Companion Model · Motor Control
Motor Control Economics
VFD vs Soft Starter vs DOL — lifecycle cost, payback and load physics
Interactive model comparing capex, annual energy cost and 10-year lifecycle NPV for the three basic starting options on any AC motor. Companion to the motor-control primer.
SCOPE · AC VFD ARCHITECTURE
This model prices the traditional AC VFD cabinet. Inside hyperscale AI data centres, DC-fed inverter architectures redistribute the drive stack: the standalone VFD cabinet simplifies to a motor-side inverter card at an estimated 60 to 70 percent of VFD $/kW, with a facility AFE allocation of roughly $8 to $15 per kW on top. Soft starters are nearly absent as a growth socket in that context. For the architectural context and the full category direction, see the DC-native data-centre companion essay.
Motor and duty inputs
LV (≤ 1 kV nominal)
MV (2.3–13.8 kV)
Per IEC 61800. LV covers 400 / 480 / 690 V standard, with some product families extending to ~1.2 kV for mining and marine niches. The 1.2 to 2.3 kV gap reflects real product-family conventions: standard AC motors above 1.2 kV are rare, and semiconductor voltage classes jump from 1700 V IGBTs (used in LV) to 3.3 kV IGBTs / IGCTs (used in MV) with nothing between.
IE1
IE2
IE3
IE4
IE5
Actual efficiency recomputed from IEC 60034-30-1:2025 anchor tables based on the motor rating above (4-pole 50 Hz baseline; pole-count effects are not modelled explicitly, and actual IE values vary modestly between 2-, 4- and 6-pole designs across the rating range). The 2025 edition introduced normative IE5 limits. EU Ecodesign 2019/1781 mandates IE3 for most three-phase motors above 0.75 kW since July 2021 and IE4 for qualifying 75 to 200 kW two-, four- and six-pole motors since July 2023 (with exclusions for brake motors, some explosion-protected motors and specific niche duties).
Centrifugal (P ∝ N³)
Constant torque (P ∝ N)
Constant power (P = const)
Load physics decides most of the VFD economic case:
- Centrifugal (pumps, fans, blowers) — cubic affinity law; 70% speed cuts shaft power to about 34%.
- Constant torque (conveyors, screw compressors, extruders) — power scales linearly; 70% speed cuts shaft power to 70%.
- Constant power (traction, winders, machine spindles) — power is roughly invariant with speed; no VFD energy saving on this duty.
Fraction of rated power the motor draws when running at rated speed.
The remaining share of operating time runs at rated speed.
C&I tariff component. Illustrative potential demand-charge saving assumes the motor's partial-speed operation contributes to the site's billed peak. Actual savings depend on the tariff's demand interval and whether this motor sets the site peak. In process and cooling plants, peak ambient heat often coincides with peak-tariff hours; partial-speed savings during that window are smaller than at average load.
Include
Exclude
Default $/kW/year below. Override to match your service contract or local labour rate.
Advanced: DC-native architecture (bus voltage, breaker topology, AFE allocation)
Illustrative architecture economics. Enables the DC-fed inverter (DCI) scenario against a facility DC bus. Default parameters here (DCI capex ~60-70% of VFD $/kW, AFE allocation $8-18/kW, 800 VDC install uplift -7%, SSCB +0.3 pp loss) are working assumptions rather than empirically anchored vendor data — override via the sliders below to match specific project economics. For a conventional AC VFD comparison, leave this panel collapsed; the baseline DOL / SS / VFD scenarios compute without it.
400 VDC
800 VDC
800 VDC halves the busbar current at the same power. Install uplift drops ~7% versus 400 VDC (copper + conduit savings) but switching losses rise ~0.1 pp as SiC devices operate closer to their voltage rating. Governs the DCI scenario only.
Fuse + contactor
Hybrid
SSCB (SiC)
Fuse-based protection is cheap but single-use and tolerates ms-scale interruption only. Hybrid uses the mechanical path at steady state and semiconductor path on fault (sub-ms interruption, low conduction loss). SSCB interrupts in microseconds but adds ~0.3 pp conduction loss. Each preset auto-sets the AFE allocation and the DCI energy penalty; override the AFE slider manually if needed.
Allocation per kW of mechanical plant. Auto-set by the breaker topology toggle above (fuse $8, hybrid $12, SSCB $18); drag to override.
The core insight
The delta between DOL, SS and VFD comes from the load-physics saving on partial-speed hours, not from the motor class. Pick the right load type and duty profile first; the IE class is a secondary lever.
Lowest modelled 10-year cost
n/a
Set inputs to see the result.
