On the Development of a Test Bench Dedicated for the Investigation of the Features of a Delta-inverter Fed BDCM Drive

Transcription

1 MARCH 5-8 On the Development of a Test Bench Dedated for the Investigation of the Features of a Delta-inverter Fed BDCM Drive A. Driss, A. Ben Rhouma, B. El Badsi, A. Guermazi, and A. Masmoudi Research Unit on Renewable Energies and Electr Vehles University of Sfax, Sfax Engineering School, PO Box W, 8 Sfax, Tunis ameni driss@yahoo.fr, asma.benrhouma@laposte.net Copyright c 1 MCD & MITI Abstract: Reduced structure inverter fed electr motor drives are attractive for hybrid propulsion systems as far as the cost-effectiveness is concerned. Within this statement, the paper is aimed at the development of a test bench devoted to the investigation of the features of a delta-inverter fed brushless DC motor (BDCM) drive. The delta-inverter includes three switches rather than six in conventional inverters. Furthermore, the BDCM requires a low-resolution encoder. These represent crucl cost benefits for propulsion applations. Keywords: Brushless DC machines, delta-inverter, test bench, current control, torque estimator, maximum torque. 1. Introduction Till the sixtieth, automotive manufacturers did not worry about the cost of fuel. They have never heard of air pollution, and they have never thought about life cycle. Ease of operation with reduced maintenance costs meant everything back then. In recent years, clean air mandates are driving the market to embrace new propulsion systems built around an internal combustion engine and an electr motor drive, yielding the so-called hybrid propulsion systems (HPS). Besides the automotive industry, many research teams worldwide are deeply involved in R&D projects focused towards the design of effient, cost-effective and relble HPS. Within this trend, the electr drive unit has been the subject of intensive works. Thus, thanks to the signifant advances in the power electron converters, a novel era of electr machine technology has been evolving, based on the principle that the best machine design is the one that the simply produces the optimum match between the machine and the assocted converter. Recent tendencies in HPS are focused towards the design of specif electr motor drives, in an attempt to: reach a high flexility in the control of the power flow through the propulsion system, and to achieve effient operating cycles, improve the cost-effectiveness and the compactness whh could be gained thanks to reduced structure inverters feeding the electr motors. Of partular interest are: the four-switch three-phase inverter (FSTPI) [1, ], the three-switch three-phase inverter (TSTPI), also known delta-inverter [, 4]. These have been assocted to brushless DC as well as to induction motors [5]. Within the last item, the paper deals with the development of a test bench dedated to the investigation of the features of a delta-inverter fed brushless DC motor drive.

2 . Basis of TSTPI Fed BDCM Drives Figure 1 shows the topology of the delta-shaped TSTPI. Each leg of the inverter includes the third of the battery pack in series with a set made up of an IGBT and an antiparallelconnected diode. The so-arranged legs are delta-connected, with the summits feeding the BDCM. The six operating sequences of the BDCM with a TSTPI in the armature are achieved as shown in figure 1, where thk lines indate the conducting circuits. Both active and regenerative subsequences, yielded by bang-bang current regulation of the IGBTs, are illustrated. Va Vb Vc BDCM Udc i TSTPI S Udc Figure 1: TSTPI fed BDCM drive connections A characterization of the BDCM six operating sequences is summarized in table 1. Table 1: Characterization of BDCM operating sequences. Legend: Seq a i and Seqr i indate the active subsequence and the regenerative one, respectively, of sequence i, with 1 i Sub- Conducting phase voltages sequence switch(es) V a V b V c Seq a 1 S Seq r 1 & Seq a & S Seq r Seq a Seq r & Seq a 4 & Seq r 4 Seq a 5 Seq r 5 & Seq a & S Seq r a S i S S S i sequence 1 sequence 4 S i S S S i sequence sequence 5 S S i S i S sequence sequence Figure : The six operating sequences of the BDCM fed by a TSTPI under bang-bang current regulation. Legend: (left): active subsequences, (right): regenerative subsequences

3 . Dedated Control Strategy The present section deals with the implementation of a dedated self strategy in the TSTPIfed BDCM drive. Bascally, it consists of an abc-control strategy, where the self-control angle ψ is kept null, so that the electromagnet torque turns to be maximum: T ψ= em = p Φ pm I a (1) where p is the pole pair number and where Φ pm and I a are the rms-values of the PM flux and of the armature current, respectively. Compared to the control strategy considered in the case of a conventional six-switch threephase inverter in the armature, the proposed control scheme includes a torque loop, thanks to whh, the high torque ripples during sequenceto-sequence commutations have been reduced. These are due to the commutations of the BDCM power supply from 1/ to / (and veversa) of the battery pack []. Figure shows the block dgram of the proposed control strategy. The introduction of the torque loop requires the implementation of a torque estimator, whh at a first glance would result in a CPU-time consuming procedure. This drawback has been discarded thanks to a simple formulation of the electromagnet torque based on the normalized back EMF, as follows []: T em = K t (f a i a + f b i b + f c i c ) () where K t is the torque constant, and where f a,b,c and i a,b,c are the a,b,c-phase normalized trapezoidal back EMF and 1 -rectangular current, respectively. Furthermore, unlike conventional inverters, the control of each of the TSTPI IGBTs does not mean the control of the current feeding one of the BDCM phases. Therefore, the outputs of the current bang-bang controllers could not be directly used to to achieve the control of the IGBTs of the delta-inverter. In order to solve this problem, logal functions have been inserted between the outputs of the current controllers and the control inputs of the IGBTs []. Figure : Block dgram of the control strategy implemented in the TSTPI-fed BDCM drive

