Improvement of Output Voltage Waveform in Dual Inverter Fed Open-winding Induction Motor

at Low Speed Area

Akihito Mizukoshi, Hitoshi Haga Nagaoka University of Technology

Department of Electrical, Electronics and Information Engineering Nagaoka, Niigata, Japan

akihito_mizukoshi@stn.nagaokaut.ac.jp, hagah@vos.nagaokaut.ac.jp

Abstract— this paper proposes an improvement method of the output voltage waveform in dual inverter fed open-winding induction motor focusing of low speed operation. The system consists of two isolated DC power supplies, an open-end winding induction motor, and two voltage source inverters which are respectively connected to the opposite terminals of the open-end winding with unequal DC link voltages. In the proposed control method, the use of the voltage difference output from the inverters constituting the dual inverter lowers the peak voltage at low speed conditions. For realization of the proposed method, this paper proposes the commutation sequence of PWM pattern considering the dead-time of the dual inverter. The proposed method is verified experimentally by driving the dual inverter fed open-winding induction motor. Experimental results confirm that the harmonic component of the output voltage, which is related to the switching frequency (10 kHz), reduces from 1.03 to 0.74 by the proposed method at the motor speed of 300 rpm and the load torque of 0.48 Nm. Furthermore, the proposed method clarifies that the motor loss is reduced by 0.5 W at 300 rpm and 0.2 Nm as compared with the conventional method.

Keywords— open-end winding induction motor, induction motor, space vector modulation, dual inverter, low modulation index, field-oriented cntrol (FOC).

I. INTRODUCTION

Some motor drive systems, which are operated by a voltage source inverter, are widely used for various applications such as electric vehicles and air conditioners. For the electric vehicles applications, the motor is required to have high-efficiency driving characteristics in a wide speed range. Currently, various approaches are being pursued to solve this requirement. Multilevel inverters are an effective solution for improving the efficiency of motor drive systems [1]. Since the multilevel inverter outputs a stair-step output voltage (dv/dt is small), the harmonic content of the output voltage is small as compared with the 2-level inverter. Therefore, the motor drive by the inverter has the feature that the harmonic loss of the motor is reduced. As another approaches, reducing the peak value of the pulse width modulated (PWM) voltage by using the chopper is also effective for improving the motor efficiency [2]. The multilevel waveforms and lower peak value of the voltage can reduce the output harmonics and the motor’s harmonic losses [1], [2].

A motor drive system using a dual inverter is also effective for improving the waveform of the winding voltage with multilevel operation [3], [4]. The dual inverter system consists of two voltage source inverters which are connected respectively to the opposite terminals of the open-end winding motor. The multi-level operation is possible due to the high redundancy of switching patterns [4]. This paper discusses the control method to achieve the high efficiency operation of the motor focusing on the dual inverter.

For the dual inverter, various circuit topologies and PWM methods have been considered such as, achieving the multilevel operation [5]-[15], reducing the current ripple in the winding [16], [17], and common mode voltage reduction [18]. In particular, the dual inverter topology, which consists of two 2-level inverters having isolated DC link voltages in each inverter, can generate a for-level waveform by keeping each DC link voltages in a ratio of 2:1 [5]-[9]. Through these previous researches, motor drive systems can achieve a high efficiency at rated power condition due to generating the multilevel waveform. In the case of the high modulation index, the PWM pattern, which operates the either inverter, is introduced [5]. However, the PWM modulation index is low at low motor speeds, the harmonic distortion of the inverter’s output voltage is high because of an increase of the switching frequency harmonic compared with the fundamental component [2]. The voltage distortion is the cause of the increase in motor loss. Hence, the motor efficiency at low speeds and at partial load is low. This paper proposes high efficiency control method in the low speed region of the motor driven by the dual inverter.

Figure 1 shows the motor drive system which is the subject of this paper. The system consists of two isolated DC power supplies, an open-end winding induction motor, and a dual inverter. This paper proposes an improvement method of the output voltage waveform focusing on low motor speeds. In the proposed method, the output voltage difference of the inverters constituting the dual inverter lowers the peak value of the output voltage at low motor speeds. This paper also shows the commutation sequence of the PWM pattern considering the dead-time of the dual inverter to realize the proposed method, which is reported in [8]. The effectiveness of the proposed method is shown by experimental results.

Fig. 1. The configuration of dual inverter fed open-end winding induction motor drive.

