Kawarwa na Ƙwarewa na Tsakiya
Mawakin AC Induction na da cikakken yadda ake amfani da su wajen kula da hanyar tushen masu shirya, gaskiya da kuma zama da ake iya yanayi. Ana amfani da su a wasu muhimman abubuwa daga tushen shirya a kasashen aiki zuwa tushen kayan ado. Amma, amfani da mawakin induction a matsayin da ya fi kyau shi shi ne abin da ba a zama saboda modello mai girma da kyau da alamar sautin kananan. Wannan nuna cewa za a yi aiki a kan mawaki a kan tsarin kontrol mai kyau kamar vector control.
Tsarin Field Oriented Control
Tsarin kontrol scalar kamar “V/Hz” tana da muhimmanci saboda kyautar yadda ake amfani da su. Tsarin kontrol scalar na mawaki induction na gina jirgin kuli da ake amfani da su. Saboda haka, don samun inganci na yawan aiki, zai bukata tsarin kontrol mai kyau waɗanda zai iya haɗa kuli da tushen torque da flux. Da ake amfani da micro-controllers, digital signal processors da FGPA, ana iya yi aiki da adadin tsari mai kyau don haɗa kuli da tushen torque da flux a mawaki AC induction. Wannan haɗa kuli da torque da flux na ake kira rotor Flux Oriented Control (FOC).
Field Oriented Control na nufin yadda ake amfani da kontrol ta torque da speed a kan halin electromagnet a mawaki, kamar yadda ake amfani da mawaki DC. FOC shi ne babban tattalin kontrol “real” variables of torque da flux. Daga bane da haɗa kuli da tushen stator current (magnetizing flux da torque), an iya kontrol tushen torque na ake amfani da su a kan stator flux daɗi. Don haɗa kuli da kontrol, a wajen low speeds, zai iya ci gaba da state of magnetization a mawaki a kan level da ke da shi, da kuma torque zai iya kontrol don regulate speed.
“FOC an yi amfani da ita kawai wajen high-performance motor applications wadanda zai iya yi aiki da kyau a kan wajen wide speed range, zai iya produce full torque a zero speed, da kuma zai iya yi acceleration da deceleration da kyau.”
Sabbin Tsarin Field Oriented Control
A field oriented control na nufin kontrol stator currents da ake sanya da vector. Tsarin kontrol na na amsa a kan projections wadanda suke haɗa kuli da three phase time and speed dependent system zuwa two coordinate (d and q frame) time invariant system. Wannan haɗa kuli da projections na nuna structure kamar yadda ake amfani da DC machine control. FOC machines sun buƙata biyu na constant as input references: tushen torque component (aligned with the q coordinate) da flux component (aligned with d coordinate).
The three-phase voltages, currents and fluxes of AC-motors can be analyzed in terms of complex space vectors. If we take ia, ib, ic as instantaneous currents in the stator phases, then the stator current vector is defined as follow:
Where, (a, b, c) are the axes of three phase system.
This current space vector represents the three phase sinusoidal system. It needs to be transformed into a two time invariant coordinate system. This transformation can be divided into two steps:
(a, b, c) → (α, β) (the Clarke transformation), which gives outputs of two coordinate time variant system.
(a, β) → (d, q) (the Park transformation), which gives outputs of two coordinate time invariant system.
The (a, b, c) → (α, β) Projection (Clarke transformation)
Three-phase quantities either voltages or currents, varying in time along the axes a, b, and c can be mathematically transformed into two-phase voltages or currents, varying in time along the axes α and β by the following transformation matrix:
Assuming that the axis a and the axis α are along same direction and β is orthogonal to them, we have the following vector diagram:
The above projection modifies the three phase system into the (α, β) two dimension orthogonal system as stated below:
But these two phase (α, β) currents still depends upon time and speed.
The (α, β) → (d.q) projection (Park transformation)
This is the most important transformation in the FOC. In fact, this projection modifies the two phase fixed orthogonal system (α, β) into d, q rotating reference system. The transformation matrix is given below:
Where, θ is the angle between the rotating and fixed coordinate system.
If you consider the d axis aligned with the rotor flux, Figure 2 shows the relationship from the two reference frames for the current vector:
Where, θ is the rotor flux position. The torque and flux components of the current vector are determined by the following equations:
These components depend on the current vector (α, β) components and on the rotor flux position. If you know the accurate rotor flux position then, by above equation, the d, q component can be easily calculated. At this instant, the torque can be controlled directly because flux component (isd) and torque component (isq) are independent now.
Basic Module for Field Oriented Control
Stator phase currents are measured. These measured currents are fed into the Clarke transformation block. The outputs of this projection are entitled isα and isβ. These two components of the current enter into the Park transformation block that provide the current in the d, q reference frame. The isd and isq components are contrasted to the references: isdref (the flux reference) and isqref (the torque reference). At this instant, the control structure has an advantage: it can be used to control either synchronous or induction machines by simply changing the flux reference and tracking rotor flux position. In case of PMSM the rotor flux is fixed determined by the magnets so there is no need to create one. Therefore, while controlling a PMSM, isdref should be equal to zero. As induction motors need a rotor flux creation in order to operate, the flux reference must not be equal to zero. This easily eliminates one of the major shortcomings of the “classic” control structures: the portability from asynchronous to synchronous drives. The outputs of the PI controllers are Vsdref and Vsqref. They are applied to the inverse Park transformation block. The outputs of this projection are Vsαref and Vsβref are fed to the space vector pulse width modulation (SVPWM) algorithm block. The outputs of this block provide signals that drive the inverter. Here both Park and inverse Park transformations need the rotor flux position. Hence rotor flux position is essence of FOC.
The evaluation of the rotor flux position is different if we consider the synchronous or induction motor.
In case of synchronous motor(s), the rotor speed is equal to the rotor flux speed. Then rotor flux position is directly determined by position sensor or by integration of rotor speed.
In case of asynchronous motor(s), the rotor speed is not equal to the rotor flux speed because of slip; therefore a particular method is used to evaluate rotor flux position (θ). This method utilizes current model, which needs two equations of the induction motor model in d,q rotating reference frame.
