Control system, electronically commutated motor system, draft inducer apparatus and method

• US 5,075,608 A
• Filed: 02/20/1990
• Issued: 12/24/1991
• Est. Priority Date: 06/24/1974
• Status: Expired due to Term

• Current Assignee: David M. Erdman, Mark A. Brattoli
• Original Assignee: David M. Erdman, Mark A. Brattoli
• Inventors: Erdman, David M., Brattoli, Mark A.
• Primary Examiner(s): Ro, Bentsu
• Application Number:
  US07/483,329
• Time in Patent Office: 672 Days
• Field of Search: 318/138, 318/254, 318/439, 318/599, 126/516, 126/517, 126/521, 126/530, 126/533, 126/110 R, 126/110 A, 126/110 D, 126/104 A
• US Class Current: 318/599
• CPC Class Codes: F23N 2233/04 with variable speed
• CPC Class Codes: F23N 3/082 using electronic means
• CPC Class Codes: F25B 2600/021 Inverters therefor
• CPC Class Codes: F25B 49/025 Motor control arrangements
• CPC Class Codes: G05D 23/19 characterised by the use of electric means G05D23/1393 takes precedence
• CPC Class Codes: H02K 29/06 with position sensing devices H02K29/03 takes precedence
• CPC Class Codes: H02K 29/10 using light effect devices
• CPC Class Codes: H02P 6/16 Circuit arrangements for detecting position
• CPC Class Codes: H02P 6/182 using back-emf in windings

Abstract

A control system and method for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly. The control system is adapted to receive control pulses having a duty cycle which is a function of a desired operating torque or speed for the motor. The control system and method are responsive to the motor current for generating a pluse width modulated (PWM) series of pulses having a pulse repetition rate having a duty cycle which is a function of the duty cycle of the control pulses. The PWM series of pulses are supplied to the commutating circuit as a pulsed signal whereby the operating torque or speed of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage. The control system may be part of a draft inducer apparatus or method for use with a combustion chamber having an exhaust outlet including a fan for moving air through the exhaust outlet and thereby to induce a draft in the combustion chamber. A pressure sensor connected to the motor control apparatus may be located within the exhaust outlet.

Claims

1. A control system for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, and which control system is adapted to receive control pulses having a duty cycle representing a desired operating torque for the motor, the control system for use with a commutating circuit for controlling power switching devices for applying the voltage to one or more of the winding stages at a time having a duty cycle which is a function of a pulsed signal and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly, the control system comprising:

means responsive to the motor current for generating a pulse width modulated (PWM) series of pulses having a pulse repetition rate having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PMW series of pulses to the commutating circuit as the pulsed signal;

means for comparing the applied voltage to a reference voltage; and

means, responsive to the means for comparing, for increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage and for decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage whereby the operating torque of the motor is a function of the duty cycle of the control pulse and is substantially independent of variations in the magnitude of the applied voltage.

2. The system of claim 1 wherein said means for comparing comprises a comparator adapted to compare the applied voltage to an oscillating voltage signal and to provide a comparator output signal in response thereto, and a latch, responsive to the comparator output signal, adapted to inhibit the PWM series of pulses during the period that the oscillating voltage signal is greater than the applied voltage.

3. The system of claim 1 further comprising means for generating a sum of a first voltage having a magnitude which is a function of the duty cycle of the control pulses and a second voltage having a magnitude which is a function of the motor current, and means for inhibiting the PWM series of pulses during the period that the sum is greater than the reference voltage.

4. The system of claim 1 wherein said means for generating comprises means for comparing the motor current and a signal representing the duty cycle of the control pulses and providing an output signal in response thereto, and means for providing a series of pulses as the PWM series of pulses, each pulse having a width which is a function of the output signal of the means for comparing the motor current.

5. The system of claim 4 wherein said means for comparing the motor current comprises means for generating a first voltage having a magnitude which is a function of the duty cycle of the control pulses, means for generating a second voltage having a magnitude which is a function of the motor current, means for summing the first and second voltages, and means for comparing the sum to a reference voltage to provide an output signal in response thereto as the PWM series of pulses.

