Starting methods for BLDC motors applied on reciprocating compressors

Abstract

The present invention refers to a starting method with two-levels modified and discrete fixed-time or fixed-frequency hysteresis current controller, under high load conditions, of a BLDC motor (10) with trapezoidal-shaped induced voltages, wherein the BLDC motor (10) is driven by an inverter bridge (30) with only one current sensor (20) positioned in the busbar, controlled by means of a processing unit associated to an analog-digital converter, wherein the novelty basically comprises the fact that: the intensity of the BLDC motor current (Im) is controlled by means of the two-levels and discrete fixed-time or fixed-frequency hysteresis current controller.

Claims

1. A starting method with two-levels discrete fixed-time or fixed frequency hysteresis current control, under high load conditions, of a BLDC motor (10) with trapezoidal shaped induced voltages, the BLDC motor (10) being driven by an inverter bridge (30) with only one current sensor (20) positioned in the busbar, controlled by means of a processing unit associated to an analog-digital converter, comprising: driving the BLDC motor (10) by a six-step driving technique, wherein only two phases are driven at a certain electric position; applying, by means of a closed mesh of current control, a current in the BLDC motor (10) of maximum intensity in order to generate a maximum torque in said BLDC motor (10); applying the unipolar switching pattern of an ON_PWM type, wherein each switch is driven during 120 electrical in the following way: each switch is maintained switched on continuously during a first 60 electrical and is modulated by pulse width modulation during a final 60 electrical; wherein the method further comprises: controlling, by means of the two-levels discrete fixed-time or fixed frequency hysteresis current control, an intensity of the BLDC motor current (I.sub.m), wherein a first protecting level comprises: turning off a first switch, said first switch belonging to a portion of the inverter bridge associated with the two phases driven at a certain electric position, during a first protecting time (T.sub.off1), when the BLDC motor current (I.sub.m), measured in a measuring instant (X), surpasses a first maximum current limit (I.sub.max1); and keeping the first switch turned off and turning off a second switch, said second switch belonging to the portion of the inverter bridge associated with the two phases driven at a certain electric position, during a second protecting time (T.sub.off2), in the end of the first protecting time (T.sub.off1); wherein a second protecting level comprises: turning off the first switch and the second switch, during a third protecting time (T.sub.off3), when the BLDC motor current (I.sub.m), measured in a measuring instant (X), surpasses a second maximum current limit (I.sub.max2), wherein the first maximum current limit (I.sub.max1) is lower than the second maximum current limit (I.sub.max2); and wherein the second protecting level actuates after the actuation of first protecting level.

2. The method, according to the claim 1, characterized in that the second maximum current limit (I.sub.max2) is lower than a maximum current allowed (I.sub.desmag).

3. The method, according to claim 1, characterized in that the second maximum current limit (I.sub.max2) is set by subtracting a maximum current allowed (I.sub.desmag) by a second maximum error (I.sub.max2), and the first maximum current limit (I.sub.max1) is set by subtracting the second maximum current limit (I.sub.max2) by a first maximum error (I.sub.max1).

4. The method, according to claim 1, characterized in that a difference between a value of the first maximum current limit (I.sub.max1) and a value of the second maximum current limit (I.sub.max2) is higher than a value of a first maximum error (I.sub.max1).

5. The method, according to claim 1, characterized in that a difference between a value of the second maximum current limit (I.sub.max2) and a value of a maximum current allowed (I.sub.desmag) is higher than a value of a second maximum error (.sub.Imax2).

6. The method, according to claim 1, characterized in that the first protecting time (T.sub.off1) guarantees that a reduction of the BLDC motor current (I.sub.m) is higher than a first maximum error (I.sub.max1) in the case of a successful start.

7. The method, according to claim 1, characterized in that the second protecting time (T.sub.off2) must be lower than or equal to a sampling period (T.sub.s).

8. The method, according to claim 7, characterized in that the second protecting time (T.sub.off2) is gradually extinct when the BLDC motor (10) reaches of the nominal speed or carries out a complete mechanical turn.

9. The method, according to claim 8, characterized in that the gradual extinction of the second protecting time (T.sub.off2) must be done in approximately one second.

