Novel Position Sensorless Starting Method of BLDC Motor for Reciprocating Compressor

Transcription

1 Novel Position Sensorless Starting Method of BLDC Motor for Reciprocating Compressor Dae-kyong Kim 1, Duck-shik Shin 1, Sang-Taek Lee 1,2, Hee-Jun Kim 2, Byung-Il Kwon 2, Byung-Taek Kim 3 and Kwang-Woon Lee 4 1 Digital Convergence R&D Center, Korea Electronics Technology Institute, Korea 2 Dept. of Electrical Engineering, Hanynag University, Korea 3 Dept. of Electrical Engineering, Kunsan National University, Korea 4 Div. of Maritime Electronics and Communication Engineering, Mokpo National Maritime University, Korea dkkim@keti.re.kr Abstract The magnitude of output torque in a BLDC (Brushless DC) motor depends on torque angle so that the exact initial position of rotor is essentially required for good starting performance. This paper describes starting performance for smooth starting in position sensorless BLDC motor drives for reciprocating compressor of refrigerator. In addition, position of rotor is determined from current response characteristics at align mode and level of the measured phase currents at the open-loop acceleration mode. The experimental results have suggested the effectiveness of starting method as starting load variations of reciprocating compressor. I. INTRDUCTIN During the last decades, energy saving has been one of the important issues in home appliances [1]. Compressor is the device which consumes most of electrical energy in airconditioner and refrigerator. Therefore, a lot of efforts have been made to improve the energy efficiency of the compressor. For these reasons, the BLDC motor is expanding its application in compressor markets. The rotor position sensing devices can not be used in the BLDC motor driven compressor because environment of high temperature and pressure inside the compressor can cause permanent damage. Thus, sensorless controlled BLDC motors have been used for driving compressors [1-11]. Among various sensorless control strategies for the BLDC motor, the sensorless control method based on the detection of zero crossing point (ZCP) of back-emf has been widely used for low cost applications because of its simplicity [7]. However, the ZCP of the back-emf can not be obtained when the BLDC motor is at standstill or operating at nearly zero speed. Therefore, a special starting control is required for smooth starting and reliable transfer to sensorless control. Therefore, a few researchers have studied to improve starting performance of sensorless BLDC motor drives for reciprocating compressor. In [8-9], the peak currents during the forced synchronous acceleration mode have been limited by doing commutation according to the level of the measured phase currents. In [10-11], the forced alignment success or fail has been obtained from the current response characteristic during the forced alignment process. This paper proposes novel forced alignment method that uses both peak current limitation method [8-9] and improved forced alignment method [10-11] during starting. It is shown that the effectiveness of starting method is validated by experiments using the load variation system of reciprocating compressor. II. SENSRLESS CNTRL F BLDC MTRS The back-emf s e a, e b and e c of BLDC motor have trapezoidal shapes and the currents i a, i b and i c needed to produce a steady state torque without torque pulsations are shown in figure 1. In a position sensorless BLDC motor drive, commutation points of the inverter can be obtained by knowing the zerocross-point (ZCP) of the back-emf and a speed dependent period of time delay [1]. The phase back-emf induced in the stator windings of a BLDC motor is trapezoidal so that the ZCP of the back-emf can be detected by monitoring the terminal voltage waveform of a silent phase. The instance when the terminal voltage of the silent phase match with the half DC link voltage, during which point switching devices are turned on, is the zero crossing point of the back-emf. Fig. 1. Current, back-emf and torque waveforms of BLDC motor /09/$ IEEE

2 (a) Fig. 3. Conventional start-up sequence of a BLDC motor driven reciprocating compressor (b) Fig. 4. Starting current waveforms of the conventional method when pressure difference between suction and discharge port of the compressor is unbalance (c) (d) Fig. 2. (a) configuration of a BLDC motor drive, (b) switching pattern, (c) terminal voltage sensing circuit and (d) terminal voltage waveforms The commutation points are estimated like this: T cmt T 1 = Tzcp + TZCP (1) 2 = T T ( k 1) ZCP ZCP ZCP (2) where, T cmt (k) is the commutation time and T ZCP (k) is the zero crossing time of the back-emf. Fig. 2(a) depicts the configuration of a position sensorless BLDC motor drive, fig. 2(b) the switching pattern of the outgoing phase unipolar PWM, fig. 2(c) the terminal voltage sensing circuit, and fig. 2(d) the motor terminal voltage waveforms when the out-going phase unipolar PWM is applied. The terminal voltage sensing circuit in fig. 2(c) compares the terminal voltage of each phase with the half DC link voltage. In fig. 2(c), capacitor C is used to reduce switching noises included in the terminal voltage. The sensorless control method based on terminal voltage sensing cannot be used during low speeds since the back-emf is zero at rest and proportional to the speed. III. CNVENTINAL STARTING METHD The reciprocating compressor usually starts its operation when a pressure difference between suction and discharge port is nearly zero. In order to restart the compressor after stop its operation, a temporary pause of starting is required until the pressure of the suction and the discharge port goes to an equilibrium state since the presence of the pressure difference between the suction and the discharge port acts as an excessive load to the motor for driving the compressor. The waiting time required for the balance of the pressure between the suction and the discharge port varies depending on surrounding conditions such as temperature and atmospheric pressure. Generally, the temporary waiting time for restarting is fixed to 5 minutes in a commercial refrigerator so that the compressor may start its operation before the pressure between the suction and the discharge port reaches a balanced state. In this case, a large starting torque, obtained by exact commutation, is required for smooth starting of the compressor. The conventional starting sequence of the BLDC motor driven reciprocating compressor for refrigerator is shown in figure 3. It aligns rotor to the specified position by energizing any two phases of the stator windings, and then it linearly accelerates the rotor to the speed where the stable detection of the ZCP of the back-emf is available. During the open loop acceleration mode, the stator windings are commutated regardless of the real rotor position. In addition, the conventional starting method of the BLDC motor driven reciprocating compressor is not known if forced alignment state is success. The load torque applied to the BLDC motor for driving the compressor depends on the pressure difference of the suction and the discharge port and the position of the piston. Figure 4 shows the current waveform of the conventional method when pressure difference between the suction and the discharge port of the compressor is unbalance. In this case, the conventional starting method could not start the reciprocating compressor and peak currents over 6 [A] were generated. Therefore, large instantaneous peak currents, which cause