6. The system of claim 5 further comprising means, responsive to the means for comparing the sum, for selectively inhibiting the PWM series of pulses during the period that the sum is greater than the reference voltage.

7. The system of claim 6 wherein said means for comparing the sum comprises a comparator adapted to compare the sum to the reference voltage and to provide a comparator output signal in response thereto, and said means for selectively inhibiting comprises means, responsive to the comparator output signal, for conducting the PWM voltage away from said commutating circuit during the period that the sum is less than the reference voltage.

8. The system of claim 7 wherein said means for generating a first voltage comprises a dc level converter, said means for generating a second voltage comprises a motor current shunt, said means for summing comprises a summer and said means for conducting comprises a diode.

9. The system, of claim 8 wherein said means for increasing and decreasing comprises means, responsive to the means for comparing the applied voltage to a reference voltage, for increasing the duty cycle of the PWM series of pulses when the applied voltage decreases and for decreasing the duty cycle of the PWM series of pulses when the applied voltage increases.

10. The system of claim 9 wherein said means for comparing the applied voltage to a reference voltage comprises a comparator adapted to compare the applied voltage to an oscillating voltage signal and to provide a comparator output signal in response thereto, and a latch, responsive to the comparator output signal, adapted to inhibit the PWM series of pulses during the period that the oscillating voltage signal is greater than the applied voltage.

11. A control system for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, and which control system is adapted to receive control pulses having a duty cycle representing a desired operating torque for the motor, the control system for use with a commutating circuit for controlling power switching devices for applying the voltage to one or more of the winding stages at a time having a duty cycle which is a function of a pulsed signal and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly, the control system comprising:

means for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the commutating circuit as the pulsed signal;

means for comparing the applied voltage to a reference voltage; and

means, responsive to the means for comparing, for increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage and for decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage whereby the operating speed of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

12. The system of claim 11 wherein said means for comparing comprises a comparator adapted to compare the applied voltage to an oscillating voltage signal and to provide a comparator output signal in response thereto, and a latch, responsive to the comparator output signal, adapted to inhibit the PWM series of pulses during the period that the oscillating voltage signal is greater than the applied voltage.

13. The system of claim 11 wherein said means for generating comprises means for comparing an oscillating reference signal and a signal representing the duty cycle of the control pulses and providing an output signal in response thereto as the PWM series of pulses.

14. The system of claim 13 wherein said means for comparing the applied voltage to a reference voltage comprises means for generating a first voltage having a magnitude which is a function of the duty cycle of the control pulses, means for generating a second voltage having a magnitude which is a function of the oscillating reference signal, means for comparing the first and second voltages and generating an output signal representative thereof as the PWM series of pulses.

15. The system of claim 14 wherein said means for comparing the first and second voltages comprises a comparator adapted to compare the magnitudes of the first and second voltages and adapted to provide a comparator output signal in response thereto.

16. The system of claim 15 wherein said means for generating a first voltage comprises a dc level converter and said means for generating a second voltage comprises a motor voltage sensor.

17. The system of claim 11 wherein said means for generating is responsive to the motor current.

18. The system of claim 17 wherein said means for comparing comprises a comparator adapted to compare the applied voltage to an oscillating voltage signal and to provide a comparator output signal in response thereto, and a latch, responsive to the comparator output signal, adapted to inhibit the PWM series of pulses during the period that the oscillating voltage signal is greater than the applied voltage.

19. The system of claim 18 wherein said means for generating comprises means for comparing the motor current and a current reference signal and providing an output signal in response thereto to inhibit the PWM series of pulses whenever the the motor current is greater than the current reference signal.

20. The system of claim 19 wherein said means for comparing the applied voltage to a reference voltage comprises means for generating a first voltage having a magnitude which is a function of the current reference signal, means for generating a second voltage having a magnitude which is a function of the motor current, means for summing the first and second voltages, means for comparing the sum to a reference providing an output signal in response thereto as the PWM series of pulses.

21. The system of claim 20 further comprising means, responsive to the means for comparing the sum, for inhibiting the PWM series of pulses during the period that the sum is greater than the reference voltage.

