CN103475296B - Permanent-magnet synchronous DC brushless motor low frequency control method - Google Patents

Abstract

The present invention relates to air conditioner controlling technology.The present invention is directed to the estimation of compressor operating timing-axis fluctuation inaccurate, compressor position probing step-out, propose permanent-magnet synchronous DC brushless motor low frequency control method, system gathers the phase current of motor by current detecting unit, be transferred to coordinate transformation unit, obtain the d shaft current component I of phase current _dand q shaft current component I _qafter be transferred to axis error estimation unit, system is according to gained current component I _dand I _qetc. parameter, calculate the error delta θ of physical location with estimated position of rotor, system is by motor present operating frequency f and first frequency f ₁and second frequency f ₂relatively, calculate speed estimating unit input value △ θ _pLL, and calculate the current estimation rotational speed omega of rotor _c, then the current estimated location θ of rotor is obtained by phase estimating unit _c.The present inventor, for reducing axis error △ θ, is equivalent to carry out amplitude limiting processing to axis error, reaches the effect of control convergence.Be applicable to permanent-magnet synchronous DC brushless motor.

Description

Permanent-magnet synchronous DC brushless motor low frequency control method

Technical field

The present invention relates to air conditioner controlling technology, particularly permanent-magnet synchronous DC brushless motor low frequency control method.

Background technology

Permanent-magnet synchronous DC brushless motor non-position sensor vector control method, one of key technology is wherein exactly the position probing of rotor and the presumption of running frequency, particularly during low frequency, the problem such as hydrops and the insufficient bearing lubrication caused of oil return due to compressor can make the fluctuation of service of compressor, cause detection current fluctuation, cause axis error △ θ to estimate inaccurate.The rotor position presuming value θ that motor is current _cit is the rotational speed omega utilizing axis error △ θ to obtain the current presumption of motor _c, ω _cby phase estimating unit according to formula:

θ＝∫ωdt

After carrying out integral adjustment calculating, then obtain.

During due to low frequency, axis error △ θ is inaccurate, and bigger than normal, just causes the rotational speed omega of the current presumption of motor _cinaccurate, frequency f _calso inaccurate (ω _c=2 π f _c), also just cause the phase theta of current presumption simultaneously _calso inaccurate.During the operation of the direct current machine pulse width modulation (PWM) ripple adopting inaccurate controling parameters to produce by three phase inverter bridge control circui compressor, the current detection value returned from current detecting unit is also just inaccurate, the estimation affecting again △ θ is conversely inaccurate, last result is, the very big fluctuation of the running frequency of axis error presumption, control not restrain, cause control out of control.

Summary of the invention

Technical problem to be solved by this invention, be exactly in prior art during compressor low-frequency operation, because inaccurate and control problems such as the compressor position probing step-out of appearance bigger than normal are estimated in axis error estimation, permanent-magnet synchronous DC brushless motor low frequency control method is provided, by when low frequency, artificial reduction axis error, is equivalent to carry out amplitude limiting processing to axis error, reaches the effect of control convergence.

The present invention solve the technical problem, the technical scheme adopted is, permanent-magnet synchronous DC brushless motor low frequency control system, comprise permanent-magnet synchronous DC brushless motor, also comprise current conversion unit, axis error estimation unit, position detection unit, three-phase inversion bridge control circuit and vector control and direct current machine pulse width modulation (PWM) ripple control unit, described permanent-magnet synchronous DC brushless motor is connected with current conversion unit and three-phase inversion bridge control circuit respectively, current conversion unit respectively with axis error estimation unit, position detection unit is connected with vector control and direct current machine pulse width modulation (PWM) ripple control unit, axis error estimation unit is connected with position detection unit and vector control and direct current machine pulse width modulation (PWM) ripple control unit respectively, position detection unit is connected with vector control and direct current machine pulse width modulation (PWM) ripple control unit, vector control and direct current machine pulse width modulation (PWM) ripple control unit are connected with three-phase inversion bridge control circuit.

