proceeds current reversal therein;

comparing the monitoring signal with a reference signal;

deriving an error signal indicative of a difference between said monitoring signal and said reference signal;

comparing said ramp waveform from said ramp generating means with said error signal; and

generating said trigger signals in response to an output of the second comparator,

whereby the said ramp waveform is used both to derive said monitoring signal and is used to generate said trigger signals.

10. A power controller for an induction motor which is coupled to an alternating current supply, comprising:

at least one SCR device, each said SCR device connected between a phase of said alternating current supply and the induction motor to be energized from the supply, one SCR device being provided for each phase of the supply, each said SCR device also including a trigger input for trigger signals for turning on said SCR device;

current detecting means for detecting a current in at least one of said SCR devices;

voltage detecting means for detecting a supply voltage applied to said at least one SCR device;

monitor means, coupled to said at least one SCR device and to said current detecting means and said voltage detecting means, for deriving a monitor signal representative of a phase lag of said detected current in said at least one SCR device, behind said detected supply voltage thereof;

reference generating means for producing a reference signal;

error-detection means, connected to said monitor means and said reference generating means, for deriving an error signal indicative of a relationship between said monitor signal and said reference signal;

trigger pulse generator means, connected to said SCR devices and said error detection means, for generating said trigger signals for said SCR devices in response to said error signal;

detector means, connected to said SCR devices, for detecting a back e.m.f. of the motor;

comparison means, connected to said detection means, for comparing said back e.m.f. with a threshold value representative of a back e.m.f. value when the motor tends to stall; and

override means, connected to said detector means and said comparison means and actuated by said back e.m.f., for lengthening a conduction period of said SCR device when said motor back e.m.f. falls below said threshold value.

16. A method of controlling an induction motor which is coupled to an alternating current supply, comprising the steps of:

connecting one SCR device including a trigger input for trigger signals for turning on said SCR device for each phase of the supply between a phase of said alternating current supply and the induction motor to be energized from the supply, one SCR device being provided for each phase of the supply;

detecting a current in at least one of said SCR devices;

detecting a supply voltage applied to said at least one SCR device;

deriving a monitor signal representative of a phase lag of said detected current in said at least one SCR device behind said detected supply voltage thereof;

producing a reference signal;

deriving an error signal indicative of a relationship between said monitor signal and said reference signal;

comparing said back e.m.f. with a threshold value representative of a back e.m.f. value when the motor tends to stall; and

lengthening a conduction period of said SCR device when said motor back e.m.f. falls below said threshold value.

17. A power controller for a three-phase induction motor supplied by three wires comprising:

three switching means for connection between an alternating current electrical supply and an induction motor which is to be energized from the supply, there being one switching means for each phase of the supply, and each switching means becoming conductive when a primary trigger signal is applied to that switching means and remaining conductive until said current supplied thereto ceases, each of said three switching means, in operation, connected between a phase of a three phase supply and a respective terminal of a star or delta connected induction motor,

means for generating a ramp waveform which has a repetition frequency twice that of the supply and has minima synchonized with zero crossings in the voltage waveform of one phase of the supply;

monitor means for deriving a monitoring signal representative of respective intervals between a zero in the voltage waveform of said one phase of the supply and the cessation of current in that phase which precedes current reversal therein, the monitoring means including means for sampling the magnitude of said ramp waveform each time said cessation of current occurs to provide the monitoring signal,

means for deriving an error signal from the monitoring signal using a reference signal,

control means for generating the trigger signals, said control means being responsive to the error signal to change the time relationship between the supply waveform and the trigger signals in that sense which shortens the conduction period of the switching means when said interval tends to increase and lengthens the said conduction period when said interval tends to decrease;

said control means including means for generating secondary trigger signals at the same time as said primary trigger signals, each secondary trigger signal being applied to that switching means which received the last primary trigger signal; and

a power controller including means for suppressing at least use of the monitoring signals whenever current through a switching means ceases but next flows again in the same direction.