22. The system of claim 21 wherein said means for comparing the sum comprises a comparator adapted to compare the sum to the reference voltage and to provide a comparator output signal in response thereto, and said means for inhibiting comprises means, responsive to the comparator output signal, for conducting the PWM series of pulses away from said commutating circuit during the period that the sum is less than the reference voltage.

23. The system of claim 22 wherein said means for generating a first voltage comprises a dc voltage source, said means for generating a second voltage comprises a motor current shunt, said means for summing comprises a summer and said means for conducting comprises a diode.

24. The system of claim 11 further comprising a motor control apparatus adapted to control the motor by generating the control pulses having a duty cycle which is a function of a desired operating torque for the motor and the speed of the motor and further including means for providing a tachometer signal representative of the motor speed to the motor control apparatus whereby the control pulses are a function of the tachometer signal.

25. The system of claim 11 wherein the control pulses represent a desired operating torque or speed of the motor;

further comprising means for selecting the motor current or the motor voltage as a control parameter; and

wherein said means for generating is responsive to the selected motor operating condition.

26. The system of claim 11 further comprising means for generating a sum of a first voltage having a magnitude which is a function of the duty cycle of the control pulses and a second voltage having a magnitude which is a function of the motor current, and means for inhibiting the PWM series of pulses during the period that the sum is greater than the reference voltage.

27. A control system for power switching devices of an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having rotatable assembly, and which control system is adapted to receive control pulses having a duty cycle representing a desired operating torque for the motor, the control system comprising:

means for controlling the power switching devices to apply a voltage having a duty cycle which is a function of a pulsed signal to one or more of the winding stages at a time and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly;

means responsive to the motor current for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the controlling means as the pulsed signal;

means for comparing the applied voltage to a reference voltage; and

means, responsive to the means for comparing, for increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage and for decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage whereby the operating torque of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

28. The system of claim 27 wherein said means for generating comprises means for comparing the motor current and a signal representing the duty cycle of the control pulses and providing an output signal in response thereto, and means for providing a series of pulses as the PWM series of pulses, each pulse having a width which is a function of the output signal of the means for comparing the motor current.

29. The system of claim 27 further comprising means for generating a sum of a first voltage having a magnitude which is a function of the duty cycle of the control pulses and a second voltage having a magnitude which is a function of the motor current, and means for inhibiting the PWM series of pulses during the period that the sum is greater than the reference voltage.

30. A control system for power switching devices of an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to applications of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, and which control system is adapted to receive control pulses having a duty cycle representing a desired operating speed for the motor, the control system comprising:

means for controlling the power switching devices to apply a voltage having a duty cycle which is a function of a pulsed signal to one or more of the winding stages at a time and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly;

means for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the controlling means as the pulsed signal;

means for comparing the applied voltage to a reference voltage; and

means, responsive to the means for comparing, for increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage and for decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage whereby the operating speed of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

31. The system of claim 30 wherein said means for generating comprises means for comparing an oscillating reference signal and a signal representing the duty cycle of the control pulses and providing an output signal in response thereto as the PWM series of pulses.

32. The system of claim 30 wherein said means for generating is responsive to the motor current.

33. The system of claim 32 wherein said means for generating comprises means for comparing the motor current and a current reference signal and providing an output signal in response thereto to inhibit the PWM series of pulses whenever the the motor current is greater than the current reference signal.

34. A control system for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, which control system is responsive to a motor control apparatus adapted to control the motor by generating control pulses having a duty cycle which is a function of a desired operating torque for the motor and the speed of the motor, the control system comprising:

means for applying a voltage having a duty cycle which is a function of a pulsed signal to one or more of the winding stages at a time and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly;

means responsive to the motor current for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the applying means as the pulsed signal; and

means for providing a tachometer signal representative of the motor speed to the motor control apparatus.

35. The system of claim 34 wherein the tachometer signal providing means comprises an isolator adapted to receive commutation pulses from said generating means and adapted to provide the tachometer signal in response to the commutation pulses.

36. The system of claim 35 further including means for varying the duty cycle of the applied voltage inversely as a function of variations in the magnitude of the applied voltage.