Described current conversion unit, realizes coordinate transform by the electric current of detection after electric current provide current component for realizing detecting, and be transferred to axis error estimation unit;

Described axis error estimation unit, for calculating axis error △ θ, and is transferred to presumption unit;

Described position detection unit, the axis error △ θ for being exported by axis error estimation unit is estimated as the estimation rotational speed omega of motor _cand the estimated position θ of rotor _c;

Described vector control and direct current machine pulse width modulation (PWM) ripple control unit are used for the estimated position θ based on rotor _cproduce corresponding direct current machine pulse width modulation (PWM) ripple, and then be adjusted to the second tachometer value based on the rotating speed that described direct current machine pulse width modulation (PWM) ripple controls described rotor by the first tachometer value;

Described three-phase inversion bridge control circuit, the direct current machine pulse width modulation (PWM) ripple inversion for the variable duty ratio produced according to vector control and direct current machine pulse width modulation (PWM) ripple control unit is the three-phase alternating current of motor, controls the running of motor.

Concrete, described current conversion unit comprises current detecting unit and coordinate transformation unit, current detecting unit is connected with coordinate transformation unit, described current detecting unit is connected with permanent-magnet synchronous DC brushless motor and three-phase inversion bridge control circuit respectively, and described coordinate transformation unit is connected with axis error estimation unit and vector control and direct current machine pulse width modulation (PWM) ripple control unit respectively;

Described current detecting unit, for realizing current detecting, and is transferred to coordinate transformation unit;

Described coordinate transformation unit, the electric current for being detected by current detecting unit realizes coordinate transform and provides current component, and is transferred to axis error estimation unit.

Concrete, described position detection unit comprises speed estimating unit and phase estimating unit, speed estimating unit is connected with phase estimating unit, link is connected with vector control and direct current machine pulse width modulation (PWM) ripple control unit, speed estimating unit is connected with axis error estimation unit, and phase estimating unit is connected with vector control and direct current machine pulse width modulation (PWM) ripple control unit and coordinate transformation unit respectively;

Described speed estimating unit, the axis error △ θ for being exported by axis error estimation unit is estimated as the current estimation rotational speed omega of rotor _c, and be transferred to phase estimating unit;

Described phase estimating unit, regulates computing by the current estimation rotational speed omega of rotor for passing through _cbe converted to the current estimated location θ of rotor _c, and input to vector control and direct current machine pulse width modulation (PWM) ripple control unit and current conversion unit.

Permanent-magnet synchronous DC brushless motor low frequency control method, comprises following step:

Step 1, system gather the phase current of permanent-magnet synchronous DC brushless motor by current detecting unit, comprise first-phase electric current I _u, second-phase electric current I _vand third phase electric current I _w, and be transferred to coordinate transformation unit;

The phase current of permanent-magnet synchronous DC brushless motor is carried out coordinate transform by coordinate transformation unit by step 2, system, obtains the d shaft current component I of this permanent-magnet synchronous DC brushless motor phase current _dand q shaft current component I _q, and be transferred to axis error estimation unit;

Step 3, system pass through axis error estimation unit, according to the d shaft current component I of gained phase current _dand the q shaft current component I of phase current _qthe physical location calculating rotor, with the error of estimated position, counts axis error △ θ;

Step 4, system are by electric permanent-magnet synchronous DC brushless motor present operating frequency f and first frequency f ₁and second frequency f ₂relatively, calculate speed estimating unit input value, count △ θ _pLL, first frequency f ₁large lower limit frequency value is become, second frequency f for making axis error △ θ ₂for making axis error △ θ become large upper limit frequency value, and by speed estimating unit input value △ θ _pLLwith given speed estimating unit input value △ θ _pLL0=0 compares, and calculates the current estimation rotational speed omega of rotor _c;

Step 5, system are according to the current estimation rotational speed omega of rotor _c, the current estimated location θ of rotor is obtained by phase estimating unit _c.

