MAE/ECE 5320 Brushless DC Motor(Actuator Slides) Daniel Aguirre Utah State University HW-1

Electric MOTOR:  An Electric motor takes a current of electricity and transfers that energy into rotational mechanical energy (Torque or speed.)  Generally turning a shaft.  The torque or speed that comes from the shaft is limitless in its possible uses.  pumps  vacuums  elevators  compactors  electric cars  RC cars  robotic manipulators  and many more (limited only by imagination) Ref. 1

DC Motor  The DC motor is believed to be one of the earliest forms of the electric motor (ref. 2, pg 486).  Still widely used for many applications because;  high torque  speed controllability over a wide range  portability (batteries tend to be DC)  well-behaved speed-torque characteristics,  easy and accurate modeling  adaptability to various types of control methods

DC Motor Diagram Ref. 3

Commutator and Brushes  In order to keep the torque and direction constant as the inner coil (armature coil) rotates and not reverse as it enters the other side of the magnetic field a device called a commutator reverses the current at that point maintaining the direction of the armature spin and torque.  Generally a split ring device that rotates with the armature.  In order to maintain voltage to the armature stationary conductors remain in contact with the rotating armature, called brushes.

Brushes  They are called brushes because initially the were made of highly conductive copper strands spanning the distance between the spinning commutator ring and the voltage supply. (arranged much like a brush)  Copper brushes are not common today, and have been replaced mostly by conductive blocks made of graphite.  maintain sliding contact with the commutator ring.  reduced sparks from arcing  cheaper  more durable  more contact area Ref. 3

Shortcomings of Brushes and Ring Commutator  Rapid wear out  though the carbon blocks last longer than the copper brush they still wear  brushes need replaced  commutator ring needs resurfacing  the faster the rotation the faster the wear  added mechanism to keep worn brushes moving closer to armature to maintain contact  spring loaded  overhaul and reposition of brushes  frequent adjustment  arcing because of inaccurate spacing  heat from friction  energy loss to overcome friction  noise  oxidation

Go brushless then. A brushless motor has the same basic principle as the brushed motor simply uses a different method of reversing the current at the other side of the magnetic field.  In a brushless motor the permanent magnet is the rotating armature and the current reversing happens in the stationary portion of the motor.  A switch reverses the current around the outer magnets to maintain the torque or speed direction.

Brushless Motor Diagram Voltage Supply Ref. 4

‘Permanent-magnet’ motors Both brushed and brushless DC motors have permanent magnets but the placement of this magnet is the fundamental difference between the two.  brushed permanent magnet is surrounding (stator or field circuit) the rotating portion (rotor or armature circuit)  brushless the permanent magnet is the rotating portion (rotor or armature circuit) and is surrounded by a switching current (stator or field circuit)

The Benefits of Brushless Brushless DC motors have several benefits over Brushed DC motors:  more torque per weight  increased efficiency  more torque per watt  absence of loss due to friction with brushes  longer lifetime  no brush degradation  no commutator ring erosion  no brush to armature arcing

‘Every Rose Has Its Thorn’ (Ref. 5) Shortcomings of Brushless The apparent main disadvantage of a brushless DC motor is the same disadvantage of most upgrades to anything: $$$COST$$$ The cost comes from the struggles that come with trying to control and manufacture brushless motor.  Drive Electronics are required to keep the torque and speed in the same direction as the magnetic field is passed thorugh.  Generally the stator coils are wound by hand, because the use of brushless motors is new enough limited manufacturers have generated machinery to do so though this is becoming less of a factor. Ref. 6

Motor Sizing  ‘Motor Sizing’ is the phrase used to mean the procedure of matching a motor to the desired specific application (speed, torque, input, output, etc…)  Generally speed and torque are the two primary considerations in choosing a motor for a particular application.  speed torque curves are generally available from a manufacturer  depending on required ‘trust’ in the motor, the same curves should be verified by experimentation of the user before ‘trust’ isgiven to the motor.  if human life, expensive machinery/product, reputation of employer/employee are reliant on motor great lengths should be used to prove motor before implementation.  if simple ‘widget’ the extensive testing might not be required but reliance on manufacturer information will be enough.

Motor Sizing Along with speed and torque many other motor characteristic values are of importance.  Mechanical Data  Electrical Data  General Data For each category there are many data points that might drive the decision of which motor to use for the specific application.

Sizing Procedure The required load for the motor can be given in many ways, generally as a load curve. A ‘region’ of the speed-torque curve that meets the requirements of the application. Ref. 2 pg 507

2 General Constructions of Brushless Motors (Ref. 9) 1. Inrunner  permanent magnets are part of the rotor  three or more (generally 3) stator windings surround the rotor (Ref. 7)

2 General Constructions of Brushless Motors (Ref. 9) 2. outrunner  permanent magnets spin within a rotor that surrounds the stator core  stator coils form the center or core of the motor (Ref. 8)

General Winding Configurations (Ref. 9) 1. Delta Configuration  Connects three windings together in series  winding represented by resistances a,b, and c  Triangle configuration shape much like a delta character  Power is applied at each of the connections(N 1, N 2, & N 3 )  low torque at low speed but higher top speeds

General Winding Configurations (Ref. 9) 1. Wye Configuration  Connects three windings together parallel  winding represented by resistances a,b, and c  split configuration shape much like a ‘Y’ character  Power is applied at each of the ends of the winding (N 1, N 2, & N 3 )  high torque at low speed and lower top speeds

References Mechatronics A Foundation Course, Clarence W. de Silva, CRC Press; Taylor & Francis Group ‘Every Rose Has Its Thorn’; Open up and Say…Ahh!! Poison, motors/inrunners/ motors/inrunners/ 8. motors/outrunners/ motors/outrunners/ 9.