37. The system of claim 36 wherein said means for varying comprises means for comparing the applied voltage to a reference voltage, and means, responsive to the means for comparing, for increasing the duty cycle of the PWM series of pulses when the applied voltage decreases and for decreasing the duty cycle of the PWM series of pulses when the applied voltage increases.

38. The system of claim 37 wherein said means for generating comprises means for comparing the motor current and a signal representing the duty cycle of the control pulses and providing an output signal in response thereto, and means for providing a series of pulses as the PWM series of pulses, each pulse having a width which is a function, of the output signal of the means for comparing the motor current.

39. The system of claim 35 wherein said means for generating comprises means for comparing the motor current and a signal representing the duty cycle of the control pulses and providing an output signal in response thereto, and means for providing a series of pulses as the PWM series of pulses, each pulse having a width which is a function of the output signal of the means for comparing.

40. The system of claim 34 further comprising means for generating a sum of a first voltage having a magnitude which is a function of the duty cycle of the control pulses and a second voltage having a magnitude which is a function of the motor current, and means for inhibiting the PWM series of pulses during the period that the sum is greater than a reference voltage.

41. A control system for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, which control system is responsive to a motor control apparatus adapted to control the motor by generating control pulses having a duty cycle which is a function of a desired operating torque for the motor and the speed of the motor, the control system comprising:

means for applying a voltage having a duty cycle which is a function of a pulsed signal to one or more of the winding stages at a time and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly;

means for varying the duty cycle of the applied voltage as an inverse function of the magnitude of the applied voltage;

means for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the applying means as the pulsed signal; and

means for providing a tachometer signal representative of the motor speed to the motor control apparatus.

42. The system of claim 41 wherein said means for varying comprises means for comparing the applied voltage to a reference voltage, and means, responsive to the means for comparing, for increasing the duty cycle of the PWM series of pulses when the applied voltage decreases and for decreasing the duty cycle of the PWM series of pulses when the applied voltage increases.

43. The system of claim 42 wherein said means for generating comprises means for comparing an oscillating reference signal and a signal representing the duty cycle of the control pulses and providing an output signal in response thereto as the PWM series of pulses.

44. The system of claim 42 wherein said means for generating is responsive to the motor current.

45. The system of claim 44 wherein said means for generating comprises means for comparing the motor current and a current reference signal and providing an output signal in response thereto to inhibit the PWM series of pulses whenever the the motor current is greater than the current reference signal.

46. The system of claim 41 further comprising means for generating a sum of a first voltage having a magnitude which is a function of the duty cycle of the control pulses and a second voltage having a magnitude which is a function of the motor current, and means for inhibiting the PWM series of pulses during the period that the sum is greater than a reference voltage.

47. A control system for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current and further having a rotatable assembly, and which control system is adapted to receive control pulses having a duty cycle representing a desired operating torque or speed for the motor, the control system being adapted for use with a commutating circuit for applying a voltage to one or more of the winding stages at a time in accordance with a pulsed signal and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly, the control system comprising:

means for selecting the motor current or the motor voltage as a control parameter said selecting means including means for sensing the motor current and means for providing a reference;

means responsive to the selected motor operating parameter for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the commutating circuit as the pulsed signal whereby the torque of the motor is a function of the duty cycle of the control pulses when the motor current is selected as the control parameter and the speed of the motor is a function of the duty cycle of the control pulses when the motor voltage is selected as the control parameter.

48. The system of claim 47 wherein said means for generating comprises means responsive to the motor current for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses when the motor current is selected as the control parameter by said means for selecting, the generating means adapted to supply the PWM series of pulses to the commutating circuit as the pulsed signal whereby the operating torque of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

49. The system of claim 47 wherein said means for generating comprises means for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control parameter by said means for selecting, the generating means adapted to supply the PWM series of pulses to the commutating circuit as the pulsed signal whereby the operating speed of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

50. The system of claim 47 further comprising means for providing a tachometer signal representative of the motor speed and wherein said means for generating is responsive to said means for providing a tachometer signal to provide the PWM series of pulses having a duty cycle varying as a function of the tachometer signal.

