New Trends in Brushless DC Motor Drives

Brushless DC (BLDC) motors have experienced a rapid adoption in the marketplace and over a broad range of motion control applications due to the distinct benefits they have over traditional brushed DC motors: less maintenance, higher operating speeds, compactness, less electrical noise, better torque-to-weight ratios, to name a few.1 Despite these benefits, BLDC motors have cost more than traditional DC motors since they require a motor drive controller (for electronic commutation) plus a rotor position sensor.2 But new trends in the design of BLDC motor drives are overcoming not only the cost issue, but also are providing BLDC motor drives performance capabilities not possible with traditional DC motors. Implementing sensorless motor drives that do not require a rotor position feedback sensor is one of the newest trends in BLDC motor drive design that can obtain cost savings. Changes in the PWM switching strategy within the motor drive promises to eliminate the problems associated with torque ripple. Another major trend in BLDC motor drive technology is integrating the BLDC motor and drive electronics into a single package to simplify the system, minimize interconnection cabling, reduce noise, and solve motor-drive compatibility issues.3

Sensorless Control

While primarily considered a cost savings benefit, sensorless control technology also can improve system reliability, reduce the number of electrical connections, eliminate problems with mechanical alignment, and reduce the motor’s size and weight. In general, the definition of sensorless control is “the operation of the BLDC motor without its usually required rotor position sensor.”4 Eliminating the rotor position sensor (e.g., optical encoder, Hall Effect sensor, resolver, cabling and decoding circuitry) adds up to decreased manufacturing costs with improved reliability and durability.5

Sensored, BLDC motor drives use a 3-phase PWM inverter with a rotor position sensor to “perform phase commutation and/or current control.”6 However, there are different ways of obtaining rotor position information. In sensorless control technology, “rotor position information is determined by indirectly sensing the back EMF (electromotive force) from one of the three motor terminal voltages.”7 Since only two of the three BLDC motor phase windings are conducting at a time, the third non-conducting phase carries the back

EMF where rotor position and velocity can be indirectly calculated.8 At the present time, sensorless technology does not have widespread adoption; however, in the future, it is expected to be the primary BLDC motor control methodology.

Torque Ripple Reductions

Despite the many advantages BLDC motors, they possess a limiting factor: the tendency to exhibit torque pulsations.9 These pulsations can cause acoustical noise and vibration, and can “severely limit the performance of the system, specifically in high precision and high stabilization applications.”10 In high speed applications, torque pulsations can be filtered out by the inertia of the load. However, at low speeds, when they are most noticeable, torque pulsations can greatly limit performance.11 Torque ripple is caused by both the BLDC motor and PWM drive controller design and include “geometric imperfections in the motor, imprecise commutation, fidelity of current driving waveforms, phase delays, friction, and the magnetic hysteresis in the motors. They may be reduced through better motor design or the use of better drive controllers.”12

Torque ripple is divided into two general categories: cogging torque and commutation torque. “Cogging torque is produced by reluctance variation due to the stator slot openings as the rotor rotates. Cogging torque can be reduced by changes in motor design such as skewing of the stator slots, choosing a fractional slots/pole motor design, or choosing a magnet width relative to the slot pitch.”13 Commutation torque ripple is caused by the drive’s PWM inverter and is due to electrical current hysteresis or the inverter generating high frequency current ripple. During commutation, “as one phase turns off, another turns on, [so] the rise and fall rates of the respective phase currents are not equivalent and thus the torque generated by the two currents during commutation does not instantaneously add to the value of torque of one fully excited phase, which would allow a smooth torque over the commutation interval.”14

To minimize commutation torque ripple, improvements in BLDC drives are required. There are several methods used in motor drive designs to minimize commutation torque ripple. One way is by adding inductor-capacitor (L-C) filters to “reduce the high frequency component of the inverter’s output to the motor. 15 But a key factor in reducing torque ripple is eliminating torque harmonics by “adjusting the conducting phase of the windings so they compensate appropriately.”16 There are four other methods being introduced to achieve torque ripple reductions: (1) using direct torque control, (2) dynamically changing the input voltage, (3) adding torque estimation circuitry, and (4) employing artificial neural networks and active disturbance rejection control.17

Integrated BLDC Motor and Drive

The integrated BLDC motor and drive is one of the newest trends and promises cost reductions, reliability, and compactness. In the future, integrated BLDC motor-drives are expected to be the marketplace standard. In 2010, integrated BLDC motors grew by 47.7% and are quickly replacing AC servo motors. This change is the result of an “increasing number of brushless DC integrated motors [being] shipped with advanced feedback. In addition, an 11.6% compound annual growth rate (CAGR) is expected for integrated BLDC motors with position control from 2009 to 2015.”18 The integration of the motor and drive electronics is the result of two new developments: (1) the efficiency of electronic components is increasing which is causing the size of power electronics to reduce19 and (2) the rare earth permanent magnets of BLDC motor eliminate a heat source at the rotor so “the internal temperature rise is smaller than that of a traditional DC motors, which allows the inverter control to be installed into the motor.”20 At the present time, integrated BLDC motor and drives are in the 100 watt range.21 But as the market and size range of integrated BLDC motors increase, they will create simpler motor-drive installations, and eliminate the need for inverter control rooms, ventilation equipment and cabling between the motor and inverters.22

