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New Era University College of Engineering and Technology Department of Electrical Engineering

EE572 MW 10:00-11:30AM

Electrical Machine Design

Design 1-B Design of Commutator and Brushes

Item 1 - 39 440kW, 450V

Name: Torrente, Arvin Joseph K. Rating: __________ Course: BS Electrical Engineer Date Started: July 30, 2015 Year: 5th year Date Submitted: August 2015

Engr. Reynaldo M. Dela Cruz

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Specifications: 440kW, 450V I. Objective: - To follow the design procedure from item 1 - 39 II. Materials/Instruments: - Drawing Materials - Laptop - Calculator - Printer - Book III. Procedure: Item 40 : Diameter of Commutator. Refer to article 14. A diameter of commutator not exceeding 80 percent of the armature core diameter is generally found practically although a reasonably good rule to follow is to make Dc = D /2 + 4. Thus, Dc = 19/2 +4 = 13.5 inches This is 70.9 percent of the core diameter and makes the peripheral velocity of 0.709 x 5970 = 4235 fpm near the upper limit. This dimension is subjected to correction if the thickness of the individual bars does not work out satisfactory. Item 41 to 43 : Numbers of commutator bars. The potential difference between adjacent commutator segments in a 250 volt machine might be anything between 4 to 12 volts. The average per turn of armature winding is

�

��

2���

=300

�4502(6)

�= ��

If the commutator is constructed with the same number of segment as slots there will be two turn between adjacent segments, because for a lap winding and four effective commutator segment will, moreover, be desirable to use 150 commutator segment, with a resulting improvement in commutator and a more suitable, narrower segment. Item 44 and 45 : Width Commutator segment. The bar pitch is 3.14 x 13.5 /222 = 0.40 inch and with mica 0.03 inch the bar width 0.325 – 0.03 = 0.295 inch at the commutator surface. Item 46 : Radial Depth of segment. The proper depth of copper in the cross section of commutator bar is usually determined by mechanical considerations (chapter 13). It must be sufficient to prevent appreciable deflection under the action of centrifugal force. For peripheral velocities up to about 4500 fpm the radial depth of the commutator segment should be about h=(Dc+15)/15; for higher speeds the depth should be increased in portion to the square of velocity. Thus,

ℎ = � 19. +15

15� �

4235�

4500�� = � ������

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Item 47 to 51: Dimension of Brushes. Unless a very soft quality of carbon is used the current density over the brush-contact surface is about 30 to 50 A/sq. Taking 40 as a preliminary value to be modified later, if necessary to accommodate a standard size of brush, the contact surface of one set of brushes will be (6 x 117.7) / (3x40) = 5.885 sq. in. A 1 inch width of brush, Art 34. Will cover a little over three bars which is reasonable for an armature with 150 segment. The total length for brushes per set, measured in a direction parallel to the axis of the machine will be 5.885 or say 6 which can be made up of eight brushes 8 ¼ x 1. The current density will thus be increased slightly to 40 x(5.885/6) = 39.23 amp per sq. in. Item 52 : Length of Armature. In addition to the 8 inches which must be provided for 6 1 ¼ x 1 inches carbon brushes axial length of the commutator face must allow the following:

a. Brush holders and clearance 8 x 5/16 = 2 1/2 b. Staggering of (+) (-) = 5/8 c. End clearance for brushes = 1 inch d. End play = 3/8

The total length will, therefor be Lc = 7 + 2 1/2 + 5/8 + 1 + 3/8 = 11.5 inches Item 53 : Brush Contact Drop. Referring to Figure 38 the brush contact drop for hard carbon at about 40 amps per sq Inch is 2.08. Allowing 10 percent for roughness, chipping and irregularities this drop will be about 2.3V. Item 54 to 56: Brush Losses. The brush contact loss will be 2.3 x (6x117.7) = 1625 watts. The brush-friction loss may be calculated by using the formula (48), page 143, where Wf = c PANDc x 3.14 x 746 / 12 x 33 000 Using c = 0.25 for hard carbon, P = 1.5 lb for a peripheral speed greater 4000fpm

�� =0.25 × 1.5 × 6 × 5 × 1200 × 13.5 × 3.14 × 746

12 × 33000= ����. ��

Total Brush Loss = 1078.60 + 1625 = 2703.60 watts Item 57 : The illustration or Figure 18 gives the leading dimensions of armature and commutator as worked out in this design. The diameter of the shaft supporting the armature may be calculated by formula 160 in chapter 13 treating of mechanical features of the design of electrical machinery. This is 0.084 = 4.7 or 5 inches

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IV. Circuit Diagram

Section of core and commutator for illustrative example of Art. 18. Assembly of armature core and commutator for small dynamo or motor

Armature and commutator for d-c machine of large outpu

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Small commutator

High-speed commutator of large size Illustrating calculations for strength and stiffness of long commutator bars.

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V. DATA AND RESULT DESIGN SHEET FOR ARMATURE OF D-C GENERATOR

Item No.

