Journal of Power Electronics

The Korean Institute of Power Electronics (전력전자학회)
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Hybrid Phase Excitation Method for Improving Efficiency of 7-Phase BLDC Motors for Ship Propulsion Systems

  • Park, Hyung-Seok (Department of Electrical and Computer Engineering, Pusan National University) ;
  • Park, Sang-Woo (Appliance Control Research Team, LG Electronics) ;
  • Kim, Dong-Youn (Department of Electrical and Computer Engineering, Pusan National University) ;
  • Kim, Jang-Mok (Department of Electrical and Computer Engineering, Pusan National University)
  • Received : 2018.12.23
  • Accepted : 2019.02.06
  • Published : 2019.05.20
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Abstract

This paper proposes a hybrid phase windings excitation method for improving the efficiency of a 7-phase brushless DC (BLDC) motor in the electric propulsion system of a ship. The electrical losses of a BLDC motor system depend on the operating region and the number of excited phase windings (2-phase, 4-phase or general 6-phase windings). In this paper the operating region and torque/speed characteristics according to the motor rotation speed and propeller load are analyzed for a number of excitation methods. In addition, it analyzes the electrical losses of the system under each of the excitation methods in the entire operating region of the motor. In every sampling time, the proposed control method calculates the electrical loss of the system for each of the excitation methods and operates a 7-phase BLDC motor by selecting the excitation method that results a decreased electrical loss at the operating speed. The usefulness of the proposed control algorithm is verified through experimental results.

Keywords

I. INTRODUCTION

Recently, the use of electric propulsion systems has been gradually increasing in ship and electric vehicle systems [1]-[5]. It can achieve higher efficiency and power as well as lower noise and vibration when compared to conventional fuel engine systems [6], [7]. Brushless DC (BLDC) motors are widely used for electric propulsion systems due to their features such as high power density, lower mass and volume, high torque, high efficiency, simple control, simple hardware and software structures, and lower maintenance [8]. In particular, multi-phase BLDC motors have many advantages over traditional 3-phase BLDC motors. These advantages are as follows [9], [10].

1) Better dynamic performance

2) Higher efficiency and reliability

3) Lower ripple of the dc-link current and commutation torque

4) Reductions in the power ratings of several devices

5) Improved system reliability by bypassing during failures

To achieve both a constant maximum torque output and a high power density in a multi-phase BLDC motor with a trapezoidal back EMF, the phase windings of the steady back EMF are excited with a maximum number except for the one non-exciting phase in which the back EMF crosses zero [10]-[14]. Generally, a 7-phase BLDC motor is operated by the 6-phase excitation (6/7π conduction) method [11], [12]. However, although the operating range is reduced due to a decreased output torque, it is possible to operate a 7-phase BLDC motor by using the 2-phase (2/7π conduction) or 4-phase (4/7π conduction) winding excitation methods. Fig. 1 shows an equivalent circuit of a 7-phase BLDC motor inverter system.

E1PWAX_2019_v19n3_761_f0001.png 이미지

Fig. 1. Equivalent circuit of a 7-phase BLDC motor system.

High efficiency operation is very important in electric propulsion systems that use energy storage devices such as batteries. However, under the same load conditions, the electrical loss of a multi-phase BLDC motor depends on the number of the excited windings and the respective instantaneous phase current values. The smaller the number of the excited phase windings, the smaller the electric losses of the motor. However, the operating area is also reduced. Therefore, in order to increase the efficiency, it is necessary to appropriately select a phase winding excitation method with decreased electric loss according to the operation region.

This paper proposes a hybrid phase winding excitation method for improving the efficiency of a 7-phase BLDC motor for ship propulsion systems. The operating range for each of the phase windings excitation methods is analyzed using the torque/speed characteristics and taking the propeller load torque in to consideration. The calculation of the electrical losses according to the speed and torque conditions for each of the phase winding excitation methods are also performed. Using analyzed studies and normalized equations, the control algorithm operates a 7-phase BLDC motor by selectively using one of the 2-phase, 4-phase or 6-phase winding excitation (2/7π, 4/7π, and 6/7π conduction) methods according to the operating region. An experimental set up of 2kW and 450kW 7-phase BLDC motor drive systems were designed and built. Experimental results obtained from field tests are included to verify the proposed control method.

