Electrical Machine Design

WEEK 13: SYNCHRONOUS GENERATOR: ROTOR DESIGN

Sections: Magnetic Circuit | Air Gap | MMF | Slot Flux

The Current Density in the field magnet winding of a salient pole machine ranges from 1,100 to 1,699 amperes per sq. in.

The approximate values of the leakage coefficient used for estimating the flux in poles and yoke ring:

High values: 1.35 to 1.45
used when pole pitch is small, and radial length of pole core is great in proportion to width

Average values: 1.25 to 1.35
used for pole pitch 8 to 12 in. and length of winding space about equal to width of pole core.

Low values: 1.15 to 1.25
used for large pole pitch and small radial lenth of pole core.

For high leakage factor, the cross section of poles and yoke maybe calculated for a flux density of 85,000 to 100,000 lines per sq. in.

Sections: Magnetic Circuit | Air Gap | MMF | Slot Flux

84. Equivalent Air Gap

Equivalent Air Gap: use the formula:

Equivalent air gap d, under the pole at any other point is equal to d = de / (cos a), where a is the angle in electrical space degrees betweent he center of the pole and the point considered.

Sections: Magnetic Circuit | Air Gap | MMF | Slot Flux

86. Armature MMF

Referring to the figure:

The Ampere-conductors in the space is equal to:

The Ampere-conductors in the space OC is equal to:

The maximum value of the armature ampere-turns is obtained when q = p / 2, thus:

Where:
C_s = the number of series conductors per slot.
p = the number of poles
n = the number of phases
n_s = the number of slots per pole per phase
Z' = nn_sC_sp

Considering the number of slots per pole, thus:

Sections: Magnetic Circuit | Air Gap | MMF | Slot Flux

88. Slot Leakage Flux

88. Slot-leakge Flux. IfF_s is the maximum value of leakage flux in one slot and n = the number of phases, then the average value of slot flux is:

The maximum value of slot flux would be:

89. Equivalent Slot Flux.

F = 0.4p C_si_s

Where:
C_s = the number of conductors per slot.
i_s = ampere per conductors.
F = total slot MMF
l_a = the gross lenth of armature in centimeters
P_es = the equivalent permeance per centimeter length of slot.

Refer to the Notes for the Illustrative Example: Design of Field Magnet (Rotor) of A-C Generator, p. 231-247.