05-07-2012, 10:20 AM
brush less permanent magnet dc motor
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INTRODUCTION
In a conventional (brushed) DC motor, the brushes make mechanical contact with a set of electrical contacts on the rotor (called the commutator), forming an electrical circuit between the DC electrical source and the armature coil-windings. As the armature rotates on axis, the stationary brushes come into contact with different sections of the rotating commutator.
The rotor assembly may be internal or external to the stator in a DC brushless motor. The combination of an inner permanent magnet rotor and outer windings offers the advantages of lower rotor inertia and more efficient heat dissipation than DC brush-type construction. The elimination of brushes reduces maintenance, increases life and reliability and reduces noise and EMI generation.
The possible number of phases and winding arrangements for the DC brushless stator are quite varied. As in the case of brush type DC motors, increasing the number of phases reduces torque. However, an important practical consideration for DC brushless motors is the number of electronic switches required to commutate the phases. The winding arrangement for a three phase motor may be either a Y or a ∆configuration. The most efficient operation of the motor requires current flow in more than one phase at any instant and current reversal in each of the phases at some point during 360 electrical degrees of rotation. Rotor position sensing is essential for proper commutation of DC brushless motors. Magnetic sensing with inexpensive Hall Effect switches is frequently adequate. The devises require little space and can easily be placed within the motor.
You understand AC motor operation, even a DC motor is relatively easy to figure out. However brushless motor operation is still “medieval science”. The following will lead you out of the dark ages and into today’s modem world – the use of brushless technology.
BRUSHLESS DC MOTORS
Conventional dc motors are highly efficient and they can effectively be used as servomotors. However, due to the presence of commutator and brushes it is subjected to mechanical wear n tear and hence requires maintenance. As time passed the functions of commutator and brushes were implemented by solid-state switches, thus maintenance-free motors were realized. These motors on further development gave birth to a special electrical machine known as brushless dc motors. Although the conventional dc motors and the brushless dc motors look seemingly similar in their static characteristics there exist remarkable differences in some aspects. A brushless DC motor (BLDC) is a synchronous electric motor electronically controlled commutation system. The unconventional presence of Hall Effect sensors makes it possible for us to adjust the rotor position by which the torque, speed and efficiency are kept track of.
This paper brings out the differences and similarities between the conventional...
Brushless motor rotation relies on the identical theory as for AC and DC motors. That is, two magnetic fields interact, which result in movement. In the case of AC motors, the stator winding sets up one magnetic field while inducing the second interacting field onto the squirrel cage rotor. With DC motors, the permanent magnet stator sets up the first magnetic field, and the rotor windings produce the second field.
These two magnetic fields interacting, results in rotation. In the DC motor, the two fields try to align. However the commutator continually switches power from winding to winding. Thus, maintaining the two magnetic fields at a 90 degree relationship. If they did indeed align, motor rotation would not occur. Compared to DC motors, brushless technology has been termed an “inside out” design. That is, the permanent magnets are on the rotor, and the stator consists of windings. The design still consists of two magnetic fields interacting.
COMPARISON OF CONVENTIONAL AND BRUSHLESS DC MOTORS
BLDC motors are considered to be more efficient than brushed DC motors. This means that for the same input power, a BLDC motor will convert more electrical power into mechanical power than a brushed motor, mostly due to the absence of friction of brushes. Under high mechanical loads, BLDC motors and high-quality brushed motors are comparable in efficiency. In a BLDC motor, the electromagnets do not move; instead, the permanent magnets rotate and the armature remains static. In a BLDC the brush-system / commutator assembly is replaced by an intelligent electronic controller. The controller performs the same power distribution found in a brushed DC motor, but using a solid-state circuit rather than a commutator/brush system.
BLDC motors offer several advantages over brushed DC motors this includes higher efficiency and reliability, reduced noise, longer lifetime (no brush erosion), elimination of ionizing sparks from the commutator, and overall reduction of electromagnetic interference (EMI). With no windings on the rotor, they are not subjected to centrifugal forces, and because the electromagnets are located around the perimeter, the electromagnets can be cooled by conduction to the motor casing, requiring no airflow inside the motor for cooling. This in turn means that the motor's internals can be entirely enclosed and protected from dirt or other foreign matter BLDC motors are considered to be more efficient than brushed DC motors. Hence a BLDC motor will convert more electrical power into mechanical power than a brushed motor, mostly due to the absence of friction of brushes