II. PHASE EXCITATION METHODS

Fig. 2 shows the operation principle of a 6-phase winding excitation method for a 7-phase BLDC motor and PWM inverter. The inverter is composed of a DC-link capacitor and fourteen switching devices each having an IGBT connected with anti-parallel freewheeling diode. It is driven by applying the 6-stator currents in every position (PST). The position of the 7-phase BLDC motor is divided into 14 sections according to hall effect sensor signals. With the position signals, the motor phase windings are excited sequentially to produce a desired torque and speed. One winding is turned on at the same time that the next winding, which is just leaving the polar section, is turned off [10]-[14].

E1PWAX_2019_v19n3_761_f0003.png 이미지

Fig. 2. Operation principle of a 7-phase BLDC motor inverter system using a 6-phase excitation method.

In addition to the 6-phase excitation method, the 7-phase BLDC motor can also be driven by applying 2 stator or 4 stator currents in every position. Fig. 3 shows the timing of the motor phases, the switching pattern, the trapezoidal back EMF due to the PM flux and the currents that must be commanded by the position sensor (ideally, in phase with the back-EMF voltages) for each of the excitation methods. The conductive angles are ‘2π/7’, ‘4π/7’ and ‘6π/7’ in the 2-phase, 4-phase and 6-phase excitation methods, respectively. Fig. 4 shows commutation circuits of the PST-1 switching pattern for each of the excitation methods as an example.

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Fig. 3. Back-EMFs, phase current waveforms and switching patterns for each excitation method (2-/4-/6-phase excitation methods).

E1PWAX_2019_v19n3_761_f0004.png 이미지

Fig. 4. Commutation circuits of the position-1 switching pattern for each excitation method (2-/4-/6-phase excitation methods).

The normalized output power equation and the electric torque equation for each of the phase excitation methods of a multi-phase BLDC motor are expressed as:

\(P_{e(N)}=\sum_{n=a}^{g}\left(e_{n} i_{n}\right)=N E_{s} I_{s(N)}\)       (1)

\(T_{e(N)}=\frac{P_{e(N)}}{\omega_{m}}=\frac{N E_{s} I_{s}}{\omega_{m}}=\frac{N k_{e} \omega_{m} I_{s(N)}}{\omega_{m}}=N k_{e} I_{s(N)}\)       (2)

where N is the number of excited phase windings (2, 4 or 6), ES and IS are the constant peak values of the back-EMF and phase current, ke is the back-EMF constant, and ωm is the rotor angle speed [13], [14].

The back-EMF and phase current for all of the excited phase windings are equal and constant. Under the same load conditions, the smaller the number of excited phase windings, the greater the magnitude of the phase current.

III. TORQUE CHARACTERISTICS AND OPERATING RANGE FOR EACH OF THE EXCITATION METHODS

In order to apply the load torque of a 7-phase BLDC motor for ship propulsion, a mathematical model of the propeller load torque should be considered. Such a model is explained in [15], [16]. The load torque is proportional to the square of the motor rotational speed and can be simplified as:

\(T_{L}=k_{m} \omega_{m}^{2}\)       (3)

where km is the equivalent proportionality constant of various coefficients depending on the mechanical and hydrodynamic characteristics of the propulsion system.

From the normalized electrical torque equation and the mechanical motion equation with an applied load torque, the rated torque for each of the excitation methods can be expressed as:

\(T_{e(N) rated }=N k_{e} I_{s(N) rated }\)       (4)

\(T_{e(N) rated}=J_{m} \frac{d \omega_{m(N) rated}}{d t}+B_{m} \omega_{m(N) rated}+k_{m} \omega_{m(N) rated}^{2}\)       (5)

where N is the number of excited phase windings, and Te(N)rated, Is(N)rated and ωn_max are the rated torque, rated current, and rated angular speed of an N-phase winding excitation method, respectively. Jm is the moment of inertia, and Bm is the viscous friction coefficient.

By using (4) and (5), the torque/speed curves for each of the phase excitation methods and the load torque/speed curve are shown in Fig. 5. The rated operating speed points for each of the excitation methods in the constant torque region can be determined [4]. The operable region differs depending on the excitation method. When the number of excited phase windings decreases under the same load conditions, the magnitude of the required phase current increases and the operable range decreases due to the limit of the output torque in the inverter system.

