WO2005025050A1

WO2005025050A1 - Driving method and driving apparatus of permanent magnet synchronous motor for extending flux weakening region

Info

Publication number: WO2005025050A1
Application number: PCT/JP2003/011337
Authority: WO
Inventors: Shunsuke Unehara; Subrata Saha; Koji Hamaoka; Hidehisa Tanaka; Kazushige Narazaki
Original assignee: Matsushita Electric Industrial Co., Ltd.
Priority date: 2003-09-05
Filing date: 2003-09-05
Publication date: 2005-03-17
Also published as: AU2003259571A1; JP4467520B2; JP2007519384A

Abstract

In a driving method of a permanent magnet synchronous motor, which is capable of extending the flux-weakening region of a wide conduction angle drive, the motor is started with a 120o conduction drive in closed loop. The motor drive mode is changed from the 120° conduction drive mode to a 150o conduction drive mode in closed loop, when PWM duty for the 120o conduction drive becomes greater than 95 %, during acceleration. The motor drive is changed from the 150o conduction drive to a forced drive in open loop, during further acceleration, when the PWM duty of the 150o conduction drive becomes greater than 95 %. The motor drive is changed from the forced drive to the 150o conduction drive, when the zero-cross of the back emf is detected again continuously, during deceleration. The motor drive is changed from the 150o conduction drive to the 120o conduction drive during further deceleration.

Description

DESCRIPTION

DRIVING METHOD AND DRIVING APPARATUS OF PERMANENT MAGNET SYNCHRONOUS MOTOR FOR EXTENDING FLUX WEAKENING REGION

Technical Field The present invention relates to a driving technique for a permanent magnet synchronous motor (i.e. surface permanent magnet motor or interior permanent magnet motor) without using any position and current sensors, and capable of extending the flux-weakening region of a wide conduction angle drive (i.e. more than 120° and less than 180° conduction angle).

Background Art Development of a 120° conduction pulse width modulated

(PWM) drive for a surface permanent magnet (SPM) or an interior permanent magnet (IPM) synchronous motor by sensing the induced back electromotive force (emf) and without using any current and position sensors is well reported ("Microcomputer control for sensorless brushless motor", lizuka, H.Uzuhashi, M.Kano, T.Endo and T.Mohri, IEEE transactions on Industry Applications, Vol. IA-21 , No.4, May/June 1985, pp 595-691 ). A typical inverter configuration is shown in Fig. 1. The inverter is provided with a voltage-divider resistor consisting of R1 and R2 at each of the three phase terminals for sensing the induced back emf (electromotive force) of the SPM or the IPM synchronous motor. The values of the voltage divider resistor consisting of R1 and R2 are chosen so that the induced back emf of the motor is reduced to appropriate voltage levels compatible with the analog and digital circuits or the A/D port of a micro-controller. In the first scheme, for a traditional 120° conduction drive, the PWM base drive waveforms for the three phases and the induced back emf waveforms at the three phase terminals U, V and W are shown in Fig. 2, where the upper transistors T1 , T3 and T5 are turned on and off with duty δ in a PWM mode and the lower transistors T2, T4 and T6 are fully turned on during the 120° conduction period. In the second scheme, another set of typical PWM base drive waveforms for the three phases and the induced back emf waveforms at the three phase terminals are shown in Fig. 3, where both the upper transistors T1 , T3 and T5 and the lower transistors T2, T4 and T6 are fully turned on for the first 60° and then both are turned on and off with duty δ in a PWM mode in the next 60° during the 120° conduction period. As shown in Figs. 2 and 3, during the current zero-cross period of a particular phase, the corresponding induced back emf waveform is trapezoidal in nature having a center E/2, where E is the main supply voltage. This center-line E/2 can be referred as the zero-cross of the back emf. The present control technique includes detecting the zero-cross of the back emf of the three phases every 60° and subsequently giving a pre-calculated delay (D) corresponding to the speed before starting the conduction of a new phase. Detection of the zero-cross from the induced back emf voltages of the three phases can be done by hardware with simple analog and digital circuits and is shown in Fig. 4. The detected zero-cross signal is given as an input to a single channel A/D port of the micro-controller for controlling the motor. On the other hand, the induced back emf voltages of the three phases are directly given as inputs into the three channels of the A/D ports of a micro-controller which are enabled sequentially to generate the zero-cross detecting signals by software within the micro-controller itself for controlling the motor and is shown in Fig. 5. The block diagram for the closed loop control is shown in Fig. 6. The incremental rotor position Δ θ corresponding to every cycle of the PWM carrier frequency is estimated within the micro-controller by dividing 60° with the total number of PWM interrupts between two consecutive zero-crossings of the back emf. The speed ω is estimated from Δ θ . The estimated speed ω is first passed through a low pass filter (LPF) and then compared with the reference speed ω ^*. The error between ω and ω ^* is processed in a proportional and integral (PI) controller to generate the PWM turn-on duty δ . Recently, a wide angle 150° conduction drive sensing the induced back emf and without using any current and position sensors has been proposed (Japanese Patent Laid-Open Publication Nos. 2002-078373 and 2002-359991). The motor is started as a 120° conduction drive and between speed ω ₁ and ω 2 the 120° PWM base drive waveforms for the three phases are extended in the left direction by 30° to make it wide angle 150° conduction drive as shown in Figs. 7 and 8. The induced back emf waveforms at the three phase terminals U, V and W for the wide angle 150° conduction drive are also shown in Figs. 7 and 8. During the change from 120° to 150° conduction angle drive the PWM duty δ is always less than 95 %. 2005/025050