Concrete, in step 2, the d shaft current component I of system-computed permanent-magnet synchronous DC brushless motor phase current _dand q shaft current component I _qtime, first adopt the conversion of energy invariant coordinates, calculate the electric current I under α, β coordinate system _αand I _β, formula is:

$\left[\begin{array}{c}{I}_{\mathrm{\α}}\\ I_{\mathrm{\β}}\end{array}\right]=\sqrt{\frac{2}{3}}\left[\begin{array}{ccc}1& -\frac{1}{2}& -\frac{1}{2}\\ 0& \frac{\sqrt{3}}{2}& -\frac{\sqrt{3}}{2}\end{array}\right]\left[\begin{array}{c}{I}_u\\ I_v\\ I_w\end{array}\right]$

Wherein, I _ufor first-phase electric current, I _vfor second-phase electric current, I _wfor third phase electric current;

The coordinate transform formulae discovery that recycling α, β coordinate is tied to d, q axis coordinate system goes out the d shaft current component I of phase current _dand q shaft current component I _q, coordinate transform formula is:

$\left[\begin{array}{c}{I}_d\\ I_q\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& sin\mathrm{\θ}\\ -sin\mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{I}_{\mathrm{\α}}\\ I_{\mathrm{\β}}\end{array}\right]$

Wherein, θ is the current phase angle of rotor.

Concrete, in step 3, the physical location of system-computed rotor and the error of rotor estimated position, axis error △ θ, the axis error computing formula adopted is:

$\mathrm{\Δ}\mathrm{\θ}=\frac{V_d-{\mathrm{RI}}_d+L_q{\mathrm{\ωI}}_q}{\[K_E+(L_d-L_q)I_d\]\mathrm{\ω}}$

Wherein, R is the resistance of permanent-magnet synchronous DC brushless motor, K _efor induced voltage constant, L _dfor given d axle inductance value, L _qfor given q axle inductance value, V _dfor given motor d shaft voltage value, ω is the rotating speed of current motor, I _dfor phase current d shaft current component, I _qfor phase current q shaft current component.

Concrete, in step 4, the current estimation rotational speed omega of system-computed rotor _c, adopt formula to be:

ω _c＝K _p(0-Δθ _PLL)+T _sK _I(0-Δθ _PLL)

Wherein, K _pfor the scale parameter that proportional integral regulates PI to regulate, K _ifor the integral parameter that proportional integral regulates PI to regulate, T _sfor computing cycle, △ θ _pLLfor speed estimating unit input value.

Further, when permanent-magnet synchronous DC brushless motor present operating frequency f is lower than first frequency f ₁time, error delta θ is constant for system retainer shaft, that is:

Δθ _PLL＝Δθ

Calculate the current estimation rotational speed omega of rotor _c, its computing formula is:

${\mathrm{\ω}}_c=\frac{K_P}{a}(0-{\mathrm{\Δ\θ}}_{PLL})+T_s\frac{K_I}{a}(0-{\mathrm{\Δ\θ}}_{PLL})$

Wherein, a be greater than 1 constant, K _pfor the scale parameter that proportional integral regulates PI to regulate, K _ifor the integral parameter that proportional integral regulates PI to regulate, T _sfor computing cycle, △ θ _pLLfor speed estimating unit input value.

Concrete, in step 4, when permanent-magnet synchronous DC brushless motor present operating frequency f is lower than first frequency f ₁time, axis error △ θ reduces by system, as the input △ θ of speed estimating unit _pLL, calculate the current estimation rotational speed omega of rotor _c.

