design of commutator and brushes

TOPIC:-DESIGN OF COMMUTATOR AND

BRUSHES

COMMUTATOR Function?Rectify alternating current Shape?Cylindrically shaped Essentials of construction?No. of copper bars or segments are separated from one another. What is Risers?Connect segments to the armature coils.

Segments? Insulation?

mica or micanite The number of commutator segments is equal to the number of

active armature coils. The diameter of the commutator will generally be about (60 to

80)% of the armature diameter.Dc =(0.6 to 0.8)D The commutator peripheral velocity vc = ∏ DC N / 60 The commutator segment pitch ƮC = (outside width of one

segment + mica insulation between segments)= ∏ DC / Number of segments The outer surface width of commutator segment lies between 4

and 20 mm in practice.

The temperature rise of the commutator can be calculated by using the following empirical formula.

Ѳ°C=

The different losses that are responsible for the temperature rise of the commutator are

a. brush contact loss andb. brush frictional loss.

120 × watt loss / cm of dissipating surface π DC Lc

1 + 0.1 vC

Brush contact loss = voltage drop / brush set × Ia

Brush frictional loss (due to all the brush arms) = frictional torque in Nm × angular velocity

= frictional force in Newton x distance in metre × (2 ∏ N/60)

= 9.81 μ Pb Aball× (DC / 2) ×(2 ∏ N/60)= 9.81 μ Pb Aball vc

Aball =Ab× number of brush arms

BRUSH DETAILS

Following Points are considered when brushes are

1. designed current collected by each brush arm = (2Ia/p)

2. the cross-sectional area of the brush or brush arm or a holder Ab = (2Ia/p δa)

3. The current density δa depends on the brush material

In order to ensure a continuous supply of power and cost of replacement of damaged or worn out brushes is cheaper, a number of subdivided brushes are used instead of one single brush.

Thus ifi. tb is the thickness of the brushii. wb is the width of the brush andiii. nb is the number of sub divided brushes.

Then Ab = tb wb nb

The number of brushes / spindle can be found out by assuming a standard brush width or a maximum current / sub divided brush.

Standard brush width can be 1.6, 2.2 or 3.2 cm.

Current/subdivided brush should not be more than 70A.

Width of each brush wb =Ab

Nb tb

STEP BY STEP DESIGN PROCEDURE OF COMMUTATOR AND BRUSHES

1. Diameter of the commutator Dc = (0.6 to 0.8) D and must be such that the peripheral velocity of the commutator vC = π Dc N / 60 is not more than 15 m/s as far as possible.

2. The commutator segment pitch Ʈc = π Dc / Number of segments should not be less than 4 mm from the mechanical strength point of view.

3. The number of commutator segments is equal to number of active armature coils.

4. Length of the commutator Lc = (width of the brush + brush box thickness 0.5 cm) number of brushes / spindle nb + end clearance 2 to 4 cm + clearance for risers 2 to 4 cm + clearance for staggering of brushes 2 to 4 cm.

5. Cross-sectional area of the brush / spindle or arm or holder Ab= (2Ia / p δa) cm²

6. Maximum thickness of the brush tb max= 4 Ʈc for machines greater than 50 kW= 5 Ʈc for machines less than 50 kW7. With standard brush width Wb assumed, the number of

brushes / spindle

nb=

8. Total commutator losses = Brush contact loss + Brush frictional loss= voltage drop / brush set × Ia + 9.81 μ Pb Aball vc

A b t b Wb

9. Temperature rise of the commutator Ѳ°C= Cooling coefficient x watt loss / dissipating surface

= × watt loss / cm² of dissipating surface π Dc Lc

10. Temperature rise should be less than about 55°C.

1201 + 0.1 vc

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