4 Different modulation schemes of the base drive waveforms are also proposed for the wide angle 150° conduction drive for low vibration and efficient operation of the drive system. Modulation of the base drive waveforms guarantees perfect zero-cross detection of the induced back emf. In one such modulation scheme, the turn-on periods ( ) in the first 30° is increased by k% whereas the turn-on periods in the last 30° is decreased by k% as shown in Fig. 9 ("A novel sensorless control drive for an interior permanent magnet motor", S.Saha, T.Tazawa, T.lijima, KNarazaki, H.Murakami and Y.Honda:, in IEEE-IECON conference at Denever, USA, Nov. 2001 , pp 1655-1660"). The value of k is increased from 0 % to ki % as the motor changes from 120° to 150° conduction angle between speed ω i and ω₂. In 150° conduction angle mode the value of k is decreased linearly with the speed ω of the motor from ki % to 0 %. The major advantage of the wide angle 150° conduction drive compared to the 120° conduction drive is the extension of the flux-weakening region and is shown in Fig. 10. This is because in a position sensorless 120° conduction drive the effective conduction angle of the machine is not fully utilized leading to its saturated flux condition at a much lower speed. The closed loop block diagram of the wide angle 150° conduction drive with PWM base drive modulation is shown in Fig. 11. For maximum efficiency operation, the present control technique for the wide angle 150° conduction drive is similar to the 120° conduction drive which demand the detection of the zero-cross of the back emf E/2 and an appropriate constant delay angle (D) between an instant of the back emf zero-cross and the turn on of the next suitable phase. Thus, with the present sensorless control technique of the 150° conduction drive the value of the load angle ε between the fundamental phase voltage and the phase magnetic back emf is always indirectly fixed and therefore, further extension of the flux-weakening region is not possible.

Disclosure of the invention The present invention is directed to provide a driving method of a permanent magnet synchronous motor capable of extending the flux-weakening region of a position sensorless and a current sensorless wide angle conduction drive. The flux-weakening region of a position sensorless and a current sensorless wide angle 150° conduction drive for a surface permanent magnet (SPM) or an interior permanent magnet (IPM) synchronous motor can be extended if the control strategy of the wide angle 150° conduction drive is changed from closed loop by sensing the back emf of the motor to open loop by forced drive technique without sensing the back emf of the motor. Thus, the speed and torque limit of the wide angle position sensorless and current sensorless 150° conduction drive is enhanced due to the forced drive technique. The forced drive mode does not need to detect the back emf and hence, the current zero-cross period as in the 150° conduction drive is not required. Therefore, the reverse diodes in the three phase inverter starts conducting giving a 180° phase current conduction in the forced drive mode which is the first basic reason for an extended flux-weakening region. As the speed or the torque of the drive system is increased during the forced drive mode the reverse diodes conducts at appropriate instants so that the phase voltage is more advanced with respect to the phase back emf giving an increased value of load angle ε which further extends the flux-weakening region. This automatic adjustment of the load angle ε is an inherent characteristics of the permanent magnet synchronous motor drive system to attain stability and can be termed as self-synchronization. The motor drive system loses stability and goes out of synchronization when the load angle ε exceeds 90° for a SPM synchronous motor or 70° to 85° (depending on the value of q-axis reactance X_q and the d-axis reactance X ) for an IPM synchronous motor. If the speed response and the torque response are kept suitably low considering the high inertia of the motor drive system, the phenomenon of self-synchronization is guaranteed and the load angle ε never exceeds a stable value in a permanent magnet synchronous motor during the forced drive mode. In a control method of a permanent magnet synchronous motor according to the invention, the motor is driven with a closed loop drive mode by sensing the back emf of the motor. When a driving status reaches a predetermined condition, the motor drive mode is changed from the closed loop drive mode to an open loop drive mode by a forced drive technique without sensing the back emf of the motor. This allows a forced drive system to be stable and the flux-weakening region of the permanent magnet synchronous motor to be extended. The control of the motor during the forced drive period can be carried out from the values of the estimated rotor position and the speed derived from the information of the zero-cross of the last back emf detection and the predetermined acceleration and deceleration of the motor. Since the back emf zero-cross is always perfectly detected before entering into the forced drive mode, an initial very stable load angle ε between the fundamental phase voltage and the phase back emf is always confirmed. In the first aspect of the invention, a wide angle drive strategy I is provided. In the strategy I, the motor is started with a first drive mode in closed loop with a first conduction angle. During acceleration, the motor drive mode is changed from the first drive mode to a second drive mode in closed loop with a second conduction angle wider than the first conduction angle, at the instant when the PWM duty δ for the first drive mode becomes greater than 95 %. During further acceleration, the motor drive mode is changed from the second drive mode to a forced drive mode in open loop at the instant when the PWM duty δ of the second drive mode becomes greater than 95 %. This allows a forced drive system to be stable and the flux-weakening region of the permanent magnet synchronous motor to be extended. During deceleration, the motor drive mode is changed from the forced drive mode to the second drive mode at the instant when the zero-cross of the back emf is detected again continuously. During further deceleration, the motor drive mode is changed from the second drive mode to the first drive mode at the instant when the PWM duty δ of the wide angle drive becomes less than 80 %. In the second aspect of the invention, a wide angle drive strategy II is provided. In the strategy II, the motor is started with a first drive mode with a first conduction angle between 120° and 150° in closed loop. During acceleration, the motor drive mode is changed from the first drive mode to a second drive mode in closed loop with a second conduction angle wider than the first conduction angle, at the instant when the PWM duty δ for the first drive mode becomes greater than 95 %. During further acceleration, the motor drive mode is changed from the second drive mode to a forced drive mode in open loop, at the instant when the PWM duty δ of the second drive mode becomes greater than 95 %. Thus a forced drive system is stable and the flux -weakening region of the permanent magnet synchronous motor can be extended. During deceleration, the motor drive mode is changed from the forced drive mode to the second drive mode, at the instant when the zero-cross of the back emf is detected again continuously. During further deceleration, the motor drive mode is changed from the second drive mode to the first drive mode at the instant when the PWM duty δ of the second drive mode becomes less than 80 %. In the third aspect of the invention, a wide angle drive strategy