10. The method, according to claim 1, characterized in that the third protecting time (T.sub.off3) guarantees that a reduction of the BLDC motor current (I.sub.m) is bigger than a value of the second maximum error (I.sub.max2) even in the case of a starting failure.

11. The method, according to claim 1, characterized in that a third protecting time (T.sub.off3) is replaceable when: the two switches are switched on again when the motor current (I.sub.m), measured in a measuring instant (X), is lower than the first maximum current limit (I.sub.max1).

12. The method, according to claim 1, characterized in that the third protecting time (T.sub.off3) is replaceable as follows: the starting is aborted when the motor current (I.sub.m) surpasses a second maximum current limit (I.sub.max2).

13. The method, according to claim 1, characterized in that the first protecting time (T.sub.off1) is adjustable as a function of the speed of the motor.

14. The method, according to claim 1, characterized in that the step of turning off a first switch, during the first protecting time (T.sub.off1), when the BLDC motor current (I.sub.m), measured in a measuring instant (X), surpasses a first maximum current limit (I.sub.max1) is replaced by: turning off a first switch when the BLDC motor current (I.sub.m), measured in a measuring instant (X), surpasses a first maximum current limit (I.sub.max1), and only switching the first switch on again in the beginning of a next modified period of the pulse width modulation (T.sub.MPWM).

15. The method, according to claim 1, characterized in that the first switch to be turned off is the switch associated to the phase kept driven after a change of electrical position.

16. The method, according to claim 1, wherein the method comprises utilizing opening of the two switches, at an end of the second protecting time T.sub.off2, to carry out monitoring of a largest current circulating in the BLDC motor 10.

17. The method, according to claim 1, wherein the method is performed to protect against exceeding the first maximum current limit or the second maximum current limit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The objectives and advantages of the present invention will become clearer in the following detailed description of the examples and non-limiting drawings shown at the end of this document:

(2) FIG. 1 depicts an electronic circuit and a BLDC motor associated to said electronic circuit of the prior art;

(3) FIG. 2 depicts the induced voltage waveforms, of the desired current and the command signs of the six-step driving method of a BLDC motor of the prior art;

(4) FIGS. 3A and 3B depict the difference between the switching types: (A) bipolar and (b) unipolar for driving a BLDC motor of the prior art;

(5) FIG. 4A depicts the PWM_ON pattern being applied on switches of the prior art;

(6) FIG. 4B depicts the ON_PWM pattern being applied on switches of the prior art;

(7) FIG. 5 depicts the sampling impact on the discrete hysteresis controllers associated to the unipolar switching of prior art;

(8) FIG. 6 depicts the command signs of a fixed-time hysteresis current controller of the prior art;

(9) FIGS. 7A, 7B and 7C depict the relationships between the phase currents of the motor and the busbar current when only two phases are conducting of the prior art;

(10) FIG. 8 depicts a model equivalent of the BLDC motor during the driving of phases A and B of the prior art;

(11) FIG. 9 depicts the command signs of a modified hysteresis current controller according to the present invention;

(12) FIG. 10 depicts the result of applying protection of the current by two-levels modified hysteresis current controller according to the present invention;

(13) FIGS. 11A and 11B depict the relationships between the phase currents of the motor and the busbar current when the three phases are conducting according to the present invention.

DETAILED DESCRIPTION

(14) The present invention describes a starting method of a BLDC motor 10 with the aid of a two-levels discrete and modified fixed-time or fixed frequency hysteresis current controller, using only a current sensor 20 in the busbar.

(15) As noted in FIG. 1, the current control is reached by means of a closed mesh, that associates operatively the current sensor 20 in the busbar, the A/D converter, the current controller and the BLDC motor 10, wherein the voltage applied to the BLDC motor 10 consists of the difference between the desired applied current and the real applied current.

(16) As shown in FIG. 9, the starting method with two-levels fixed time modified and discrete hysteresis current controller, object of the present invention, sets a third step with a second protecting time T.sub.off2, wherein two switches are open, for example, S.sub.1 and S.sub.4. Thus, the of BLDC motor current I.sub.m is forced to pass in the opposite direction by the busbar, applying a negative voltage on the BLDC motor 10 and forcing a reduction of such current, such as shown in FIG. 7C.

