SMC Networks Amplifier manual Brushless versus Brush-typeComparison, Application Guide

Application Guide

Brushless versus Brush-type Comparison

There are two basic types of motor design that are used for high-performance motion control systems: Brush-Type PM (permanent magnet), and Brushless-Type PM. As you can see in the figure, a brush- type motor has windings on the rotor (shaft) and magnets in the stator (frame). In a brushless-type mo- tor, the magnets are on the rotor and the windings are in the stator.

To produce optimal torque in a motor, it is necessary to direct the flow of current to the appropriate windings with respect to the magnetic fields of the permanent magnets. In a Brush-Type motor, this is accomplished by using a commutator and brushes. The brushes, which are mounted in the stator, are connected to the motor wires, and the commutator contacts, which are mounted on the rotor, are con- nected to the windings. As the rotor turns, the brushes switch the current flow to the windings which are optimally oriented with respect to the magnetic field, which in turn produces maximum torque.

In a brushless motor there is no commutator to direct the current flow through the windings. Instead, an encoder, hall sensors or a resolver on the motor shaft senses the rotor position ( and thus the mag- net orientation). The position data is fed to the amplifier which in turn commutates the motor electroni- cally by directing the current through the appropriate windings to produce maximum torque. The effect is analogous to a string of sequencing Christmas lights: the lights seem to chase each other around the string. In this case, the magnets on rotor “chase” the magnetic fields of the windings as the fields “move” around the stator.

The brushless motors are more reliable as Brush maintenance is eliminated and no brush dust is gen- erated. The brushless motor can be driven to much higher RPM limits and typically have lower inertia. The brushless motor also dissipates heat more efficiently since the stator windings are thermally con- nected to the outside of the motor case. It is also safer for explosive atmospheres and quieter and less electrical noise generated as there is no brush arcing in a brushless motor.

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