Further, when permanent-magnet synchronous DC brushless motor present operating frequency f is lower than first frequency f ₁time, the method that axis error △ θ reduces to adopt is, by following formulae discovery by system:

${\mathrm{\Δ\θ}}_{PLL}=\frac{\mathrm{\Δ}\mathrm{\θ}}{a}$

Wherein, a be greater than 1 constant, △ θ is axis error;

System is according to the input value △ θ of this speed estimating unit _pLL, calculate the current estimation rotational speed omega of rotor _c.

Concrete, in step 4, when permanent-magnet synchronous DC brushless motor present operating frequency f is higher than second frequency f ₂time, system uses axis error △ θ as the input △ θ of speed estimating unit _pLL, i.e. △ θ _pLL=△ θ, calculates the current estimation rotational speed omega of rotor _c.

Concrete, in step 4, when permanent-magnet synchronous DC brushless motor present operating frequency f is between first frequency f ₁with second frequency f ₂between time, wherein f ₂> f ₁, the input △ θ of system-computed phase-locked loop speed estimating unit _pLLformula be:

${\mathrm{\Δ\θ}}_{PLL}=\frac{\mathrm{\Δ}\mathrm{\θ}(f_2-f_1)}{(f_2-f_1)+(a-1)(f_2-f)}$

Wherein, a is a constant being greater than 1, f ₁for first frequency, f ₂for second frequency, △ θ is axis error;

And according to the input value △ θ of speed estimating unit _pLL, calculate the current estimation rotational speed omega of rotor _c.

Concrete, step 5, the current estimated location θ of system-computed rotor _c, the formula of employing is:

θ _c＝θ _c-1+ω _cT _c

Wherein, T _cfor computing cycle, θ _c-1for a upper phase angle, ω _cfor the current estimation rotating speed of rotor.

The invention has the beneficial effects as follows, by above-mentioned permanent-magnet synchronous DC brushless motor low frequency control system and method, when can make low frequency, the axis error △ θ of compressor fluctuates and reduces, the absolute value of axis error △ θ diminishes, the compressor actual frequency fluctuation of microprocessor detect and control reduces, and permanent-magnet synchronous DC brushless motor actual current reduces, and improves efficiency, reduce system power, system stability, control convergence, improves low frequency control precision.

Accompanying drawing explanation

Fig. 1 is permanent-magnet synchronous DC brushless motor low frequency control system block diagram in the embodiment of the present invention;

Fig. 2 is the schematic diagram of rotor-position under dq coordinate system of embodiment of the present invention rotor physical location and presumption.

Embodiment

Technical scheme of the present invention is described in detail below in conjunction with drawings and Examples:

When the present invention is directed to compressor low-frequency operation in prior art, because inaccurate and control problems such as the compressor position probing step-out of appearance bigger than normal are estimated in axis error estimation, permanent-magnet synchronous DC brushless motor low frequency control method is provided, comprise: first, system gathers the phase current of permanent-magnet synchronous DC brushless motor by current detecting unit, comprises first-phase electric current I _u, second-phase electric current I _vand third phase electric current I _w, and be transferred to coordinate transformation unit.Secondly, the phase current of permanent-magnet synchronous DC brushless motor is carried out coordinate transform by coordinate transformation unit by system, obtains the d shaft current component I of this permanent-magnet synchronous DC brushless motor phase current _dand q shaft current component I _q, and be transferred to axis error estimation unit.Then, system passes through axis error estimation unit, according to the d shaft current component I of gained phase current _dand the q shaft current component I of phase current _qthe physical location calculating rotor, with the error of estimated position, counts axis error △ θ.Subsequently, system is by electric permanent-magnet synchronous DC brushless motor present operating frequency f and first frequency f ₁and second frequency f ₂relatively, calculate speed estimating unit input value, count △ θ _pLL, described first frequency f ₁for making axis error △ θ become large lower limit frequency value, described second frequency f ₂for making axis error △ θ become large upper limit frequency value, and by speed estimating unit input value △ θ _pLLwith given speed estimating unit input value △ θ _pLL0=0 compares, and calculates the current estimation rotational speed omega of rotor _c.Finally, system is according to the current estimation rotational speed omega of rotor _c, the current estimated location θ of rotor is obtained by phase estimating unit _c.By when low frequency, artificially reduce axis error, be equivalent to, to axis error amplitude limiting processing, reach the effect of control convergence.

