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Posted on 01 September 2019

2-phase Switched Reluctance Motor Drives

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SRM is used mainly in vacuum cleaners

There is growing attention about the use of energy-efficient devices to preserve the environment. The concern for energy efficiency is forcing the industry to find new solutions without compromising the application requirements.

By Byoungchul Cho, Sungil Yong, Motion Control System Group, Fairchild Semiconductor, Korea

 

Switched Reluctance Motors have been considered to be an alternative to AC drives because of their high efficiency, high power per unit volume and weight, high torque-cost ratio, controllability over a wide speed range and good reliability for high speed operation. In the home appliance industry, the SRM is used mainly in vacuum cleaners because it operates at high speeds - tens of thousands of rpm – and requires high torque to generate strong suction. Since the main source of audible noise in vacuum cleaners is its fan, the noise caused by SRM is significantly reduced. Moreover, controversy over the harmful effects of carbon dust generated by the brush in the more widely used universal motor vacuum cleaners is spurring the development of an inverter-type vacuum cleaner. In recent years, single-phase SRMs for vacuum cleaners have been developed to minimize the price of SRM drive circuitry. To meet these needs, Fairchild Semiconductor has already developed a single- phase SRM power module to meet market demand and growing trends in the industry.

The reluctance motor operates on the principle that a magnetically salient rotor is free to move to a position of minimum reluctance to the flow of flux in a magnetic circuit. Therefore, it can operate with any number of phase windings, however for conventional machines there are some guidelines governing the choice of stator and rotor pole numbers. The main factors concerned with the choice of phase number are the required starting performance and the rated output power (the maximum speed). These two factors influence the choice of phase number. Single-phase motors are less attractive when the required rated power is significantly greater than 1kW, because of discontinuous nature of its torque, which leads high peak current of device and the size of the DC link capacitor. Single-phase motors must also be ruled out where difficult loads are concerned, for instance loads which combine direct-drive and high friction. Two phase motors can be started under difficult load conditions and they will generally be cheaper than three phase equivalent, due to the lower component count in the power electronic drive.

Fairchild semiconductor developed two phase SRM module for its prominent merit over single-phase. Unlike to the previous single-phase SRM, this module is SIP (Single-In-line Package), which helps to make compact PCB layout and system. In this paper, the structure, introduction, distinctive features and the convenience provided by this module are shown. In addition, the application considerations are described with typical operating waveforms.

Overview of the SRM Module

Figure 1 is an external view of the power module. Figure 2 is a block diagram of an SRM module. The IGBTs and FRDs have been selected according to their practical application. The IGBTs are designed to optimize the trade-off between conduction and switching losses over the drive conditions. The typical Vce(sat) of IGBT is 2.1V at 25°C, 30A condition. The high-speed built-in HVIC enables the use of a single power supply without a photo coupler. The HVIC drive circuitry is designed to optimize the switching characteristics and to minimize system noise.

External view of 2-phase SRM module

The 2-phase SRM module in SIP package provides two main protective functions. One is control supply under-voltage lockout.

Internal block diagram of 2-phase SRM module

When VBS or VCC are below its detect level, the internal gating signal is blocked by each drive IC, which prevents abnormal operation. Once the supply voltage rises again over the reset level, the module is operated by the command signals. The other function is short-circuit current protection. A 2-phase SRM module can monitor the inverter leg current by using an external shunt resistor. This current information is inputted to Csc pin and when this voltage is over 0.5V, the internal gating signal is blocked by low-side drive IC and the fault-out signal is generated to send this situation to micro controller.

The SRM power module not only offers space efficiency but also enhances productivity and cost effectiveness in mass production. Four NPT-IGBTs, four FRDs, three drive ICs and discrete bootstrap diodes in the module are used in the main power circuits of 2-phase SRM drive systems employing asymmetric converter topologies. The SRM module has all these components and an additional thermistor. By placing a thermistor inside the module rather than on the heat sink, the actual silicon temperature can be tracked with a smaller time constant and verified with lower margin of error. The typical thermal resistance between IGBT junction and thermistor, Rth(j-th), is 3.2 (°C/W). In addition to these features, the discrete bootstrap diodes are used in the module rather than embedded one in the HVIC to enhance the reliability.

The package is designed to guarantee the best heat transfer from the power chips to the outer heat-sink by using IMS package. The IMS substrate uses an aluminum plate at the base. The upper side of the substrate consists of a thermally conductive dielectric layer and a copper cladding on which the cir- cuit etched. This takes advantage of high thermal conductivity and simple manufacturing. Figure 3 shows the cross sectional structure of 2-phase SRM module. The total thickness of the molding is 5.5mm.

Cross sectional structure

The 2-phase SRM module provides two types of lead forms shown in Figure 4. One is Y-form lead and the other is L-form lead. It takes advantage of the free arrangement of heat-sink as well as PCB design flexibility. For the design of 2-phase SRM module, isolation distances of pin-to-pin and pin-to-heat sink should be considered. As shown in Figure 5(a), clearance and creepage distances of pin-to-heat sink are 3.10mm and 4.08mm respectively. As shown in Figure 5(b), clearance and creepage distances of pin-to-pin are 2.35mm and 4.15mm respectively.

Lead forms of-2 phase SRM module

Isolation distance pin-to-Heatsink

Isolation distance pin-to-pin

Electrical Characteristics of the SRM Module

The NPT IGBT and ultra fast recovery diodes used in 2-phase SRM are designed for optimized power rating. Because 2- phase SRM drive for vacuum cleaner has a small switching frequency around 3kHz, conduction loss is more important to make small power loss than switching loss. Figure 6 shows DC&AC characteristics of proposed module. Figure 7 shows the phase current for vacuum cleaner using 2-phase SRM.

DC & AC characteristics

Phase current for vacuum cleaner

Conclusions

2-phase SRM module for symmetric bridge converter is introduced. In addition to the single-phase SRM module, this will provide the design flexibility, easy assembly and particularly excellent cost-effectiveness. By using optimized NPT IGBT, the total power loss is minimized to be suited in the application, where the conduction loss is dominant compared to the switching loss.

 

 

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