Vector Control (Field Oriented Control, Direct Torque Control) Referents: Prof. Dr. Ing. Ralph Kennel (ralph.kennel@tum.de) Technische Universität München Arcisstraße 21 80333 München Germany 1
The General Idea of Field Oriented Control (FOC) as well as Direct Torque Control (DTC) is to operate AC machines/motors directly by the physical law of Lorentz force (that means, the torque is produced by an electrical current in a magnetic field) magnetic field and armature current (for torque production) are controlled separately and independently 2
Force Production in Synchronous Machines in principle the magnetic conditions in a synchronous machines are equal to DC machines I f B B 3
Force Production in Synchronous Machines even in high speed condition the mechanical motion is so slow that Maxwell s equations can be applied in the same way (there is no energy radiation there is no Displacement Current) I f B B 4
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The General Idea voltage and frequency are irrelevant for this type of control under this assumption any AC machine/motor behaves like a DC motor 11
Torque and Power Characteristics of Electrical Machines constant torque constant power 12
Torque and Power Characteristics of Electrical Machines base constant speed torque range constant field weakening power range 13
Vector Control (Field Oriented Control, Direct Torque Control) characteristic Referents: Prof. Dr. Ing. Hans Georg Herzog (hg.herzog@tum.de) Prof. Dr. Ing. Ralph Kennel (ralph.kennel@tum.de) Technische Universität München Arcisstraße 21 80333 München Germany 14
(Linear) Control Loop control error e(t) controller controlling element controlled system advantages of (linear) control loops well accepted compromise between stability and dynamics simple optimization procedures (step response ) acceptable robustness against parameter variations (limited) implicit linearization auf non-linear components 15
Step Responses overshoot compromise lower dynamics 16
Cascade Structure s* Lageregelung position controller speed controller Drehzahlregelung n* i* - - - s n torque/current Drehmoment-/Strom controller Regelung i servo motor M 3~ Kommutierungssignale commutation signals tacho generator Tacho position encoder Lagegeber cascaded control with 3 control loops 17
Field Oriented Control (FOC) controls the magnetic field by a current (Ampère s law) in d-direction and and the torque by a quadrature (armature) current in q-direction the position of the magnetic field is needed to perform FOC this can be obtained by a position sensor or so-called sensorless control 18
Field Oriented Control (FOC) speed M-Regler controller current Stromcontrollers Regler n* i* q - * i* d - e j M 3~ field weakening Feldschwächung - - Feld-Regler field controller machine Maschinenmodell for asynchronous machines e -j e -j i u Encoder for synchronous machines 19
Field Oriented Control Rotor flux orientation PI-controllers Cascaded structure Limitation in the dynamic response Average voltage control * w w * r PI * i sq * i sd i sq PI id * v sd * v sq dq * vs * vs abc * v * v b * v c S 1 S 2 S 3 r PI w i sd PI iq s 20 20
Direct Torque Control (DTC) controls the magnetic field by the stator voltage (Faraday s law) and the torque by the stator current (Lorentz force) DTC does not need a position/speed sensor unless position or speed have to be controlled 21
Direct Torque Control Stator flux and torque tontrol Hysteresis controllers No current regulation No modulator Switching table * w w * T * T 22 22
Results DTC: Stator flux distortion magnetic flux is integral of stator voltage magnetic flux moves in direction of stator voltage v 3 s k 1 v 3 s k 1 s k s k 23 23
Results DTC: Torque and flux hysteresis bands Stator flux path control Current waveform Number of commutations Direct Self Control (DSC) increasing 24 24
Final Compariosn: Steady state Tn=20[Nm],w=1000[RPM] 30 Torque [Nm] 20 10 FOC 0 0.9 0.92 0.94 0.96 0.98 1 30 Torque [Nm] 20 10 DTC 0 0.9 0.92 0.94 0.96 0.98 1 30 Torque [Nm] 20 10 PTCk1 0 0.9 0.92 0.94 0.96 0.98 1 Torque [Nm] 30 20 10 PTCk2 0 0.9 0.92 0.94 0.96 0.98 1 Time [s] 25 25
Final Compariosn: Steady state Tn=20[Nm],w=1000[RPM] THD Amplitude [A] 30 15 0-15 FOC -30 0.9 0.92 0.94 0.96 0.98 1 2% Amplitude [A] 30 15 0-15 -30 0.9 0.92 0.94 0.96 0.98 1 9.5% DTC Amplitude [A] Amplitude [A] 30 15 0-15 -30 0.9 0.92 0.94 0.96 0.98 1 30 15 0-15 PTCk1 6% PTCk2 3.5% -30 0.9 0.92 0.94 0.96 0.98 1 Time [s] 26 26
