Selecting the right motor ensures reliable operation, energy efficiency, and long service life. Undersizing causes overheating and premature failure; oversizing wastes capital and runs inefficiently at partial load. This guide covers the key factors in motor selection.
Calculating Required Horsepower
The fundamental relationship:
HP = (Torque × RPM) / 5252
Where torque is in lb-ft. For loads defined by force and velocity:
HP = (Force × Velocity) / 33,000
Where force is in pounds and velocity is in ft/min.
Common Application Formulas
Conveyor
HP = (Belt speed × Belt tension) / 33,000
Pump
HP = (GPM × Head × SG) / (3960 × Efficiency)
Fan/Blower
HP = (CFM × Pressure) / (6356 × Efficiency)
Calculate Required Motor HP
Enter your load parameters to find the right motor size. Includes service factor recommendations.
Service Factor (SF)
Service factor is the allowable continuous overload. A 10 HP motor with SF 1.15 can run continuously at 11.5 HP:
- SF 1.0: No overload capacity; run at nameplate or below
- SF 1.15: Standard for most industrial motors
- SF 1.25: Some special-purpose motors
Use service factor as a margin, not as rated capacity. Running at SF continuously shortens motor life.
Motor Speed Selection
Standard speeds at 60 Hz:
| Poles | Synchronous | Full Load (typical) |
|---|---|---|
| 2 | 3600 RPM | 3450 RPM |
| 4 | 1800 RPM | 1750 RPM |
| 6 | 1200 RPM | 1150 RPM |
| 8 | 900 RPM | 870 RPM |
Higher speeds = smaller, less expensive motors. Use gear reducers or belt drives to match output speed to application needs.
NEMA Motor Designs
- Design A: High efficiency, high starting current
- Design B: Most common; general purpose
- Design C: High starting torque (conveyors, crushers)
- Design D: Very high starting torque, high slip (hoists, punch presses)
Enclosure Types
- ODP (Open Drip Proof): Indoor, clean environments
- TEFC (Totally Enclosed Fan Cooled): Standard industrial, dust/moisture resistant
- TENV: No external fan, for variable speed applications
- TEBC: Blower cooled for VFD applications
- XP (Explosion Proof): Hazardous locations
VFD Considerations
When using Variable Frequency Drives:
- Use inverter-duty rated motors
- Cooling is reduced at low speeds—derate or add forced cooling
- Consider 4:1 or 10:1 turndown ratio requirements
- Constant torque vs. variable torque loads have different sizing
Starting Requirements
Motors draw 5-8× full load current during starting. Consider:
- Across-the-line: Simple but high inrush current
- Soft starter: Reduces inrush by 50-70%
- VFD: Lowest inrush, best control
- Wye-delta: Reduces voltage during start
Check with your electrical utility for inrush limits and demand charges.
Common Mistakes
- Ignoring duty cycle: Intermittent duty may allow smaller motors
- Wrong voltage: 230V vs 460V selection affects efficiency and cost
- Oversizing for starting: VFDs often eliminate the need to oversize
- Ignoring power factor: Oversized motors run at poor power factor
Motor selection involves balancing performance, efficiency, reliability, and cost. When in doubt, consult motor manufacturers' selection guides or application engineers for your specific requirements.
