Robot and Servo Drive Lab. Department of Electrical Engineering Southern Taiwan University of Science and Technology 2016/6/14 Commutation Control for the Low-Commutation Torque Ripple in the Position Sensorless Drive of the Low- Voltage Brushless DC Motor IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 11, NOVEMBER 2014 Sang-Yong Jung, Member, IEEE, Yong-Jae Kim, Member, IEEE, Jungmoon Jae, and Jaehong Kim, Member, IEEE Teacher: Prof. Ming-Syhan Wang Student (presented): Ika Noer Syamsiana

Department of Electrical Engineering Southern Taiwan University OUTLINE : 2016/6/14 Robot and Servo Drive Lab. 2 Abstract Introduction Phase advancing and overlapping control Analysis of the torque ripple in the PAO method Analysis of the commutation torque ripple with various commutation control methods Simulation and experimental results Conclusion Reference

Department of Electrical Engineering Southern Taiwan University Abstract This Journal discusses a commutation control method aimed : Reducing the commutation torque ripple in sensorless drive of brushless direct current motors BLDC motors are generally used for low-cost applications because of relatively high efficiency and low manufacturing cost On the other hand, they show a high torque ripple characteristic caused by nonideal commutation currents In order to minimize torque ripple for the entire speed range, a comprehensive analysis of commutation torque ripple was made according to three commutation control methods whereupon an optimal current vector trajectory for low torque ripple was devised

Department of Electrical Engineering Southern Taiwan University INTRODUCTION  The Advantages of BLDC Motor :  High reliability  Simple frame  Straightforward control  Low friction  Compared to (PMSMs) :high-speed adjusting performance and power density 2016/6/14 Robot and Servo Drive Lab. 4 The most important part of BLDC drive : Commutation Control

Department of Electrical Engineering Southern Taiwan University The previous research : NoProposedDisadvantage 1 Six-step commutation with sensorless  cost effective solution that requires a simple control frame and robust High torque ripple caused by non ideal commutation current that limits wider use 2A BLDC motor with trapezoidal back EMF fed by ideal rectangular current generates smooth instantaneous torque Ideal rectangular current shapes cannot be realized in practice due to the phase inductance and finite inverter voltage 3Using PWMWhen it has different PWM patterns, occurs commutation torque ripples 4A Hysterisis and deadbeat current control to minimize commutation torque ripple by using inner current loop. In order to keep incoming and outgoing phase currents changing at the same rate during commutation, the duty cycle is regulated at low speed and the deadbeat current control is adopted at high speed. PWM duty ratio was modified to compensate voltage disturbance caused by commutation current in the sensorless drive of the BLDC motor, an overlapping technique, which extends the phase conduction period over 120 electrical degree, was adopted to reduce the torque spike by exciting a new conducting phase in advance

Department of Electrical Engineering Southern Taiwan University The previous research : NoProposedDisadvantage 5The direct torque control (DTC) schemeIt needs arithmetic calculations for the extracting torque and flux compensation term that can add further computational overload to low-cost CPUs 6The duty ratio compensating torque fluctuation in PWM_ON _PWM method It needs real-time measurements and calculation of phase current, angular position, and speed 7A buck converter was used with a new modulation pattern to reduce the commutation torque ripple The bandwidth of the buck converter was not considered, so this structure can only handle torque pulsation at the low speed 8A super-lift Luo topology and SEPIC converterThese structures need complex control or additional power switches

Department of Electrical Engineering Southern Taiwan University The proposed method of this Journal The proposed method uses three commutation control methods for full speed range operation : Conventional six-step and phase-advancing (PA) methods are adopted below the base speed The phase-advancing with overlapping (PAO) method is used for over the base speed to obtain higher speed operation with low torque ripple

Department of Electrical Engineering Southern Taiwan University PHASE ADVANCING AND OVERLAPPING CONTROL A. System Configuration Fig 1. Power Circuit Configuration 13V sensorless BLDC fan motor drive system. The BLDC motor is driven by a conventional three-phase inverter. DC power is supplied by the battery

Department of Electrical Engineering Southern Taiwan University Single loop control configuration of the sensorless BLDC motor drive system Speed control output is directly fed to the PWM module as the duty ratio The commutation detection block enables sensorless operation of the motor, comparing the measured back EMF with half dc-link voltage Mode number m is transferred to the PWM module to determine commutation status, and actual speed is measured by comparing the mode change instance with internal timer register value,  t m, of the digital signal processor (DSP) Fig 2a. Whole system configuration

Department of Electrical Engineering Southern Taiwan University Fig 2b. phase-to-phase model of BLDC motor drive system

Department of Electrical Engineering Southern Taiwan University During two-phase conduction the inverter phase-to-phase output voltage is directly proportional to the duty ratio. The system transfer function is calculated as :

Department of Electrical Engineering Southern Taiwan University PI gains :  In this case  = 1.11 and  n = 2 

