DC Motors in Mobile HVAC Applications

Permanent magnet DC motors (PMDC) are widely specified for motion control functions in mobile HVAC applications 1 such as fans, blowers, pumps, condensers and compressors that are used in RVs, mobile homes, buses, trucks, and passenger cars, among others. Energy efficiency, reliability, compact size and cost effectiveness are the primary benefits of these motors. There are two general categories of PMDC motors: brushed and brushless. Brushed PMDC motors 2 exhibit better energy efficiency than wound-field (electromagnetically excited) motors, higher power density and better dynamic performance in a simplified, compact construction. 3 Despite their small market share, brushless DC motors (BLDC) 4 (also known as ECPMs or electronically commutated permanent magnet motors) are gaining interest in mobile HVAC applications because they “offer variable-speed capability at negligible additional cost while achieving benefits of improved efficiency and reliability.” 5

DC Permanent Magnet DC Motors: Brushed Type

Commonly used for vehicle air conditioning blowers, brushed PMDC motors are a popular fractional horsepower drive with a high efficiency, a wide range of variations and inexpensive manufacturing costs. 6 They are usually the best type of motor if reliable and consistent performance with high startup torque, good motor speed properties, compactness and safety are a concern. Since energy efficiency is critical for battery-powered applications and motors directly affect electrical power consumption, optimizing the energy efficiency of these motors is critical for the normal use of mobile HVAC appliances. To this end, PMDC motors are designed with “low loss steel laminations, high temperature polyester varnish impregnated armatures, long life sealed ball bearings with high temperature grease, and precision dynamic balancing for excellent heat dissipation” 7 to generate efficient performance. Historically, the drawback of these motors has been the wear associated with their brushes and mechanical commutator; however, improvements in both motor and brush design have expanded brush life. 8 In addition, the maintenance associated with brush wear has been simplified with externally accessed brushes as well as brush wear indicators. Yet, despite the simplicity of brush-type PMDC motors, brushless DC (BLDC) motors eliminate brushes and their maintenance or replacement by using electronic commutation. 9

Permanent Magnet DC Motors: Brushless Type

Used in mobile, battery-powered HVAC equipment such as fans, pumps, blowers and compressors, BLDC motors exhibit attractive characteristics such as virtually maintenance-free, quiet operation, variable speed capability, high operating speeds, superior energy efficiency, compact size and fast response. 10 They are constructed with a permanent magnet rotor and phase windings (armature) wound on a slotted, stator core. BLDC motors do not have a mechanical commutator or brush assembly since the motor is electronically commutated with an electronic controller using sensored or sensorless control. 11 Sensored control uses a Hall Effect12 position-sensing, feedback device. Sensored control requires more wiring to install making this type more costly. Sensorless control uses the back-EMF of the motor to estimate the position of the rotor. 13 Sensorless control is frequently used in variable speed applications in refrigeration and air conditioning (fans, pumps and compressors). One of the major advantages of BLDC motors is their variable speed capability; they can be operated efficiently at partial loads which “avoids cycling losses caused by on/off operation and throttling losses generated by flow throttling valves or dampers. Since a BLDC motor is an “inherently a variable speed motor, [it] can reduce seasonal energy consumption in most blower applications by at least 30%, relative to a single speed induction motor.” 14

  1. Frank Kreith, D. Yogi Goswami, and Bela I. Sandor. The CRC Handbook of Mechanical Engineering, Second Edition. CRC Press, 2004. Page 9-2.
  2. Ohio Electric Motors. Permanent Magnet DC Motors. Ohio Elecric Motors, 2011.
  3. Jacek F. Gieras, Mitchell Wing, editors. Permanent Magnet Motor Technology: Design and Applications, Second Edition. CRC Press, 2002. Page 1.
  4. Ohio Electric Motors. Brushless DC Motors: Low Maintenance and High Efficiency. Ohio Elecric Motors, 2011.
  5. Kurt W. Roth, Ph.D., Allan Chertok, P.E., John Dieckmann, P.E. and James Brodrick, Ph.D. Electronically Commutated Permanent Magnet Motors. ASHRAE Journal, March 2004. Page 75.
  6. Hans-Dieter Stoelting, Eberhard Kallenbach, Wolfgang Amrhein, editors. Handbook of Fractional-Horsepower Drives. Springer, 2008. Page 14.
  7. Ohio Electric Motors. Permanent Magnet DC Motors. Ohio Elecric Motors, 2011.
  8. Ohio Electric Motors. Permanent Magnet DC Motors. Robert J. Hamilton. Brush Life in DC Motors. PlantServices.com, 2012.
  9. Clarence W. de Silva. Mechatronics: An Integrated Approach. CRC Press, 2004. Page 768.
  10. Ohio Electric Motors. Brushless DC Motors: Low Maintenance and High Efficiency. Ohio Elecric Motors, 2011.
  11. Musa Jouaneh. Fundamentals of Mechatronics, Si Edition. Cengage Learning, 2012. Page 269.
  12. Edward Ramsden. Hall-Effect Sensors: Theory and Applications. Newnes, 2006. Page 183
  13. Chang-liang Xia. Permanent Magnet Brushless DC Motor Drives and Controls. John Wiley & Sons, 2012. Page 168.
  14. Kurt W. Roth, Ph.D., Allan Chertok, P.E., John Dieckmann, P.E. and James Brodrick, Ph.D. Electronically Commutated Permanent Magnet Motors. ASHRAE Journal, March 2004. Page 75.
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