Embodiment

In this example, permanent-magnet synchronous DC brushless motor low frequency control system, as shown in Figure 1, permanent-magnet synchronous DC brushless motor is connected with three-phase inversion bridge control circuit and current detecting unit respectively, current detecting unit is connected with coordinate transformation unit, coordinate transformation unit respectively with axis error estimation unit, phase estimating unit is connected with vector control and direct current machine pulse width modulation (PWM) ripple control unit, axis error estimation unit is connected with speed estimating unit and vector control and direct current machine pulse width modulation (PWM) ripple control unit respectively, speed estimating unit is connected with phase estimating unit and vector control and direct current machine pulse width modulation (PWM) ripple control unit respectively, vector control and direct current machine pulse width modulation (PWM) ripple control unit are connected with three-phase inversion bridge control circuit.

During work, system, by the current sensor of current detecting unit, detects the first-phase electric current I of permanent-magnet synchronous DC brushless motor _u, second-phase electric current I _vand third phase electric current I _wafter, as shown in Figure 2, set up d, q axle rectangular coordinate system rotated with rotor, is overlapped by abscissa d axle with the current location of rotor, ordinate is q axle and vertical with d axle, and first employing energy invariant coordinates converts, and calculates electric current I under α, β coordinate system _αand I _β, formula is:

$\left[\begin{array}{c}{I}_{\mathrm{\α}}\\ I_{\mathrm{\β}}\end{array}\right]=\sqrt{\frac{2}{3}}\left[\begin{array}{ccc}1& -\frac{1}{2}& -\frac{1}{2}\\ 0& \frac{\sqrt{3}}{2}& -\frac{\sqrt{3}}{2}\end{array}\right]\left[\begin{array}{c}{I}_u\\ I_v\\ I_w\end{array}\right]$

Wherein, Iu is first-phase electric current, Iv is second-phase electric current, Iw is third phase electric current.

Recycling α, β coordinate is tied to the coordinate transform of d, q axis coordinate system, calculates the d shaft current component I of phase current _dand q shaft current component I _q, adopt following coordinate transform formula:

$\left[\begin{array}{c}{I}_d\\ I_q\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& sin\mathrm{\θ}\\ -sin\mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{I}_{\mathrm{\α}}\\ I_{\mathrm{\β}}\end{array}\right]$

Wherein, θ is the current phase angle of rotor.

Obtain the d shaft current component I of phase current _dand q shaft current component I _qafter, meanwhile, utilize the rotational speed omega of current permanent-magnet synchronous DC brushless motor, by axis error estimation unit, can obtain axis error △ θ, computing formula is:

$\mathrm{\Δ}\mathrm{\θ}=\frac{V_d-{\mathrm{RI}}_d+L_q{\mathrm{\ωI}}_q}{\[K_E+(L_d-L_q)I_d\]\mathrm{\ω}}$

Wherein, R is the resistance of permanent-magnet synchronous DC brushless motor, K _efor induced voltage constant, L _dfor given d axle inductance value, L _qfor given q axle inductance value, V _dfor given motor d shaft voltage value, ω is the current rotating speed of rotor, I _dfor phase current d shaft current component, I _qfor phase current q shaft current component.

