Performance Analysis Of Switched Reluctance Engineering Essay

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Switched reluctance motor has many advantages, such as rigid structure, wide speed range, suitable for high-speed operation, high acceleration capability, and high power density. Furthermore, the SRM converter circuit is simple [6,7,13] and free from arm feed-through short. However, the SRM still has many disadvantages, such as nonlinear torque dynamics, high torque ripple, and high mechanical vibration and acoustic noise, etc. Consequently, the torque generating capability is reduced.

Till now, more researches have been conducted to improve the performance of a SRM, such as: (i) converter circuits and their switching controls [1-5]; (ii) DC-link voltage boosting circuits [6,10], they can effectively increase winding current rising rate at high speed; (iii) soft switching

circuits [3] to reduce switching loss and stress; (iv) commutation instant tuning to improve torque generating capability [8,11];(v) reduction of mechanical vibration and acoustic noise [12-14]; (vi) current profiling control [13]; (vii) speed control [4,9]; (viii) position servo driving control ; (ix) front-end converters [1,2] and switch-mode rectifiers , which are placed between the power source and the SRM converter to boost the DC-link voltage and charge the battery reversely with good line power quality.

FRONT-END CONVERTER WITH DC INPUT

The main purpose of a DC-DC converter is to supply a regulated DC output voltage to a variable load resistance from an unstable DC input voltage. DC-DC converters are commonly used in applications requiring regulated DC power, such as computers, medical instrumentation and communication devices. DC-DC converters are also used to provide a stable variable DC voltage for DC motor speed control applications.

There are three types of DC-DC converters in use today, linear converters, switched capacitor converters (also known as charge pumps), and switched converters. Linear converters can only generate lower output voltage from the higher input voltage. Their conversion efficiency is never greater than Vout / Vin. In practice most linear converters operate with typical conversion efficiencies of only 30%. Basic switched DC-DC converter is shown in Fig.1

Fig.1. Basic Switched DC-DC Converter

A. DC/DC Boost Converter

A boost converter topology is obtained by rearranging the components of a buck converter. During the time the switch is closed energy is transferred to the inductor while the diode is preventing the capacitor to discharge through the switch. When the switch opens current through the inductor continues to flow in the same direction as during the previous cycle. This forward biases the diode and both the input voltage source and the inductor are transferring energy to the load. Hence, a voltage boost occurs across the load, which causes the output voltage to be higher than the input voltage. The capacitor must be large enough to keep the output voltage approximately constant.

Fig.2. Two Quadrant Boost Converter

The layout of the bidirectional dc-dc converter is accomplished by connecting in antiparallel a dc-dc step-up stage and a dc-dc step-down stage. For motoring operations of the motor drive the converter step-up stage is used to step up the battery voltage and control the inverter input voltage in order to minimize the ripple of the motor current waveform.

B. Modes of Operation

Thereby, the M1 and M2 are never operated at the same time, being the switch M2 always off during motoring operation, whereas the switch M1 is kept off continuously whenever the regenerative braking operation is commanded. For the transition from M1 switching operation to M2 switching operation a delay blanking time between the gating signals of the two switches is used, in order to avoid a "shoot through" or cross-conduction current through both the converter output capacitor and the two switches. For motoring operations the switch M1 is operated at constant switching frequency and variable duty-cycle, in order to step up the battery voltage at a voltage level which is slightly greater than the peak value of the motor phase-to-phase EMF. Fig.3.a) shows the circuit states which occur along one time period of the converter switching frequency. When the switch M1 is on, the battery supplies energy to the inductor L. When the switch M1 is off, the output capacitor C receives energy from the inductor as well as from the battery. Thereby, the voltage Vdc at the output capacitor terminals can be regulated accordingly with the motor speed by adjusting the duty-cycle of the switch M1. Whenever either the reference value for the motoring current is set to zero, the switch M1 is turned off and kept in this state till the braking command is removed and a reference value greater than zero is commanded for the motoring current.

Fig.3. a) Motoring Operation

Fig.3. b) Regenerative Braking Operation

After a fixed blanking time interval the switch M2 is turned on to allow the reversal of the power flow. Thereafter, the switch M2 is operated at constant switching frequency and variable duty-cycle in order to keep at the desired value the braking current flowing in the battery. Fig.3.b) shows the converter modes of conduction related to the regenerative braking operation. When the switch M2 is on, the battery receives energy from the capacitor C as well as from the machine, which operates as a generator. By turning off the switch M2 the battery is isolated from such an energy supply, and thereby the control of the average value of the braking current flowing in the battery can be accomplished by regulating the duty cycle of the switch M2. Such a control of the braking current reduces stressing of the battery due to the regenerative braking operation.

