Motor Power Basics
Electric motor ratings tell you shaft output (mechanical power). The electrical input is higher because no motor is 100% efficient. Knowing both prevents tripped breakers and undersized wiring.
Power Conversions
1 hp = 745.7 W = 0.7457 kW
1 kW = 1.341 hp
1 kVA = 1000 VA (apparent, AC circuits)
Shaft power: P_mech = τ × ω (N·m × rad/s)
P_mech = τ × 2π × n/60 (n in RPM)Electrical Input from Shaft Output
Single-phase: I = P_mech / (V × PF × η)
Three-phase: I = P_mech / (√3 × V_L × PF × η)
5 hp motor, 230V 3-phase, PF=0.85, η=0.90:
P_input = 5 × 745.7 / 0.90 = 4143 W
I = 4143 / (1.732 × 230 × 0.85) = 12.2 ASelecting the Right Motor Size
- Calculate load torque and speed → get required kW
- Add 20-25% service factor for intermittent peak loads
- Choose next standard frame size above calculated kW
- IE3 (premium efficiency) motors required in most industrial applications (EU, US)
Convert motor power: Free Motor Power Calculator
Motor Power Formulas
- Mechanical output power: P = T × ω = T × (2πN/60), where T = torque (N·m), N = speed (RPM)
- Electrical input power (3-phase): P_in = √3 × V × I × PF
- Motor efficiency: η = P_out / P_in × 100%
- Starting current: Typically 6–8 × full load current for DOL (direct on line) starting
Motor Selection Criteria
Motor selection involves more than matching rated power to load. Service factor (SF): standard motors have SF = 1.0–1.15, meaning occasional overloads up to 15% are permitted. Duty cycle: S1 = continuous; S3 = intermittent periodic; S6 = continuous periodic. Enclosure type: IP54 (protected from dust and water splash) is standard industrial; IP65 is fully dust-tight and water jet protected; ATEX certified for explosive atmospheres. Insulation class: F (155°C) is standard; H (180°C) for high ambient temperature. Starting method: DOL (full voltage start) is simplest but draws high starting current; star-delta, autotransformer, or VSD (variable speed drive) reduce starting current and mechanical shock.
Frequently Asked Questions
What does motor efficiency class IE1, IE2, IE3 mean?
IEC 60034-30 defines International Efficiency (IE) classes: IE1 = Standard efficiency (~90% at 4 kW), IE2 = High efficiency (~91.5%), IE3 = Premium efficiency (~92.4%), IE4 = Super premium. EU regulations mandate IE3 for most industrial motors above 0.75 kW since 2021. Higher efficiency reduces operating costs: a 4 kW motor running 8,000 h/year at €0.15/kWh saves ~€70/year upgrading from IE1 to IE3 — typically paying back in 1–3 years.
What is slip in an induction motor?
Slip = (synchronous speed - actual speed) / synchronous speed × 100%. Synchronous speed = 120f/p where f = supply frequency (Hz), p = number of poles. A 4-pole motor on 50 Hz has synchronous speed 1,500 RPM; actual nameplate speed might be 1,460 RPM; slip = (1500-1460)/1500 = 2.7%. Slip increases under load — it is the mechanism by which an induction motor develops torque. Slip is small (1–5%) in normal operation.
When should I use a VSD (variable speed drive)?
VSD saves energy when load power varies with speed cubed (pumps, fans, compressors) — reducing speed to 75% cuts power to ~42%. Also use when: soft starting reduces mechanical shock and extends belt/coupling life; speed control is required for the process; motor size must be reduced by limiting starting current. VSD adds cost (£500–£5,000+ depending on size) but pays back quickly in energy-intensive applications with partial loading.