Commutation Process in Detail

 Commutation process

• Just before the armature coil reaches the brush, it carries current (= Ia / A ) in one direction after coil has traversed brush width, current gets reversed to ( -Ia / A ). This reversal of current is called Commutation. Here, A represents number of parallel paths.

• Good commutation means no sparking at brushes and commutator surface remains unaffected.

• So, current in coil reduces from Ic to zero and then increase in negative direction to Ic again.

• Under commutation : Tcommutation > Tc

• Over commutation : Tcommutation < Tc

• Commutation period, Tc = Brush width / Commutator peripheral speed

Resistance Commutation :-

Rc = coil resistance

r1 = Resistance between bar1 and brush

r2 = Resistance between bar2 and brush

If no emf is induced in commutated coil, then applying KVL in brush bar1 and bar2

(2Ic - I2)r1 + (Ic - I2)Rc - I2.r2 = 0

I2 = (Rc + 2.r1)Ic / (Rc + r1 + r2)Ic

Current in coil 1

ic = Ic - I2 = Ic[(r1 - r2) / ( RC + r1 + r2 ) ] = Ic[1 - {2r1/(r1 + r2)}] / [1 + {Rc/(r1 + r2)}]

Neglecting, Rc/(r1 + r2)

ic =  Ic[1- {2.r / (r1 + r2)}]

r ∝ 1/A

ic =  Ic[1- {(2/A1) / (1/A1 + 1/A2)} ]

With rotation to right A1 decreases and A2 increases linearly. In fractional kW DC machines, resistance commutation provides good commutation.

Delayed commutation :-

• During commutation period, an emf is induced in coil due to self-inductance of coil.

ec = Lc.dic/dt

According to Lenz's law, induced emf opposes the cause so it oppose change in "ic" and thus delays the commutation.