III is provided. In the strategy III, the motor is started with a first drive mode with a first conduction angle in closed loop. During acceleration, the motor drive mode is changed from the first drive mode to a second drive mode having a second conduction angle wider than the first conduction angle with a speed between the first speed ω \ and the second speed 012, at the instant when the PWM duty δ lies at a value between 10 % and 95 %. During further acceleration, the motor drive mode is changed from the second drive mode to a forced drive mode in open loop, at the instant when the PWM duty δ of the second drive mode becomes greater than 95 %, to provide a forced drive system which is stable and thus extending the flux weakening region of the permanent magnet synchronous motor. During deceleration the motor drive mode is changed from the forced drive mode to the second drive mode, at the instant when the zero-cross of the back emf is detected again continuously. During further deceleration, once the speed reaches ω₂, the conduction angle starts decreasing and at speed ω i the conduction angle returns to the first conduction angle. In the fourth aspect of the invention, a wide angle drive strategy IV is provided. In the strategy IV, the motor is started with a first drive mode with a first conduction angle in closed loop. During acceleration, the motor drive mode is changed from the first drive mode to a second drive mode having a second conduction angle wider than the first conduction angle in closed loop with a speed between a first speed ω ₁ and a second speed ω ₂ when the PWM duty δ lies at a value between 10 % and 95 %. During further acceleration, the motor drive mode is changed from the second drive mode to a forced drive mode in open loop at speed 013 when the PWM duty δ always lies between 10% and 95%, thus providing a forced drive system which is stable and extending the flux weakening region of the permanent magnet synchronous motor. When the motor drive mode changes to the forced drive mode, the PWM turn-on duty δ is fixed. During deceleration, the motor drive mode is changed to the second drive mode after successfully detecting the back emf, when the motor speed reaches ω z. During further deceleration, the motor drive mode is changed from the second drive mode to the first drive mode in a speed between the first speed ω ₁ and the second speed ω₂. For the strategy I, II, III or IV, the forced drive mode can be carried out from values of the estimated rotor position and the speed derived from the information of the zero-cross value of the back electromotive force detected just before entering into the forced drive mode and the predetermined acceleration and deceleration of the motor. For the strategy I, II, III or IV, in the first drive mode, the closed loop control of the motor may be carried out by detecting the zero-cross of the back emf and giving a delay varying between 0 ° and 30°. In the second drive mode, the closed loop control of the motor may be carried out by detecting the zero-cross of the back emf and giving a delay equal to 0°, if modulation of the PWM base drive waveform does not take place. If modulation of the PWM base drive waveform takes place, the closed loop control of the motor may be carried out by detecting the zero-cross of the back emf and giving a delay varying between 0° and 15°. In the strategy I, III or IV, the first conduction angle may be equal to 120° and the second conduction angle may be equal to 150°. In the strategy I, II, III or IV, the second conduction angle may be laid between 120° and 180°. The proposed wide angle drive system with the forced drive technique is ideal for the compressor drive of refrigerators, electric driven car air-conditioners and domestic air-conditioners. Since the speed and torque response for the compressor drives are relatively slow, the proposed forced drive technique is feasible.

