Electrical Machine Design

Legend: : Formula | : Limits, Range | : Assumption, etc.

Item 33. Efficiency at Unity power factor at n load, %eff_n
%eff_n = 1 - [ (W_Fe + nxW_Cu) / (nVA + W_Fe + nW_Cu ) .
Where: nVA = the output, watts
n = load factor: 1.25, 1.00, 0.75, 0.50, and 0.25 of load.
Compare these values to Guaranteed Efficiencies. See table on Item A

Maximum efficiency, %eff_max
Maximum efficiency occus when the total copper losses, W_Cu is equal to the total core loss, W_Fe, under which the rated load is KVA x Ö (W_Fe/W_Cu). Apply this load to get the efficiency using the formula in item 33, above.

Item 34. All-day efficiency (4-hr full-load), %eff_all-day
%eff_all-day = VA x 4 / [(VA x 4) + (W_Fe x 24) + (W_Cu x 4)].
Where VA = KVA x 1,000, watts
All-day efficiency is taken for 4-hour full load and 20-hr no load operations.

Ampere-Turns at Core, (TI)_core
Refer to the value of wt/lb and B" used in Item 31 and estimate the approximate value of Ampere-Turns per inch, TI/inch_Fig.161 using the BH curve on Fig. 161
Use: (TI) = TI/inch_Fig.161 x mean length
Where the mean length_H.T. = H x 2.
For the mean length_L.T. = 2 [D +2L].

Ampere-Turns at Joints, (TI)_joints
Refer to the value of wt/lb and B" used in Item 31 and estimate the approximate value of Ampere-Turns per inch in the joints, TI/inch_Fig.162 using the BH curve on Fig. 162
Use: (TI) = TI/inch_Fig.162 x #joints
Where the # joints = 2 for both joints in the limbs and in the ends.

Ampere-Turns at Joints, (TI)_total
(TI)_total = (TI)_core + (TI)_joints.

Item 35. Primary exciting current, amp, I_c
I_c = (TI)_total / Ö2 x T_H.T..

"Energy" Component, amp, I_w
I_w = W_Fe / E_H.T..

Total Exciting Current, amp, I_{c total}
I_{c total} = Ö [(I_c)² + (I_w)²].

Items 36 - 42