1. UNIT 4 BRUSHLESS DC MOTOR

2. Outline – Reference list – To probe further – Major applications – Basic working principle illustrated – A typical sample configuration in application (application notes) – Major specifications – Limitations – And many more relevant issues in applications (such as, how to choose, cost information, where to buy etc.)

3. Major applications In theory, Brushless DC Motors can potentially be used in any type of application where Brushed DC Motors are currently employed. Certain limitations (like cost primarily), however prevent them from replacing brushed motors in common areas of use. Areas in which Brushless DC Motors currently dominate include specialized applications where higher torque, longer product life or even more precision is required, such as computer hard drives, CD/DVD players and PC cooling fans, mechanisms within office products like laser printers and photocopiers as well as industrial automation equipment.

4. It should be noted that because of their ease of operation, when torque / high precision is required (such as servomotor construction), Brushless DC Motors are the growing choice when designing this type of applications. And since the ratio of torque delivered to the size of the motor is higher than in other motor types, the Brushless DC Motor is very useful in applications where space and weight are critical factors.

5. Major applications Here are some detailed example of DC Brushless Motor applications: Industrial automation Fans (i.e. PC & Ceiling) Electrical Power Steering Electric vehicle traction drive Office automation equipment Servo drives HVAC systems (air handling equipment) Refrigerator Washing machines Blowers

6. Basic working principle illustrated To understand how Brushless DC Motors operate, first it is necessary to understand the working principle of conventional (brushed DC Motors). In these motors a voltage applied to the rotor (rotating part of the motor), generates a magnetic field that opposes that of the stator (stationary part of the motor), inducing the movement. In other words, the magnetic field induced by the polarity of the applied voltage causes the rotor to realign itself in respect to the stator magnetic field.

7. Basic working principle illustrated Brushes make mechanical contact with the commutator, and each turn assures the required change of polarity. A circuit is then created between the power supply and the contacts on the rotor, by means of the stator coil windings. As each side of the commutator (shaped as a two sided ring) turns, the brushes change the applied polarity, inverting the magnetic field. This is due to the fact that when the coil is perpendicular to the magnetic field, the current must be reversed in direction for the torque to remain in the same direction.

8. Basic working principle illustrated The brushless motor system consists of a wound stator, a permanent magnet rotor, a rotor position sensor, and a solid state switching assembly. (2) On the Brushless DC motors, the commutation is controlled electronically. For rotation to be achieved, the stator windings have to be energized in a sequence. The must common type of Brushless DC motors includes 3 stator windings. Every sequence has one of the windings energized to positive, the second to negative and the third is a non-energized condition. Images: References 1 & 2

9. Basic working principle illustrated In place of a stator with brushes and a rotating ring, the effect is achieved as with every sequence two windings are energized (through multi-phase Inverter controllers ), and the a rotation occurs (except in this case, the windings are energized: there are no brushes). Torque is then generated from the interaction between the magnetic field (from the stator coils, according to the current input) and the permanent magnets. The peak torque occurs when two fields are at 90° to each other and falls off as the fields move together. To keep the motor running, the magnetic field should shift along side the rotor, hence the sequencing. From everything stated, it can be inferred that the Brushless DC Motor isn’t really a direct current device, but rather actually a a multi-phase AC motor with position feedback that can be control like a DC motor.

10. Basic working principle illustrated The adjacent image gives the schematic representation of a two- phase brushless motor (See Reference 2). The torque output of phase A is: T A = I A (ZΦ/2 π) sin(p θ/2) = I A K T sin (p θ/2) Where: I A = current in phase A; K T = (ZΦ/2 π) = torque constant motor; p = number of poles; θ = angular position of the rotor. T B = I B K T cos (p θ/2) The total torque is: T = T A + T b = = I B K T [(sin 2 (p θ/2) + cos 2 (p θ/2)]

11. Basic working principle illustrated An animation of a two pole, 3 phase Brushless DC Motor is provided from the Sensor Magnetics webpage. From this animation, the entire described process can be viewed in real time, as well as step by step.

12. A typical sample configuration in application Brushless DC motors are normally driven using a three- phase bridge. In this circuit example, hall- effect sensors are used for position feedback. A commutation pattern is used to select which of the six transistors are used.