Secondly, system is by permanent-magnet synchronous DC brushless motor present operating frequency f, first frequency f ₁and second frequency f ₂relatively, when given frequency f is less than first frequency f ₁time, axis error △ θ reduces by system, as the input △ θ of speed estimating unit _pLL, be not limited only to proportionally reduce, also can segmentation reduce, or amplitude limiting processing etc., can formula be adopted in this example:

Δθ _PLL＝Δθ

Wherein, a be greater than 1 constant, △ θ is axis error;

And by speed estimating unit input value △ θ _pLLwith the input △ θ of given speed estimating unit _pLL=0 compares, and then according to following formula, calculates the current estimation rotational speed omega of rotor _c:

ω _c＝K _p(0-Δθ _PLL)+T _sK _I(0-Δθ _PLL)

Wherein, K _pfor the scale parameter that PI regulates, K _ifor the integral parameter that PI regulates, T _sfor computing cycle, △ θ _pLLfor speed estimating unit input value.

When permanent-magnet synchronous DC brushless motor present operating frequency f is less than first frequency f ₁time, this example provides another method, and error delta θ is constant for system retainer shaft, that is:

Δθ _PLL＝Δθ

Regulated by PI, calculate the current estimation rotational speed omega of rotor _c, its computing formula is:

${\mathrm{\ω}}_c=\frac{K_P}{a}(0-{\mathrm{\Δ\θ}}_{PLL})+T_s\frac{K_I}{a}(0-{\mathrm{\Δ\θ}}_{PLL})$

Wherein, a be greater than 1 constant, K _pfor the scale parameter that PI regulates, K _ifor the integral parameter that PI regulates, T _sfor computing cycle, △ θ _pLLfor speed estimating unit input value.

When permanent-magnet synchronous DC brushless motor present operating frequency f is higher than second frequency f ₂time, system directly uses axis error △ θ as the input △ θ of speed estimating unit _pLL, i.e. △ θ _pLL=△ θ, by △ θ _pLLwith given △ θ _pLL=0 compares, and then according to following formula, calculates the current estimation rotational speed omega of rotor _c:

ω _c＝K _p(0-Δθ _PLL)+T _sK _I(0-Δθ _PLL)

Wherein, K _pfor the scale parameter that PI regulates, K _ifor the integral parameter that PI regulates, T _sfor computing cycle, △ θ _pLLfor speed estimating unit input value.

When permanent-magnet synchronous DC brushless motor present operating frequency f is between first frequency f ₁with second frequency f ₂between time (f ₂>f ₁), the input △ θ of system-computed phase-locked loop speed estimating unit _pLLformula be:

${\mathrm{\Δ\θ}}_{PLL}=\frac{\mathrm{\Δ}\mathrm{\θ}(f_2-f_1)}{(f_2-f_1)+(a-1)(f_2-f)}$

Wherein, a is a constant being greater than 1, f ₁for first frequency, f ₂for second frequency;

Then, by speed estimating unit input value △ θ _pLLwith the input △ θ of given speed estimating unit _pLL=0 compares, and then according to following formula, calculates the current estimation rotational speed omega of rotor _c:

ω _c＝K _p(0-Δθ _PLL)+T _sK _I(0-Δθ _PLL)

Wherein, K _pfor the scale parameter that PI regulates, K _ifor the integral parameter that PI regulates, T _sfor computing cycle, △ θ _pLLfor speed estimating unit input value.

Subsequently, system is according to the current estimation rotational speed omega of rotor _c, calculate the current estimated location θ of rotor _c, formula used is:

θ _c＝θ _c-1+ω _cT _c

Wherein, T _cfor computing cycle, θ _c-1for a upper phase angle, ω _cfor the current estimation rotating speed of rotor.

θ _cthe current estimated location of rotor.By ω _cwith the current estimated location θ of the current rotational speed omega of given rotor, rotor _cand the d shaft current component I of phase current that current detection arrives _dand q shaft current component I _q, be input to vector control and direct current machine pulse width modulation (PWM) ripple control unit, this element is except exporting given voltage V _dthere is provided outside axis error estimation unit, the three-phase DC motor pulse width modulation (PWM) ripple that also output duty cycle is adjustable, under the control of three phase inverter bridge, control the rotating speed of motor, realize controlling permanent-magnet synchronous DC brushless motor, and make axis error △ θ fluctuate to reduce when low frequency, system stability, control convergence.

