0093-9994 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2017.2737398, IEEE Transactions on Industry Applications Active Phase-Converter for Operation of Three-Phase Induction Motors on Single-Phase Grid Anil K Adapa, Vinod John Department of Electrical Engineering, Indian Institute of Science Email: aniladapa@ee.iisc.ernet.in, vjohn@ee.iisc.ernet.in Abstract—The operation of a three-phase induction machine on single-phase supply has been an approach used for electrome- chanical energy conversion in rural communities with limited or no access to the three-phase grid. Such single-phase to three- phase conversion can be achieved by passive and active means. In passive methods, fixed capacitors are used for starting and running the motor. Single-phase to three-phase conversion with reduced switch count is desirable, as this leads to a lower cost. However, it is a challenge to start an induction motor using such a power converter and to maintain balanced three-phase voltage under all loading conditions. It is shown in this work that balanced excitation can not be achieved under all conditions with variable capacitor emulation method. An active phase- converter configuration and its control are proposed that ensures balanced three-phase power supply at the motor terminals under all operating conditions. Also, this approach has the ability to soft-start the motor. It is shown that this can be done without drawing excessive currents from the power converter or from the single-phase grid. A design methodology is presented for selection of the autotransformer taps to limit start-up transient currents to the desired level, based on the analytical results. Simulation and experimental results are presented that validate the proposed topology and control. Keywords—active phase-converter, auxiliary capacitor, reduced rated power converter, factor of unbalance, tap changing auto transformer, soft-start of induction motor. I. I NTRODUCTION Low cost, superior electrical and mechanical performance, and large power to weight ratio are the main reasons favouring the use of three-phase machines over their single-phase counter parts in many applications [1]. In remote areas and rural communities where only a single-phase supply is available, application of standard three-phase motors is limited. A simple method of operating three-phase induction machine from a single-phase power supply is by employing a passive phase converter as shown in Fig. 1(a), which provides a reasonable phase split to run a three-phase induction motor on single- phase grid. Although generating balanced three-phase supply is not possible with just a capacitor, the degree of unbalance can be reduced considerably for a specific operating speed by proper choice of the capacitance [2], [3]. Ideally, the phase balancing capacitor should vary continu- ously with speed of the motor since the impedance of the motor is also a function of its speed, but in practice, it is limited to the This work was supported by CPRI, Ministry of Power, Government of India, under the project Power conversion, control, and protection technologies for microgrid. prudent choice of two capacitor values, one for starting and the other for running at normal speed [4]. Moreover, if the speed of the motor is changed for any reason, such as for a change in the loading conditions, another value of running capacitor is necessary to have a reasonable performance at that particular load condition. The configuration shown in Fig. 1(b) constitut- ing a variable capacitance that can be electronically emulated [5]. This method can give three-phase supply but attaining perfect balance under all operating conditions is not possible. This is because there are two requirements for providing a balanced three-phase supply (1) magnitudes of the line-to-line voltages should be the same (2) appropriate phase shift of 120 ◦ should be maintained between line-to-line voltages. Achieving both these requirements simultaneously with a single element is not feasible under all operating conditions of the motor. It is possible to produce perfectly balanced conditions at any speed by using additional passive circuit elements of appropriate value [1]. These configurations increase size and complexity of the system. Traditional approaches like a back- to-back connected front end converter and inverter system can generate perfectly balanced three-phase supply at the cost of two full power rated converters [6]. A large number of fixed frequency single-phase to three-phase converter topologies and their control strategies are studied with an objective to reduce the switch count in the aforementioned applications [6]–[10], [10]–[15]. However, in practice these converter topologies introduce challenges while line starting the motor as current drawn from such a converter or grid can be as large as 6 pu, leading to the need for overrating the converter, so as to prevent tripping of the converter [15]. Some of the reduced switch topology based single-phase to three-phase converters have the ability to soft-start the motor by V/F control. But these topologies have disadvantages such as the circulation of low- frequency fundamental current through the dc-link mid point, increased dc link voltage in-order to produce required line- to-line voltages and need for semiconductor devices of higher voltage rating [8]–[11], [15]. In this paper, a criterion based on the factor of unbalance is used to select an optimal variable capacitance. It is shown that even with such an optimal capacitance, unbalance can not be eliminated under all loading conditions. Fig. 1(c) shows the proposed converter based on the single-phase to three- phase power conversion topologies [6], [14]. The proposed circuit configuration and control strategy improve the running performance of the machine over variable capacitor emulation techniques by providing balanced three-phase supply under all