E1PWAX_2019_v19n3_761_f0005.png 이미지

Fig. 5. Divided speed region by the controllable speed according to the phase excitation methods.

IV. LOSS ANALYSIS OF A 7-PHASE BLDC MOTOR ACCORDING TO THE EXCITATION METHOD

The losses in the motor drive system can be classified into the electrical losses in the inverter, electrical losses and mechanical losses such as friction, stray losses, and so on. All of them are calculated by using well known equations and loss analysis data from several papers and IGBT application notes [17]-[28].

The electrical loss of the inverter system is the sum of the IGBT conduction loss, the inverse diode conduction loss and the switching loss. The total inverter losses according to the phase excitation method are given by (6) [17]-[22]. Where N is the number of excited phase windings, I(N) is the constant phase current under the N-phase excitation method, VIGBT is the on-drop voltage of the IGBT, RIGBT is the on-state resistance, Vdiode is the forward voltage of the freewheeling diode, Rdiode is the diode forward resistance, Rs is the stator resistance, Vdc is the voltage of the DC link, fsw is the switching frequency, and Etotal is the sum of the turn-on and off energy losses.

\(P_{\text {inerter loss}}(N)=\left(N I_{(N)} V_{I G B T}+N I_{(N)}^{2} R_{I G B T}\right)\left(\frac{1}{2}+\frac{R_{s} I_{(N)}}{V_{d c}}+\frac{k_{e}}{V_{d c}} \sqrt{\frac{N k_{e} I_{(N)}}{k_{m}}}\right)+\left(N I_{(N)} V_{diode}+N I_{(N)}^{2} R_{diode}\right)\left(\frac{1}{2}-\frac{R_{s} I_{(N)}}{V_{d c}}+\frac{k_{e}}{V_{d}} \sqrt{\frac{n k_{e} I_{(N)}}{k_{m}}}\right)+N f_{sw} E_{total}\)       (6)

The electrical loss of a motor is divided into the copper loss and core loss, which can be represented as (7) [23]-[28]. Where kh is the hysteresis loss coefficient, kc is the eddy current loss coefficient, ke is the excess core loss coefficient, and Bm is the maximum value of the magnetic flux density. Since the variation of the electrical parameters according to the excitation method is negligibly small, the left-side equation of Equ. (7) can be simplified to the right-side equation. This means that the copper loss and core loss can vary with the excitation method and the operating point of the motor.

\(P_{\text {motor loss}}(N)=N I_{n}^{2} R_{s}+k_{f}\left[k_{h} B_{m}^{2} f_{m}+\frac{\pi^{2} \sigma d^{2}}{6}\left(B_{m} f_{m}\right)^{2}+8.67 k_{e}\left(B_{m} f_{m}\right)^{\frac{3}{2}}\right] \cong N I_{n}^{2} R_{s}+k_{1} \omega\)       (7)

Since the mechanical parameters and operating conditions are the same in all of the excitation methods and the main purpose of the loss analysis is a relative loss comparison for each of the excitation methods, Eqns. (6) and (7) can be expressed as equations for the speed, phase current and number of excited phase windings. In addition, the mechanical loss is also neglected.

Fig. 6 shows simulation results of the calculated electrical losses for each of the excitation methods according to the speed and load conditions. The electrical losses are different due to changes in the number of excited phase windings and phase current magnitude of the applied excitation method. Within the rated operating range for each of the excitation methods, when the number of excited phase windings decreases, the electrical losses are also reduced. However, the operating region exceeding the rating points analyzed in terms of the efficiency for each of the excitation methods is reduced in section 3.

E1PWAX_2019_v19n3_761_f0006.png 이미지

Fig. 6. Electrical losses for each of the phase excitation methods according to the speed region of a 7-phase BLDC motor. (a) Curves of the electrical losses in the full speed range. (b) Curves of the electrical losses in the enlarged speed range.

Therefore, based on the results of the operating range comparison analyzed in section 3 and the results of the electrical loss comparison analyzed in this section, the phase windings excitation method with a lower number within the rated operating range should be used. This means that it is preferable to use the 2-phase, 4-phase and 6-phase excitation methods in regions 1, 2 and 3, respectively.

