Electric motor

01

Size the motor for the load it will actually run at

The US Department of Energy's motor fact sheet states that "most electric motors are designed to run at 50% to 100% of rated load" and that "maximum efficiency is usually near 75% of rated load"; a 10 hp motor therefore suits loads of about 5–10 hp. Efficiency drops significantly as load falls into the underloaded range, and DOE says "power factor tends to drop off sooner, but less steeply than efficiency, as load decreases". [1]

Oversizing can come from covering an occasional peak; DOE notes that "far too often motors are mismatched—or oversized—for the load they are intended to serve". DOE suggests two-speed motors, adjustable-speed drives or load management as alternatives for variable loads. [1]

DOE explains service factor: a 10 hp motor with a 1.15 service factor can carry 11.5 hp for short periods without significant damage, but running continuously above rating reduces efficiency and life, and a motor should never be overloaded when voltage is low or cooling is impaired by altitude, high ambient temperature or dirt. [1]

Give the supplier the driven machine's power at its normal and peak duty, starting conditions and hours per year. A nameplate copied from the old motor repeats its sizing mistakes.

02

Check the efficiency class required where the motor will be used

In the European Union, Regulation (EU) 2019/1781 has required, from 1 July 2021, that three-phase 2-, 4-, 6- and 8-pole motors from 0.75 kW to 1,000 kW meet at least IE3, and those from 0.12 kW to below 0.75 kW at least IE2 (Ex eb increased-safety motors excluded). From 1 July 2023 it added IE4 for three-phase 2-, 4- and 6-pole motors from 75 kW to 200 kW (with listed exclusions), and IE2 for single-phase motors of 0.12 kW and above and for Ex eb motors. [2]

Other markets set their own minimum efficiency rules. State the market where the machine or motor will be placed, and ask the supplier for the efficiency class, the tested efficiency at rated load and the test standard on the nameplate and data sheet.

DOE notes that whether a premium-efficiency motor pays back depends on running hours, electricity price and the price premium; it suggests replacing significantly oversized motors at the next opportunity and properly sized standard-efficiency motors when they fail. [1]

03

Measure the load of an existing motor before replacing it

DOE describes three ways to estimate load: measured input power compared with full-load input, a voltage-corrected current ratio, or the slip method. It recommends surveying all motors that run more than 1,000 hours a year. [1]

Current varies roughly linearly with load down to about 50% load; below that, magnetising current makes current a poor guide. Nameplate full-load current applies only at rated voltage, so measured current must be corrected for actual voltage. [1]

The slip method uses speed. An induction motor's synchronous speed depends on supply frequency and pole count (for example 1,800 rpm for 4 poles at 60 Hz), and slip below synchronous speed is roughly proportional to load: at 50% load the speed is halfway between full-load and synchronous speed. As a calculation example only, a 4-pole 60 Hz motor rated 1,750 rpm at full load and measured at 1,775 rpm is running at about (1,800 − 1,775) ÷ (1,800 − 1,750) = 50% load. [1]

04

Match motor type, supply and enclosure to the application

Suppliers linked to this page offer three-phase and single-phase AC motors, DC motors and gear motors, and stainless-steel motors for washdown. [3] [4] [5]

Hydraflu, for example, lists 12/24 V DC motors for hydraulic power units on mobile machines and single- and three-phase AC motors from 100–120 V to 380–415 V for indoor units. Sunrise lists brushless DC and permanent-magnet DC motors, with gearboxes. [4] [5]

Specify supply voltage and frequency, single- or three-phase, the starting method (direct-on-line, soft starter or variable-speed drive), enclosure and IP rating, mounting (foot, flange or both), frame size, duty cycle and ambient temperature and altitude.

05

Trace the motor components

This is a functional reference built from the sources cited; it is not one maker's parts list. Materials and ratings come from the data sheet of the motor you order.

06

Fill the specification fields with values for your drive

A motor quotation needs drive-specific values. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the load and site data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Load: [driven machine; normal and peak power; starting torque; duty cycle; hours per year]
Speed: [required speed; pole count; fixed or variable speed]
Supply: [voltage; frequency; single- or three-phase; starting method or drive]
Efficiency: [market of use; required IE class; tested efficiency]
Enclosure: [IP rating; cooling; stainless or washdown; hazardous area]
Mounting: [foot, flange or both; frame size; shaft dimensions]
Environment: [ambient temperature; altitude; dust; humidity]
Accessories: [thermal protection; anti-condensation heater; brake; encoder]
Documents: [data sheet; efficiency certificate; dimension drawing; test report]

See manufacturer records Send an enquiry through CNFX