(17) Optionally, still according to FIG. 9, the starting method uses fixed-frequency, setting a modified period of pulse width modulation T.sub.MPWM comprising of first, second and third steps. During the modified period of pulse width modulation T.sub.MPWM, both switches, for example, S.sub.1 and S.sub.4, are open in the end of each modified period T.sub.MPWM by a time T.sub.off2. Additionally, one of the switches, for example, the switch S.sub.1 can be open in a previous moment, for example, when the current surpasses the first maximum current limit I.sub.max1 and will only be turned on again in the beginning of the next modified period T.sub.MPWM.

(18) The difference between the fixed-time hysteresis current controller and the fixed-frequency hysteresis current controller is the fact that in the fixed-time, T.sub.off1 is fixed and T.sub.MPWM is variable, while in the fixed-frequency, T.sub.MPWM is fixed and T.sub.off1 is variable. In both cases, the time T.sub.on is variable and depends on the operating condition and the time T.sub.off2 is fixed.

(19) The current equation during the third step, (equation i3(t)); the approximating equation (equation 3) of this current by a straight line with the inclination defined by the derivative of the current i3(t) in the initial instant (t=0); and the current variation equation in the third step (equation i3) are defined as below:

(20) $i_3\left(t\right)=\frac{-V_{\mathrm{CC}}-E}{R}\left(1-e^{-\frac{t}{}}\right)+I_3{e}^{-\frac{t}{}}{}_3=\frac{{\mathrm{di}}_3\left(t\right)}{\mathrm{dt}}{.Math.}_{t=0}=\frac{-V_{\mathrm{CC}}-E}{L}-\frac{I_{3}R}{L}{i}_3={}_3.Math.T_{\mathrm{off}2}=\left(\frac{-V_{\mathrm{CC}}-E}{L}-\frac{I_{3}R}{L}\right).Math.T_{\mathrm{Off}2}$

(21) Thus, even in the case of very low revolutions, such as in the beginning of the start or in the case of a start failure, if we take into account the very low or practically null induced voltage E of the BLDC motor 10, and the busbar voltage V.sub.cc much higher than the voltage drop in the resistances R, due to the current I.sub.1 circulating int the BLDC motor 10, the increment in the current variation in the first step i.sub.i can be compensated by the decrement in the current variation in the third step i.sub.3, provided that the second protecting time T.sub.off2 is equal to the first time period T.sub.on. Thus, taking into account the worst case when the current is already above the first maximum current limit I.sub.max1, wherein the first time period T.sub.on is equal to the sampling period T.sub.s, the second protecting time T.sub.off2 must be adjusted in order to be equal to the sampling period T.sub.s. Thus, it is possible to guarantee that, even in such conditions, the current variation i, resulting from three steps i1, i2 and i3, is less than zero and it is possible to prevent the overcurrent, such as shown by the equations below:

(22) $i=i_1+i_2+i_{3}i=\left(\frac{V_{\mathrm{CC}}-I_{1}R}{L}\right).Math.T_{\mathrm{On}}-\frac{I_{2}R}{L}.Math.T_{\mathrm{Off}1}+\left(\frac{-V_{\mathrm{CC}}-I_{3}R}{L}\right).Math.T_{\mathrm{Off}2}i=\frac{V_{\mathrm{CC}}}{L}.Math.\left(T_1-T_2\right)-\frac{I_{2}R.Math.T_{\mathrm{pwm}}}{L}i=-\frac{I_{2}R.Math.T_{\mathrm{pwm}}}{L}$

(23) In the development above, the portions I.sub.1R/L.Math.T.sub.On, I.sub.2R/L.Math.T.sub.Off1 and I.sub.2R/L.Math.T.sub.Off2, which correspond to the natural answer of the RL circuit, were simplified taking as a base the higher current among I.sub.1, I.sub.2 and I.sub.3, in other words I.sub.2, and grouping the time portions t.sub.On, T.sub.off1 and T.sub.off2, totaling the period of pulse width modulation T.sub.pwm.

