Regenerative Braking of BLDC Motors

Transcription

1 Reenerative Brakin of BLC Motors By aniel Torres, Applications Enineer Patrick Heath, Marketin Manaer Hih-Performance Microcontroller ivision Microchip Technoloy Inc.

2 ifferent electrical brakin scheme Kinetic Enery OR ynamic Brakin Kinetic Enery Reenerative Brakin 2

3 Reenerative Brake in BLC motor Kinetic Enery BLC Hub Motor used in e-bike application While brakin, enery is stored in the battery Reenerative brakin stores enery back into the battery, while increasin the life of friction pads on brake shoe. However, to brin the bike to a complete stop, the mechanical brakes are required. 3

4 Confiuration of a 3-Phase Rectifier usin imulink Continuous powerui cope 1563 RPM 1/9.55 RPM to rad /sec HURT MOTOR m w A B C - v V_AB - v V_BC - v V_CA i - I_A i - I_B i - I_C The Hurst BLC motor runnin at 1563 RPM, enerates the EMF, which is rectified by a 3 phase diode confiuration and filtered to C to chare the battery. iode iode3 iode1 iode4 iode2 iode5 Filter I_C_BU i - V_C_BU - v 4

5 3-Phase Rectifier imulation Results Voltae constant of this motor = 7.24 Vpeak/ Krpm For a speed of 1563 RPM, the voltae = 11.3 Vpeak Filtered C Bus Voltae 5

6 MOFET Bride as a 3-Phase Rectifier (usin imulink) Continuous powerui cope 1563 RPM 1/9.55 RPM to rad /sec HURT MOTOR w A m B C - v V_AB - v V_BC - v V_CA i - I_A i - I_B Constant 3 Mosfet3 i - Constant 4 Mosfet4 Constant 5 Mosfet5 Filter V_C_BU - v I_C Constant Constant 1 Constant 2 Body iode of MOFET acts as rectifier Mosfet2 Mosfet Mosfet1 I_C_BU i - All MOFETs are turned OFF 6

7 3-Phase MOFET Bride Rectifier imulation Results Filtered C Bus Voltae The C bus voltae results for the 3-phase MOFET bride are similar to the rectified 3-phase diode circuit. 7

8 4-Quadrant Motor Operation Forward Brakin E I V peed E I V Forward Motorin E > V 2 1 V > E Reverse Motorin E V > E 3 4 V E E > V V Torque Reverse Brakin I I For a BLC motor to operate in 2nd quadrant, the value of the back EMF enerated by the BLC motor should be reater than the battery voltae (C bus voltae). This ensures that the direction of the current reverses, while the motor still runs in the forward direction. 8

9 Enery Flow Motorin (Current from Battery to Motor) Generatin (Brakin) (Current from Motor to Battery) For current to flow into battery, the bus voltae should be hiher than the battery terminal voltae. Hence we have to boost the voltae developed from motor hiher than the battery. 9

10 Limitation of a irect Connection ince this motor is rated for 24-Volts, the battery terminal voltae would be 24-Volts. To enerate 24-Volts from the motor (or hiher voltae), the motor should run at a speed of 3,4 RPM or hiher. Hence we have to fiure out ways to boost the back EMF enerated by the motor so that even at lower speeds, the motor can work as brake. 1

11 imple Boost Converter (usin imulink) Continuous powerui Boost _Converter The output voltae is proportional to the duty cycle of the MOFET. cope iode eries RLC Branch Filter Load V_C_BU - v V_INPUT v - C Voltae ource Pulse Generator Mosfet2 I_C_BU i - 11

12 imple Boost Converter imulation Results Input voltae = 12 volts C Boost voltae = 3 volts C Boost current = 2.5 Amps 12

13 Boost Converter Based on a 3-phase MOFET Bride BRAKE_MOEL _3 Continuous powerui cope 2 cope 2 RPM HURT MOTOR m 1/9.55 Gain w A B C - v V_AB - v V_BC - v V_CA i - I_A i - I_B RPM 1 Mosfet i - RPM2 Mosfet1 RPM 3 Mosfet2 Filter V_C_BU v - By varyin the duty cycle, the output voltae can be boosted to different manitude. Pulse Generator I_C cope 1 Mosfet3 Mosfet4 Mosfet5 I_C_BU i - 13

14 Boost Converter 3-phase MOFET Bride imulation Results C Output Voltae ~26 2RPM and 5% duty cycle 14

15 Boost Converter 3-phase MOFET Bride imulation Results C Output Voltae ~38 2RPM and 7% duty cycle 15

16 Picture of Test etup 16

17 Test Result of Boosted Voltae while runnin Motor at 25 RPM No Load voltae vs uty (Tested on Hurst Motor) Voltae (V) RPM 25 RPM 3 RPM uty (%) This slide shows the output voltae of the motor after boost, for different speed and duty cycles. The manitude of the voltae will increase proportional to the shaft speed. Another point evident from the plots is that the output voltae ets boosted proportionally to the duty cycle. 17

18 Test Versus imulation Results at 2 RPM For low value duty cycles, the boosted voltae is low. Hence no current flows into the battery. At around 3% duty cycle, the voltae beins to boost and the current flows into the battery. This is the point where reenerative brakin starts. Current (A) Current vs uty (Tested on Hurst Motor) eries1 The peak current from simulation is 7% duty cycle. This translates to 24V *.5 Amps = 12 Watts. ince brake force is proportional to the current, this is the point of maximum brake force. Current (A) uty (%) Current vs uty (imulation) Current (A) Beyond that point, the current starts to fall, mainly because of the motor construction (resistance and inductance drops) uty (%) 18

19 Efficiency imulation Results Efficiency vs uty (imulation) Efficiency (%) Efficiency (%) uty (%) This slide shows the efficiency curve of the brake setup, which ives a maximum efficiency of 55% at 5% duty cycle. ince this is a simulated result, the actual fiure of efficiency miht be lower. From the plot, it can be seen that the maximum efficiency point and maximum brake force points do not coincide: Max brake 7% duty cycle Max 5% duty cycle Hence, the brakin alorithm can be desined to operate at either maximum efficiency or at maximum brake force points. 19

20 PI Control for Constant Brake Force Analo voltae proportional to the required brake force The PI loop will try to maintain a constant brake force at different motor speeds. Hence the user will et a linear response of brake force. Wheel peed Required Brake force Current command Error PI uty Cycle uty Cycle limit for over voltae protection MOFET BLC Motor Current Feedback 2

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