Page 401 - Power Electronics Handbook

390 Power semiconductor circuit applications
(ii) Sparking reduces the life of the commutator so that conventional d.c.
machines need periodic servicing and brush replacement and cannot
be mounted in inaccessible positions.
(iii) Usually the commutator is mounted on the rotating member, since an
inverted motor would require the brushes to rotate. This limits the
maximum speed at which it can run and therefore the power output
from a given frame size.
(iv) The commutator also limits the machine design since it requires a
large ratio of diameter to length of armature iron.
(v) The commutator is made from good-quality copper which softens at
high temperatures and limits the machine-operating temperature.
Dirt and moisture also attack the commutator and the machine needs
to be specially protected in hazardous atmospheres.
(vi) Rubbing of the brushes on the commutator and clatter of the brushes
in their holder give rise to a considerable amount of audible noise.
An electronic commutator motor maintains all the advantages of a
traditional d.c. machine, such as high starting torque and excellent
efficiency at all speeds, whilst avoiding the disadvantages of a mechanical
commutator. In addition it can also produce smoother torques in small
motors, due to the fact that the number of commutator segments in a small
conventional motor is usually limited by physical size to six or eight, giving
a relatively poor torque-position characteristic. An electronic commutator
using four segments, on the other hand, can produce a much smoother
torque by using its switches in an amplification mode.
The d.c. commutatorless motor gives a performance similar to that
obtained from an induction motor driven by an inverter. From the
hardware aspect both these systems are also very similar, the induction
motor drive being slightly simpler due to the absence of the rotor-position
sensors. There is, however, one important difference. The frequency at
which the stator switches operate in an electronic commutator motor is
determined by the rotor speed, as sensed by the position sensors, this
speed being in turn fixed by the winding current. For an inverter drive, on
the other hand, the switch frequency is fixed by an internal oscillator in the
inverter. The motor merely follows this rotating field and speed control is
achieved by controlling the inverter frequency and not the supply voltage.
The following further differences can be noted between the two systems:

(i) The inverter of an induction motor drive could be located at some
distance from the motor. This would require only two wires
connecting the driver to the motor. An electronic commutator motor
would need the sensing leads to be brought out as well, these leads
then being exposed to pick-up effects.
(ii) Permanent magnet motors take a larger starting current than
corresponding induction machines, which could be a serious problem
since a current surge can cause demagnetisation of the permanent
magnet. Smaller motors, with higher-impedance windings, are less
prone to this effect.
(iii) Since the inverter can operate independently of the motor in an
inverter drive, several motors can be run in parallel off the same
inverter.