Download Commutator

Design of Commutator and Brushes
 Commutator and brush arrangement are used to convert the bidirectional current to
unidirectional current
 Brushes are located at the magnetic neutral axis ( mid way between two adjacent
poles)
 The phenomenon of commutation is affected by resistance of the brush, reactance
emf induced by leakage flux, emf induced by armature flux.
 Classification of commutation process
 Resistance commutation
 Retarded commutation
 Accelerated commutation
 Sinusoidal commutation
 Commutator is of cylindrical in shape and placed at one end of the armature
 Consists of number of copper bars or segments separated from one another by a
suitable insulating material of thickness of 0.5 to 1mm
 Number of commutator segments = no. of coils in the armature
 Materials used :
 Commutator segments: Hard Drawn Copper or Aluminum Copper
 Insulation :Mica, Resin Bonded Asbestos
 Brushes :Natural Graphite, Hard Carbon , Electro Graphite, Metal Graphite
Design formulae
1. No. of commutator segments, C = ½ u.S a
where, u – coils sides/slot
Sa – no. of armature slots
2. Minimum no. of segments = Ep/15
3. Commutator segment pitch = βc = πDc/C
where,
Commutator Diameter Dc – 60% to 80% of diameter of armature
βc ≥ 4mm
4. Current carried by each brush Ib= 2Ia/P
for lap winding
Ib= Ia
 for wave winding
5. Total brush contact area/spindle Ab= Ib/δb
6. Number of brush locations are decided by the type of winding
Lap winding: No of brush location = no. of poles
Wave winding : No of brush location =2
7. Area of each individual brush should be chosen such that , it does not carry more
than 70A
Let ,
ab – Contact area of each brush
nb – Number of brushes / spindle
 Contact area of brushes in a spindle, Ab = nb. ab
also ab = wb.tb
Ab = nb. wb.tb
Usually, tb = (1 to 3) βc
wb = Ab/ nb. Tb = ab/tb
8. Lc – depends on space required for mounting the brushes and to dissipate the heat
generated by commutator losses
Lc = nb(wb + Cb) + C1
+ C2
where, Cb - Clearnace between brushes (5mm)
C1 -
Clearance allowed for staggering of brushes (10mm, 30mm)
C2 – Clearance for allowing end play (10 to 25 mm)
9. Losses :
 Brush contact losses: depends on material, condition, quality of
commutation
 Brush friction losses
Brush friction loss Pbf = μ pb AB.Vc
μ – Coefficient of friction
pb-Brush contact pressure on commutator (N/m 2)
AB - Total contact area of all brushes (m2)
AB =P Ab (for lap winding)
= 2 Ab (for wave winding)
Vc – Peripheral speed of commutator (m/s)
Design of Interpoles
 Interpoles: Small poles placed between main poles
 Materials Used: Cast steel (or) Punched from sheet steel without pole shoes
 Purposes:
 To neutralize cross magnetizing armature MMF
 To produce flux density required to generate rotational voltage in the coil
undergoing commutation to cancel the reactance voltage.
 Since both effects related to armature current, interpole winding should be
connected in series with armature winding
 Average reactance voltage of coil by Pitchelmayer’s Equation is, Erav = 2Tc ac Va.L .λ
Inductance of a coil in armature =2Tc2 .L .λ
 Normally, Length of interpole = length of main pole
Flux density under interpole, Bgi = ac. λ .(L/Lip)
where, Lip- length of interpole
In general,
Bgi = 2 Iz. Zs. (L/Lip). (1/Va.Tc).λ
mmf required to  mmf required to over come 
ATi  



establish Bgi
 armature reaction
MMF required to establish Bgi = 800000Bgi.Kgi.lgi
MMF required to 
 Iz .Z
overcome

2P
armature reaction
 ( without compensating winding)
I .Z
 (1 -  )  z
AT
2P
No.of turns  i
Ia
 ( with compensating winding)
Current density in 
2
, δi  2.5 to 4 A/mm
interpole winding 
Area of X - section of 
Ia
A ip 
interpole conductor, 
δi
Losses and efficiency :
1. Iron Loss - i)Eddy current loss ii) Hysteresis loss
2. Rotational losses - Windage and friction losses
3. Variable or copper loss
Condition for maximum efficiency :
Constant Loss= Variable Loss