3 Fig. 3. Conventional start-up sequence of a BLDC motor driven reciprocating compressor Fig. 5. Current waveforms as the relation of the rotor position and the commutation point (upper: proper commutation, middle: lagged commutation, lower: leaded commutation) Fig. 6. Relation of average current (TH) and over current value (TH_over) Fig. 4. Starting current waveforms of the conventional method when pressure difference between suction and discharge port of the compressor is unbalance serious demagnetization of the permanent magnets in the rotor and undesirable vibrations of the compressor, can be generated owing to improper commutation time. IV. PRPSED METHD The proposed method uses both peak current limitation method [8-9] and improved forced alignment method [10-11] during starting. A. Peak Current Limitation Method during Starting [8-9] In the conventional align and go starting method of the BLDC motor driven compressor, the BLDC motor is commutated according to the predetermined pattern regardless of the real rotor position during open loop acceleration mode. During the open loop acceleration mode, current waveforms are affected by commutation point. Figure 5 shows the relation between current waveform and commutation point when the voltage applied to the BLDC motor is constant. If the commutation point lags behind proper commutation point, the phase current before commutation goes up since the back-emf of the corresponding phase after exact commutation point is falling down. n the other hand, the phase current after commutation goes up in the case of leaded commutation. By using these phenomena, the large instantaneous peak current, caused by improper commutation point in the conventional starting method, during open loop acceleration mode can be reduced. When the acceleration speed is set to slightly slower than the mechanical response speed of the reciprocating compressor, the commutation points lag behind the correct commutation points. In this case, the phase current waveform is shown as figure 6. The average current value (TH) is obtained by averaging non-commutated phase currents after transient period of commutation. The over current value (TH_over) is set to the 110~120% of TH and δt is the time of measured TH_over. If the phase current goes over TH_over then it can be assumed that the correct commutation point is elapsed. The peak current limitation method commutates the BLDC motor during open loop acceleration mode depending on the level of the phase current. n the other hand, the conventional starting method commutates the BLDC motor according to the predetermined commutation period. Therefore, the peak current limitation method can considerably reduce the error of commutation point compared to the conventional starting method. B. Improved Forced Alignment Method During Starting [10-11] When rotor is made an attempt to align, back-emf is generated by moving rotor. However, when forced alignment is success state, back-emf is not generated if rotor is made an attempt to align again. It is known to grasp the forced alignment success or fail from the current response characteristic during the forced alignment process by using the physics. Figure 7 shows flowchart of the improved forced alignment method. The rotor is made an attempt to align from current control in conditions where bandwidth of current controller is

4 Try to align btain the maximum value of absolute value of current error during align No Emax < Th Yes Fig. 8. Experimental test-bed test Synchronous acceleration mode Transfer synchronous acceleration Mode to sensorless mode Sensorless operation Fig. 7.. Flowchart of the improved forced alignment method set to 2 [Hz]. In this time, the error is obtained from current response for current reference during the forced alignment and the maximum value (Emax) is obtained from absolute value of the error. If Emax is under the set level (Th), it can be assumed that the forced alignment is not success state. And then the forced alignment is retriedd until success state. V. EPERIMENTAL RESULTS Figure 8 shows the experimental test-bed bed for the simplified refrigeration system that consists of BLDC reciprocating compressor taking a SPM-type BLDC motor with ferrite-type magnet. In this experiment, a low-cost fixed-point fixed digital signal processor (DSP), TMS320LF2406A, implements the proposed starting method. Fig. 9.. Starting current waveform of the proposed method at 3.0 kgf/cm2 /cm2 pressure difference between suction and discharge port of the compressor [2A/div., 0.5sec/div.] Figure 9 shows starting current waveforms of the proposed method at 3.0 kgf/cm2 pressure difference between suction and discharge port of the compressor. (a) is the forced for alignment mode, (b) is the synchronous acceleration mode and (c) is the sensorless control mode, respectively. VI. CNCLUSIN This paper proposes a Comparison of Starting Method for Position Sensorless BLDC Motor Driven Reciprocating Compressor.. The experimental results resu show that the proposed method is the best solution for the good starting performance of reciprocating compressor. Table I shows starting success or fail at 3.0 to 12.0 kgf/cm2 pressure difference between suction and discharge port of the compressor. ACKNWLEDGMENT It is noted that the improved forced alignment method remarkably has the good starting performance of reciprocating compressor at 12.0 kgf/cm2 pressure difference between suction and nd discharge port of the compressor. This paper has been supported by KITECH (Development of Application System Technology echnology for Eco-friendly Eco Smart Home Appliances). Table I. Starting success or fail Pressure difference between suction and discharge port of the compressor [kgf/cm^2] Pressure Conventional method Peak current limitation method Improved forced alignment method Proposed method

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