Investigation on the Bipolar-Starting and Unipolar-Running Method to Drive a Brushless DC Motor at High Speed with Large Starting Torque PREM, Department of Mechanical Engineering Hanyang University, Korea Myung-Gyu Kim

Contents Motivation Objective & Methodology Driving methods of BLDC motor Torque-speed-current relationship of BLDC motor Torque nonlinearity of BLDC motor Bipolar-starting and unipolar-running method New Inverter Topology System Implementation Experiment Conclusion

Motivation The trend of brushless DC (BLDC) motor –High efficiency and good controllability over a wide range of speed  High speed applications of electromechanical systems. The characteristics of BLDC motor –Small starting torque and long transient period in order to run the motor at high speed  One of the drawbacks of a BLDC motor in high speed applications. The research of high-speed BLDC motor focus on the driving method that considers the starting torque.

Objective & Methodology Investigate (a) the method to drive a BLDC motor at high speed with large starting torque. (b) the new inverter topology. (b) the effectiveness of the new inverter topology and the bipolar-starting and unipolar-running method experimentally. Investigation procedure The winding pattern and the driving method of BLDC motor The DSP-based BLDC motor controller The torque-speed-current relationship of BLDC motor The effectiveness of the bipolar-starting and unipolar-running method The new inverter topology

Driving Methods of BLDC motor (1) Table 1. Commutation sequence of bipolar and unipolar drive Fig 1. Inverter circuits (a) bipolar drive (b) unipolar drive + V S A+B+C+ A-B-C- A C B (a) (b) +V S A+B+ A C B C+ A-B- C- -V S Bipolar drive  Not using the neutral point. Unipolar drive  Using the neutral point.

Driving Methods of BLDC motor (2) -The phase difference of 30 electrical degrees between the commutation sequences of bipolar and unipolar drive. Fig 2. Torque curves (a) bipolar drive (b) unipolar drive

Bipolar-starting and unipolar-running method (1) -The slope of eqn (4) is independent of the terminal voltage and speed. -The torque decreases linearly as the speed increase. -The slope of eqn (4) is independent of the terminal voltage and speed. -The torque decreases linearly as the speed increase.

Bipolar-starting and unipolar-running method (2) Difference between bipolar and unipolar drive Bipolar driveUnipolar drive Torque constant Resistant Torque-speed Relationship Starting torque No-load speed Design variables Table 2. Major design variables of a BLDC motor driven by bipolar and unipolar drive

Bipolar-starting and unipolar-running method (3) Bipolar-starting and unipolar-running method - Starting torque of bipolar drive - Maximum speed of unipolar drive - Suitable for driving method of high-speed BLDC motor Bipolar-starting and unipolar-running method - Starting torque of bipolar drive - Maximum speed of unipolar drive - Suitable for driving method of high-speed BLDC motor Occur the torque nonlinearity of BLDC motor in practice - At same terminal voltage Input current Reduction of torque constant Starting torque Bipolar drive < Unipolar drive Bipolar drive > Unipolar drive Magnetic effect of stator current Magnetic saturation due to large input current The reduction of torque constant due to large input current

New Inverter Topology (1) The basic inverter topology +V S A+B+C+ A-B-C- A C B SW1 SW2 -V S DC link Fig 3. Basic Inverter circuit for bipolar-starting and unipolar-running drive - Proposed by PREM - For switching from bipolar to unipolar drive, switch 1 : open  ground, switch 2 : ground  -12V - Problem  Need the additional input power

New Inverter Topology (2) The theoretical inverter topology +V S A+B+C+ A-B-C- A C B N+ N- Fig 4. Theoretical Inverter circuit for bipolar-starting and unipolar-running drive - Mentioned by SGS-Thomson Microelectronics, Western Digital Table 3. State of the theoretical inverter circuit

New Inverter Topology (3) The problem of the theoretical inverter topology Generated voltages due to the interaction of a rotating flux and a stationary coil This back emf would drive current around the freewheel diode path. The current would build up in an uncontrolled fashion. The current would contribute to losses and would produce negative torque. Back-emf Electrical angle, deg. 0°0°60°120°180°240°300°360° A+B-C-B+A-C+ ABC ①②③④ Fig 5. Ideal back-emf waveform

New Inverter Topology (4) - Proposed by PREM - Energized current path is similar with that of the theoretical inverter topology Use the additional sub-TR to control the current driven around the freewheeling diode path by back-EMF The new inverter topology +V S A+B+C+ A-B-C- A C B N+ N- AS+AS+BS+BS+CS+CS+ AS-AS-BS-BS-CS-CS- Fig 6. New Inverter circuit for bipolar-starting and unipolar-running drive Inverter-TR Freewheeling diode Additional sub-TR

New Inverter Topology (5) The state of new inverter circuit and freewheeling current Table 4. State of the new inverter circuit +V S A+ B+C+ A-B-C- A C B N+ N- AS+AS+BS+BS+CS+CS+ AS-AS-BS-BS-CS-CS- Fig 7. Freewheeling current of unipolar drive Commutation mode A+  B- · A+ diode : not used. · A- : Freewheeling diode. · Closed loop 형성.

