Brushless DC Motors: Low Maintenance and High Efficiency

The need for reliable electrical contact (i.e., no brush sparking), no electrical interference (common to A.C. induction motors) and a long working life ¹ spawned the aerospace industry to develop the first brushless D.C. motors in the 1960s ². While they have not totally replaced the brushed type, the advantages of brushless D.C. motors are apparent in terms of speed, response, reliability and power density. ³

The primary advantages of brushless D.C. motors are:

• Low Maintenance
• No Brush Sparking
• High Operating Speeds
• High Efficiency
• Compact Size
• Fast Response

But brushless D.C. motors are not without their drawbacks. They are more expensive due to their complex control system, the use of rare earth magnets and the need for rotor position sensors. (Higher costs are overcome by a longer life and lower maintenance costs.) ⁴ They are primarily used in fractional horsepower applications; however, they have been used as drives in electric vehicles up to 100KW. ⁵

Construction

Brushless D.C. motors are most commonly constructed with a radially magnetized permanent magnet rotor, mounted on a steel cylinder, and phase windings wound on a slotted, laminated, non-salient stator. There are usually multiple phase windings; however, a single winding can be wound so that it’s distributed over the stator core. ⁶ This is the reverse of brushed D.C. motors (rotating armature and stationary field) and allows brushless D.C. motors to have less internal resistance and much better heat dissipation in the stator coils ⁷ resulting in higher operating efficiencies since heat can more efficiently dissipate via the stationary motor housing. ⁸

Brushless D.C. motors do not use carbon brushes or a mechanical commutator; rather, commutation is done via a complex electronic controller used in conjunction with a rotor position sensor (e.g., photo transistor-LED, electromagnetic or Hall Effect generators). ⁹ This is the primary reason why brushless D.C. motors are low maintenance and non-sparking. In addition, without brushes or a mechanical commutator, they have less shaft friction or inertia, less audible noise and much better torque to weight ratios (power density).¹⁰ Hence, they are much smaller in size than a comparable brushed D.C. motor.

Operation

Brushless D.C. motors operate in conjunction with an electronic controller and a rotor position feedback sensor. Based upon the actual rotor position, the controller sequentially energizes or switches “on” the stator’s phase windings so that torque is continuously generated as the permanent magnet (PM) rotor rotates. ¹¹ This switching action is called electronic commutation. To sense the rotor’s angular position, position sensors are used. When the PM rotor passes one set of phase windings, a signal from the position sensor is sent to the controller which then sends a signal to switch “on” the next set of phase windings so that the magnetic fields of the rotor and the stator’s phase windings remain synchronized. The torque/speed characteristic of the motor is determined by the magnitude of the signal and the switching rate of the controller. ¹²

For example, in a 2-phase motor, when the phase 1 winding is energized, the PM rotor will rotate to align itself with magnetic field produced by the phase 1 winding. When the phase 1 winding is turned “off”, the phase 2 winding is turned “on” and the rotor will continue to rotate to align itself with the magnetic field of the phase 2 winding. This “on” and “off” switching of the phase windings will maintain torque of the PM rotor. ¹³

The number of windings and the type of switching sequence determines the phase winding utilization and the torque response of the motor. Fewer windings and fewer switching pulses give the motor less winding utilization, more inertia to overcome and a poorer torque response. The most common configuration of a brushless D.C. motor is “three phase six pulse.” ¹⁴ In this configuration, the stator has three phase windings connected in a delta or star configuration with no neutral. The windings are excited by 6 pulses sequentially with each winding being switched by pulses of opposite polarity. This delivers a linear torque speed characteristic, similar to a brushed D.C. motor. ¹⁵

Applications

Brushless D.C. motors are widely used in the fractional horsepower range for disk or tape drives ¹⁶ for computer peripherals as well as in CD/DVD players, cooling fans, laser printers and photocopiers. ¹⁷ In addition, they are often used as spindle drives ¹⁸ in machine tools because they can be driven at very high speeds with fast acceleration, deceleration and reversing responses. The larger horsepower sizes have found application in electric vehicles. Since they do not have brushes, hence, are not susceptible to arcing, brushless D.C motors are safe for environments with flammable gases, such as in the petrochemical industry. ¹⁹