Department of Electrical Engineering Southern Taiwan University B. Phase-Advancing and Overlapping Control of the Low- Voltage Sensorless BLDC Motor Drive  A three-phase BLDC motor is generally driven by 120° two-phase conduction switching, which is called the six-step commutation method.  The six-step commutation in phase with the trapezoidal back EMF is the best choice for high efficiency and the low torque ripple.  This commutation method is quite similar to the maximum torque per ampere control in the SM-PMSM drive below the base speed because the stator flux always leads the rotor flux by 90 electrical degrees  The PA technique, which is quite similar to the conventional field-weakening control in the SM-PMSM drive above the base speed, is a good solution to obtain a wider speed range  Another approach to obtain a wider speed range is the so-called overlapping method that increases the conduction period over 120°, i.e., 150° or 180°. That is the case of  ou > 0 and  ol = 0.

Department of Electrical Engineering Southern Taiwan University That is the cases  ou =  ol > 0 : Fig 3. Voltage and current waveforms according to the commutation control method: (a)three-phase back- EMFs and (b)a-phase currents This can reach 50% of the average torque with conventional six-step commutation method

Department of Electrical Engineering Southern Taiwan University ANALYSIS OF THE TORQUE RIPPLE IN THE PAO METHOD 2016/6/14 Robot and Servo Drive Lab. 15 A. Modeling of the Commutation Torque Ripple From the assumptions of symmetrical windings, general voltage equations for the three-phase BLDC motor are :

Department of Electrical Engineering Southern Taiwan University Commutation torque ripples are presented in the three-phase conduction states as : Fig 4. Waveforms of phase currents, electric torque, and phase back-EMFs when PAO is applied near the maximum speed. The electrical angular speed  c is assumed to be constant during the commutation period t1-t2, due to relatively large mechanical time constant. During t1- t2, eb = Vep and ec = -Vep, where Vep is the peak value od the phase back EMF

Department of Electrical Engineering Southern Taiwan University The rate of change of torque is given by conventional power equation as  to be constant during the commutation period 2. Large mechanical time constant is assumed : 1. The torque equation changes to :

Department of Electrical Engineering Southern Taiwan University During the commutation period are represented in the state space form as : And : The offset voltage 

Department of Electrical Engineering Southern Taiwan University Fig. 5. Equivalent circuit of period t1-t2 in Fig. 4. Fig. 4

Department of Electrical Engineering Southern Taiwan University The duty ratio making :

Department of Electrical Engineering Southern Taiwan University B. Torque Ripple's Dependence on the Load Current In the middle- and high-speed operations, ohmic voltage drop in the stator winding is negligible Commutation time : Thus, the total torque variation caused by the commutation current is calculated as

Department of Electrical Engineering Southern Taiwan University At steady state :  the commutation torque ripple mostly depends on Ia1 Vep is obviously a speed dependent variable (DVcd-Vep) is fixed when the load torque is not varying. It should be pointed out that just a little variation in magnitude of the commutation torque ripple is shown with respect to the Vep variation a. When Vep varies a (DVcd-Vep) is fixed b. When Ia1 varies and Dvdc and Vep are fixed c. When Vep varies Ia1 and Dvdc are fixed Fig. 6. Magnitude of the commutation torque ripple calculated from

Department of Electrical Engineering Southern Taiwan University ANALYSIS OF THE COMMUTATION TORQUE RIPPLE WITH VARIOUS COMMUTATION CONTROL METHODS 2016/6/14 Robot and Servo Drive Lab. 23 A. Commutation Torque Ripple in the PAO Method The offset voltage : a-phase current during the overlapping period is directly calculated

Department of Electrical Engineering Southern Taiwan University B. Commutation Torque Ripple in the PA Method During commutation period, the offset voltage :

Department of Electrical Engineering Southern Taiwan University When e b (t1) at the start of commutation reduces from Vep, corresponding magnitude. Fig. 8. Variation of the commutation torque ripple with respect to eb variation.

Department of Electrical Engineering Southern Taiwan University C. Commutation Torque Ripple in the Conventional Overlapping Control In this case, the time derivative of the commutation torque ripple becomes :

Department of Electrical Engineering Southern Taiwan University DESIRED CURRENT TRAJECTORY FOR THELOW COMMUTATION TORQUE RIPPLE The speed is controlled by the PWM duty ratio at this region (first trajectory) Fig 9. Theoretical full trajectory of the current vector for low- commutation torque ripple

Department of Electrical Engineering Southern Taiwan University Above the base speed, the d-axis reactive current should be injected to weaken the air-gap field, and higher average voltage should be applied to overcome the large back EMF The PAO, which increases both current angle and average voltage applied, would be a more suitable choice here.. The position sensorless drive of the BLDC motor generally measures three- phase voltages. The voltage of the nonconducting phase is detected to calculate rotor position during two-phase conduction instance. Therefore, two-phase conduction instances should appear six times during a period. Otherwise, the controller fails to detect the rotor position. Thus, the angle of the current vector cannot exceed 5  /24. Practically is much less than  /3 for high-load conditions because the tail current is enlarged depending on the load current and thus the commutation period extends.