SWITCHED RELUCTANCE MOTOR DRIVE

A schematic representation of the lamination pattern of two phase, three phase and four phase switched reluctance motors is shown in Figure 3.1. In each of the motors shown in Figure 3.1 a coil is wound around each stator pole and is connected, usually in series with the coil on the diametrically opposite stator pole to form a phase winding. The reluctance of the flux path between the two diametrically opposite stator poles varies as a pair of rotor poles rotates into and out of alignment. Since inductance is inversely proportional to reluctance, the inductance of a phase winding is a maximum when the rotor is in the aligned position and a minimum when the rotor is in the non- aligned position. A pulse of positive torque is produced if current flows in a phase winding as the inductance of that phase winding is increasing. A negative torque contribution is avoided if the current is reduced to zero before the inductance starts to decrease again. The rotor speed can be varied by changing the frequency of the phase current pulses while retaining synchronism with the rotor position.

a) 2 Phase b) 3 Phase c) 4 Phase

Fig .4. Schematic of Switched Reluctance Laminations all with Two Poles per Phase

Fig.5. Single Phase Equivalent Circuit of SRM

An elementary equivalent circuit for the SRM can be derived neglecting the mutual inductance between the phases as follows. The applied voltage to a phase is equal to the sum of the resistive voltage drop and the rate of the flux linkages and is given as:

where Rs is the resistance per phase, and λ the flux linkage per phase given by:

where L is the inductance dependent on the rotor position and phase current. Then, the phase voltage is:

In this equation, the three terms on the right-hand side represent the resistive voltage drop, inductive voltage drop and induced emf, respectively. The induced emf, e, is expressed as:

A. Power Converter

Unlike induction motors or d.c motors the reluctance motor cannot run directly from an a.c or d.c supply. A certain amount of control and power electronics must be present. The power converter is the electronic commutator, controlling the phase currents to produce continuous motion. The purpose of the power converter circuit is to provide some means of increasing and decreasing the supply of current to the phase winding. Many different power converter circuits have been proposed for the switched reluctance motor. By far the most common power converter for the switched drive is the asymmetric half-bridge, shown in Fig.6. a four phase motor. Each asymmetric half-bridge has three main modes of operation. The first, a positive voltage loop, occurs when both switching devices associated with a phase winding are turned on. The supply voltage is connected across the phase winding and the current in the phase winding increases rapidly, supplying energy to the motor. The second mode of operation is a zero voltage loop. This occurs if either of the two switching devices is turned off while current is flowing in a phase winding. In this case the current continues to flow through one switching device and one diode. The voltage across the phase winding during this time is equal to the sum of the on-state voltages of the two semiconductor devices. This voltage is very small compared to the supply voltage and so the current in the phase winding decays very slowly.

Fig.6. Power Converter for Four Phase Motor with Asymmetric Half Bridge

The final mode of operation is a negative voltage loop. Both the switching devices are turned off. The current is forced to flow through both the freewheel diodes. The current in the phase winding decreases rapidly as energy is returned from the motor to the supply. The asymmetric half-bridge thus offers three very flexible modes for current control. The zero voltage loop is very important in minimising the current ripple at any given switching frequency.

B. SRM Power Converter Operation

In SRM converter, two switches and two diodes are used for per phase windings of the SRM as shown in Fig.7.

Fig.7. Single Phase Converter Operation

When both the switches M1 and M2 are ON, then the winding is in energizing mode. When both the switches M1 and M2 are OFF, then the winding is in de-energized mode. When any one of the switches is ON and another switch is OFF, then the winding is in current regulation mode.

Fig.8. Modes of Operation for the Classical Converter (a) Energization

(b) Zero Loop (c) De-Energization

IV. CONVERTER FOR CONTROLLED DEMAGNETIZATION VOLTAGE

The main reason for using a higher demagnetization voltage is to reduce the phase current to zero in as short duration as possible after the aligned position. This leads to higher positive torque, since the current turn-off can be initiated closer to the aligned position, and lower negative torque, since the current decays faster in the negative torque zone(after the aligned position). Thus at a particular speed, the maximum average torque can be increased. Using similar reasoning for the same value of desired torque, the rms motor current will be lower with higher demagnetization voltage.

The need for high demagnetization voltage is only at high speeds, hence only the single pulse mode of operation needs to be considered. For a given speed, input dc voltage, Vdc and demagnetization voltage, Vdm the average torque depends on the ON and OFF angles for each phase. The maximum torque obtainable, based on the values of θon and θoff, then depends on the maximum current allowable for the power converter. Thus, for specified peak converter current, Imax, θon and θoff can be derived for calculating maximum possible torque. For each value of θon considered, θoff is chosen such that the maximum possible average torque is obtained.