51. The system of claim 47 wherein said means for selecting comprises means for switching between said means for sensing the motor current and said means for providing a reference;

and said means for generating comprises;

means for comparing a signal representative of the duty cycle of the control pulses to a signal representative of the motor current when said means for switching is connected to said means for sensing the motor current; and

means for comparing a signal representative of the duty cycle of the control pulses to an oscillating reference signal when said means for switching is connected to said means for providing a reference.

52. The system of claim 51 wherein said means for generating further comprises means for comparing a reference signal representative of the maximum motor current to a signal representative of the motor current when said means for switching is connected to said means for sensing the motor current.

53. The system of claim 47 wherein said means for generating comprises means for comparing a signal representative of the duty cycle of the control pulses to a signal representative of the motor current when the motor current is selected as the control parameter by said means for selecting and means for comparing a signal representative of the duty cycle of the control pulses to an oscillating reference signal representative of the maximum duty cycle of the voltage to be applied to the motor when the motor voltage is selected as the control parameter by said means for selecting.

54. Draft inducer apparatus for use with a combustion chamber having an exhaust outlet comprising:

a fan for moving air through the exhaust outlet and thereby to induce a draft in the combustion chamber;

an electronically commutated motor including a stationary armature having a core and at least two energizable winding stages arranged to establish a predetermined number of magnetic poles, and a permanent magnet rotor coupled to said fan and adapted to rotate in response to the magnetic poles established by said winding stages;

means for generating a pulse width modulated (PWM) series of pulses having a duty cycle representing a desired torque or speed of the motor;

power switching devices for applying a voltage to one or more of said winding stages at a time; and

means for controlling said power switching devices in accordance with the PWM series of pulses and commutating said winding stages in a preselected sequence to rotate said permanent magnet rotor and said fan;

sensor means for sensing pressure within the exhaust outlet, said means for generating further comprising means connected to said sensor means for generating the PWM series of pulses with a duty cycle varying as a function of the sensed pressure.

55. The system of claim 54 wherein said means for generating comprises means for generating control pulses having a duty cycle representing a desired operating torque for the motor;

means for varying the duty cycle of the applied voltage inversely as a function of variations in the magnitude of the applied voltage; and

means responsive to the motor current for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the power switching devices as the pulsed signal whereby the operating torque of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

56. The system of claim 54 wherein said means for generating comprises means for generating control pulses having a duty cycle representing a desired operating speed for the motor;

means for varying the duty cycle of the applied voltage varies inversely as a function of variations in the magnitude of the applied voltage; and

means for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the power switching devices as the pulsed signal whereby the operating speed of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

57. The system of claim 54 wherein said generating means generates the PWM series of pulses when the pressure is above a preselected limit, whereby a draft is induced in the combustion chamber when the pressure is above the preselected limit.

58. The apparatus of claim 54 further comprising means for providing a tachometer signal representative of the motor speed and wherein said means for generating is responsive to said means for providing a tachometer signal to provide the series of pulses having a duty cycle varying as a function of the sensed pressure and the tachometer signal.

59. The system of claim 54 further comprising means for generating a sum of a first voltage having a magnitude which is a function of the duty cycle of the control pulses and a second voltage having a magnitude which is a function of the motor current, and means for inhibiting the PWM series of pulses during the period that the sum is greater than a reference voltage.

60. Draft inducer apparatus for use with a combustion chamber having an exhaust outlet, comprising:

a fan for moving air through the exhaust outlet and thereby to induce a draft in the combustion chamber;

an electronically commutated motor including a stationary armature having a core and at least two energizable winding stages arranged to establish a predetermined number of magnetic poles, and a permanent magnet motor coupled to said fan and adapted to rotate in response to the magnetic poles established by said winding stages;

means for providing a tachometer signal representative of the motor speed;

storage means for storing a speed/torque profile of the motor;

means, responsive to said means for providing a tachometer signal and said storage means, for generating a pulse width modulated (PWM) series of pulses having a duty cycle varying as a function of the motor speed/torque profile and the tachometer signal;

power switching devices for applying a voltage to one or more of said winding stages at a time;

means for controlling said power switching devices in accordance with the PWM series of pulses and commutating said winding stages in a preselected sequence to rotate said permanent magnet rotor and said fan.