Claims (9)

1. permanent-magnet synchronous DC brushless motor low frequency control method, is characterized in that, comprises following step:

Step 1, system gather the phase current of permanent-magnet synchronous DC brushless motor by current detecting unit, comprise first-phase electric current I _u, second-phase electric current I _vand third phase electric current I _w, and be transferred to coordinate transformation unit;

The phase current of permanent-magnet synchronous DC brushless motor is carried out coordinate transform by coordinate transformation unit by step 2, system, obtains the d shaft current component I of this permanent-magnet synchronous DC brushless motor phase current _dand q shaft current component I _q, and be transferred to axis error estimation unit;

Step 3, system pass through axis error estimation unit, according to the d shaft current component I of gained phase current _dand the q shaft current component I of phase current _qcalculate the physical location of rotor with the error of estimated position, count axis error △ θ, the axis error computing formula adopted is:

$\mathrm{\Δ}\mathrm{\θ}=\frac{V_d-{\mathrm{RI}}_d+L_q{\mathrm{\ωI}}_q}{\[K_E+(L_d-L_q)I_d\]\mathrm{\ω}}$

Wherein, R is the resistance of permanent-magnet synchronous DC brushless motor, K _efor induced voltage constant, L _dfor given d axle inductance value, L _qfor given q axle inductance value, V _dfor given motor d shaft voltage value, ω is the rotating speed of current motor, I _dfor phase current d shaft current component, I _qfor phase current q shaft current component;

Step 4, system are by permanent-magnet synchronous DC brushless motor present operating frequency f and first frequency f ₁and second frequency f ₂relatively, calculate speed estimating unit input value, count △ θ _pLL, first frequency f ₁large lower limit frequency value is become, second frequency f for making axis error △ θ ₂for making axis error △ θ become large upper limit frequency value, and by speed estimating unit input value △ θ _pLLwith given speed estimating unit input value △ θ _pLL0=0 compares, and calculates the current estimation rotational speed omega of rotor _c;

Step 5, system are according to the current estimation rotational speed omega of rotor _c, the current estimated location θ of rotor is obtained by phase estimating unit _c.

2. permanent-magnet synchronous DC brushless motor low frequency control method according to claim 1, is characterized in that, in step 2, and the d shaft current component I of system-computed permanent-magnet synchronous DC brushless motor phase current _dand q shaft current component I _qtime, first adopt the conversion of energy invariant coordinates, calculate the electric current I under α, β coordinate system _αand I _β, formula is:

$\left[\begin{array}{c}{I}_{\mathrm{\α}}\\ I_{\mathrm{\β}}\end{array}\right]=\sqrt{\frac{2}{3}}\left[\begin{array}{ccc}1& -\frac{1}{2}& -\frac{1}{2}\\ 0& \frac{\sqrt{3}}{2}& -\frac{\sqrt{3}}{2}\end{array}\right]\left[\begin{array}{c}{I}_u\\ I_v\\ I_w\end{array}\right]$

Wherein, I _ufor first-phase electric current, I _vfor second-phase electric current, I _wfor third phase electric current;

The coordinate transform formulae discovery that recycling α, β coordinate is tied to d, q axis coordinate system goes out the d shaft current component I of phase current _dand q shaft current component I _q, coordinate transform formula is:

$\left[\begin{array}{c}{I}_d\\ I_q\end{array}\right]=\left[\begin{array}{cc}cos\mathrm{\θ}& sin\mathrm{\θ}\\ -\sin \mathrm{\θ}& \cos \mathrm{\θ}\end{array}\right]\left[\begin{array}{c}{I}_{\mathrm{\α}}\\ I_{\mathrm{\β}}\end{array}\right]$

Wherein, θ is the current phase angle of rotor.

