There are many advantages of using high frequency PWM (in the range of 50 to 100 kHz) in motor drive applications. High motor efficiency, fast control response, lower motor torque ripple, close to ideal sinusoidal motor current waveform, smaller filter size, lower cost filter, etc. are a few of the advantages. However, higher frequency PWM is also associated with severe voltage reflection and motor insulation breakdown issues at the motor terminals. If standard Si IGBT based inverters are employed, losses in the switches make it difficult to overcome significant drop in efficiency of converting electrical power to mechanical power. Work on SiC and GaN based inverter has progressed and variable frequency drives (VFDs) can now be operated efficiently at carrier frequencies in the 50 to 200 kHz range, using these devices. Using soft magnetic material, the overall efficiency of filtering can be improved. The switching characteristics of SiC and GaN devices are such that even at high switching frequency, the turn on and turn off losses are minimal. Hence, there is not much penalty in increasing the carrier frequency of the VFD. Losses in AC motors due to PWM waveform are significantly reduced. All the above features put together improves system efficiency. This paper presents results obtained on using a 6-in-1 GaN module for VFD application, operating at a carrier frequency of 100 kHz with an output sine wave filter. Experimental results show the improvement in motor efficiency and system efficiency on using a GaN based VFD in comparison to the standard Si IGBT based VFD.
SiC devices are gaining acceptance in the motor drive industry. This paper compares the power loss and efficiency between two options that can be used with SiC based Variable Frequency Drives (VFDs). In the first option, the SiC VFD is equipped with an output sine wave filter with carrier frequency at 50 kHz. A dv/dt filter is used for the second option with the carrier frequency reduced to 8 kHz. Both the options are compared with a standard Si IGBT VFD operating at a carrier frequency of 8 kHz with no output filter. The focus of the paper is to present different filtering options for SiC VFDs.The dv/dt filter is designed to meet the same specification as that of the standard Si IGBT VFD with no output filter, so as to present a fair comparison between a standard Si IGBT VFD and the next generation SiC VFD. Results using a 460V, 11kW system, show that the SiC VFD with output sine wave filter has lower efficiency compared to SiC VFD with a dv/dt filter. Influence of the various filtering options on leakage current in the motor drive system has also been studied and the results are presented in this paper.Index Terms: High-speed power semiconductor devices, Filtering options for high speed power switches, dv/dt filter for SiC VFD, Sine filter for SiC VFD, Efficiency comparison between SiC and Si-IGBT VFDs I. 0093-9994 (c)
In many applications, it is often needed to extend the speed range of an electric motor. In Permanent Magnet AC motors (PMAC), the maximum speed is limited by the available voltage for a given counter EMF value. Extension of the speed range can be achieved to some extent by using the field weakening principle resulting in constant power characteristics, but is limited by machine parameters and inverter rating. Salient pole PMAC machines (inset or buried magnet types) are better suited for field weakening compared to surface mount PMAC machines.In induction motor-based spindle drives for machine tool applications, the wye/delta switchover method was adopted and still remains popular for extending the constant power range without sacrificing the torque capability at higher speeds. Conventionally, the wye/delta method employs mechanical contactors that have limited life and are associated with dead time in the range of tens to hundreds of milliseconds due to mechanical constraints.Power electronic versions of the contactors have been proposed, but they are rather complicated and involve many switching devices. This paper describes a novel and simple switching technique involving only two three-phase diode bridges and two IGBT switches to extend the speed range of AC machines (induction as well as permanent magnet type). The proposed method requires a center tap to be placed in each phase of the motor. The start, finish, and the center tap should be accessible thereby making the motor a nine-lead machine. Adopting the proposed method, results in an AC machine with dual-torque characteristics. When used in conjunction with field weakening principle, the highspeed range can be extended in PMAC machines. The circuit used to extend the speed range is described and supported by test results carried out on a non-salient type PMAC motor and an induction motor.
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