Department of Electrical Engineering Southern Taiwan University After the speed is saturated with 5  /24current angle and full duty, more average pole voltage can be applied by reducing  ol to zero Though the commutation torque ripple is enlarged a little, the maximum possible speed is obtained with  ol. So, the final current angle approaches  /6 at the maximum speed operation The magnitude of average current is defined as where T p denotes one electric period

Department of Electrical Engineering Southern Taiwan University SIMULATION AND EXPERIMENTAL RESULTS 2016/6/14 Robot and Servo Drive Lab. 30 MATLAB Simulink was used for simulation, and a 6-channel PWM module in dsPIC33FJ32MC204 for experiments Parameters for Simulation and Experiment

Department of Electrical Engineering Southern Taiwan University SIMULATION AND EXPERIMENTAL RESULTS Comparing the simulated current, and torque ripples in three commutation controls. The load torque of 1Nm was applied when the motor was operating at 1900 r/min for all cases 2016/6/14 Robot and Servo Drive Lab. 31 a.Six-step commutation b.PA   ou =  ol =  /12 c.PAO   ou =  /12,  ol = 0 Fig. 10. Current and torque waveforms according to the commutation control ( Vdc=13 V)

Department of Electrical Engineering Southern Taiwan University d.PA   ou =  ol =  /4 e.PAO   ou =  /4,  ol = 0 Fig. 10. Current and torque waveforms according to the commutation control ( Vdc=13 V)

Department of Electrical Engineering Southern Taiwan University  A BLDC motor was built for automotive fan application experiments. The PWM frequency was set to 20kHz, the speed control loop operated once in a mode, and the sensorless position estimation was implemented with the back EMF-based method.  The six-step commutation method is adopted, i.e.,  ou =  ol = 0, at the start Fig. 11. Waveforms of the phase currents at start operation

Department of Electrical Engineering Southern Taiwan University The experimental waveforms of the pole voltages and phase currents with respect to the commutation control are shown in figure Fig 12. Pole voltage, and current waveforms according to the commutation control (Vdc=13 V,  r = 2500 r/min, Te = 1 Nm): (a) line-to-line back-EMF, (b) six-step commutation, (c) PA, and (d) PAO.

Department of Electrical Engineering Southern Taiwan University Fig 12. Pole voltage, and current waveforms according to the commutation control ( Vdc=13 V,  r = 2500 r/min, Te = 1 Nm): (d) PAO.

Department of Electrical Engineering Southern Taiwan University Fig 13. Measured waveforms of mechanical vibration (Vdc = 13 V,  r = 2500 r/min, Te =1 Nm): (a) six-step commutation, (b) PA, and (c) PAO

Department of Electrical Engineering Southern Taiwan University Fig 13. Measured waveforms of mechanical vibration (Vdc = 13 V,  r = 2500 r/min, Te =1 Nm): (c) PAO

Department of Electrical Engineering Southern Taiwan University Fig 14. Measured waveforms of mechanical vibration (Vdc = 13 V,  r = 2500 r/min, Te =1 Nm): a. six-step commutation, b. PA, c. PAO

Department of Electrical Engineering Southern Taiwan University They are calculated from the measured voltage and current waveforms. Note that  ou =  ol in PA, whereas  ol = 0 in PAO. The commutation torque ripple reduces maximally 15% for (  ou =  ol )  [0,  /6] with PA Over  /6 the commutation torque ripple becomes severe with PA and thus it is not suitable for this area. On the other hand PAO slightly increases the commutation torque ripple, maximally 13%, compared with six-step commutation. Moreover the PAO reveals the highest maximum speed available for the same  ou

Department of Electrical Engineering Southern Taiwan University Fig 15. The magnitude of commutation torque ripple (D= 0.95, Te=1.13 Nm, Vdc=13 V)

Department of Electrical Engineering Southern Taiwan University Fig 16. Maximum speed measured (D=0.95, Te=1.13 Nm, Vdc =13 V)

Department of Electrical Engineering Southern Taiwan University CONCLUSION A commutation control method aimed at reducing the commutation torque ripple for low- to high-speed operation in the low-voltage sensorless drive of the BLDC motor has been discussed. In order to minimize the torque ripple through the entire speed range, a comprehensive analysis of the commutation torque ripple was made depending on common three commutation control methods. An optimal current vector trajectory for the low torque ripple in the entire speed range was designed based on the analysis. The proposed sensorless drive method for the low torque ripple was implemented for automotive fan applications 2016/6/14 Robot and Servo Drive Lab. 42

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Department of Electrical Engineering Southern Taiwan University