61. The apparatus of claim 60 further comprising sensor means for sensing pressure within the exhaust outlet and wherein said means for generating comprises means connected to said sensor means for generating a pulse width modulated (PWM) series of pulses having a duty cycle varying as a function of the pressure.

62. The system of claim 61 wherein said generating means generates the PWM series of pulses when the pressure is above a preselected limit, whereby a draft is induced in the combustion chamber when the pressure is above the preselected limit.

63. The apparatus of claim 61 wherein said means for generating is responsive to said means for providing a tachometer signal to provide the series of pulses having a duty cycle varying as a function of the pressure and the tachometer signal.

64. The system of claim 60 wherein said means for generating comprises means for generating control pulses having a duty cycle representing a desired operating torque for the motor;

means for varying the duty cycle of the applied voltage inversely as a function of variations in the magnitude of the applied voltage; and

means responsive to the motor current for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the power switching devices as the pulsed signal whereby the operating torque of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

65. The system of claim 60 wherein said means for generating comprises means for generating control pulses having a duty cycle representing a desired operating speed for the motor;

means for varying the duty cycle of the applied voltage varies inversely as a function of variations in the magnitude of the applied voltage; and

means for generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating means adapted to supply the PWM series of pulses to the power switching devices as the pulsed signal whereby the operating speed of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

66. The system of claim 60 further comprising means for generating a sum of a first voltage having a magnitude which is a function of the duty cycle of the control pulses and a second voltage having a magnitude which is a function of the motor current, and means for inhibiting the PWM series of pulses during the period that the sum is greater than a reference voltage.

67. A control method for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a rotatable assembly, and which control method is responsive to control pulses having a duty cycle representing a desired operating torque for the motor, the control method for use with a commutating circuit for controlling power switching devices for applying the voltage to one or more of the winding stages at a time having a duty cycle which is a function of a pulsed signal and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly, the control method comprising the steps of:

generating a response to the motor current a pulse width modulated (PWM) series of pulses having a pulse repetition rat having a duty cycle which is a function of the duty cycle of the control pulses, the generating step adapted to supply the PWM series of pulses to the commutating circuit as the pulsed signal;

comparing the applied voltage to a reference voltage;

increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage; and

decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage whereby the operating torque of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

68. A control method for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, and which control method is responsive to control pulses having a duty cycle representing a desired operating torque for the motor, the control method for use with a commutating circuit for controlling power switching devices for applying the voltage to one or more of the winding stages at a time having a duty cycle which is a function of a pulsed signal and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly, the control method comprising the steps of:

generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating step adapted to supply the PWM series of pulses to the commutating circuit as the pulsed signal;

comparing the applied voltage to a reference voltage;

increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage; and

decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage whereby the operating speed of the motor is a function of the duty cycle and is substantially independent of variations in the magnitude of the applied voltage.

69. A control method for power switching devices of an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, and which control method is responsive to control pulses having a duty cycle representing a desired operating torque for the motor, the control method comprising the steps of:

controlling the power switching devices to apply a voltage having a duty cycle which is a function of a pulsed signal to one or more of the winding stages at a time;

commutating the winding stages in a preselected sequence to rotate the rotatable assembly;

generating in response to the motor current a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating step adapted to supply the PWM series of pulses as the pulsed signal;

comparing the applied voltage to a reference voltage;

increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage; and

decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage whereby the operating torque of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

70. A control method for power switching devices of an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, and which control method is responsive to control pulses having a duty cycle representing a desired operating speed for the motor, the control method comprising the steps of:

controlling the power switching devices to apply a voltage having a duty cycle which is a function of a pulsed signal to one or more of the winding stages at a time;

commutating the winding stages in a preselected sequence to rotate the rotatable assembly;

generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating step adapted to supply the PWM series of pulses as the pulsed signal;

comparing the applied voltage to a reference voltage;

increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage; and

decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage whereby the operating speed of the motor is a function of the duty cycle of the control pulses and is substantially independent of variations in the magnitude of the applied voltage.