13. A typical sample configuration in application (Additional) Brushless DC Motor Control (Multi-Phase Inverter) – Freescale Semiconductors Industrial Application Voltage Motor PowerSpeed Range 100 - 240 Vac 50 -2,200 W0 - 20,000 RPM Automotive Application Voltage Motor PowerSpeed Range 12/42 V Several Watts to 1kW0 - 15,000 RPM

14. CD-ROM Motors A typical sample configuration in application (additional) This webpage features some hobbyists tinkering with CD- Rom Brushless DC Motors

15. Major specifications Brushless DC Motors are built in a fashion very similar to stepper motors. Commonly, Brushless motors use a permanent magnet rotor (external), three phases of driving coils, one or more Hall effect devices to sense the position of the rotor, and a control electronics subsystem. The Hall effect sensors (used in commutation), can provide a very convenient tachometer signal for closed-loop control applications (basically servocontrol). Since Brushless DC motors lack a proper physical commutator (or brushes for that matter), they can be used in in electrically sensitive devices like audio equipment or computers (turns out that mechanical commutation tends to cause a great deal of electrical and RF noise).

16. Major specifications Brushless DC motors can be easily synchronized to internal or external clocks, leading to a more precise speed control. Brushless DC motors are considered more efficient than Brushed DC motors: for the same input power, Brushless motors convert more electrical power into mechanical power than their Brushed counterparts (especially in the no-load and low-load region of the motor's performance curve). Under high mechanical loads, though, they are comparable in efficiency. Other advantages Brushless DC motors have over brushed DC motors and induction motors, include: higher reliability, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI). (See the comparison charts at the end of this file).

17. Major specifications Summary of some of the major specs of Brushless DC motors: They offer high efficiency Provide full torque at zero speed. Better performance at high speeds Like DC motors turned inside out, commutation done on windings Medium construction complexity, multiple fields, delicate magnets High reliability (no brush wear), even at very high achievable speeds Driven by multi-phase Inverter controllers Sensorless speed control possible Higher total system cost than for DC motors Low EMI

18. Limitations The first and most important limitation comes from the higher cost associated with Brushless DC motors, due to the high-power MOSFET devices used in the fabrication of the electronic speed controller. (Whereas Brushed DC motors can be regulated by a potentiometer or a rheostat, relatively inefficient but cost- sensitive). The cost also substantially increases due to the manufacturing techniques employed in the construction of Brushless DC motor, since most designs require manual-labor, to hand-wind the stator coils. (brushed motors use armature coils which can be machine- wound). Another limitation is related to torque ripple, which requires the motor to be controlled with advanced techniques (i.e. a more complex controller).

19. Selection Selecting the right type of motor for the given application is very important. Based on the load characteristics, the motor must be selected with the proper rating. (See Reference 1). Three physical parameters determine the motor selection for the given application. They are: – Peak torque required for the application: can be calculated by summing the load torque (TL), torque due to inertia (TJ) and the torque required to overcome the friction (TF). A 20% safety margin is recommended. TP = (TL + TJ + TF) * 1.2 Where: TL = Load torque TJ = Inertial Torque (TJ = JL + M * α ) TF = Frictional torque JL + M is the sum of the load and rotor inertia and α is the required acceleration

20. Selection – RMS torque required: can be roughly translated to the average continuous torque required for the application and is given by: TRMS = √ [{TP2 TA + (TL + TF) 2 TR + (TJ – TL – TF)2 TD}/(TA + TR + TD)] Where: TP = Peak torque TL = Load torque TJ = Inertial Torque TF = Frictional torque TA = Acceleration time TR = Run time TD = Deceleration time

21. Selection – The operating speed range: motor speed required to drive the application and is determined by the type of application. The higher operating speed can be accounted for the components of the trapezoidal speed curve, resulting in an average speed equal to the movement speed. Image: See Reference 1

22. Selection Additional parameters may include indirect ones, related to the actual performance of the Motor. These include: – Cost / Overhead – Noise (Brushless Motors -> lower noise). – Size (Brushless are smaller) – Product Lifespan (No brush maintenance) – Temperature (No SCRs, higher temperature treshold than brushed motors)

23. Comparison (Reference 1)