Digital Signal Processors

Most of the new trends for BLDC motor drives cited in this article require a high performance controller with a high speed microprocessor and a high density programmable logical controller (PLC) technology to realize them.23 Called “digital signal processors” (DSPs), these high performance controllers are rapidly being adopted in the marketplace, which has propelled their steady fall in price. Even low cost DSPs can “execute sophisticated algorithms to improve motor performance in areas such as noise control, variable speed, energy efficiency.”24 In the past, basic digital controllers with an 8-bit microcontroller had an adequate amount of bandwidth for basic speed control. “But, as the complexity of the algorithms in motor control [have] increase[ed], the need for higher performance and more programmable solutions also has increased. DSPs provide much of the bandwidth and programmability required for such applications.”25 Specifically, the bandwidth usage of a low cost DSP realizes the following features:

  • Proportional–Integral–Derivative (PID) control for greater precision
  • Sensorless algorithms to eliminate the expensive speed and current sensors
  • Random pulse width modulations (PWMs) to reduce noise and input filter sizes
  • Ripple compensation algorithms to reduce the drive’s DC link capacitor size
  • Power factor correction (PFC) to eliminate a dedicated PFC controller

Perhaps the greatest long-term benefits of DSPs are the standardization of the drive’s interface, the complete digitalization of the drive system, and an easier means for “data transmission with the upper level and remote control systems which facilitates the monitoring and diagnosis of system failures.”26

  1. J. David Irwin. The Industrial Electronics Handbook. CRC Press, 1997. Page 752.
  2. Hamid A. Toliyat and Gerald B. Kliman. Handbook of Electric Motors. CRC Press, 2004. Page 763.
  3. Chang-liang Xia. Permanent Magnet Brushless DC Motor Drives and Controls. John Wiley & Sons, 2012. Page 15.
  4. J. Ali Emadi. Handbook of Automotive Power Electronics and Motor Drives. CRC Press, 2005. Page 535.
  5. J. Ali Emadi. Handbook of Automotive Power Electronics and Motor Drives. CRC Press, 2005. Page 535.
  6. Gui-Jia Su and John W. McKeever. Low-Cost Sensorless Control of Brushless dc Motors with Improved Speed Range. Oak Ridge National Laboratory, National Transportation Research Center, 2001. Page 1.
  7. S.Joshuwa, E.Sathishkumar and S.Ramsankar. Advanced Rotor Position Detection Technique for Sensorless BLDC Motor Control. International Journal of Soft Computing and Engineering (IJSCE) ISSN: 2231-2307, Volume-2, Issue-1, March 2012. Page 1.
  8. Paul Kettle, Aengus Murray & Finbarr Moynihan. Sensorless Control of a Brushless DC motor using an Extended Kalman estimator, Analog Devices, Motion Control Group, PCIM’98 Intelligent Motion. May 1998 Proceedings. Page 385.
  9. Timothy L. Skvarenina. The Power Electronics Handbook. CRC Press, 2002. Page 10-11.
  10. Chang-liang Xia. Permanent Magnet Brushless DC Motor Drives and Controls. John Wiley & Sons, 2012. Page 5.0.
  11. Timothy L. Skvarenina. The Power Electronics Handbook. CRC Press, 2002. Page 10-11.
  12. Farshad Khorrami, Prashanth Krishnamurthy, and Hemant Melkote. Modeling and Adaptive Nonlinear Control of Electric Motors. Springer, 2003. Page 383.
  13. Timothy L. Skvarenina. The Power Electronics Handbook. CRC Press, 2002. Page 10-11.
  14. Ali Emadi. Handbook of Automotive Power Electronics and Motor Drives. CRC Press, 2005. Page 524.
  15. Vinu V. Das, Janahallal Stephen, and Nessy Thankachan. Power Electronics and Instrumentation Engineering:

    International Conference, PEIE 2010. Springer, 2011. Page 53.

  16. Chang-liang Xia. Permanent Magnet Brushless DC Motor Drives and Controls. John Wiley & Sons, 2012. Page 11.
  17. Chang-liang Xia. Permanent Magnet Brushless DC Motor Drives and Controls. John Wiley & Sons, 2012. Page 11.
  18. Jenelea Howell. Integrated Brushless DC Motors Becoming Major Competitors for AC Servo Integrated Motors. IMS Research Analyst Blog, 2012.
  19. Peter Vas. Artificial-Intelligence-based Electrical Machines and Drives. Oxford University Press, 1999. Page 14.
  20. Chang-liang Xia. Permanent Magnet Brushless DC Motor Drives and Controls. John Wiley & Sons, 2012. Page 15.
  21. Austin Hughes. Electric Motors and Drives: Fundamentals, Types, and Applications. Newnes, 2006. Page 357.
  22. Peter Vas. Artificial-Intelligence-based Electrical Machines and Drives. Oxford University Press, 1999. Page 14.
  23. Chang-liang Xia. Permanent Magnet Brushless DC Motor Drives and Controls. John Wiley & Sons, 2012. Page 15.
  24. F. Acar Savaci. Editor. Artificial Intelligence and Neural Networks. Springer, 2006. Page 108.
  25. Robert Oshana. DSP Software Development Techniques for Embedded and Real-Time Systems. Newnes, 2006. Page 9.
  26. Chang-liang Xia. Permanent Magnet Brushless DC Motor Drives and Controls. John Wiley & Sons, 2012. Page 15.
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