TABLE I EXCITATION METHOD WITH THE LOWEST LOSS ACCORDING TO THE SPEED

E1PWAX_2019_v19n3_761_t0001.png 이미지

V. OVERALL CONFIGURATION OF THE PROPOSED HYBRID EXCITATION METHOD

Fig. 7 shows the overall configuration of the proposed hybrid phase winding excitation method for improving the efficiency of a 7-phase BLDC motor of a ship propulsion system. Part (A) of Fig. 7 is the conventional speed control and measurement system in a 7-phase BLDC motor drive system.

E1PWAX_2019_v19n3_761_f0007.png 이미지

Fig. 7. Overall block diagram of the proposed hybrid excitation method for a 7-phase BLDC drive system.

The proposed control algorithm is divided into three regions, namely (B), (C) and (D). The electrical losses for each of the excitation methods are calculated from the measured phase currents and angular speed in region (B). In region (C), the excitation method with lowest loss is determined by comparing the calculated losses for each of the excitation methods and confirming the rated operating region.

Finally, in region (D), the operation characteristics of the selected excitation method from region (C) are applied to the conventional control algorithm (the excited phase windings and conductive angle according to the hall position). Accordingly, the 7-phase BLDC motor drive system can be operated with increased efficiency by using the proposed hybrid excitation method.

VI. EXPERIMENTAL RESULTS

The experimental setup shown in Fig. 8 is a 2kW small-scale system, which was manufactured and installed for the safe verification of the proposed control algorithm. It is composed of a 7-phase BLDC motor and inverter system, a digital control system and a load motor drive system. The electrical specifications of the 2kW 7-phase BLDC motor are shown in Table II, and the proposed algorithm is operated with a 14kHz switching frequency by a DSP+FPGA control board.

E1PWAX_2019_v19n3_761_f0008.png 이미지

Fig. 8. Experiment setup of a 2kW reduced model.

TABLE II ELECTRICAL SPECIFICATIONS OF THE 2KW 7-PHASE BLDC MOTOR

E1PWAX_2019_v19n3_761_t0002.png 이미지

Fig. 9 and Fig. 10 show experimental results of the speed control for the 7-phase BLDC motor using each of the excitation methods and the proposed hybrid excitation method, respectively. As shown in Fig. 10, the phase excitation method is changed from a 2-phase excitation method to a 4-phase excitation method at 500r/min. Then it is changed from a 4-phase excitation method to a 6-phase excitation method at 1,500r/min. Considering the system electrical capacity, the phase current was limited to 8A. The 7-phase BLDC motor was operated using the proposed method from the transient state to the steady state.

E1PWAX_2019_v19n3_761_f0009.png 이미지

Fig. 9. Experimental results for the speed control of a 7-phase BLDC motor by each of the excitation methods. (a) 2-phase windings excitation method. (b) 4-phase windings excitation method. (c) 6-phase windings excitation method.

E1PWAX_2019_v19n3_761_f0010.png 이미지

Fig. 10. Experimental results for the speed control of a 7-phase BLDC motor by the proposed control method. (a) Speed and current waveforms under the proposed method. (b) Around changing point 1 (500 r/min). (c) Around changing point 2 (1,500 r/min).

The experimental setup of a 450kW 7-phase BLDC motor drive system for a high-power electric propulsion system was built as shown in Fig. 11. It is composed of a 7-phase BLDC motor and inverter system, a DSP+FPGA control system, a load system, a power supply system and a water-cooling system. The parameters of the 450kW 7-phase BLDC motor are listed in Table III.

E1PWAX_2019_v19n3_761_f0011.png 이미지

Fig. 11. Experimental set of a 450kW 7-phase BLDC motor drive system for an electric propulsion system.

TABLE III ELECTRICAL SPECIFICATIONS OF THE 450KW 7-PHASE BLDC MOTOR

E1PWAX_2019_v19n3_761_t0003.png 이미지

Field tests are conducted for the measurement and comparison of the electrical losses for each of the excitation methods according to the speed region. From the obtained results, the necessity of the proposed excitation method and the loss analysis according to the speed region in the previous section can be verified.

