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CHAPTER 2 SRM DRIVE SYSTEM WITH CURRENT FEEDBACK LOOP
Operating Principle of the Switched Reluctance Motor
Switched Reluctance Motor is an electric motor in which the torque is produced by the tendency of its moveable part shifting to a position where the inductance of the excited winding is maximized [10]. Both stator and rotor of the SRM have salient poles, but no winding on the rotor. The number of poles on the stator is usually unequal to the number on the rotor to avoid the eventuality of the rotor being in a state of producing no initial torque, which occurs when all the rotor poles are locked in with the stator poles. The winding on the stator is wound on the opposite poles and connected in series or parallel to consist a number of electrically separate circuit or phases. These phase windings can be excited separately or together depending on the control scheme. Fig. 2.1 shows the cross-section of a 12/8 SRM motor, which has 12 stators and 8 rotors. This is a three-phase motor, each phase comprises three coils wound on opposite poles.
The mechanical movement of SRM is achieved by exciting each phase in a sequence that depends on the direction of movement. For example, if the clockwise rotation is desired, each phase is energized in the a-b-c sequence. For counterclockwise rotation, the reverse sequence is executed. During the movement, there are three relative positions between the stator and rotor. The first one is so called unaligned position; the second one is the intermediate position; and the third one is the aligned position. When any pair of the rotor poles is exactly aligned with stator poles of a particular phase, that phase is said to be in an aligned position. Similarly, if the inter-polar axis of the rotor is aligned with the stator poles of a particular phase, that phase is said to be in an unaligned position. The position between the aligned and unaligned position is called the intermediate position.
The inductance of each phase varies widely with both the rotor position and phase current. Fig. 2.2 indicates that the inductance of each phase in 12/8 SRM varies with the rotor position. Each curve is repeated in a period of τ = 2Nrπ . Nr is the number of the rotor poles. The motor used in this project has eight rotor poles, thus each phase inductance repeats in 45 mechanical degrees.
The inductance of each phase varies from minimum value Lu0 to maximum value La0 when the rotor position changes from unaligned position to aligned position.
In the unaligned position, the phase inductance reaches the minimum value Lu0 because the magnetic reluctance of the flux path is lowest. When the rotor is in the aligned position, the magnetic reluctance of the flux path is highest; as a result, the phase inductance reaches the maximum value La0.
There are two operation modes with the SRM. One is the motoring mode; another is the generating mode. When current flows in a phase, the torque is always produced to pull the rotor to the aligned position no matter the direction of the phase current. So, the torque for the motoring mode can be produced between the unaligned and aligned positions. In other words, the positive torque can be produced during the inductance-increasing period. For the motoring mode, when the phase current is firing in a particular sequence, the rotor should move in the desired direction. When the motor operates in the generating mode, the negative torque can be produced during the inductance-decreasing period.
The SRM makes use of the basic electromagnetic principle that converts electric energy to the mechanical energy. However, not all the energy provided by the power supply is converted to the mechanical work. Some of them are stored in the magnetic field during the “fluxing” process, and then returned to the power supply during “de-fluxing” period. Fig. 2.3 shows the energy conversion loop during one “fluxing” and “de-fluxing” cycle [10].
Where C is the commutation point from one phase to another phase. Area w1+w2 represents the co-energy, which contributes to the mechanical movement. W1 represents the energy produced during the fluxing process; w2 represents the energy produced during the “de-fluxing” period; and Z represents the energy returned to the power supply. Z+W1+W2 represents the total energy provided by the power supply. During the derivation of the energy curve and the energy balance, constant supply voltage Vs and rotor speed ω are assumed.
Where R represents the phase resistance, which increases with the rotor speed; L (θ, i) indicates the instant inductance value. In (2.2), the three terms on the right hand side represent the resistive, inductive and back EMF terms, respectively. If the phase resistance R is small, the flux-linkage will increases linearly with the rotor position. In terms of the inductance profile {see Fig. 2.2}, due to constant inductance around the unaligned position, the current increases linearly at first. However, when the rotor pole overlaps with the stator pole, the inductance increases with the rotor position θ and the back EMF starts to build up. As a result, the current rising rate is decreased. When the back EMF is larger than the input voltage Vs, the current starts to decrease. During “de-fluxing” period, the supply voltage reverses and the phase current drops to zero very quickly.
The torque equation can be derived based on the energy balance in the SRM. During the derivation process, no saturation and straight magnetization curves are assumed. Also, constant supply voltage Vs and rotor speed ω are assumed.
The power in the SRM is represented by the three terms in (2.6), which are the copper losses, air-gap power and rate of change of stored energy in the phase winding, respectively. The air-gap power is used to convert the electric energy to the mechanical movement, which equals to air-gap torque times the rotor speed.
In terms of the equation (2.8) it is obvious that the torque produced in the SRM depends on the phase current and rotor position. When the rotor is around the unaligned or aligned position, no torque is produced. If the phase current is constant, the torque is constant during the rotor is in the overlapped or intermediate position. Furthermore, positive torque is produced when the phase winding is excited during the rising inductance, and negative torque is produced during the falling inductance. Because the torque changes with the rotor position and current, although the current flows to phase windings continually from the power supply, there is still a dip in the torque waveform at the commutation instant from one phase to the next one. There are two reasons for this. One is that each phase current always starts from zero; another one is that the change of the inductance with rotor position is small at the commutation instant
Chapter 1 INTRODUCTION.
1.1 SWITCHED RELUCTANCE MOTOR
1.2 AC SMALL SIGNAL MODELING TECHNIQUE IN SRM
1.3 THESIS OBJECTIVE AND OVERVIEW
Chapter 2 SRM DRIVE SYSTEM WITH CURRENT FEEDBACK LOOP
2.1 OPERATING PRINCIPLE OF THE SWITCHED RELUCTANCE MOTOR
2.2 SRM DRIVE SYSTEM WITH CURRENT FEEDBACK LOOP
Chapter 3 DERIVATION OF AC SMALL SINGNAL MODEL FOR SRM WITH PWM CURRENT CONTROLLER
3.1 AC SMALL SIGNAL MODEL FOR SRM CONVERTER
3.2 AC SMALL SIGNAL MODEL FOR CURRENT FEEDBACK LOOP
3.3 DESIGN OF THE CURRENT CONTROLLER FOR SRM
Chapter 4 SIMULATION RESULTS
4.1 GATE SIGNAL AND PHASE CURRENT
4.2 FFT ANALYSIS OF PHASE CURRENT
Chapter 5 IMPLEMENTATION OF SRM DRIVE SYSTEM WITH PWM CURRENT CONTROLLER
5.1 HARDWARE IMPLEMENTATION
5.2 SOFTWARE LAYOUT
Chapter 6 EXPERIMENTAL RESULTS
6.1 FIRING TIMING
6.2 GATE DRIVE SIGNAL
Chapter 7 CONCLUSION AND FUTURE WORK
REFERENCES
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