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 control, which is supplying power and running the motor.

Electronic Commutation Principles

To illustrate and explain electronic commutation, a brushless motor consisting of a rotor with a small feedback device will be used. The feedback device consists of a small magnet (located on the rotor shaft) and three Hall sensors (mounted on the stator). Edward Hall in 1879 discovered that a sensor would alter its output when subjected to a passing magnetic field.

Thus as the rotor shaft turns, the small magnet, with north and south poles, passes by and causes the three Hall devices to turn on and off. This provides information about the rotor position. Of course, other feedback devices may be used to detect shaft/rotor position, however this was chosen for simplification. This rotor position information is fed into the logic circuitry of the control.

The control's logic circuitry uses this information to turn "on" specific power devices applying power to the motor windings thus maintaining the magnetic field at the optimum angle for rotation.

Torque Development

A brushless motor's back-emf displays only the positive portion of the waveform. When power is applied to the first winding, the shaft will normally rotate to the 180 electrical degree position and stop. However, if power is removed from the first winding "R" and applied to the second winding "S" when at the 120 electrical degree position, the shaft will continue to rotate. This process of electronic commutation is repeated (power is applied next to the third winding "T" at 180 degrees) and rotation continues.

Note that each winding is conducting for 60 degrees, thus this would yield an effective or average back-emf (Ke) of:

AC and DC Drive Schemes for Brushless Motors in text equation

About the Author:
John Mazurkiewicz' experience began in research and development of servo control systems, and then progressed into application engineering, technical marketing and new product development. John has held key positions, the most recent as Product Marketing Manager for Baldor Electric (Ft Smith, AR) in which he was responsible for introducing numerous new products. He has recently retired from Baldor.

AC and DC Drive Schemes for Brushless Motors chart 1

AC and DC Drive Schemes for Brushless Motors chart 2