System Implementation (1) -Drive circuit  Control the inverter-TR and additional sub-TR -DSP  All operating for driving the motor. -Motor Controller  Run the motor with bipolar or unipolar driving method and switch from one method to another at any speed -Drive circuit  Control the inverter-TR and additional sub-TR -DSP  All operating for driving the motor. -Motor Controller  Run the motor with bipolar or unipolar driving method and switch from one method to another at any speed Fig 8. Developed DSP-based BLDC motor controller Inverter Circuit +V S A+B+ C+ A-B- C- A B C DSP / Drive Circuit Speed feedback / Switching signal BLDC motor Digital I/O AS-AS-BS-BS- CS-CS- AS+AS+BS+BS+ CS+CS+ N+ N- PC

System Implementation (2) (a)(b) Fig 9. (a) New Inverter circuit (b) Driver circuit Freewheeling Diode Inverter Transistor Additional sub Transistor Part I : Switch signal for inverter Part II : Switch for neutral point Part III : Switch for sub-TR

System Implementation (3) +V S Neutral point DSP Input signal 2k 0V Photocoupler sub-TR (P-channel) Freewheeling diode Phase terminal voltage (off signal) on signal Gate signal Protect reverse voltage (a) Neutral point DSP Input signal +V S sub-TR (P-channel) Freewheeling diode Gate signal Phase terminal voltage (off signal) on signal Protect reverse voltage (b) Fig 10. Switch for the additional sub-TR (a) upper part (b) lower part -Variation of phase terminal voltage : -20 ~ 30V -Use the photocoupler for the behavior of additional sub-TR -Off signal : use the phase terminal voltage directly. -On signal : 12V, 0V -Rising time I) Inverter Transistor : ns II) Photocoupler : µs

Experiment (1) Motor Analyzer DSP Emulator PC Motor Torque meter Current probe Switching Oscilloscope Fig 11. Experimental setup to measure toque-speed-current characteristics -BLDC motor spec. used in hard disk drive. Y-winding, 8 poles, 12slots The rated operating speed of 5400rpm -Torque meter Load torque : 0.5mN·m

Experiment (2) Fig 12. Terminal and neutral voltage of bipolar drive Fig 13. Terminal voltage and current of bipolar drive -Use the new inverter circuit for bipolar drive -Traditional waveform of bipolar drive -Maximum speed : 6900rpm

Experiment (3) Fig 14. Terminal and neutral voltage of unipolar drive Fig 15. Terminal voltage and current of unipolar drive -Use the theoretical inverter circuit for unipolar drive. -The current would produce negative torque. -Maximum speed : 7000rpm Back-emf Current by back-emf

Experiment (4) Fig 16. Terminal and neutral voltage of unipolar drive Fig 17. Terminal voltage and current of unipolar drive -Use the new inverter circuit for unipolar drive. -No current which would be produce negative torque. -Maximum speed : 11500rpm Back-emf No current A B Current ripple

Experiment (5) Fig 18. Terminal voltage and gate signal of unipolar drive Fig 19. Terminal voltage and current of unipolar drive -Current ripple A. -Difference of rising time between inverter transistor and photocoupler. -Need for the freewheeling current.

Experiment (6) Fig 20. Terminal voltage and gate signal of unipolar drive Fig 21. Terminal voltage and current of unipolar drive -Current ripple B -Difference of rising time between inverter transistor and photocoupler -Also Appeared by the basic inverter circuit.

Experiment (7) Fig 22. Torque-speed curve of bipolar and unipolar drive -Nonlinear torque-speed relation. -Bipolar drive generates a large starting torque. -Unipolar drive runs the motor higher than the speed of a bipolar drive. -Switch the driving method at 1500rpm. -Starting torque :  14.98mN·m  15% Maximum speed : 6900  11500rpm  67%

Experiment (8) Fig 23. Speed variation of a BLDC motor with 1 disk -Bipolar drive speed up a little more rapidly than unipolar drive

Experiment (9) Fig 24. Speed variation of a BLDC motor with no disk -Switch time : 4000rpm. -Max. speed of the bipolar driving and bipolar-starting and unipolar-running driving methods : 11500rpm. -The same start motion between bipolar drive and bipolar-starting and unipolar running.

Experiment (10) (a) (b) (c) Fig 25. Variation of phase current of a BLDC motor (a) bipolar drive (b) unipolar drive (c) bipolar-starting and unipolar running drive -No load condition -Bipolar drive start up the motor with 1.3A 6900rpm with 0.15A -Unipolar drive start up the motor with 2.1A 11500rpm with 0.3A -Bipolar-starting and unipolar- running drive switched at 4000prm start up the motor with 1.3A 11500rpm with 0.3A

Experiment (11) Fig 12. Variation of phase A current from bipolar to unipolar drive Fig 13. Variation of phase C current from bipolar to unipolar drive 30° -The phase difference of 30 electrical degrees between the commutation sequences of bipolar and unipolar drive. -Switched from bipolar drive and unipolar drive smoothly.

Conclusion The bipolar-starting and unipolar running method of BLDC motor. –It runs the motor to high speed with large starting torque. –It reduce the rising time of the motor. –It can protects the inverter circuit by reducing large input current during start-up –The effectiveness of the this method is verified by experimentally. This method can be effectively applied to drive a BLDC motor under large load conditions to high speed.

Future work Performance degradation occurred by the current ripple of the new inverter circuit. –Speed, Torque, Efficiency, etc. –Comparison between the basic inverter circuit and new inverter circuit. Optimal switching time from a efficiency point of view. Another driving method to improve the starting torque of BLDC motor. –Tripolar driving method, 12-step driving method, etc.