II. DUAL INVERTER FED OPEN-END WINDING INDUCTION

MOTOR

A. Circuit Configuration of the Dual Inverter

Figure 1 shows the configuration of the open-end winding induction motor fed by a dual inverter. The output voltage difference between the inverters is supplied to the stator windings. The U-phase winding voltage vu (as well as V-phase voltage vv and W-phase voltage vw) is written as follows;

vu = vu1-vu2 (1)

Here, vu1 and vu2 are the output voltage of the each inverter. From the equation, a higher or lower voltage than that of each inverter is supplied to the motor by using the phase difference between the two inverters. The dual inverter outputs the PWM waveform by using both of the Vdc1 and Vdc2 at high speeds (high modulation index). Then, the peak voltage of the PWM will be 2(Vdc1+Vdc2)/3.

B. Space Vectors of the Dual Inverter

This paper applies Space Vector Modulation (SVM) on the individual 2-level inverters in Figure 2. In this paper, the switching states of INV.1 and INV.2 are respectively numbered as 0, 1, 2, ..., 7 and 0 ', 1', 2, ..., 7 '. Here, a “+” means that the top switch of a leg is turned on, while a “-” means that the bottom switch of the leg is turned on. Thus, the dual inverter, which consists of two two-level inverters, has 64 possible switching states [3]. The vector combinations of the dual inverter are shown in Figure 3 when the ratio of the DC link voltages is Vdc1:Vdc2 = 2:1. Figure 3 shows that the four-level waveform generates due to the high redundancy of the switching pattern when the modulation index is high [5]-[7]. In the conventional method, which is reported in [5], the switching loss can be reduced when the motor is driven at low speeds (low

Fig. 2. Space voltage vectors of INV.1 and INV.2.

Fig. 3. Space vector combinations of the dual inverter when the DC voltageratio is Vdc1: Vdc2 = 2:1.

modulation index) because of the dual inverter outputting the PWM waveform by using only the INV. 1. However, the winding voltage distortion increases because the peak voltage of PWM is fixed at 2Vdc1/3.

III. PROPOSED PWM METHOD FOR DUAL INVERTER

A. Proposed Space Vector Modulation

In the proposed system, the dual inverter and the bi-directional chopper regulate the voltage Vdc2 depending on the modulation index. The DC-link voltage of the left-side inverter (INV. 1) is higher than the right-side inverter (INV. 2). The dual inverter outputs the PWM waveform by using both Vdc1 and Vdc2 at low speeds (low modulation index) in the proposed method as shown in Figure 4. Then, the peak value of the PWM supplied to the winding will be (Vdc1 - Vdc2) by outputting synchronized pulses in both inverters. Therefore, the proposed method improves the voltage distortion because the peak value of the PWM is reduced compared with the conventional method. Hence, the proposed method improves the motor loss at low speeds.

Since the voltage applied to the winding is the difference between the output voltages of INV. 1 and INV. 2 from equation (1), the shorter vectors, which magnitude is defined as (Vdc1 - Vdc2), is used by selecting the same vectors in each inverter such as 1 (+--) and 1’ (+--) as in Figure 5. In the proposed method, switching states 00’, 11’, 22’, 33’, 44’, 55’, 66’, and 77’

Fig. 4. An image of the Proposed PWM waveforms.

Fig. 5. Output voltage vectors of the proposed method.

Fig. 6. The effect of the dead-time on switching state.

are only used. Using these switching states, the peak winding voltage value can be related to only the difference between each DC-link voltage of INV.1 and INV.2. Therefore, this proposed method is able to output the equivalent winding voltage as with a single 2-level inverter, having the DC-link voltage as (Vdc1 - Vdc2).

B. Motor Current Commutation for Proposed Method

In the proposed PWM switching method, the peak value of the winding voltage pulses are reduced by outputting synchronized pulses in both inverters. In fact, the dead-time, which switches off the legs of both inverters, is required when the switching state is changed from top switch on to bottom switch on and vice versa. Then, if all the switches of the legs of both inverters are turned off in the dead-time, an error voltage is applied to the winding due to the freewheeling diodes Dn1 and Dp2 (or Dp1 and Dn2), which start conducting (in Figure 6).

Figure 6 shows the effect of the dead-time on switching states, when the switching states of both inverters are changed from “+” to “-”. The effect occurs at each phase of the dual inverter, and the diodes which turn on change according to the

Fig. 7. An example of switching sequence when the current direction is positive.

direction of the phase current. Therefore, the current commutation is considered to avoid the effect of the dead-time reported in [8] as shown in Figure 7. The commutation pattern is achieved by detecting each phase current and it is performed in 3-steps operation. On the first step, Sp2 is turned off as the

Fig. 8. An example of the current commutation sequence to avoid the effect of the dead-time.

Fig. 9. Control diagram of the dual inverter based on slip frequency type vector control.

freewheeling diodes start conducting. The switching states of both inverters are not changed because the freewheeling diode is still conducting. On the second step, Sp1 is switched off and Sn2 is turned on at the same time. The switching states of both inverters are changed because the freewheeling diode, which is located on the opposite side of Sp1, starts conducting in this step. Finally, the commutation operation is finished by turning on Sn1. Figure 8 shows an example of the current commutation sequence when the switching states of both inverters are changed from “+” to “-”.

