Build Calculations

How to Calculate Inverter, Motor and Power Module Build Metrics Step by Step

A step-by-step walkthrough of the five formulas that run a power electronics and motor line, with real units, inputs, and worked numbers.

Every power electronics and motor line runs on five recurring calculations: assembly cost, winding labor, test time, first-pass yield, and good-unit capacity. Each one has a clean formula, a handful of inputs pulled from shop-floor logs, and a units trap that bites the unwary. This guide walks each formula end to end with worked numbers so you can reproduce them by hand or in the matching calculator. Keep your inputs measured at steady state, not nameplate, because a winder rated at 15 operations per minute that sustains 12 will throw every downstream number off by a quarter if you use the spec sheet.

Start with assembly cost, the base of any inverter quote. The Inverter Assembly Cost formula is variable cost equals assemblies times per-assembly cost times capture rate, then add a fixed program adder. Take 100 assemblies at 45 dollars each with 80 percent capture: 100 times 45 times 0.80 gives 3,600 dollars variable. Add a 250 dollar fixed adder for fixturing and ICT programming and total is 3,850 dollars, or 38.50 per unit. The per-unit figure lands below the 45 nominal only because capture trims the variable base before the fixed adder spreads back across all 100 units. Keep high-value IGBT and SiC modules out of the per-assembly blend if you track them as separate BOM lines.

Winding labor is the second formula and usually the pacing operation. Motor Winding Labor is base time equals operations divided by completion rate, then multiplied by an allowance factor. One operation is a completed coil or slot fill, not a single turn, which is the classic units error: entering turns per minute understates time by an order of magnitude. Run 120 operations at 12 operations per minute for 10 minutes of pure winding, then apply a 10 percent setup and handling allowance for threading, indexing, and lead dressing to get 11 required minutes. That effective pace of 10.9 operations per minute is what you schedule and staff against, not the raw 12.

Drive test time uses the same shape but different meaning. Drive Test Time is drives divided by test completion rate, scaled by a setup and retest allowance. For 120 drives at 12 per minute the base is 10 minutes; a 10 percent allowance gives 11 minutes, which is about 5.5 seconds of station time per drive. The trap here is burn-in. If your entered rate does not include the hours-long thermal soak, model burn-in separately, because folding a functional-test rate and a burn-in dwell into one number is meaningless. When first-pass yield is weak, the real allowance climbs toward 25 to 30 percent as retests eat the station.

First-pass yield is a ratio, not a rate, and it drives everything downstream. Power Module First-Pass Yield is modules passed first time divided by total tested, times 100, with the gap being target minus actual. Count only parts that passed with no rework or retest; including reworked units inflates the number and hides your rework burden. If 240 of 250 modules pass clean, yield is 96.0 percent, one point over a 95 percent target. When yield craters, check test limits, fixture contact, and calibration before scrapping, because a mis-set parametric limit fails good parts wholesale and looks identical to a real excursion on the chart.

Good-unit capacity ties throughput to reality by discounting gross output for uptime and yield. Stator Winding Capacity is stators per cycle times planned cycles for gross, then times uptime times first-pass yield for good output. Take 4 stators per cycle across 480 cycles for 1,920 gross, then multiply by 0.90 uptime and 0.97 yield to get 1,676 good stators. That 244-unit gap, about 13 percent, is exactly why committing to gross capacity leaves winding lines short of plan. Inverter Burn-in Capacity uses the identical structure with inverters per chamber load, so the same discipline applies to sizing burn-in racks.

The five formulas interlock, so feed outputs forward rather than recomputing from scratch. First-pass yield from the module line becomes the yield term in Stator Winding Capacity and Inverter Burn-in Capacity. Winding labor minutes and drive test minutes feed the touch and test burden inside the per-assembly cost you plug into Inverter Assembly Cost. When you change one input, trace it through: a 3-point yield drop cuts good capacity, raises the effective test allowance as retests grow, and lifts per-unit cost as the fixed adder spreads across fewer good units. Consistent units, steady-state rates, and clean first-pass counting are the three habits that keep the whole chain trustworthy.

Published 2026-07-01.