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Magnetic loading = PΦ Where P = Number of poles Φ = Flux per pole |
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Electrical loading = IZ × Z Where IZ = Current in each conductor Z = Number of stator conductors |
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Specific magnetic loading BAV = PΦ / πDL = Φ /
τL Where D = Diameter of stator bore L = Length of stator core τ = Pole pitch = P / πD |
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Output equation of DC Machine = ( Total magnetic loading )( Total electrical
loading ) ( Speed in
rps ) |
|
Output power = C0D2Lns Where C0 = π2 ( BAV ) (
ac ) × 10 – 3 = Output
co-efficient BAV = Average magnetic loading ac = Specific electrical loading ns = Speed in revolution per second |
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Output equation of AC machine Q = C0D2Lns Where C0 = 11 ( BAV ) ( ac ) ( Kw
) × 10 – 3 = Output
co-efficient Kw = Winding factor |
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The volume of active part of DC and AC machines
decreases with increases in speed and vice versa. ( π/4 )D2L
α ( 1 / Ns ) |
|
The volume of active part is inversely proportional
to value of output co-efficient. The size and cost of machine increases with
increase of specific electrical loading and specific magnetic loading. |
|
The flux density in the teeth is inversely
proportional to specific magnetic loading or average flux density in the air.
The maximum value of flux density in the teeth occurs where the teeth width
is smallest. |
|
The ratio of Bt / Bav is large
at the section where the teeth have a smallest width and therefore Bav
must be reduced in small machines. The small machines is designed with lower
value of specific magnetic loading due to above reason. |
|
The mmf required for air gap is directly
proportional to specific magnetic loading. The area of iron part of magnetic
circuit α ( 1/ B ) |
|
Two machines having linear dimension of x : 1 Core loss x3 : 1 Percentage core loss ( x3 / x4
) = 1 / x Therefore the percentage core loss decreases
with increase in machine size. |
|
The heat dissipation per unit area is proportional
to specific electrical loading. |
|
The specific electrical loading is directly
proportional to space factor considering that fixed ratio of slot width to
slot pitch and fixed value of depth of slot and current density. ( ac ) α Sf
( Ws, ys,
δ are constant ) Where δ = Current density Ws = Width of slot ys = Slot pitch = πD / S S = Number of armature slots D = Diameter of armature |
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The volume of active part of machine is directly
proportional to torque. |
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Two machines have linear dimensions x : 1 Output x4 : 1 ( for same speed, current density and
flux density ) Losses x3 : 1 ( for same speed, current density and
flux density ) Efficiency = Output / ( Output + Losses ) = x4 / ( x4 + x3 ) = 1 / ( 1 + 1 / x ) Therefore the efficiency increases with
increase in the dimension of machines. |
|
Temperature rise α C θ Where C = Cooling co-efficient |
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