IV. EXPERIMENTAL RESULTS

A. Experimental Setup and Conditions

Figure 9 shows the control block diagram of the proposed dual inverter fed induction motor at low speed. The control method is based on the slip frequency type Field Oriented Control (FOC). The switching patterns of the dual inverter are selected by the switching table depending on the current direction of each phase (Figure 9). The experimental setup is shown in Figure 10 as well as the experimental conditions and parameters of the open-end winding induction motor in Table 1-2. For the verification of the proposed method, the open-end winding induction motor is driven at a constant speed by the dual inverter, and is loaded with constant torque using the load motor, which windings are Y-connected, connected through the rotor shaft.

Fig. 10. Experimental setup

Table 1. Experimental condition.

Drive Method Conventional Proposed

INV.1 DC Voltage: Vdc1 360 V 180 V

INV.2 DC Voltage: Vdc2 180 V 120 V

Rotor speed: N 300rpm

Torque: T 0.2Nm-1.0Nm

Frequency: f 50Hz

Carrier Frequency: fc 10kHz

Dead time: Td 2us

Table 2. Parameters of the open-end winding induction motor.

Rated power 750W Poles 4

Rated voltage 200V Rated frequency 50Hz

Rated current 3.5A Rated speed 1410rpm

Stator resistance 2.74Ω Leakage inductance 10.5mH

Rotor resistance 2.08Ω Mutual inductance 0.195H

Fig. 11. Motor loss – Mechanical output characteristics.

B. Motor Loss and Mechanical Output Characteristic

In this experiment, the INV. 2 DC-link voltage Vdc2 is set to 120V, while the INV. 1 DC-link voltage Vdc1 is set to 180V. In other words, the voltage difference between Vdc1 and Vdc2 is 60V in the proposed method. The motor loss and mechanical output characteristics are shown in Figure 11 by the torque varied from 0.2Nm to 1.0Nm. The motor loss reduction of 0.5W is verified compared with the conventional method at 300rpm and 0.2Nm.

C. Voltage Waveforms and Harmonics Analysis

Figure 12 shows the winding voltage waveforms of the U-phase with the conventional method, which operates to use only INV. 1, having the DC-link voltage at 360V (Figure 12 (a)), and with the proposed method having the difference of the DC-link voltage at 60V (Figure 12 (b)). It was verified that the proposed method reduced the peak voltage from 240V to 40V. Figure 13 shows the results of the harmonics analysis of the U-phase voltage shown in Figure12. The harmonic components of the output voltage, which are related to the switching frequency (10 kHz), was reduced from 1.03 to 0.74 by the proposed method compared with the conventional method. Therefore, the total harmonic distortion (THD) of the phase voltage was reduced from 320% to 134%.

V. CONCLUSIONS

This paper proposes a control method for the dual inverter fed open-end winding induction motor at low motor speed conditions. The proposed control method improves the output voltage waveform by reducing the peak value of output voltage. Experimental results confirm that the harmonic component of the output voltage, which is related to the switching frequency (10 kHz), can be reduced from 1.03 to 0.74 by the proposed method at a motor speed of 300 rpm and a load torque of 0.48 Nm. Furthermore, a 0.5 W motor loss reduction is verified compared with the conventional method at 300rpm and 0.2Nm.

(a)

(b)

Fig. 12. Waveforms of phase voltage with (a) conventional method, (b) proposed method.

(a)

(b)

Fig. 13. Harmonics analysis of phase voltage with (a) conventional method, (b) proposed method.

REFERENCES [1] A. Nabae, I. Takahashi, and H. Akagi, “A New Neutral-Point-Clamped

PWM Inverter”, IEEE Transactions on Industry Applications, vol.17, no.5 pp.518-523, Sept. 1981.

[2] M. Morimoto, K. Sumito, S. Sato, K. Oshitani, M. Ishida, S. Okuma, “High efficiency, unity power factor VVVF drive system of an induction motor”, IEEE Transactions on Power Electronics, vol. 6, no. 3, pp. 498-503, July. 1991.

[3] Y. Kawabata, M. Nasu, T. Nomoto, E. C. Ejiogu, and T. Kawabata, “High-Efficiency and Low Acoustic Noise Drive System Using Open-end winding AC Motor and Two Space-Vector-Modulated Inverters”, IEEE Transactions on Industrial Electronics, vol.49, no.4, pp.783-789, Aug. 2002.

[4] K.A. Corzine, S.D. Sudhoff and C.A. Whitcomb, “Performance Characteristics of a Cascaded Two-Level Converter”, IEEE Transactions on Energy Conversion, vol.14, no.3, pp.433-439, Sept. 1999.