3. permanent-magnet synchronous DC brushless motor low frequency control method according to claim 1, is characterized in that, in step 4, and the current estimation rotational speed omega of system-computed rotor _c, adopt formula to be:

ω _c＝K _p(0-△θ _PLL)+T _sK _I(0-△θ _PLL)

Wherein, K _pfor the scale parameter that proportional integral regulates PI to regulate, K _ifor the integral parameter that proportional integral regulates PI to regulate, T _sfor computing cycle, △ θ _pLLfor speed estimating unit input value.

4. permanent-magnet synchronous DC brushless motor low frequency control method according to claim 3, is characterized in that, when permanent-magnet synchronous DC brushless motor present operating frequency f is lower than first frequency f ₁time, error delta θ is constant for system retainer shaft, that is:

△θ _PLL＝+θ

Calculate the current estimation rotational speed omega of rotor _c, its computing formula is:

${\mathrm{\ω}}_c=\frac{K_P}{a}(0-{\mathrm{\Δ\θ}}_{PLL})+T_s\frac{K_I}{a}(0-{\mathrm{\Δ\θ}}_{PLL})$

Wherein, a be greater than 1 constant, K _pfor the scale parameter that proportional integral regulates PI to regulate, K _ifor the integral parameter that proportional integral regulates PI to regulate, T _sfor computing cycle, △ θ _pLLfor speed estimating unit input value.

5. permanent-magnet synchronous DC brushless motor low frequency control method according to claim 1, is characterized in that, in described step 4, when permanent-magnet synchronous DC brushless motor present operating frequency f is lower than first frequency f ₁time, axis error △ θ reduces by system, as the input △ θ of speed estimating unit _pLL, calculate the current estimation rotational speed omega of rotor _c.

6. permanent-magnet synchronous DC brushless motor low frequency control method according to claim 5, is characterized in that, when permanent-magnet synchronous DC brushless motor present operating frequency f is lower than first frequency f ₁time, the method that axis error △ θ reduces to adopt is, by following formulae discovery by system:

${\mathrm{\Δ\θ}}_{PLL}=\frac{\mathrm{\Δ}\mathrm{\θ}}{a}$

Wherein, a be greater than 1 constant, △ θ is axis error;

System is according to the input value △ θ of this speed estimating unit _pLL, calculate the current estimation rotational speed omega of rotor _c.

7. permanent-magnet synchronous DC brushless motor low frequency control method according to claim 1, is characterized in that, in described step 4, when permanent-magnet synchronous DC brushless motor present operating frequency f is higher than second frequency f ₂time, system uses axis error △ θ as the input △ θ of speed estimating unit _pLL, i.e. △ θ _pLL=△ θ, calculates the current estimation rotational speed omega of rotor _c.

8. permanent-magnet synchronous DC brushless motor low frequency control method according to claim 1, is characterized in that, in described step 4, when permanent-magnet synchronous DC brushless motor present operating frequency f is between first frequency f ₁with second frequency f ₂between time, wherein f ₂> f ₁, the input value △ θ of system-computed speed estimating unit _pLLformula be:

${\mathrm{\Δ\θ}}_{PLL}=\frac{\mathrm{\Δ}\mathrm{\θ}(f_2-f_1)}{(f_2-f_1)+(a-1)(f_2-f)}$

Wherein, a is a constant being greater than 1, f ₁for first frequency, f ₂for second frequency, △ θ is axis error;

And according to the input value △ θ of speed estimating unit _pLL, calculate the current estimation rotational speed omega of rotor _c.

9. permanent-magnet synchronous DC brushless motor low frequency control method according to claim 1, is characterized in that, described step 5, the current estimated location θ of system-computed rotor _c, the formula of employing is:

θ _c＝θ _c-1+ω _cT _c

Wherein, T _cfor computing cycle, θ _c-1for a upper phase angle, ω _cfor the current estimation rotating speed of rotor.