Permanent magnet DC motors

From Combat Robot

A brushed permanent magnet direct current (PMDC) motor has a rotating 'armature' of wire coils, typically wound around iron pole cores. The armature is positioned in a field generated by stationary permanent magnets. Electrical power is transferred to and correctly switched between the armature windings as they rotate by stationary brushes pressing against a set of contacts on the armature (the 'commutator'). The brushes sliding across the commutator create friction, wear, and inefficiency.

A brushless PMDC motor has stationary wire coils and rotating permanent magnets. Since the coils are stationary, sliding brushes and commutator are not required -- however, the switching of electrical power to the correct coil windings must be handled by an 'intelligent' motor controller that senses the position of the rotating permanent magnet field. If the rotating magnets are outside the wire coils, the motor is an 'outrunner' or 'rotating can' design. If the rotating magnets are surrounded by the wire coils, it is called an 'inrunner'.

Speed controllers for brushed and brushless motors are very different in design and cannot be interchanged.

A few things handy to know about permanent magnet DC motors:

• They generate their maximum torque at zero speed (stall torque);
• The amperage drawn is proportional to torque;
• Maximum horsepower comes at 1/2 the maximum (unloaded) RPM;