Figure 4.24: Torque produced by the DC EM system with 3 phase coils T a :
torque produced by coil-AX, T b : torque produced by coil-BY, T c :torquepro-
duced by coil-CZ
The mechanical commutation system has been successfully used in DC
motors for many years. But it has some inherent problems which are of serious
concern in many applications. Such problems include,
1. Spikes: Commutation process results in sudden jumps in the current
waveform (Figure 4.22). These jumpscausessparksbetweenthesurfaces
of commutator and brushes inducing electromagnetic interference (EMI)
as the magnetic fi eld energy stored in the inductance of the coils must
be released in a very short time. The sparks can damage the contact
surfaces of the brushes and commutator, and reduce the lifespan of the
2. Speed: The commutator is formed by many segments as shown in Fig-
ure 4.25. Due to centrifugal force, such mechanical structure cannot
withstand high speed rotation. In addition, the presence of brushes also
puts limit on the maximum surface speed of the commutator as the brush
material cannot stand the intensive wear and tear caused by high speed.
3. Vibration and noise: The brushes must contact the commutator surface
with certain pressure. In high speed operation, such mechanical contact
can induce severe acoustic noise and vibration.
4. Maintenance: The brush must be in contact with the commutator sur-
face during operation of motor, making wear and tear of brushes and
commutator inevitable. Consequently, regular maintenance is necessary
which may not be feasible in many applications, e.g., HDD.