(24) According to FIG. 9, the first maximum error I.sub.max1 in the current limitation, due to the sign sampling carried out by the one-level modified and discrete fixed-time or fixed-frequency hysteresis current controller, can be obtained taking into account the equating of the first step for a first time period T.sub.on equal to a sampling period T.sub.s, and nulled induced voltage E, according to the equation below:

(25) $I_{\max 1}=\left(\frac{V_{\mathrm{CC}}}{L}-\frac{I_{\max 1}R}{L}\right).Math.T_S\frac{V_{\mathrm{CC}}}{L}.Math.T_S$

(26) Thus, the first maximum current limit I.sub.max1 must be lower than the maximum current allowed I.sub.desmag minus the first maximum error I.sub.max1.

(27) However, such first protecting level does not impede that, in case of occurring a starting failure with reversion of the rotation direction, typical of reciprocating compressors, the current does not continue increasing even after the opening of the first switch with the first protecting level. In this case, if we take into account the induced voltage E of the BLDC motor 10 as negative, which we will rename it as E.sub.rev due to the reversion of rotation direction, the BLDC motor current I.sub.m can increase, instead of diminishing, during a switching period. This happens whenever the reversed induced voltage E.sub.rev of the BLDC motor 10 is higher than the voltage drop in the resistances R of the BLDC motor 10, such as shown by the equations below:

(28) $i=i_1.Math.i_2.Math.i_{3}i=\left(\frac{V_{\mathrm{CC}}.Math.E_{\mathrm{rev}}{I}_{3}R}{L}\right).Math.T_{\mathrm{On}}+\frac{E_{\mathrm{rev}}{I}_{2}R}{L}.Math.T_{\mathrm{Off}1}+\left(\frac{E_{\mathrm{rev}}{V}_{\mathrm{CC}}{I}_{3}R}{L}\right).Math.T_{\mathrm{Off}2}i=\frac{V_{\mathrm{CC}}}{L}\left(T_3-T_3\right)+\frac{R_{\mathrm{rev}}}{L}.Math.T_{\mathrm{pwm}}-\frac{I_{3}R}{L}.Math.T_{\mathrm{pwm}}i=\frac{\left(E_{\mathrm{rev}}-I_{3}R\right)}{L}.Math.T_{\mathrm{pwm}}$

(29) In the development above, the same simplifying procedure was applied to the portions relating to the natural answer of the circuit RL and the portions relating to the reverse induced voltage E.sub.rev, where the portions of time t.sub.On, T.sub.off1 and T.sub.off2 are grouped, totaling the pulse width modulation period T.sub.pwm.

(30) Thus, the present invention also proposes a second protecting level which defines a second maximum current limit I.sub.max2, said limit, when reached by the current of the BLDC motor I.sub.m, measured in a measuring instant X, opens two switches during a third protecting time T.sub.off3, as exemplified in FIG. 10, wherein the switches S.sub.1 e S.sub.4 were open. Still according to FIG. 10, the motor current I.sub.m is forced to pass in the opposite direction by the busbar, applying a negative voltage on the BLDC motor 10 and forcing a more effective reduction of such current.

(31) Therefore, the starting method with two-levels modified and discrete fixed-time or fixed-frequency hysteresis current controller, according to the present invention, as can be seen in FIG. 10, works in the following way: in a first protecting level, related to a first maximum current limit I.sub.max1, the switch S.sub.1 is open during a first protecting time T.sub.off1. In order to assure an effective current protection, the switches S.sub.1 and S.sub.4 are open during a second protecting time T.sub.off2 after the first protecting time T.sub.off1. Then, there is a second protecting level, related to a second maximum current limit I.sub.max2, where the switches S.sub.1 and S.sub.4 are open for a third protecting time T.sub.off3, and the BLDC motor current i.sub.m is forced to pass by the busbar, being reduced faster and assuring the reliability of the BLDC motor 10.

(32) It Is worth to highlight that the value of the second maximum current limit I.sub.max2 must take into account the second maximum error I.sub.max2, which can occur due to the increase of the current on account of the reversion of the rotation direction. In this case, the second maximum error I.sub.max2 depends on the reverse induced voltage E.sub.rev of the BLDC motor 10 and may be set by the equation below:

(33) $I_{max2}=\frac{E_{\mathrm{rev}}-I_{\mathrm{demag}}R}{L}.Math.T_{\mathrm{pwm}}$

(34) Thus, the value of the second maximum current limit I.sub.max2 must be lower than the maximum current allowed I.sub.desmag and must be set taking into account the second maximum error I.sub.max2.