Brief description of drawings Fig. 1 shows a typical three phase inverter configuration for a permanent magnet synchronous motor. Fig. 2 shows the traditional 120° PWM base drive waveforms for the three phases of a permanent magnet synchronous motor and the induced back emf waveforms at the three phase terminals U, V and W with scheme I. Fig. 3 shows the traditional 120° PWM base drive waveforms for the three phases of a permanent synchronous motor and the induced back emf waveforms at the three phase terminals U, V and W with scheme II. Fig. 4 shows the hardware scheme for the detection of back emf zero-cross. Fig. 5 shows the software scheme for the detection of back emf zero-cross. Fig. 6 shows the block diagram for the closed loop control of permanent magnet synchronous motor with traditional 120° PWM drive. Fig. 7 shows the wide angle 150° PWM base drive waveforms for the three phases of a permanent magnet synchronous motor and the induced back emf waveforms at the three phase terminals U, V and W with scheme I. Fig. 8 shows the wide angle 150° PWM base drive waveforms for the three phases of a permanent magnet synchronous motor and the induced back emf waveforms at the three phase terminals U, V and W with scheme II. Fig. 9 shows the unique voltage modulation scheme for the wide angle 150° PWM drive. Fig. 10 shows the speed versus torque characteristics of a permanent magnet synchronous motor with the traditional 120° and the wide angle 150° PWM drive with position and current sensorless technique, in which

CO_M is base speed for 120° conduction drive when δ is 100% and iOb2 is base speed for 150° conduction drive when δ is 100%. Fig. 11 shows the block diagram for the closed loop control of a permanent magnet synchronous motor with the wide angle 150° PWM drive with modulation. Fig. 12A shows a control block diagram of the motor control apparatus of First Embodiment according to the present invention for the extension of the flux-weakening region of a permanent magnet synchronous motor with the 150° conduction drive mode without modulation. Fig. 12B shows the per phase vector diagram of a interior permanent magnet motor (IPM) during the 150° conduction drive mode with the current and position sensorless control technique. Fig. 13A shows the U-phase terminal voltage (X), U-phase current (Y) and U-phase magnetic back emf (Z) for an interior permanent magnet synchronous motor during forced drive mode at torque Ti and speed ω

₁ respectively. Fig. 13B shows the U-phase terminal voltage (X), U-phase current (Y) and U-phase magnetic back emf (Z) for an interior permanent magnet synchronous motor during forced drive mode at torque Ti and speed ω ₂ respectively. Fig. 13C shows the U-phase terminal voltage (X), U-phase current (Y) and U-phase magnetic back emf (Z) for an interior permanent magnet synchronous motor during forced drive mode at torque T₂ and speed ω

₂ respectively. Fig. 14 shows the per phase vector diagram of the interior permanent magnet (IPM) synchronous motor during the forced mode. Fig. 15 shows the speed versus torque characteristics of a permanent magnet synchronous motor with the traditional 120°, the wide angle 150° PWM drive and the forced drive with position and current sensorless technique, in which tObi is base speed for 120° conduction drive when δ is 100% and 0Jb2 is base speed for 150° conduction drive when δ is 100%. Fig. 16A shows a control block diagram of the motor control apparatus of Second Embodiment according to the present invention. Fig. 16B shows a control block diagram of the motor control apparatus of Third Embodiment according to the present invention. Fig. 16C shows a control block diagram of the motor control apparatus of Fourth Embodiment according to the present invention. Fig. 17 shows a configuration of a compressor drive according to the present invention.

Best mode for carrying out the Invention The present invention is explained fundamentally by defining the "Self-Synchronization" principle, which is an inherent characteristic of the permanent magnet synchronous motor drive system to attain stability. The invention relates to a unique method discussed for extending the flux-weakening region of a SPM and an IPM synchronous motor for a wide angle drive with conduction angle equal to 150° without sensing the zero-cross of the induced back emf. The basic hardware configuration and the software control algorithm for the closed loop 150° conduction drive are identical with the 120° conduction drive and are already explained in the background art.

The only major difference between the 150° conduction drive and the 120° conduction drive is the shortening of the induced back emf sensing period of the motor from 60° to 30° and this is taken care by either a modified hardware or a software scheme. The instant at which the motor drive is changed from closed loop 120° conduction drive mode to the 150° conduction drive mode and vice-versa depend on the PWM duty ( δ ) and the speed ( ω ) of the motor. To extend the flux-weakening region of the motor in the 150° conduction drive mode, the operation of the motor is changed from the closed loop control mode to the forced drive open loop mode where the sensing of the zero-cross of the induced back emf is not required. During acceleration, the instant at which the motor drive is changed from the closed loop 150° conduction drive mode to an open loop forced drive mode depends on the PWM duty ( δ ) and the speed ( ω ) of the motor. Before entering the forced drive mode, the rotor position and the estimated speed of the motor from the last detected zero-cross of the back emf are noted. In the forced drive mode, from the predetermined value of acceleration and deceleration, the speed (ω ) of the motor, and thereby the rotor position ( θ ) of the motor are estimated. During, deceleration, the instant at which the motor drive is again changed from the open loop forced drive mode to the closed loop 150° conduction drive mode depends on the continuous detection of the zero-cross of the back emf once again. Hence, for the extension of the flux-weakening region of the permanent magnet synchronous motor for the 150° conduction drive mode, the software control algorithm has to be only modified keeping the hardware strategy almost same.

