Permanent Magnet Brushless Motors

Permanent magnet brushless motors provide rotary and linear motion in a variety of applications. They offer superior power, efficiency, and reliability over traditional brushed DC motors.

To control the rotor position, the drive electronics need to know the physical location of the magnets within the rotor assembly. This information is typically provided by Hall sensors. Some motors use trapezoidal commutation, requiring simple combinational logic, while others require more sophisticated sensorless controllers.

Power

As electrification accelerates across our day-to-day lives, we need electric motors that can provide a high power density with lower emissions. That’s why today most high speed motors used in cars, appliances, industrial equipment, pumps and drills are permanent magnet brushless.

In a brushed DC motor, brushes are connected to the rotor and through commutator supply is given to armature windings. Over time the brushes wear out causing sparking in the machine and increasing friction, which increases the power consumption of the motor.

A brushless motor, also known as a PMSM motor or Permanent Magnet Synchronous Motor (PMSM), eliminates this problem by electronic commutation. In a three phase brushless motor, an “H-bridge” made up of electronic switches such as transistors, IGBTs or MOSFETs, controls the direction and rotation of the magnetic field of the stator. The controller can control speed and torque by pulse width modulating the output of one of the switches.

The rotor has permanent magnets affixed to it, while the stator Permanent magnet brushless motor is a multi-phase electromagnet with a different construction for linear motors. The number of phases is determined by the number of coils in the electromagnet – which can vary from two to more than four. The motor can be constructed as an outrunner or inrunner depending on the way the magnets are arranged on the rotor and stator.

Efficiency

Brushless DC motors have a higher torque density than their traditional brushed counterparts. They produce more power per watt, have a longer lifetime through eliminating brushes and commutator wear, are quieter with no arcing or brush humming and are safer since there is no risk of scalding with hot magnets. Their size can be smaller with fewer components resulting in a lower bill of materials.

Brushed motors use mechanical brushes and a commutator to switch current between the field windings of the permanent magnet rotor and the armature of the motor. The instantaneous switching of current between these contact points generates significant electrical noise, and arcing can occur which reduces efficiency. Brushless motors eliminate this problem through electronic control.

A BLDC motor controller uses feedback to optimize both the d-axis and q-axis currents that flow through the permanent magnet rotor. This minimizes copper consumption, improves operation and maintenance, and increases efficiency at high speed.

Traditional permanent magnet brushless motors exhibit cogging, a cyclical torque disturbance caused by the interaction of the rotor magnets with slots in the stator teeth. While this can be mitigated by using skewing laminations, special mechanical modifications or by implementing compensation in the motor controller, it still exists to some extent. Recent improvements in both permanent magnet material and stator design are allowing manufacturers to achieve better performance without the need for these costly modifications.

Noise

In permanent magnet brushless motors, noise is mainly caused by the electromagnetic vibration of the stator. This vibration is related to the frequency, order and amplitude of the electromagnetic force waves generated by air gap magnetic field in the motor stator. When the frequency of these waves is close to or equal to the natural frequency of a particular order in a stator mode, resonance will occur, leading to the deformation and vibration of the motor stator system. This, in turn, leads to the generation of electromagnetic torque ripple which generates noise.

The acoustic noise of the motor is also closely related to the mechanical and electrical noise. The acoustic noise of a motor can be generated by mechanical vibration such as the rotor level impeller runout, machining errors of flange and shaft, and out-of-tolerance bearing and housing fit. The electrical noise is a result of the delay time between the electrical angle and the mechanical angle when the motor is commutated. This type of noise can be reduced by increasing the flange and shaft tolerance, reducing the form and position error of the shaft and flange, and improving the fit clearance between the rotor and bearing.

In addition, the electromagnetic radial force wave and vibration noise performance of the motor can be improved by changing the matching number of polar slots and altering the polar arc coefficient, reshaping the magnet edge with reduced arc length, or by using different magnet materials. By using these measures, the noise of a permanent magnet motor can be significantly reduced without losing power or efficiency.

Cogging

The rotor in a permanent magnet brushless motor has no brushes and instead uses electronic commutation to transfer current between electromagnetic phase wires (copper windings). The currents from each electromagnetic phase are controlled by the motor controller based on encoder feedback. When the correct current vector is applied to each phase set, the magnetic forces create torque that can be used in motion control applications.

Brushless motors can be very powerful, and that requires a high coercivity of the permanent magnets. But the higher coercivity comes at a cost. The longer the air gap between iron and the magnets the more power the copper coils have to draw to create a magnetic field, and that draws heat. The heat causes the copper to become less conductive, and that decreases efficiency.

Skewing the rotor or stator teeth is an effective way to reduce cogging for both types of permanent magnet DC motors. This is accomplished by placing the magnetic phase wire in different positions on each side of the stator teeth or slots. Celera Motion Omni Series stators use skewed stator teeth and rotor magnets to minimize the fundamental frequency of cogging torque.

Another option to reduce the effect of the air gap Waterproof DC motor is to use a Halbach magnetization profile. These magnets produce sinusoidal airgap fields, which reduces cogging torque and provides superior performance compared to radial or straight segments.