Specifications: 440KW; 450V; 1200rpm

Symbol Preliminary or

assumed values

Final values

40 Commutator and Brushes

Diameter of commutator, in...... Dc ..... 40

41 Average volts per turn of armature

winding ..... ...... 8

42 Number of turns between bars ..... ..... 1 43 Total number of commutator bars ..... ..... 150 44 Bar pitch, in............................. ..... ..... 0.40 45 Width of copper bar (on surface), in ..... ..... 0.295 46 Radial depth of bar, in.............. ..... ...... 2

47 Current density at brush-contact surface, amp per sq in...............

40 39.23

48 Contact area per brush set, sq in....... ..... 5.885 6 49 Brush arc (circumferential width), in ..... ..... 1.0

50 Axial brush length (total) per set,

in... ..... ..... 6

51 Number of brushes per set......... ..... ..... 6 52 Axial length of commutator, in... Lc ..... 11.5 53 Brush-contact drop, volts........ ..... ..... 2.3 54 Brush-contact loss, watts......... ..... ..... 1625 55 Brush-friction loss, watts.......... ..... ..... 1078.60 56 Total brush loss, watts ..... ..... 2703.60

57 Drawing to scale giving leading

dimensions of armature and commutator........................................

..... Fig. 18

VI. Question/Problems:

1. Given a dc-dynamo: developed volts=100; total number of slots = 53; number of inductors per slot = 6; number of poles = 4; speed = 1000rpm; winding, series (or wave). Calculate the flux per pole.

2. A certain four-pole simplex-lap-wound dc generator has 768 armature inductors and revolves at 20 rps. If the flux per pole = 715,000 Maxwell, calculate the generated voltage.

3. The lap-wound armature of a six-pole generator has 600 active conductors, the speed is 1000rpm. The area of each pole face is 100 sq. in., and the average air-gap flux density under the pole face is 8000gauss. Calculate the emf between positive and negative brushes.

VII. Remarks:

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V. Data and Results:

DESIGN SHEET FOR ARMATURE OF D-C GENERATOR- PART 1 Item No.

Specifications: 440kW, 450V Symbol Preliminary or

Assumed Values Final Value

1 Armature Core and Winding number of Poles p 6 6

Frequency f - 60 2 Ratio of Pole Arc to Pole Pitch r 0.64 0.64 3 Specific Loading q 900 1040 4 Apparent air-gap flux density (open-circuit) B”g 59000 55468 5 Line Current (full load) I - 978 6 Type of Winding - lap lap 7 Armature Current per Circuit IC 201 - 8 Output factor (laD2) - 6537.73 7200 9 Armature Diameter D 24 24

10 Peripheral Velocity, fpm v 7550 7550 11 Total number of face conductors z 337 390 12 Number of slots S - 78 13 Number of conductors per slot - - 5 14 Axial length of armature; gross, in. la 11.50 12.50 15 Flux per pole (open circuit) ϕ 5.11 × 10� - 16 Pole pitch, in. τ 12.56 - 17 Pole arc, in. r τ 8.04 -

18 Area covered by pole face

(r τ la), sq. in. - 79 92.13

19 Dimensions of armature conductors, in. - - 2 × (0.032 × 1.23) 20 Slot pitch, in. γ - 0.967 21 Slow width, in. s - 0.4 22 Slot depth d - 2.46

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Foot width, in. - - Top t - 0.567 Roof - 0.368

Average - 0.468 24 Number of radial ventilating ducts n - 4 25 Width of radial ducts, in. - - 0.375 26 Net length of armature core, in. ln 10.06 11.03 27 Net tooth section under pole at center, sq. in. - 39.17 42.95 28 Apparent density in teeth under pole at center, sq. in. B”t 130457 118985 29 Length per turn of armature coil, in. - - 67 30 Resistance of one turn, @ 60°C - - 0.00067

31 Resistance of armature, ohms - - 0.0037 32 IR drop in armature, volts - - 4.42 33 I2R loss in armature, volts - - 5331 34 Estimated full-load flux per pole - - 5.21 × 10� 35 Flux density in armature core below teeth - 73000 71274.15 36 Internal diameter of core stampings, in. - - 12.58 37 Weight of iron core without teeth, lb - - 500 38 Weight of iron teeth, lb - - 277.33 39 Total weight of armature stampings, lb. - - 777.33

VI. Question/Problems:

4. Given a dc-dynamo: developed volts=100; total number of slots = 53; number of inductors per slot = 6; number of poles = 4; speed = 1000rpm; winding, series (or wave). Calculate the flux per pole.

5. A certain four-pole simplex-lap-wound dc generator has 768 armature inductors and revolves at 20 rps. If the flux per pole = 715,000 Maxwell, calculate the generated voltage.

6. The lap-wound armature of a six-pole generator has 600 active conductors, the speed is 1000rpm. The area of each pole face is 100 sq. in., and the average air-gap flux density under the pole face is 8000gauss. Calculate the emf between positive and negative brushes.

VII. Remarks: