Brushless DC motor control

• US 6,034,493 A
• Filed: 02/05/1998
• Issued: 03/07/2000
• Est. Priority Date: 02/05/1997
• Status: Expired due to Term

• Current Assignee: Fisher & Paykel Limited
• Original Assignee: Fisher & Paykel Limited
• Inventors: Duncan, Gerald, Perry, Michael Thomas George, Boyd, John H.
• Primary Examiner(s): Ip, Paul
• Application Number:
  US09/019,513
• Time in Patent Office: 761 Days
• Field of Search: 318/138, 318/139, 318/245, 318/256, 318/439
• US Class Current: 318/400.31
• CPC Class Codes: H02P 6/15 Controlling commutation time
• CPC Class Codes: H02P 6/182 using back-emf in windings
• CPC Class Codes: H02P 6/28 Arrangements for controlling current H02P6/10 takes precedence

Abstract

An electronically commutated brushless DC motor primarily for fractional horsepower applications of the type where at any instant one motor winding is unpowered and used to detect back EMF zero-crossings which information is used to initiate winding commutations. The duration of the pulse produced in this winding due to dissipation of stored energy by free-wheel diodes in parallel with the commutation devices after supply of current has been removed from this winding is used to provide a measure of motor current. This allows for simplified commutation device current limiting circuits and is available for control purposes which are a function of motor torque. There is also disclosed a method for maximizing useful power output by reducing the phase angle between the motor current and the back EMF. This is accomplished by introducing a delay in commutating the motor windings beyond the occurrence of each back EMF zero-crossing, with the delay being a function of the time between commutations.

Claims

1. In an electronically commutated motor system comprising:

a brushless DC motor having an armature and a stator with at least one phase winding,

a commutation circuit including a direct current power supply, switching devices connected to said power supply to supply current to said at least one winding or to selected pairs of stator windings and unidirectional current devices which supply a current path to dissipate energy stored in each winding after supply of current through a switching device has terminated,

a digitizer circuit which senses the voltage on said at least one winding and compares said voltage to a reference signal voltage to thereby detect zero-crossings of the back EMF induced in said at least one winding by movement of the armature,

a programmed digital processor including memory and input-output ports, a first port being connected to the output of said digitizer circuit and a second group of ports being connected to said commutation circuit to supply switching control signals thereto,

software stored in said memory to cause said processor to determine a measure of motor current based on intervals between those zero crossings of said back EMF, which represent the duration of a current pulse produced in said at least one winding due to dissipation of stored energy by said unidirectional current devices after supply of current has been removed from said at least one winding.

2. In an electronically commutated motor system as claimed in claim 1 wherein said measure of motor current is used by said processor to limit the motor current to safe values.

3. An electronically commutated motor system comprising:

a brushless DC motor having a rotor and a stator with at least one phase winding,

a commutation circuit including a direct current power supply, switching devices connected to said power supply to supply current to said at least one winding or to selected pairs of stator windings and unidirectional current devices which supply a current path to dissipate energy stored in each winding after supply of current through a switching device has terminated,

a digitiser circuit which senses the voltage on said at least one winding and compares said voltage to A reference signal voltage to thereby detect zero-crossings of the back EMF induced in said at least one winding by rotation of the rotor,

a programmed digital processor including memory and input-output ports, a first port being connected to the output of said digitiser circuit and a second group of ports being connected to said commutation circuit to supply switching control signals thereto,

software stored in said memory to cause said processor to generate said switching control signals, said software including;

(a) a table which stores (i) a sequence of combinations of states for each switching device in said commutation circuit which if applied sequentially and cyclically to said switching devices will cause the stator winding to produce a rotating magnetic field, (ii) a sequence of the possible output states of said digitiser circuit for predetermined angular positions of said rotor which each correspond to one said combination of switching device states which will produce torque in the rotor to ensure continuing rotation when the rotor is in the corresponding predetermined position,

(b) a routine for selecting from said table each stored state combination and producing digital switching control signals for each switching device having logic levels corresponding to the states of the particular combination selected at any given time,

(c) a routine for pulse width modulating said switching control signals to thereby control the RMS current in the stator windings,

(d) a routine for setting the duty cycle of the pulse width modulation in accordance with a stored duty cycle value,

(e) a position determining routine which reads the output of said digitiser and determines therefrom the angular position of the rotor,

(f) a routine for calling said selecting routine to select the stored state combination corresponding to each sensed rotor position for continuing rotation thereof,

(g) a speed determining routine which reads the output of said digitiser and determines therefrom the angular speed of said rotor,

(h) a table for storing values of desired rotor speeds,

(i) a routine for comparing the determined rotor speed with the value of the desired rotor speed to produce a speed error value,

(j) a routine for receiving said speed error value and updating said stored duty cycle value so as to increase it when rotor speed is less than desired speed and decrease it when rotor speed is higher than desired speed,

(k) a pulse duration determining routine which reads the output of said digitiser and determines therefrom the duration of the pulse produced in each phase winding during dissipation of stored energy after supply of current to each winding through a switching device has been terminated,

(l) a routine for comparing the so determined pulse duration with a stored predetermined maximum value of pulse duration to produce a value for setting said stored duty cycle value to a reduced value if the so determined duration is greater than said maximum value, and

(m) a routine which reads said decrementing value and updates the currently stored duty cycle value.

4. An electronically commutated motor system according to claim 3 wherein said stator has at least three phase windings, said switching devices sequentially connect selected pairs of windings to said power supply and said digitiser is connected to each winding to detect zero crossings of the back EMF induced in all windings.

5. An electronically commutated motor system according to claim 3 wherein there is included a routine to reduce the phase angle between the current and the back EMF by delaying the switching of phase windings when said digitising circuit detects a zero-crossing.

6. An electronically commutated motor system according to claim 5 wherein the delaying routine implements a delay which is equal to half the difference between the time interval between the immediately preceding commutations and the duration of the current pulse produced in the immediately preceding commutated phase winding during dissipation of stored energy after supply of current to that winding has been terminated.

7. An electronically commutated motor system according to claim 3 wherein there is included a motor start routine which causes said selecting routine to successively select stored state combinations from said table and supply switching control signals to each switching device at a constant low frequency or a low progressively increasing frequency which is unaffected by data from said position determining routine until a predetermined speed has been attained, whereupon data from said position determining routine is then used to call said selecting routine to determine the switching device control signals.

8. An electronically commutated motor system according to claim 3 wherein there is included a routine which compares said speed error value with a stored threshold value and if the error value exceeds the threshold value the stored duty cycle value is incremented or decremented by a larger value than is the case when the error value does not exceed the threshold value.

9. An electronically commutated motor system according to claim 3 wherein there is included a routine which compares said pulse duration with a second stored predetermined value less than said predetermined maximum value to decrement said stored duty cycle value by a predetermined amount if the pulse duration is greater than said second predetermined value.

10. An electronically commutated motor system according to claim 3 wherein there is included a routine which compares the updating duty cycle value with a stored predetermined maximum value and if the updating value exceeds the maximum value the duty cycle value is set to said maximum value.

11. An electronically commutated motor system according to claim 10 wherein a second maximum value of duty cycle is stored and used when said start routine is being executed.

12. An electronically commutated motor system according to claim 3 wherein there is included a routine which controls the routine which reads the outputs of said digitiser in accordance with data passed from said selecting routine to cause outputs of the digitiser not to be read at times when there are transitions in the switching device control signals.