First Embodiment The salient features of the wide angle drive strategy I with the 150° conduction angle and the forced drive technique for the extension of the flux-weakening region are explained in sequence below. The wide-angle drive system is implemented with a micro-controller. The modified closed loop block diagram for the wide angle drive strategy I without PWM base drive waveform modulation is shown in Fig. 12A. From Fig. 12A, it can be understood that the basic control strategy of the wide angle 150° conduction drive is similar to a 120° conduction drive. (i) A surface permanent magnet (SPM) or interior permanent magnet (IPM) synchronous motor 10 is started as an open loop 120° conduction drive with an initial PWM on duty ( δ i) and an initial starting frequency ( ) of the motor 10. Back emf is detected by a sensor 21 and the zero-cross of the back-emf is detected by a module 22. After detecting the zero-cross of the back-emf six consecutive times, the motor control is changed from open loop to closed loop with 120° conduction angle. The 120° base drive waveforms can be similar to either those shown in Fig. 2 or Fig. 3. The control technique of the closed loop 120° conduction drive includes detecting the zero-cross of the back emf and giving a delay (D) before the turning on of another appropriate phase. The delay angle (D) is x° where 'x' varies between 0° and 30°. The incremental rotor position Δ θ corresponding to every cycle of the PWM carrier frequency is estimated by dividing 60° with the total number of PWM interrupts between two consecutive zero-crossings. The estimation of this Δ θ is carried out continuously from the initial starting of the motor (module 23). Now, the speed ( ω ) of the motor is estimated as, ω = Δ r3 *f_s ...(1) where, f_s is the carrier frequency (module 24). The estimated speed ( ω ) is passed through a low pass filter (LPF) 25 and then compared with the reference speed ( ω *). The error between ω and ω^* is processed in a proportional and integral (PI) controller 26 to generate the PWM turn-on duty( δ ). (ii) During acceleration, once the PWM duty δ for the 120° conduction drive reaches a value greater than 95%, the 120° PWM base drive waveforms for the three phases are extended in the left direction by x° and in the right direction by (30-x)° to make it a wide angle 150° conduction drive (module 30). Hence, the delay angle (D) after sensing the zero-cross of the induced back emf becomes 0° for the wide angle 150° conduction drive. The conduction angle is increased by Δ θ in every 1 ms interrupt until the conduction angle is increased to 150° from 120°. Δ θ corresponds to every cycle of the PWM carrier frequency. The basic control technique of the wide angle 150° conduction drive after sensing the. zero-cross of the back emf is similar to the 120° conduction drive. The per phase vector diagram of the interior permanent magnet (IPM) motor during the wide angle 150° conduction drive mode is shown in Fig. 12B assuming sinusoidal flux density distribution. The parameters shown in Fig. 12B are as follows: Lq: q-axis inductance of the motor; Ld: d-axis inductance of the motor; I: phase current; iq: q-axis current of the motor; id: d-axis current of the motor; Φm: magnetic flux; Ef: magnetic induced voltage or back emf; Φr: reluctance flux, (=Lq iq+ Ld id); ωφr: induced voltage or back emf due to reluctance flux, (=ω Lq iq+ ω Ld id=xq iq+ xd id); Φ0: total flux (=Φm+Φr); ωΦO: total induced voltage or back emf (=Ef+ωΦr); R: per phase resistance of the winding; Vt: terminal voltage; ε: load angle. (iii) During further acceleration, once the PWM duty δ of the wide angle 150° drive becomes greater than 95 %, the control of the permanent magnet synchronous motor 10 is changed to the forced drive mode to further extend the flux-weakening region (module 28). In the forced drive mode, the control operation of the motor is changed from closed loop to open loop. The back emf is not detected during the forced drive mode, and hence the current zero-cross period as in the 120° and 150° conduction drives is not required.