4 4. Description of the Developed Test Bench The bloc dgram of the developed test bench is illustrated in figure 4. It is made up of: a power system including: a BDCM (AMER) whose ratings are: power: W, speed: 5rpm, torque:.nm, DC-bus voltage: 4V, DC-bus current: A. and whose parameters are: armature resistance: r a =mω, armature inductance: L a =14mH, PM flux: Φ pm =1.mWb, inert: J =.5ms, pole pair number: p=. a TSTPI including per leg a 1V (DC) battery pack in series with a set of an IGBT and a soft recovery anti-parallel diode (IRG4PSC71UD), a control system built around a dspace 114 interface whose inputs are provided by the phase current Hall effect sensors and by the six-pulse encoder and whose outputs are controlling the three IGBTs of the delta-inverter. The circuit of the driver, enabling the control of one IGBT of the delta-inverter, is illustrated in figure 5. Experimental tests have been carried out separately on the delta-inverter and on the BDCM. Figure shows the measerements obtained in the case where the TSTPI is feeding a starconnected resistor with 18 -control of the IGBTs. In the top, is shown the load current, while in the bottom is illustrated the current circulating in the leg whose IGBT is linked to the delta-summit connected to the load phase fed by the current shown in the top. Figure 7 illustrates the waveforms of two phaseto-phase back EMFs whh are E ab and E ac. These have been selected in order to identify the one of phase a E a, as follows: if the absolute value of (E ab +E ac ) is lower than the maximum value of E ab and E ac, then: E a = (E ab + E ac ) () otherwise: if the absolute value of E ab is higher than the one of E ac, then: elsewhere: E a = E ab E a = E ac (4) (5) Power System TSTPI BDCM i a i b i c i S -pulse encoder Control System S 1 current generator computer 1 current generator dspace 114 dspace control panel adaptation board Figure 4: Bloc dgram of the developed test bench for experiments on a Delta-inverter fed BDCM drive

5 NAN R7 R4 R R5 R AND 1 R9 R8 POT C +5V HCPL R1 C8 R1 In put R1 Q1 R11 M1 NAN NAN C5 NC TC449 C4 C C R D4 m1 R1 C1 D m -15V D5 m RGon RGoff m4 D8 IGBT D C R Figure 5: Circuit of the driver controlling one IGBT of the delta-inverter Figure : Test of the TSTPI feeding a star-connected resistor with 18 -control of the IGBTs. Legend (top): load current, (bottom): current circulating in the leg of the TSTPI whose IGBT is linked to the delta-summit connected to the load phase fed by the current shown in the top

6 rpm Eab Eac Ea back emf (V) time (s) Figure 7: Test of the BDCM run as a generator under steady-state no-load operation with the speed kept constant equal 1rpm. Legend (pink): phase-to-phase back EMF E ab, (green): phase-to-phase back EMF E ac, (bleu): back EMF E a identified from E ab and E ac using the procedure descred in section 4 5. Conclusion The paper was devoted to the development of a test bench devoted to the investigation of the features of a delta-inverter fed brushless DC motor (BDCM) drive. Topologally, a delta-inverter includes three switches rather than six in conventional inverters. Moreover, the BDCM requires a low-resolution low-cost encoder. These represent crucl cost benefits for propulsion applations. The fundamentals of delta-inverter fed BDCM drives were firstly recalled. A control strategy dedated to delta inverter fed BDCM drives was proposed in a second step. Compared to conventional control schemes, the proposed one includes a torque loop, thanks to whh, a reduction of the high torque ripples during sequence-to-sequence commutations has been gained. Then the developed test bench was descred. It consists of a numerally-controlled set built around a dspace 114 interface whose inputs are (i) the BDCM phase currents provided by Hall effect sensors and (ii) the encoder signals, and whose outputs are controlling the IGBTs of the delta-inverter. Experimental tests have been carried out separately on the delta-inverter and on the BDCM. These have been attested the effectiveness of the developed test bench. This said, there is some way to go before the transient and steadystate performance of delta-inverter fed BDCM drives could be experimentally investigated. References [1] B.K. Lee and M. Ehsani, Advanced BLDC Motor Drive for Low Cost and High Performance Propulsion System in Electr and Hybrid Vehles, in CD-ROM of the IEEE Inter. Electr Machines and Drives Conference (IEMDC), Cambridge, Massachusetts, USA, June 1. [] B. EL Badsi and A. Masmoudi, DTC of a FSTPI-fed Induction Motor Drive with Extended Speed Range, Inter. Journal for Computation and Mathemats in Electral and Electron Engineering, Vol. 7, No. 5, pp , 8. [] A. Ben Rhouma, A. Masmoudi, and A. Elantably, On the Analysis and Control of a Three-switch Three-phase Inverter-fed Brushless DC-motor Drive, Inter. Journal for Computation and Mathemats in Electral and Electron Engineering, Vol., No. 1, pp. 18-, 7. [4] A. Ben Rhouma and A. Masmoudi, A DTC Strategy Dedated to Three-switch Three-phase Inverter-fed Induction Motor Drives, Inter. Journal for Computation and Mathemats in Electral and Electron Engineering, Vol. 7, No. 5, pp , 8. [5] B. El Badsi, A. Ben Rhouma, A. Driss, A. Guermazi, and A. Masmoudi, On the Potentlities of Reduced Structure Inverters Integrated in Automotive Electr Motor Drives, in CD-ROM of the Eightieth Inter. Symposium on Advanced Electromechanal Motion Systems (ELECTROMOTION), Lille, France, July 9.