Final Compariosn: Steady state Tn=20[Nm],w=1000[RPM] Voltage [V] Voltage [V] Voltage [V] Voltage [V] 400 200 0-200 -400 0.9 0.92 0.94 0.96 0.98 1 400 200 0-200 -400 0.9 0.92 0.94 0.96 0.98 1 400 200 0-200 -400 0.9 0.92 0.94 0.96 0.98 1 400 200 0-200 -400 0.9 0.92 0.94 0.96 0.98 1 Time [s] FOC DTC PTCk1 PTCk2 27 27
Final comparison: Torque response Torque reference step of 15 [Nm] 20 15 Torque [Nm] 10 5 0 FOC DTC -5 0.69 0.7 0.71 0.72 0.73 Time [s] 28 28
Final comparison: Stator flux response Flux reference step of 0.19 [Wb] 0.85 0.8 Flux [Wb] 0.75 0.7 FOC DTC 0.65 0.4 0.45 0.5 0.55 0.6 0.65 0.7 Time [s] 29 29
Final comparison: Stator flux response Stator current behavior FOC DTC 30 30
Final comparison: Starting without limitation High starting current Amplitude [A] Amplitude [A] Amplitude [A] Amplitude [A] 70 35 0-35 -70 0 0.1 0.2 0.3 70 35 0-35 -70 0 0.1 0.2 0.3 70 35 0-35 -70 0 0.1 0.2 0.3 70 35 0-35 -37.8 [A] -62.4 [A] 65 [A] 64.3 [A] FOC DTC PTCk1 PTCk2-70 0 0.1 0.2 0.3 Time [s] 31 31
Final comparison: Starting without limitation Stator flux FOC DTC 32 32
Final comparison: Starting with current limitation Limited stator current Amplitude [A] 40 20 0-20 FOC -40 0 0.1 0.2 0.3 Amplitude [A] Amplitude [A] Amplitude [A] 40 20 0-20 -40 0 0.1 0.2 0.3 40 20 0-20 -40 0 0.1 0.2 0.3 40 20 0-20 DTC PTCk1 PTCk2-40 0 0.1 0.2 0.3 Time [s] 33 33
Final comparison: Starting with current limitation Stator flux FOC DTC 34 34
characteristic AC machine with speed control any AC drive under FOC (or DTC) behaves like a synchronous drive or a DC motor with speed/position control under speed control a real speed is kept constant according to the speed reference under any load until maximum torque when exceeding maximum torque the drive is not stopping at standstill (breakdown), but reducing speed as long as the required torque can be provided 35
characteristic AC machine with speed control an electrical machine with inverter supply and speed control always provides the characteristic of a synchronous machine independant on the type of the respective electrical machine 36
torque characteristic AC machine with speed control variations of supply voltage result in a shift of the characteristic but vertically only variations of supply frequency result in a shift of the characteristic horizontally that is exactly, what speed control performs when speed reference varys load characteristic motor operation P el > 0 P mech < 0 speed operation point generator operation P el < 0 P mech > 0 37
torque characteristic AC machine with speed control here the characteristics are arbitrary definitions as well motor operation P el > 0 P mech < 0 the transition from motor to generator operation or vice versa virtually happens automatically as soon as the operation point changes from the first quadrant to the fourth quadrant (or vice versa) speed generator operation P el < 0 P mech > 0 38
characteristic AC machine with speed control limitation by maximum stator current limitation by maximum stator voltage characteristic of (synchronous) drive with speed control thermal limitation unfortunately in most data sheets the 1. quadrant is presented only source : SIEMENS 39
Vector Control (Field Oriented Control, Direct Torque Control) advantages in comparison to U/f Control Referents: Prof. Dr. Ing. Hans Georg Herzog (hg.herzog@tum.de) Prof. Dr. Ing. Ralph Kennel (ralph.kennel@tum.de) Technische Universität München Arcisstraße 21 80333 München Germany 40
advantages of FOC or DTC in comparison to U/f Control that FOC or DTC are feedback control schemes U/f control is a feedforward control scheme FOC and DTC can react on (torque) disturbances U/f control cannot 41
Comparison of Different Electrical Machines DC SM ASM Advantages - Simple control - interior ventilation simple to realize - high protection standard - small size - maintenance free - low inertia - high torque even at standstill - high dynamics - losses in the stator - high protection standard - maintenance free - high overload capability - low cost - high torque even at standstill - high speed range Disadvantages - low protection standard - mechanical wear (brushes, collector) - current limitation - standstill (collector segments) - high speed (commutation) - maximum terminal voltage of 200 V (transformer needed) - losses in the rotor (heat transfer via shaft) - high cost - limited speed range - limited overload capability (demagnetizion) - harmonic losses mainly in the rotor (heat transfer via shaft) - high inertia - field current needed (losses, size, bigger inverter) - complex control - Parameter depending control 42
Thank You!!! Any Questions? Prof. Dr.-Ing. Ralph Kennel Technische Universität München Electrical Drive Systems and Power Electronics 43