The control of the motor during the forced drive period is carried out from the estimated value of the rotor position and the speed derived from the information of the zero-cross detection of the last back emf and the predetermined acceleration of the motor. Let ωn be the estimated speed of the motor from the last detected back emf. Therefore, the initial speed of the forced drive mode at the instant t| when the motor changes from the 150° conduction drive to the forced drive mode is ω_fj. Now, at instant .₂ in the forced drive mode, the speed of the motor is given by, to_ff = ω_fi + a*(t₂ - tι) ...(2) where, 'a' is the acceleration of the motor. The incremental rotor position Δ θ corresponding to every cycle of the PWM carrier frequency is now given by, Δ θ = ω_ff / fs ...(3) This knowledge of Δ θ helps to calculate the rotor position and control the motor during the forced drive mode. In the forced drive mode, the reverse diodes in the inverter 11 starts conducting giving a 180° phase current conduction which is the first basic reason for an extended flux-weakening region. Fig. 13A shows typical U-phase current, terminal voltage and magnetic back emf waveform at torque Ti and speed ω i, whereas Fig. 13B shows typical U-phase current, terminal voltage and magnetic back emf at torque Ti and speed ω 2 of an interior permanent magnet synchronous motor during the forced drive period, where speed 01₂ is greater than ω \. In these figures, the line "X" denotes U phase terminal voltage (Vu), the line "Y" denotes U phase current, and the line z denotes U phase magnetic back emf. During torque Ti and speed ω ₁ of the forced drive mode, it is seen from Fig.13A that the zero-cross period (Co) of U-phase current (line Y) is still visible though the zero-cross of the back emf is not obtained. During torque Ti and speed ω of the forced drive mode_ι it is seen from Fig.13B that there is no U-phase current zero-cross period and there is 180° phase current conduction. From Fig. 13B it is seen that for the U-phase current there are six conduction periods Ci to Cβ possible in one cycle. These periods are as follows: Ci: During this period, diode D1 and transistor T5 conduct; C₂: During this period, diode D₂ and transistor T₄ conduct; C₃: During this period, transistor Ti and other devices conduct for 150°; C₄: During this period, diode D₂ and transistor T₆ conduct; C5: During this period, diode D1 and transistor T3 conduct; Cβ. During this period, transistor T₂ and other devices conduct for 150°. Fig. 13C shows typical U-phase current, terminal voltage and magnetic back emf waveform at torque T2 and speed ω ₂ of an interior permanent magnet synchronous motor during the forced drive mode where T₂ is greater than Ti. It is seen from Fig. 13C that the Ci period is increased whereas the C₂ period is decreased and on the other hand, the C₄ period is increased and the C₅ period is decreased compared to Fig. 13B. Thus, there is a shift of the applied fundamental voltage of the U-phase motor windings with respect to the U phase magnetic back emf during the forced drive mode as the speed or the torque of the motor is increased because of the natural conduction of the reverse diodes of the inverter at appropriate instants. The advancement of the phase voltage with respect to the phase magnetic back emf results in an increased value of load angle ε which further can extends the flux-weakening region. The per phase vector diagram of the interior permanent magnet (IPM) motor during the forced drive mode is shown in Fig. 14 where it is seen that load angle ε is increased compared to Fig. 12B for the wide angle 150° conduction drive mode. This automatic adjustment of the load angle ε is an inherent characteristics of the permanent magnet synchronous machine to attain stability and can be termed here as "self-synchronization". The speed-torque (S-T) characteristics of the forced drive is shown in Fig. 15. The torque equation of a SPM and an IPM synchronous motor is given by equations (4) and (5) respectively,

where, Vt is the phase voltage, E_f is the magnetic back emf, ω is the motor speed, ε is the load angle, x_s is the synchronous reactance of the SPM and X_d and x_q are the d-axis and q-axis reactance of the IPM respectively. From the above equations it can be understood that the pull out torque for the SPM motor occurs when the load angle ε is equal to 90° whereas the pull out torque for the IPM motor occurs when the load angle ε lies between 70° and 85° (depending on the values of q-axis reactance X_q and d-axis reactance Xd). Thus, if the load angle ε exceeds 90° for a SPM motor or 70°-85° for an IPM motor, the motor drive system loses stability and goes out of synchronization. If the speed and the torque response of the forced drive system are kept suitably low and considering the high inertia of the motor drive system the phenomenon of self-synchronisation is guaranteed and the load angle ε never exceeds a stable value in a permanent magnet synchronous motor during the forced drive mode. (iv) During deceleration, the motor changes from the open loop forced drive mode to closed loop wide angle 150° conduction drive mode, once the zero-cross of the back emf is detected again continuously. The zero-cross of the back emf is successfully detected twelve consecutive times before changing the drive system from the open loop forced drive mode to the closed loop wide angle 150° conduction drive mode. Similarly during further deceleration, once the PWM duty δ of the 150° conduction drive becomes less than 80 %, the conduction angle starts decreasing and the motor is changed from the angle 150° conduction drive mode to 120° conduction drive mode (module 29).

Second Embodiment The salient features of the wide angle drive strategy II with the 150° conduction angle and the forced drive technique for the extension of the flux-weakening region are explained in sequence below. The modified closed loop block diagram for wide angle drive strategy II is shown in Fig. 16A. (i) The surface permanent magnet (SPM) or the interior permanent magnet (IPM) synchronous motor 10 is started as an open loop wide-angle drive with conduction angle (C) which varies between 120° and 150° instead of a traditional 120° conduction drive. After detecting the zero-cross of the back-emf six consecutive times, the motor control is changed from the open loop to the closed loop wide-angle drive. The control technique for the wide-angle drive includes detecting the zero-cross of the induced back emf and giving a delay angle (D) before the starting of a new phase. The value of the delay angle (D) is x° where 0°< x°< (150-C)⁰. The closed loop control technique for estimating the incremental rotor position Δ θ , the speed ω and the PWM turn-on duty δ in the wide angle conduction mode is same as the first embodiment (by modules 23 to 26). (ii) During acceleration, once the PWM duty δ of the wide angle conduction angle drive reaches more than 95 %, the wide angle base drive waveforms for the three phases are extended in the left direction by x° and in the right direction by (150-C-x)⁰ to make it 150°. The rate of increase of conduction angle is same as the first embodiment. In this case too, the delay angle (D) after sensing the zero-cross of the induced back emf becomes 0° for the wide angle 150° conduction drive. The basic control technique of the wide angle 150° conduction drive after sensing the zero-cross of the back emf is similar to the wide angle drive with conduction angle (C). (iii) During acceleration, once the PWM duty δ of the 150° conduction drive reaches greater than 95 %, the motor drive is changed to the open loop forced drive mode. Once the motor control is changed to the forced drive mode, the technique becomes similar to the first embodiment. (iv) During deceleration, the techniques for the change from the forced drive mode to the 150° conduction drive mode and then from the 150° conduction drive to the wide angle drive with conduction angle (C) are same as the first embodiment.