Figs. 12-14 show waveforms using each of the excitation methods at the specified three points of the rotation speed and the propeller load torque (operating region 1, 2 and 3, respectively). In addition, waveforms of the rotor position, A-phase current and dc-link current by each of the excitation methods are presented. Figs. 12, 13 and 14 show experimental results at the three point of the rotation speed/load power 5,000r/min / 6.8kW, 10,000r/min / 54.6kW and 13,000r/min / 120kW, respectively. These three points are located in operating regions 1, 2 and 3, respectively.

E1PWAX_2019_v19n3_761_f0012.png 이미지

Fig. 12. Waveforms at point 1 (5,000 r/min, 6.8kW load). (a)2-phase excitation method. (b) 4-phase excitation method. (c)6-phase excitation method.

E1PWAX_2019_v19n3_761_f0013.png 이미지

Fig. 13. Waveforms at point 2 (10,000 r/min, 54.6kW load). (a)2-phase excitation method. (b) 4-phase excitation method. (c)6-phase excitation method.

E1PWAX_2019_v19n3_761_f0014.png 이미지

Fig. 14. Waveforms at point 3 (13,000 r/min, 120kW load). (a)2-phase excitation method. (b) 4-phase excitation method. (c)6-phase excitation method.

The waveforms for each of the phase currents in the experimental results of this system are distorted when compared to the 2kW small-scale system. This is due to differences in the operating conditions between the two systems (operation speed, applied load torque, influence of the back-EMF from the high speed, current delay, mechanical structure of the dynamo system, and control precision for the applied load torque), as well as the single current control structure using the average current value. The single current control structure is used to increase the control and switching frequency (to decrease the computation time) for the high-speed operation of multi-phase BLDC motors. However, this can result in distortions of the phase current in systems with a high speed and a high load torque.

From these results, the efficiency for each of the excitation methods at the three operating points are calculated using the measured input/output power, and are summarized in Table IV. As a result, in operation areas 1, 2 and 3, the operation by the 2-phase, 4-phase and 6-phase excitation methods are the most efficient, respectively. Therefore, the electrical efficiency of a 7-phase BLDC motor drive system can be improved by using the proposed hybrid excitation method.

TABLE IV EXPERIMENTAL RESULTS OF THE EFFICIENCY

E1PWAX_2019_v19n3_761_t0004.png 이미지

Experimental results of the proposed control method are presented in Fig. 15. This figure shows waveforms for the speed and phase current of the 450kW 7-phase BLDC motor. The electrical losses for each of the excitation methods were calculated and compared according to the speed and load conditions. From this, the excitation method was changed from 2-phase to 4-phase at change point 1 (around 6,250 r/min), and from 4-phase to 6-phase at change point 2 (around 10,550 r/min). In conclusion, the 7-phase BLDC motor was operated with reduced losses when the excitation method was changed at each changing point.

E1PWAX_2019_v19n3_761_f0015.png 이미지

Fig. 15. Experimental results for the speed control of a 7-phase BLDC motor by the proposed control method. (a) Speed and current waveforms under the proposed method. (b) Phase current waveform at changing point 1. (c) Phase current waveform at changing point 2.

VII. CONCLUSIONS

This paper proposed a hybrid phase windings excitation method for improving the efficiency of a 7-phase BLDC motor for a ship propulsion system. The proposed control method was implemented by changing the number of excited phase windings according to the operation region. The 7-phase BLDC motor was operated by calculating the electrical losses for each of the phase excitation methods according to the speed and load torque and by selecting the excitation method with decreased electrical losses.

In this study, the operating principle and characteristics for each of the phase excitation methods were presented. The propeller load torque characteristic and operating region for each of the phase excitation methods were analyzed. In addition, the electrical losses for each of the excitation method according to the speed and load conditions were analyzed.

The proposed control method is a technology to improve the efficiency of an electric propulsion system using a battery. The normalized equations and proposed control method for improving the efficiency of the 7-phase BLDC motor in this study can be applied to other multi-phase BLDC motors. In addition to the previous version of this research [29], actual experimental setups for 2kW and 450kW 7-phase BLDCM drive systems were built and the obtained experimental results verify the proposed control method in this paper. From these results, the validity of the analyzed contents and the proposed method were verified.

ACKNOWLEDGMENT

This work was supported by BK21PLUS, Creative Human Resource Development Program for IT Convergence.

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