Why the result moves: waterfall from direct-on-line to lowest-cost option
Starting from the DOL / IE1 reference case, green bars are savings and red bars are premiums. The IE class bar moves when you change motor efficiency; the load-physics saving is the VFD affinity-law benefit; the final bar is the winner's 10-year total.
10-year cost stack (capex + energy)
Capex (installed)
Energy over analysis period
Break-even and payback (vs direct-on-line baseline)
Direct-on-line
baseline
Reference point; no capex premium to recover.
Soft starter
n/a
Simple payback of capex premium.
VFD
n/a
Simple payback of capex premium.
DC-fed inverter
n/a
Simple payback vs DOL. Includes allocated facility AFE $/kW.
Side-by-side scenario comparison
Winner column flagged in vermillion. DCI metrics reflect the DC bus voltage and breaker topology toggles above.
Cost of saved energy (vs direct-on-line)
Capex premium divided by cumulative electricity saved over the analysis period. Below the local energy price it is cheaper to save a kWh with this option than to buy one; above it is not.
What has to be true for the winner to win
Set inputs to see the conditions.
Why the result moves
n/a
Annual operating cost (energy + demand + maintenance)
| Option | Energy MWh/yr | Total OPEX/yr | Delta vs DOL |
|---|
Capex, NPV and IRR
| Option | Installed capex | Lifetime NPV | IRR of capex premium |
|---|
Cumulative cost over analysis period
Sensitivity: which inputs move the winner's NPV most
Each bar shows how the winner's 10-year NPV changes when the input moves ±30% around the current value. Wider bars matter more for the decision.
Payback heatmap: duty cycle × load type (VFD vs DOL)
Simple payback years for VFD vs DOL across duty-cycle-at-reduced-speed (columns) and partial-speed setpoint (rows) at the current motor and price settings. Green cells pay back inside the analysis period; red cells never do. Your current setting is highlighted.
Model assumptions
Directional analytical model. Read each category below; replace defaults with project-specific inputs before using for tender-level decisions.
Capex
- Mid-band $/kW figures from the primer's Exhibit 3 (equipment only)
- Illustrative installed-cost multipliers: 1.6× VFD (cabinet, transformer for MV, harmonic filter, install), 1.25× soft starter (bypass, MCC integration), 1.2× DOL (contactor, starter, protection)
- Actual balance-of-plant varies materially by voltage, motor size, harmonic requirements, bypass arrangement and site conditions; replace with project-specific BOP for tender work
Motor efficiency
- IE class (per IEC 60034-30-1) divides every option's shaft-side energy equally
- Motor efficiency matters, but at large motor ratings the incremental IE-class energy difference is generally smaller than the saving created by changing the operating point of a variable-speed load
- The delta between DOL / SS / VFD comes from the load-physics saving on partial-speed hours, not the motor class
- IEC class efficiency is recomputed from IEC 60034-30-1:2025 anchor tables based on the motor rating selected above. The 4-pole 50 Hz baseline is used; pole-count effects are not modelled explicitly.
- VFD conversion loss is modelled as an illustrative 2.5% stacked on top of motor loss (applies to the VFD path only). Actual drive efficiency varies with load, voltage class and topology; typical LV IGBT drives run 96 to 98% at rated load, MV CHB drives run 96 to 97% including the phase-shifting transformer. Soft-starter and DOL paths add no drive loss on top of the motor.
Load-physics scaling
- Affinity-law scaling (P ∝ N³) for centrifugal loads
- Approximately linear scaling as a user-selected assumption for other loads
- Constant-torque loads scale approximately linearly with speed; VFD energy savings are therefore materially smaller than on centrifugal (affinity-law) loads. Constant-power loads show no VFD energy saving.
- Base case assumes the remaining operating time is at rated speed
Demand charges
- Reflect the C&I tariff structure (kW × $/kW/month × 12) where a large portion of the bill runs on billing demand, with energy as a smaller component
- A VFD can reduce the motor's contribution to peak demand during partial-speed hours; soft starter and DOL do not materially change running demand
- Illustrative. Actual savings depend on the site's billing-demand interval and whether this motor contributes to the site peak
Maintenance (optional)
- Included only when the maintenance toggle is set
- VFD $8/kW-year. DC-bus capacitor and cooling fan overhaul at year 7, amortised
- Soft starter $2/kW-year. Bypass contactor cycling
- DOL $1/kW-year. Starter contactor
Excluded from the model
- Control-integration cost
- Harmonic-mitigation retrofits
- Cybersecurity compliance overhead (IEC 62443 / EU CRA programme cost)
- Vendor-specific pricing (list-band inputs only)