71. A control method for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, and which control method is responsive to a motor control apparatus adapted to control the motor by generating control pulses having a duty cycle which is a function of a desired operating torque for the motor and the speed of the motor, the control method comprising the steps of:

applying a voltage having a duty cycle which is a function of a pulsed signal to one or more of the winding stages at a time;

storing a speed/torque profile of the motor;

commutating the winding stages in a preselected sequence to rotate the rotatable assembly;

generating, in response to the motor current and the motor speed, a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating step adapted to supply the PWM series of pulses as the pulsed signal;

providing a tachometer signal representative of the motor speed to the motor control apparatus; and

wherein the generating step is responsive to the storing step and the providing step for generating the PWM series of pulses with a duty cycle varying as a function of the motor speed/torque profile and the tachometer signal.

72. A control method for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current in response to application of a voltage having a magnitude subject to variations, the motor further having a rotatable assembly, and which control method is responsive to a motor control apparatus adapted to control the motor by generating control pulses having a duty cycle which is a function of a desired operating torque for the motor and the speed of the motor, the control method comprising the steps of:

applying a voltage having a duty cycle which is a function of a pulsed signal to one or more of the winding stages at a time;

commutating the winding stages in a preselected sequence to rotate the rotatable assembly;

varying the duty cycle of the applied voltage as an inverse function of the magnitude of the applied voltage;

generating a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating step adapted to supply the PWM series of pulses as the pulsed signal; and

providing a tachometer signal representative of the motor speed to the motor control apparatus.

73. A control method for an electronically commutated motor having a stationary assembly with a plurality of winding stages for carrying a motor current and further having a rotatable assembly, and which control method is responsive to control pulses having a duty cycle representing a desired operating torque or speed for the motor, the control method being adapted for use with a commutating circuit for applying a voltage to one or more of the winding stages at a time in accordance with a pulsed signal and for commutating the winding stages in a preselected sequence to rotate the rotatable assembly, the control method comprising the steps of:

selecting the motor current or the motor voltage as a control parameter;

generating in response to the selected motor operating parameter a pulse width modulated (PWM) series of pulses having a duty cycle which is a function of the duty cycle of the control pulses, the generating step adapted to supply the PWM series of pulses to the commutating circuit as the pulsed signal whereby the torque of the motor is a function of the duty cycle of the control pulses when the motor current is selected as the control parameter and the speed of the motor is a function of the duty cycle of the control pulses when the motor voltage is selected as the control parameter.

74. Method of inducing a draft in a combustion chamber having an exhaust outlet including a fan for moving air through the exhaust outlet and thereby to induce a draft in the combustion chamber and an electronically commutated motor including a stationary armature having a core and at least two energizable winding stages arranged to establish a predetermined number of magnetic poles, and a permanent magnet rotor coupled to said fan and adapted to rotate in response to the magnetic poles established by said winding stages;

said method comprising the steps of;

generating a pulse width modulated (PWM) series of pulses having a duty cycle representing a desired torque or speed of the motor;

applying a voltage to one or more of said winding stages at a time by use of power switching devices;

controlling the power switching devices in accordance with the PWM series of pulses;

commutating said winding stages in a preselected sequence to rotate said permanent magnet rotor and said fan;

comparing the applied voltage to a reference voltage;

increasing the duty cycle of the PWM series of pulses as a function of decreases in the applied voltage; and

decreasing the duty cycle of the PWM series of pulses as a function of increases in the applied voltage.

75. Method of inducing a draft in combustion chamber having an exhaust outlet including a fan for moving air through the exhaust outlet and thereby to induce a draft in the combustion chamber and an electronically commutated motor including a stationary armature having a core and at least two energizable winding stages arranged to establish a predetermined number of magnetic poles, and a permanent magnet rotor coupled to said fan and adapted to rotate in response to the magnetic poles established by said winding stages;

said method comprising the steps of;

providing a tachometer signal representative of the motor speed;

storing a speed/torque profile of the motor;

generating a pulse width modulated (PWM) series of pulses having a duty cycle varying as a function of the motor speed/torque profile and the tachometer signal;

applying a voltage to one or more of said winding stages at a time in accordance with the PWM series of pulses; and

commutating said winding stages in a preselected sequence to rotate said permanent magnet rotor and said fan.