Thoughts on DC Motors | Cypress Semiconductor
Thoughts on DC Motors
September 23, 2010
Originally DC(and universal) motors had brushes to switch (or commutate) the current through the armature windings. Explanations can be found on Google. It relies on having position base switches (commuctator) to determine the correct switching time. The problem is that the brushes wear out and can be noisy.
Later brush-less motors came out that used a Hall effect sensor to determine the position of the rotor to allow the field direction to be reversed at the proper time.
Lately people have been using the back EMF of the motor to determine the rotor position.
I have been thinking about improvements in commutators. Brushes wear out be cause they are required to switch high current inductive loads. This is called hot commutation and it requires the design of a commutator that suppresses the arcing coming from inductive switching. The brushes need to be very low in resistance and are generally made from carbon and silver. Being soft they quickly wear out. Why not cold commutation? That is use the commutator only to determine position and then switch the fields with MOSFETS. There is now no current being switched so the arcing and EMI problems go away. Also since the brushes no loner have to be low resistance, they can be made from some conductive ceramic that will never wear out. I am not a motor expert but it seems this may be a good solution for the sub 1 HP motors. Comments? Opinions.