[5] Suresh Lakhimsetty, Nagarjun Surulivel, and V. T. Somasekhar, "Improvised SVPWM Strategies for an Enhanced Performance for a Four-Level Open-End Winding Induction Motor Drive," IEEE Transactions on Industrial Electronics, vol. 64, no. 4, pp. 2750-2759, Apr. 2017.

[6] Maramreddy Harsha Vardhan Reddy, Teegala Bramhananda Reddy, Bumanapalli Ravindranath Reddy, and Munagala Surya Kalavathi, "Discontinuous PWM Technique for the Asymmetrical Dual Inverter Configuration to Eliminate the Overcharging of DC-Link Capacitor," IEEE Transactions on Industrial Electronics, vol. 65, no. 1, pp. 156-166, Jan. 2018.

[7] B. Venugopal Reddy, and V. T. Somasekhar, "An SVPWM Scheme for the Suppression of Zero-Sequence Current in a Four-Level Open-End Winding Induction Motor Drive With Nested Rectifier–Inverter," IEEE Transactions on Industrial Electronics, vol. 63, no. 5, pp. 2803-2812, May 2016.

[8] Shajjad Chowdhury, Patrick W. Wheeler, Chintan Patel, and Chris Gerada, "A Multilevel Converter With a Floating Bridge for Open-End Winding Motor Drive Applications," IEEE Transactions on Industrial Electronics, vol. 63, no. 9, pp. 5366-5375, May 2016.

[9] Shajjad Chowdhury, Patrick W. Wheeler, Chris Gerada, and Chintan Patel, "Model Predictive Control for a Dual-Active Bridge Inverter With

a Floating Bridge," IEEE Transactions on Industrial Electronics, vol. 63, no. 9, pp. 5558-5568, Sept. 2016.

[10] Michele Mengoni, Albino Amerise, Luca Zarri, Angelo Tani, Giovanni Serra, and Domenico Casadei, "Control Scheme for Open-Ended Induction Motor Drives With a Floating Capacitor Bridge Over a Wide Speed Range," IEEE Transactions on Industry Applications, vol. 53, no. 5, pp. 4504-4514, Sept. 2017.

[11] Salvatore Foti, Antonio Testa, Giacomo Scelba , Salvatore De Caro, Mario Cacciato, Giuseppe Scarcella, and Tommaso Scimone, "An Open-End Winding Motor Approach to Mitigate the Phase Voltage Distortion on Multilevel Inverters," IEEE Transactions on Power Electronics, vol. 33, no. 3, pp. 2404-2416, Mar. 2018.

[12] Italo Roger Ferreira Moreno Pinheiro da Silva, Cursino Brandão Jacobina, Alexandre Cunha Oliveira, Gregory Arthur de Almeida Carlos, and Maurício Beltrão de Rossiter Corrêa, "Hybrid Modular Multilevel DSCC Inverter for Open-End Winding Induction Motor Drives," IEEE Transactions on Industry Applications, vol. 53, no. 2, pp. 1232-1242, Mar. 2017.

[13] Mathews Boby, Sumit Pramanick, R. Sudharshan Kaarthik, S. Arun Rahul, K. Gopakumar, and L. Umanand, "Multilevel Dodecagonal Voltage Space Vector Structure Generation for Open-End Winding IM Using a Single DC Source," IEEE Transactions on Industrial Electronics, vol. 63, no. 5, pp. 2757-2765, May 2016.

[14] Abhijit Kshirsagar, R. Sudharshan Kaarthik, K. Gopakumar, Loganathan Umanand, and Kaushik Rajashekara, "Low Switch Count Nine-Level Inverter Topology for Open-End Induction Motor Drives," IEEE Transactions on Industrial Electronics, vol. 64, no. 2, pp. 1009-1017, Feb. 2017.

[15] Mohammad Sedigh Toulabi, John Salmon, and Andrew M. Knight, "Design, Control, and Experimental Test of an Open-Winding IPM Synchronous Motor," IEEE Transactions on Industrial Electronics, vol. 64, no. 4, pp. 2760-2769, Apr. 2017.

[16] K. Ramachandra Sekhar, and S. Srinivas, "Discontinuous Decoupled PWMs for Reduced Current Ripple in a Dual Two-Level Inverter Fed Open-End Winding Induction Motor Drive," IEEE Transactions on Power Electronics, vol. 28, no. 5, pp. 2493-2502, May 2013.

[17] S. Srinivas, and K. Ramachandra Sekhar, "Theoretical and Experimental Analysis for Current in a Dual-Inverter-Fed Open-End Winding Induction Motor Drive With Reduced Switching PWM," IEEE Transactions on Industrial Electronics, vol. 60, no. 10, pp. 4318-4328, 2013.