AC and DC Drive Schemes for Brushless Motors

Because they deliver an optimized torque per package size, brushless motors are widely used in many applications. As such, they are smaller in size and lighter in weight; they provide higher peak overload capacity with wider speed range capability; and provide long, reliable and maintenance-free life. If time up to speed is important in the application, brushless is eight to 10 times faster. Additional benefits can be achieved when powering a brushless motor with an ac drive scheme.

Basic Motor Operation

Motor rotation is based upon the fact that if a conductor carrying current is placed in a magnetic field, a force will act upon it. The simplest machine is the induction motor. It consists of windings on a peripheral housing, (stator assembly) and an internal assembly (rotor). The motor operates when power is applied onto the stator winding assembly resulting in current flow that sets up the first magnetic field. This in turn induces current in the rotor setting up the second field. It is the interaction of these two magnet fields which results in rotation.

The induction motor is basically a constant speed motor, with speed being dependent upon the frequency of the applied voltage. In the early days, in order to adjust an application's operating speed, various pulley diameters were initially employed.

Today microprocessors use various methods to manipulate the magnetic field to adjust speed. However, back in the early days, it took the development of the dc motor to allow speed to be adjusted quite easily - simply change the applied voltage and motor speed would vary. In the dc motor design, permanent magnets are used on the stator to set up the first magnetic field.

The rotor, which sets up the second field, consists of several windings and a commutator. Each winding consists of turns of wire set between steel laminations to concentrate the magnetic field. When power is applied onto the rotor, current passes through the windings, thus setting up the second field. As the two fields interact, resulting in rotation, they will normally align and rotation ceases.

However, the mechanical commutator switches power from winding to winding, thus maintaining the rotor magnetic field at the optimum angle with respect to the permanent magnet field to obtain maximum torque and efficiency from the motor.

In a brushless motor, the design layout incorporates the most beneficial attributes of each design to provide the best of both: the long life of the induction motor and smaller size of the dc motor.

In a brushless motor, the layout or design includes electrical windings on the outside stator housing, similar to the induction motor (however only three windings); and like the dc motor, the brushless design includes permanent magnets mounted on the rotor.

This design has intentionally eliminated the mechanical commutator; and since commutation in a permanent magnet design is the action of switching current from winding to winding for rotation to occur, the obvious question is how rotation in a brushless motor occurs.

The answer is via "electronic commutation." The motor plays no role, commutation is accomplished by the

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