Fig.9. SRM Converter for High Demagnetization Voltage

Operating modes of the converter are shown in Table. 10 Switch states and phase voltage during different operating modes is

Table 10 Operating Modes of High Demagnetization Voltage Converter

Mode

Switching states

Phase voltage

Energization

(MA1, MA2) ON

(D1, D2) OFF

Vph = Vdc - 2 Iph Rsw

Zero Voltage

(MA2, D1) ON

(MA1, D2) OFF

Vph = -(Ia Rsw + Vf)

Demagnetization- 1

(MA1, D2) ON

(D1, MA2) OFF

Vph = -(Iph Rsw + Vf + Vdm)

Demagnetization- 2

(D1, D2) ON

(MA1, MA2) OFF

Vph = -(2Vf + Vdc + Vdm)

SIMULATION MODEL

The Fig.11 shows MATLAB/SIMULINK model for switched reluctance motor drive. The SRM drive model is simulated with DC/DC boost converter and high demagnetization voltage in open loop system. The sinusoidal PWM is applied for the gate to IGBT. The front end converter is simulated in PSIM and it is coupled with SRM drive by using Sim coupler which is modeled in MATLAB.

Fig.11. Overall MATLAB Simulation Model for SRM Drive

The Fig.12 shows PSIM simulated model of power converter for SRM drive. The input dc voltage for power converter is boosted by using DC/DC boost converter. The ac voltage source is connected to power converter for initially charge the stator coil in switched reluctance motor.

Fig.12. PSIM Simulation Model of Power Converter Circuit for SRM

The Fig.13 shows PSIM simulated model with high demagnetization voltage. The demagnetization inductor (Ldm), capacitor (Cdm), diode (Ddm) and switch (Sdm) acts as a buck-boost converter end of the power converter. The demagnetization voltage is added by using demagnetization capacitor that is fed back to the source for useful utilization in SRM drive.

Fig.13. PSIM Simulation Model for power Converter with High Demagnetization Voltage

The Fig. 14 shows PSIM simulated model for DC/DC two switch boost converter with filter circuit. The filter circuit is connected to DC/DC boost converter to reduce ripple and harmonic content in the output. It is used to get a pure sinusoidal output.

Fig.14 PSIM Simulation Model for DC/DC Boost Converter with Filter Circuit

SIMULATION RESULTS & DISCUSSION

The Switched Reluctance Motor is simulated with DC/DC boost converter and high demagnetization voltage using PSIM software.

Fig.15. Sinusoidal Pulse Width Modulation for Boost Converter

The Fig.15 shows input pulses for DC/DC boost converter. The sinusoidal PWM technique is used for generating pulse with a switching frequency of 12.5 kHz.

Fig.16. Output Voltage for Boost Converter without Filter

The Fig.16 shows output voltage for DC/DC boost converter in open loop system. The input voltage is 48 V and the boosted voltage is around 64 V. It shows more ripples in output. The duty ratio is 0.25 for front end converter.

Fig.17. Output Voltage for Boost Converter with Filter

The Fig.17 shows output voltage for DC/DC boost converter with filter circuit. The output voltage is increased due to LC filter circuit and also reduced ripples in output. The output voltage is constant at a value of 70 V.

Fig.18 Voltage THD for Boost Converter

The Fig. 18 shows output voltage total harmonic distortion for DC/DC boost converter. The THD is measured across the RL load in boost converter. The THD value is 16.9%

Fig.19. Output Phase Voltage for Power Converter

The Fig.19 shows output phase voltages for four phase SRM drive. Each phase voltage is measured across the phase coil. The phase voltage is increased with the boosted voltage from DC/DC boost converter and sinusoidal PWM for the gate to IGBT. The average output voltage per phase is 102 V.

Fig.20 "Phase B" Voltage for Converter with High Demagnetization Voltage

The Fig.20 shows single phase output voltage for SRM drive with high demagnetization voltage. The increased high demagnetization voltage fed back to the source for useful utilization in SRM drive and increases efficiency.

Fig.21. Output Current for SRM

The Fig.21 shows output current for SRM drive. The output current initially increased and reaches to steady state for all three phase current at a value of 5.5 Amps.

Fig.22. Output Speed for SRM

The Fig.22 shows output speed for SRM drive. Thus at a particular speed the maximum average torque is increased by operating with high demagnetization voltage.

Fig 23 Output Flux Waveform for SRM

The Fig 23 shows output flux for SRM drive. The flux is gradually increased and attained steady state depends on the input voltage.

CONCLUSION

The performance of Switched Reluctance Motor is improved with front-end converter and high demagnetization voltage. The two quadrant front-end converter operated as a DC/DC boost converter for dynamically boosted voltage. It is connected between power source and SRM to boost voltage for charging the battery with good line power quality. To operate with boost converter for SRM drive, efficiency is increased and torque ripple is reduced. The high demagnetization voltage for motoring operation is illustrated in the form of increased torque at high speeds. At high speeds, torque output of SRM is increased with high demagnetization voltage, which enables faster reduction of phase current to zero after aligned position. Based on general requirements the classic converter is presented in this thesis has high efficiency because of demagnetization voltage fed back to the source for useful utilization.

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