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Commutator & Brush

<-- Back Items 40 - 46

Item 47. Current Density at Brush Contact Surface, D_b
Between 30 to 50 amp per sq. in
Hard Carbon & Electrolytic Graphite: 35 to 50 amp per sq. in. for surface speeds 1,000 to 3,000fpm
Soft Graphitic Carbon: 40 to 65 amp sq. in. for surface speeds 2,000 to 5,000fpm
Copper-Graphite: 90 to 95 amp per sq. in. for surface speeds 1,000 to 3,000fpm
Usual value: 40 amp per sq. in, subject for correction to accomodate standard size of brush.

Item 48. Contact Area per Brush Set, A_b
A_b = p x I_c / n_b x D_b
Where n_b, the number of segments covered by the brush, dependent on the type of winding.

Item 49. Brush Arc (circumferential width), b_arc
Usually between 5/8 to 1.25 inches.
Commonly used for design purposes: 1.00 inch.

Item 50. Axial Brush Length, b_l
b_l = A_b / b_arc.

Item 51. Number of Brushes per set, r
From item 50, it would give the approximate number of brushes and the approximate dimensions with the brush arc (circumferential width), b_arc taken as the brush thickness, per se.

Item 52. Axial Length of Commutator, l_c
Total axial length of the Commutator, l_c, is the sum of
a) Axial Brush Length, b_l,
b) Brush Holders and clearances, no. of brushes x b_h, where b_h is taken as 5/16,
c) Staggering clearances (+) and (-), 5/8 in.,
d) End Clearance, 1.00 in., usually equal to the brush thickness, per se
e) End Play, 3/8 in.

Item 53. Brush Contact drop, volts, IR_b
Refer to the graph for the potential drop due to surface resistance of carbon brushes, IR_b.
For hard carbon at about 40 amp. sq. in, this is about 2.08 A ten (10%) per cent allowance for roughness, chipping, and irregularites should be added.

Item 54. Brush Contact Loss, watts, I²R_b
Use: I²R_b = IR_b x p x I_c.

Brush Coefficient of Friction Loss, c
Hard Carbon & Electrolytic Graphite: c = 0.23 for surface speeds 1,000 to 3,000fpm
Soft Graphitic Carbon: c = 018 for surface speeds 2,000 to 5,000fpm
Copper-Graphite: c = 0.16 for surface speeds 1,000 to 3,000fpm
Usual value: 0.25 amp per sq. in for peripheral speed greated than 4,000 fpm.

Brush Pressure on Commutator, psi., P
Between 1.5 to 2 psi for large machines, and 3 to 5 psi for small machines.
Pressure of the brush, P = 2 lb per sq. in for peripheral speed greated than 4,000 fpm.

Item 55. Brush Friction Loss, watts, W_f
W_f = (c P AND_c p x 746) / (12 x 33,000).

Item 56. Total Brush Loss, watts, W_B.L.
W_B.L. = I²R_b + W_f.

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