(35) Additionally, with the addition of the second maximum current limit I.sub.max2, the first maximum current limit I.sub.max1 needs to be redefined, so that the first current limit I.sub.max1 is lower than the second maximum current limit I.sub.max2 minus the first maximum error I.sub.max1.

(36) In addition, the value of the first maximum current limit I.sub.max1 must be set for the control of the motor current I.sub.m, under normal operating conditions and, besides, it must guarantee the second maximum current limit I.sub.max2 is not surpassed in case of a successful start. Yet the value of the second maximum current limit I.sub.max2 must be set for the protection against the maximum current allowed I.sub.desmag, so that it only acts in case of a starting failure.

(37) The duration of the third protecting time can be set in function of the second maximum error I.sub.max2, according to the equation below.

(38) 0$T_{\mathrm{off}3}>\frac{I_{\max 2}.Math.L}{\left(V_{\mathrm{cc}}-E_{\mathrm{rev}}\right)+I_{\mathrm{demag}}.Math.R}$

(39) In the case of the fixed-time hysteresis controller, the duration of the third protecting time T.sub.off3 can be set by the lower modified period of the pulse width modulation T.sub.mpwm, which would be equal to the first protecting time T.sub.off1, the second protecting time T.sub.off2 and the sampling time T.sub.s, as the lower time turned on T.sub.on is equal to the sampling time T.sub.s.

(40) Alternatively, in the case of the fixed-frequency hysteresis controller, the duration of the third protecting time T.sub.off3 can be simply set by the modified period T.sub.mpwv.

(41) Finally, the third protecting time T.sub.off3 can be replaced in such a way that the start is aborted when the motor current I.sub.m surpasses the second current limit I.sub.max2, as such intensity of the current would be an indication of the revolution of the BLDC motor 10 in the opposite direction and an evidence of starting failure.

(42) Another challenge related to the use of only one current sensor 20 in the busbar would be measuring the higher current when the three phases of the BLDC motor 10 are conducting, such as, for example, in the transitory just after a change in electrical position or in a failure of the sensing of the electric position, such as it happens in a starting failure with inversion of the rotation direction of the BLDC motor 10.

(43) FIG. 11A represents a condition wherein the switches S.sub.1 and S.sub.4 are turned on, the current enters by phase A and leaves by phase B, an undesired current circulates in phase C. Such undesired current in phase C can be transitory and due to a change in electric position or may have been generated at a starting failure due to reversion of the rotation direction.

(44) Referring again to FIG. 11A, the current i.sub.bus, that passes through the current sensor 20 of the busbar, is the one similar to current i.sub.a in phase A. However, there still exists a current i.sub.c in phase C, and the current i.sub.b in phase B is characterized by the sum of the currents on the other two phases:
i.sub.b=i.sub.a+i.sub.c

(45) wherein i.sub.a>0 and i.sub.c>0, so i.sub.b>0

(46) In this sense, in order to manage to carry out the monitoring of the biggest current circulating in the BLDC motor 10, the present invention proposes taking advantage of the opening of the two switches in the end of the switching period, during the second protecting time T.sub.off2. In this case, during the time when the switches are open, the biggest current circulating in such phases passes through the current sensor 20 of the busbar. In this case, i.sub.b=i.sub.a+i.sub.c such as is illustrated in FIG. 11B.

(47) Finally, the second protecting time T.sub.off2 can be disabled as soon as a successful start is identified. A successful start may be identified when the BLDC motor 10 manages to make a complete turn or when the speed of the BLDC motor 10 reaches a minimum speed considered safe, such as of the nominal speed of the motor.

(48) The disability of the second protecting time T.sub.off2 can be done in a gradual way, gradually reducing its actuating time at each mechanical turn until its complete extinction. The gradual extinction of the second protecting time may be, for example, conducted in approximately one second.

(49) Besides the embodiments previously shown, the same inventive concept shall be applied to other alternatives or possibilities of using the invention.

(50) Though the present invention has been described in relation to certain preferred embodiments, it should be understood that similar challenges are found when 3 current sensors are used associated in series to the lower switches, instead of only one current sensor in the inverter busbar, and they can be solved with the present invention.