18. A power controller for a three-phase induction motor comprising:

three switching means for connection between an alternating current electrical supply and an induction motor which is to be energized from the supply, there being one switching means for each phase of the supply, and each switching means becoming conductive when a primary trigger signal is applied to that switching means and remaining conductive until said current supplied thereto ceases;

three ramp generator means, each for generating a ramp waveform which has a repetition frequency twice that of the supply and has minima synchronized with zero crossings in the voltage waveform of one phase of the supply;

three monitor means, one for each phase, each for deriving a monitoring signal representative of respective intervals between a zero in the voltage waveform of said one phase of the supply and the cessation of current in that phase which precedes current reversal therein, by detecting steps in the voltage across that switching means which occur when that switching means ceases to conduct, the monitoring means including means for sampling the magnitude of said ramp waveform each time said cessation of current occurs to provide the monitoring signal;

three signal-deriving means, one for each supply phase, for providing a signal which is representative of the time interval between a zero crossing in the supply voltage of that phase and the time at which the last voltage step occurs across the switching means for that phase before current reversal in that phase;

control means including three comparators, one for each respective phase, in operation, comparing the ramp signal from the ramp generator of that phase with a signal dependent thereon to provide a primary trigger signal when the signals applied to that comparator bear a predetermined magnitude relationship to one another;

direction-detection means for indicating the sequence in which the voltages of the supply phases become positive, and

secondary logic means for phase coupled to receive input signals from the direction-detection means and from said comparator which provides primary trigger signals for that phase, the secondary logic means of each phase supplying secondary trigger signals to one of the switching means of the other phases in dependence upon its input signals;

said control means being responsive to the error signal to change the time relationship between the supply waveform and the trigger signals in that sense which shortens the conduction period for the switching means when the said interval tends to increase and lengthens the said conduction period when the said interval tends to decrease; and

means for suppressing at least use of the monitoring signals of a phase when the switching means of the preceding phase in the sequence in which the phase supply voltages go positive, is not conducting.

19. A power controller for a three-phase induction motor which is star or delta connected and supplied by three wires, comprising:

three switching means each for connection between a phase of alternating current electrical supply and an induction motor which is to be energized from the supply, there being one switching means for each phase of the supply, and each switching means becoming conductive when a primary trigger signal is applied to that switching means and remaining conductive until said current supplied thereto ceases,

three ramp generating means, each for generating a ramp waveform which has a repetition frequency twice that of the supply and has minima synchronized with zero crossings in the voltage waveform of one phase of the supply,

three voltage monitoring means, each for deriving a monitoring signal representative of respective intervals between a zero in the voltage waveform of one phase of the supply and the cessation of current in that phase which precedes current reversal therein, by detecting steps in the voltage across that switching means which occur when that switching means ceases to conduct, each monitoring means including means for sampling the magnitude of said ramp waveform each time said cessation of current occurs to provide the monitoring signal,

three signal-deriving means, one for each supply phase, for providing a signal which is representative of the time interval between a zero crossing in the supply voltage of that phase and the time at which the last voltage step occurs across the switching means for that phase before current reversal in that phase;

control means for generating said primary trigger signals, including three comparators, one for each respective phase, each in operation, comparing the ramp signal from the ramp generator of that phase with a signal dependent on the monitoring signal to provide a primary trigger signal when the signals applied to that comparator bear a predetermined magnitude relationship to one another;

direction-detection means for indicating the sequence in which the voltages of the supply phases become positive,

secondary logic means for each phase coupled to receive input signals from the direction-detection means and from said comparator which provides primary trigger signals for that phase, the secondary logic means of each phase supplying secondary trigger signals to one of the switching means of the other phases in dependence upon its input signals, said control means being responsive to the error signal to change the time relationship between the supply waveform and the trigger signals in that sense which shortens the conduction period of the switching means when said interval tends to increase and lengthens said conduction period when said interval tends to decrease; and

wherein the control means includes means for generating secondary trigger signals at the same time as said primary trigger signals, each said secondary trigger signal being applied to that switching means which received that last primary trigger signal.