Pamahayag nga Nakatukod sa Field

Ang mga AC Induction motors nag-aalamin sa mga kaugnay na katangian tulad ng matibay, maasahan, at madali kangontrol. Ginagamit sila sa iba't ibang aplikasyon mula sa industriyal na sistema ng kontrol ng paggalaw hanggang sa mga aparato sa bahay. Gayunpaman, ang paggamit ng induction motor sa pinakamataas na epekibilidad ay isang mahirap na gawain dahil sa kanilang komplikadong matematikal na modelo at hindi linear na katangian sa panahon ng saturation. Ang mga katangian na ito ay nagpapahirap sa kontrol ng induction motor at nangangailangan ng paggamit ng mataas na performance na algoritmo ng kontrol tulad ng vector control.

Pagpapakilala sa Field Oriented Control

Ang scalar control tulad ng "V/Hz" strategy may limitasyon sa kanyang performance. Ang paraan ng scalar control para sa induction motors naglilikha ng mga oscillation sa naprodukta na torque. Kaya upang makamit ang mas magandang dynamic performance, kinakailangan ang mas superior na control scheme para sa Induction Motor. Sa pamamagitan ng mathematical processing capabilities na inaalamin ng micro-controllers, digital signal processors, at FGPA, maipapatupad ang advanced control strategies upang decouple ang torque generation at magnetization functions sa AC induction motor. Ang decoupled torque at magnetization flux karaniwang tinatawag na rotor Flux Oriented Control (FOC).

Field Oriented Control naglalarawan ng paraan kung paano ang kontrol ng torque at bilis ay direktang batay sa electromagnetic state ng motor, katulad ng isang DC motor. Ang FOC ang unang teknolohiya na kontrolin ang "tunay" na motor control variables ng torque at flux. Sa decoupling sa pagitan ng stator current components (magnetizing flux at torque), ang torque producing component ng stator flux maaaring kontrolin nang independente. Sa decoupled control, sa mababang bilis, ang estado ng pag-magnetize ng motor maaaring i-maintain sa angkop na antas, at ang torque maaaring kontrolin upang regulahin ang bilis.
"Ang FOC ay natutuklasan para sa high-performance motor applications na maaaring gumana nang malinis sa malawak na range ng bilis, maaaring lumikha ng buong torque sa zero speed, at kayang mabilis na i-accelerate at i-decelerate."

Prinsipyong Paggamit ng Field Oriented Control

Ang field oriented control binubuo ng kontrol sa stator currents na inirepresenta ng isang vector. Ang kontrol na ito ay batay sa mga projection na nagtransform ang three phase time at speed dependent system sa two coordinate (d at q frame) time invariant system. Ang mga transformation at projection na ito nagbibigay ng estruktura na katulad ng kontrol ng DC machine. Ang mga FOC machines nangangailangan ng dalawang constants bilang input references: ang torque component (aligned sa q coordinate) at ang flux component (aligned sa d coordinate).
Ang three-phase voltages, currents, at fluxes ng AC-motors maaaring analisin sa termino ng complex space vectors. Kung tayo ay kukunin ang ia, ib, ic bilang instantaneous currents sa stator phases, ang stator current vector ay inidefine gaya ng sumusunod:

Kung saan, (a, b, c) ang mga axes ng three phase system.

Ang 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.

1. 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.

2. 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.