Novel power factor corrected ac-dc rectifier topologies suitable for induction motor drive based elevator application are proposed. These converters make use of coupled inductor for power conversion and are capable of providing high voltage gain at low duty cycle and high efficiency. The current flowing through the coupled inductor is controlled through a feedback control loop to achieve unity power factor. The THD value of the current is observed to be approximately 4.8% which is within the limits prescribed by various standards. With the use of coupled inductor, the voltage stress of the switches operating at high frequency is reduced, which reduces switching losses. The loss comparison with the conventional converters shows a reduction of at least 22% of losses. The proposed scheme also results in reduction of the variable frequency drive’s dc link capacitance value as an ultra-capacitor bank is interfaced with the dc link through a bidirectional converter for improving efficiency and providing transient power requirements. This also helps in increasing the reliability and dynamic response of the system. The settling time for a step change in voltage reference is observed to be reduced by nearly 50%. Proposed topologies and schemes are validated through MATLAB/Simulink simulations and experiments.
- Power Factor Correction
- Ac-dc conversion
- Single phase controlled rectifier
- Three phase controlled rectifier
- Reliability and ultra-capacitor
Fig. 1 Block diagram of an elevator system
EXPECTED SIMULATION RESULTS:
Fig. 2(a) Input current and voltage of the proposed1-ph rectifier system with PFC; (b)3-ph current for PFC operation of proposed rectifier configuration; (c) The dc link voltage step changes for 10μF and 500μF dc link capacitor; and (d) Ultra-capacitor current.
Novel AC-DC PWM rectifier topologies for 1-ph and 3-ph systems, based on high voltage gain dc-dc converter principle, were proposed, analyzed and validated through experiments and simulation studies. A major advantage of these topologies is that it is possible to achieve higher voltage gain at lower duty ratio. The operation symmetry is maintained. Input power factor correction is achieved. The use of coupled inductors enhances gain, but it also increases the ripple in the input current as the turns ratio is increased. Thus, there is a trade-off between the achievable gain and the ripple.
The losses of the proposed converter are compared with the conventional ac-dc converter, and it was observed that there is a reduction of about 22% losses. The losses estimated through experimental studies also reduced from 29W to 24W when the proposed topology was used. This shows a reduction of 17% losses in experiments. Therefore, the proposed converter gives higher efficiency than the conventional ac-dc converters. It was also observed that the use of an auxiliary storage reduced the dc link capacitance value from 500 μF to 10 μF for a 1-ph system. For the 3-ph system, the auxiliary unit can be used as a support during the grid voltage sag condition thereby reducing the dc link capacitance requirement. A low value of dc link capacitance not only helps in reducing the size and improving the reliability of system, but also in improving the dynamic response of the system.
The complete system was tested in hardware and the results were presented. A detailed description of the thought process behind the development of the proposed converter was also presented. The same thought process can be extended to the development of such converter topologies. The voltage stress on switch S2 and S3 reduces to 1/8th of its value as compared to the conventional topology. But, the value of peak current increases ‘n’ times. The increase in peak current increases the high frequency current ripple in the input side. However, the duty cycle is decreased with increase in the value of ‘n’. Therefore, the overall efficiency of the converter is increased.
The ac-dc topologies proposed in this paper are unidirectional. But, they can be made bidirectional by connecting a controllable switch across the diodes. This scheme is useful for the scenarios where the loads are regenerating. These bidirectional topologies can also be used as dc-ac converters to feed power into the grid. Thus, the scope of the proposed schemes is very wide and relevant.
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