Complete Guide to Efficiency: Motors, Generators, Transformers
The Efficiency Calculator computes % efficiency from input/output power or losses. Critical for energy savings and NEC compliance.
What is Efficiency?
η = (Output Power / Input Power) × 100%
Or: η = (Input - Losses) / Input × 100%
Why Efficiency Matters
- Energy Cost: 1% gain saves thousands yearly
- Heat Reduction: Lower losses → cooler operation
- NEC 430.7: Premium motors ≥ NEMA Table 12-12
Use with Motor Load and Energy Cost calculators.
Efficiency Formulas
| Device | Formula |
|---|---|
| General | η = P_out / P_in × 100 |
| Motor | η = (746 × HP) / P_in × 100 |
| Generator | η = P_out / (P_out + Losses) × 100 |
| Transformer | η = 1 / (1 + Losses/P_out) × 100 |
NEMA Motor Efficiency Standards
| HP | Premium (%) | Standard (%) |
|---|---|---|
| 1 | 85.5 | 82.5 |
| 10 | 91.7 | 89.5 |
| 100 | 95.4 | 94.1 |
Loss Types
- Copper (I²R): Load-dependent
- Iron (Core): Voltage-dependent
- Stray, Friction, Windage
Calculation Steps
- Measure P_in, P_out: Wattmeters
- Or sum losses: No-load + load test
- Apply formula
- Compare to nameplate
Example: Motor 10 HP, 92% efficient
- P_out = 10 × 746 = 7460 W
- P_in = 7460 / 0.92 ≈ 8110 W
- Losses = 650 W
Transformer Example
- 100 kVA, 2500 W losses → η ≈ 97.6%
Efficiency vs Load
Max at 75–100% load. Drops at partial load.
Improving Efficiency
- Premium Motors: IE3/IE4
- VFDs: Match speed to load
- Proper Sizing: Avoid oversizing
- Maintenance: Clean, lubricate
Testing Methods
- IEEE 112: Motor dyno test
- No-load Test: Iron + friction
- Blocked Rotor: Copper losses
Common Mistakes
- Using kVA for motors: Use HP
- Ignoring PF: P_in = V×I×PF
- Part-load assumption: Efficiency ≠ nameplate
Energy Savings
Upgrading 10 HP motor: 89.5% → 91.7% → saves ~$300/year at $0.10/kWh, 4000 hrs.
Advanced Topics
- SEER, COP: HVAC efficiency
- IE Codes: IEC 60034-30-1
- Life Cycle Cost: Initial vs operating
FAQs
Can efficiency be >100%? No — violates thermodynamics.
Why nameplate efficiency? Full load, rated voltage.
Generator vs motor? Same formula, reverse power flow.
Conclusion
Efficiency drives cost and sustainability. Our Efficiency Calculator ensures accurate design. Optimize with Motor Load, Energy Cost, and Construction tools. Save energy with 1000 Calculators.