How to Size an Electric Motor: Step-by-Step Guide

Proper motor sizing ensures reliable operation and energy efficiency. Here's how engineers approach motor selection for common industrial applications.

Step 1: Determine the Load Type

• Constant torque: Conveyors, positive displacement pumps, compressors, hoists. Torque stays constant regardless of speed. HP varies linearly with speed.
• Variable torque: Centrifugal fans, centrifugal pumps, blowers. Torque varies with speed squared. HP varies with speed cubed — big energy savings at reduced speeds.
• Constant horsepower: Machine tool spindles, winders, some paper machines. HP is constant; torque decreases as speed increases.

Step 2: Calculate Required Torque

Torque (lb-ft) = HP x 5252 / RPM

For conveyors: Torque = (Belt pull in lbs x Pulley radius in ft) For fans: Use fan manufacturer's brake horsepower at design CFM For pumps: HP = (GPM x Head in feet x Specific Gravity) / (3960 x Pump Efficiency)

Step 3: Select Motor Speed

Standard induction motor synchronous speeds at 60 Hz: - 2-pole: 3600 RPM (actual ~3450 RPM) - 4-pole: 1800 RPM (actual ~1750 RPM) — most common - 6-pole: 1200 RPM (actual ~1150 RPM) - 8-pole: 900 RPM (actual ~870 RPM)

Choose the speed that best matches your application (with gear reduction if needed).

Step 4: Apply Service Factor

Service factor accounts for overloads, duty cycle, and environmental conditions: - Standard industrial: 1.15 minimum - Heavy-duty or high ambient temperature: 1.25 or higher - Frequent starts/stops: 1.25 or higher - Actual motor HP selected should be: Calculated HP x Service Factor

Step 5: Select Voltage and Phase

• Single-phase 115/230V: Up to 5 HP (residential, light commercial)
• Three-phase 230/460V: 1 HP and up (industrial standard)
• Three-phase 575V: Available in larger sizes for facilities with 600V service

Step 6: Determine Enclosure Type

• ODP: Indoor, clean, dry locations
• TEFC: Most industrial applications — dust, moisture, outdoor
• XPRF: Hazardous locations (Class I, Division 1 or 2)
• TENV: No external fan, used with VFDs at low speeds

Step 7: Verify Frame Size and Mounting

Match the NEMA frame size to your existing motor footprint. Key dimensions: - Shaft height, shaft diameter, shaft length - Bolt hole pattern (foot mount, C-face, D-flange) - Conduit box location

Service Bearing can help you size, select, and cross-reference motors for any application. Call us with your requirements and we'll get you the right motor at a competitive price.