How to Choose an Industrial Electric Motor

Selecting the wrong electric motor is one of the costliest mistakes in industrial equipment procurement. An oversized motor wastes energy; an undersized one fails prematurely. This guide walks through every selection parameter in order of importance, so you can specify the right motor the first time.

Step 1 — Determine the Required Power (kW)

Power is the foundational parameter. Calculate it from the mechanical load:

Conveyors: P = (F × v) / (1000 × η), where F is traction force (N), v is belt speed (m/s) and η is drivetrain efficiency.
Pumps: P = (Q × H × ρ × g) / (3600 × η_pump × η_motor), where Q is flow (m³/h) and H is head (m).
Fans: P = (Q × ΔP) / (1000 × η_fan), where Q is volumetric flow (m³/s) and ΔP is pressure rise (Pa).

Apply a service factor of 1.15–1.25 to the calculated power to account for load variations, starting conditions and future capacity increases. Always round up to the next standard power rating in the IEC series: 0.25, 0.37, 0.55, 0.75, 1.1, 1.5, 2.2, 3, 4, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90, 110, 132, 160, 200, 250, 315, 400, 500 kW.

Step 2 — Select the Speed (Number of Poles)

The number of poles determines synchronous speed at 50 Hz:

2 poles → 3000 RPM (actual: ~2900 RPM)
4 poles → 1500 RPM (actual: ~1450 RPM)
6 poles → 1000 RPM (actual: ~960 RPM)
8 poles → 750 RPM (actual: ~720 RPM)

Match the motor speed to the driven machine speed. If the driven machine speed differs significantly from any standard motor speed, a gearbox or VFD is required. Four-pole motors are the most common choice — they offer the best balance of efficiency, power density and starting torque for the majority of industrial applications.

Step 3 — Choose the IEC Frame Size

IEC frame size (e.g. 90, 112, 132, 160, 180, 200, 225, 250, 280, 315, 355) defines the shaft height and bolt hole pattern. It is determined by the motor's power and speed. For replacement motors, the frame size must match the existing installation exactly to ensure bolt-in compatibility. For new designs, the frame size follows automatically from the power/speed selection using the manufacturer's selection tables.

The IEC frame number equals the shaft centreline height above the foot mounting surface in millimetres (e.g. Frame 132 = 132 mm shaft height). This standardisation ensures interchangeability between manufacturers.

Step 4 — Select the Efficiency Class (IE1 to IE4)

The International Efficiency (IE) standard IEC 60034-30-1 defines four classes:

IE1 (Standard): Minimum efficiency. Banned for direct-on-line use in the EU above 0.75 kW.
IE2 (High Efficiency): Acceptable for VFD-driven applications in the EU from 0.75 kW.
IE3 (Premium Efficiency): Mandatory minimum for direct-on-line motors 0.75–1000 kW in the EU (Regulation 2019/1781).
IE4 (Super Premium): Lowest lifecycle cost for high-utilisation applications. Available as induction (copper rotor), PMSM or SynRM.

Specify IE3 as the absolute minimum for new EU installations. For motors running more than 4000 hours per year above 11 kW, evaluate IE4 — the energy savings often recover the price premium within 18 months.

Step 5 — Specify IP Protection and Ambient Conditions

The IP (Ingress Protection) rating has two digits: the first is solid particle protection (0–6), the second is liquid protection (0–8). Common ratings for industrial motors:

IP44: Indoor, clean environments. Light splash protection.
IP55: Standard outdoor duty. Protection against dust and directed water jets. Most common.
IP65/IP66: Washdown-duty applications (food, chemical, quarry).
IP67/IP68: Temporary or continuous submersion. Specialised pump applications.

Standard motors are rated for maximum ambient temperature of 40°C. For higher ambient temperatures, apply derating per IEC 60034-1 or specify Class H insulation motors.

Step 6 — Select the Mounting Configuration

IEC 60034-7 defines standard mounting codes (IM codes):

B3 (IM B3): Foot mounting, horizontal shaft. Most common.
B5 (IM B5): Large flange, horizontal shaft. Used for pump close-coupling.
B14 (IM B14): Small flange (face mounting). Compact, used with gearbox flanges.
B35 (IM B35): Combined foot + flange. Maximum flexibility.
V1 (IM V1): Flange mounting, vertical shaft downward. Vertical pump drives.

Select the mounting that matches your drivetrain geometry. Foot-mounted (B3) is the default for most applications; flange-mounted (B5/B14) is preferred for direct gearbox coupling.

Step 7 — Check Starting Method and Torque Requirements

Verify the motor provides sufficient starting torque for the application:

Direct-on-line (DOL): Full starting torque, high inrush current (600–800% of rated). Used below 5–7.5 kW.
Star-delta: Reduces starting current to 33% but also reduces starting torque to 33%. Suitable for low-resistance loads (centrifugal pumps, fans).
Soft starter: Electronic current ramp. Good for conveyors and compressors.
VFD: Full torque at zero speed, lowest starting current. Best for variable-speed applications.

High-inertia loads (flywheels, large fans, centrifuges) require motors with high slip (Design E) or extended acceleration time to avoid thermal damage during start-up.

Step 8 — Consider Special Options

For specific applications, include these options in your specification:

Integral brake: Spring-applied DC or AC service brake for hoists, conveyors, inclined drives.
PT100 thermistors: Winding temperature monitoring for critical process motors.
Anti-condensation heaters: Prevent moisture build-up during standstill in humid environments.
Reinforced bearings: For belt-drive or overhung load applications.
Inverter duty (VFD-rated): Reinforced insulation system for PWM drive operation.
ATEX certification: For hazardous area (Zone 1/Zone 2) installations.

Summary: Motor Selection Checklist

Calculate required power (kW) + apply service factor
Determine required speed (poles: 2/4/6/8)
Confirm IEC frame size for mechanical fit
Specify IE3 minimum efficiency (IE4 for high-run-hours)
Select IP rating for the environment (IP55 standard outdoor)
Choose mounting (B3/B5/B14/B35/V1)
Verify starting torque and current for the load type
Add special options (brake, thermistors, ATEX)

INDASTRA carries motors meeting every combination of these parameters from 700+ world manufacturers. Use our Analog Finder to cross-reference your existing motor type code against available alternatives.