Third Embodiment The salient features of the wide angle drive strategy III with the 150° conduction drive and the forced drive technique for the extension of the flux-weakening region are explained in sequence below. The modified closed loop block diagram for wide angle drive strategy III is shown in Fig. 16B. (i) The motor 10 is started as a 120° drive and rotated in closed loop with 120° conduction drive mode until the speed becomes a predetermined speed ω ₁ in a similar way described in the first embodiment. (ii) During acceleration, the motor control changes from 120° conduction drive to the 150° conduction drive between speed ω ^ and ω ₂ when the PWM duty δ lies between any predetermined value between 10 % and 95 %. Between speed ω <\ and ω₂, the 120° PWM base drive waveforms for the three phases are extended in the left direction by 30° to make it wide angle 150° as shown in Fig. 8(module 33). The striking feature of this technique is that in the 150° conduction drive mode, modulation of the PWM base drive waveform takes place. The modulation of the PWM base drive waveform can take place in the similar way as described in Fig. 9 where the turn-on periods ( δ ) in the first 30° is increased by k% whereas the turn-on periods in the last 30° is decreased by k%. The value of k is increased from 0 % to k-i % as the motor control changes from the 120° conduction drive to the 150° conduction drive between speed ω i and ω₂. In the 150° conduction drive mode, the value of k is decreased linearly with the speed ω of the motor 10 from ki % at speed ω₂ to 0 % at speed ω₃ when the PWM turn-on duty δ reaches 95%. The control technique for the 150° conduction drive includes detecting the zero-cross of the induced back emf and giving a delay (D) which is equal to x°, where 0°<x°< 15°. The closed loop control technique for estimating the incremental rotor position Δ θ , the speed ω and the PWM turn-on duty δ in the 150° conduction drive mode with base drive waveform modulation is same as the first embodiment. (iii) During further acceleration, once the PWM duty δ of the 150° drive reaches greater than 95 %, the motor control is changed to open loop forced drive mode (module 28). Once the motor control changes to the forced drive mode, the technique becomes similar to the first embodiment. (iv) During deceleration, the technique for the change from the forced drive to the 150° conduction drive is identical to the first embodiment. In the 150° conduction drive mode, during deceleration once the speed reaches o)₂, the conduction angle starts decreasing and at speed ω i the conduction angle again returns to 120°.

Fourth Embodiment The salient features of the wide angle drive strategy IV with the

150° conduction drive and the forced drive technique for the extension of the flux-weakening region are explained in sequence below. The modified closed loop block diagram for the wide angle drive strategy IV is shown in Fig. 16C. (i) The motor 10 is started as a 120° drive and rotated in closed loop in 120° conduction drive mode until speed ω ₁ in a similar way described in the first embodiment. (ii) During acceleration, the motor control changes from the 120° conduction drive to the 150° conduction drive between speed ω ₁ and ω₂ when the PWM duty δ lies between a predetermined value between 10 % and 95 %. In the wide angle 150° drive mode, modulation of the PWM base drive waveforms may either take place or may not take place, and therefore the technique during the 150° mode would be either similar to the first or third embodiment. (iii) During further acceleration, the motor control changes to the forced drive mode from the 150° conduction drive at speed ω ₃ when the PWM duty δ always lies between 10% and 95% (module 35). Once the motor control changes to the forced drive mode, the PWM turn-on duty δ is kept fixed and the control technique during the forced drive mode becomes similar to the first exemplary embodiment. (iv) During deceleration, once the motor speed reaches ω , the motor control is again changed to the 150° conduction drive mode after successfully detecting the back emf twelve consecutive times. On further deceleration, in between speed 01₂ and ω 1 the motor control changes from the

150° conduction drive mode to the 120° conduction drive mode.

Industrial Applicability The proposed wide angle 150° drive with the forced drive technique is ideal for a compressor drive of refrigerators, electric driven car air-conditioners and domestic air-conditioners. Since the speed and torque response for the compressor drives are relatively slow, the proposed forced drive technique is feasible. Fig. 17 shows an explanatory diagram of an application of the invention to a compressor drive of refrigerators, electric driven car air-conditioners and domestic air-conditioners. The compressor drive 100 is a unit to drive a SPM (or IPM) synchronous motor 10 coupled to a compressor 200, and has a configuration including the inverter 11 and the back emf sensor 21 and so on as shown in Figs. 12A and 16A to 16C.

Claims

1. A control method of a permanent magnet synchronous motor comprising: driving the motor with a closed loop drive mode by sensing the back emf of the motor; and changing the motor drive mode from the closed loop drive mode to an open loop drive mode by a forced drive technique without sensing the back emf of the motor, when a driving status reaches a predetermined condition, thus to provide a forced drive system which is stable and extension of the flux weakening region of the permanent magnet synchronous motor.

2. The method of claim 1 , wherein in the forced drive technique is carried out from values of the estimated rotor position and the speed derived from the information of the zero-cross value of the back electromotive force detected just before entering into the forced drive mode and the predetermined acceleration and deceleration of the motor.

3. A control method of a permanent magnet synchronous motor comprising: starting the motor with a first drive mode in closed loop with a first conduction angle; changing the motor drive mode from the first drive mode to a second drive mode in closed loop with a second conduction angle wider than the first conduction angle, at the instant when the PWM duty δ for the first drive mode becomes greater than 95 %, during acceleration; changing the motor drive mode from the second drive mode to a forced drive mode in open loop, during further acceleration, at the instant when the PWM duty δ of the second drive mode becomes greater than 95 %, thus to provide a forced drive system which is stable and extension of the flux weakening region of the permanent magnet synchronous motor; changing the motor drive mode from the forced drive mode to the second drive mode, at the instant when the zero-cross of the back emf is detected again continuously, during deceleration; and changing the motor drive mode from the second drive mode to the first drive mode at the instant when the PWM duty δ of the wide angle drive becomes less than 80 % during further deceleration.

4. A control method of a permanent magnet synchronous motor comprising: starting the motor with a first drive mode with a first conduction angle between 120 ° and 150 ° in closed loop; changing the motor drive mode from the first drive mode to a second drive mode in closed loop with a second conduction angle wider than the first conduction angle, at the instant when the PWM duty δ for the first drive mode becomes greater than 95 % during acceleration; changing the motor drive mode from the second drive mode to a forced drive mode in open loop, during further acceleration, at the instant when the PWM duty δ of the second drive mode becomes greater than 95 %, thus to provide a forced drive system which is stable and extension of the flux weakening region of the permanent magnet synchronous motor; changing the motor drive mode from the forced drive mode to the second drive mode, at the instant when the zero-cross of the back emf is detected again continuously during deceleration; and changing the motor drive mode from the second drive mode to the first drive mode at the instant when the PWM duty δ of the second drive mode becomes less than 80 % during further deceleration.

5. A control method of a permanent magnet synchronous motor comprising: starting the motor with a first drive mode with a first conduction angle in closed loop; changing the motor drive mode from the first drive mode to a second drive mode having a second conduction angle wider than the first conduction angle with a speed between the first speed ω 1 and the second speed ω2, at the instant when the PWM duty δ lies at a value between 10 % and 95 %, during acceleration; changing the motor drive mode from the second drive mode to a forced drive mode in open loop, during further acceleration, at the instant when the PWM duty δ of the second drive mode becomes greater than 95 %, thus to provide a forced drive system which is stable and extension of the flux weakening region of the permanent magnet synchronous motor; changing the motor drive mode from the forced drive mode to the second drive mode, at the instant when the zero-cross of the back emf is detected again continuously, during deceleration, wherein, during further deceleration, once the speed reaches ω

2, the conduction angle starts decreasing and at speed ω 1 the conduction angle returns to the first conduction angle.

6. A control method of a permanent magnet synchronous motor comprising: starting the motor with a first drive mode with a first conduction angle in closed loop; changing the motor drive mode from the first drive mode to a second drive mode having a second conduction angle wider than the first conduction angle in closed loop with a speed between a first speed ω 1 and a second speed ω2 when the PWM duty δ lies at a value between 10 % and

95 %, during acceleration; changing the motor drive mode from the second drive mode to a forced drive mode in open loop at speed ω3 when the PWM duty δ always lies between 10% and 95%, during further acceleration, thus to provide a forced drive system which is stable and extension of the flux weakening region of the permanent magnet synchronous motor; keeping the PWM turn-on duty δ fixed when the motor drive mode changes to the forced drive mode; changing the motor drive mode to the second drive mode after successfully detecting the back emf, when the motor speed reaches ω 3 during deceleration; and changing the motor drive mode from the second drive mode to the first drive mode in a speed between the first speed ω 1 and the second speed ω 2, during further deceleration.

7. The method according to any one of claims 3 to 6, wherein the forced drive mode is carried out from values of the estimated rotor position and the speed derived from the information of the zero-cross value of the back electromotive force detected just before entering into the forced drive mode and the predetermined acceleration and deceleration of the motor.

8. The method according to any one of claims 3 to 6, wherein in the first drive mode, the closed loop control of the motor is carried out by detecting the zero-cross of the back emf and giving a delay varying between 0 ° and 30 °, and in the second drive mode, the closed loop control of the motor is carried out by detecting the zero-cross of the back emf and giving a delay equal to 0 ° if modulation of the PWM base drive waveform does not take place, or if modulation of the PWM base drive waveform takes place, the closed loop control of the motor is carried out by detecting the zero-cross of the back emf and giving a delay varying between 0 ° and 15 °.

9. The method according to any one of claims 3, 5 and 6, wherein the first conduction angle is equal to 120 ° and the second conduction angle is equal to 150 °.

10. The method according to any one of claims 3 to 6, wherein the second conduction angle is laid between 120° and 180°.

11. A motor control apparatus comprising: an inverter that provides electric power to drive a permanent magnet synchronous motor; and a controller that controls the inverter by using the control method according to any one of claims 1 to 10.

12. A compressor comprising a permanent magnet synchronous motor, and the motor control apparatus according to claim 11.