How to Choose an Electric Motor: Power, Speed, Starting and Enclosure

Picking a three-phase induction motor for a pump, fan, conveyor or machine, in the order the decisions actually come: what the load needs, how fast it turns, what supply you have, how it starts, and where it lives. Each step ends on the chart that answers it.

1. Know what kind of load it is

Before any number, decide how the load's torque behaves with speed. It changes the motor size, the starter and whether a drive saves money.

Load type	Examples	What it means
Variable torque	Centrifugal pumps, fans, blowers	Torque rises with speed squared, power with speed cubed. Easy to start; slowing it with a drive saves a lot of energy.
Constant torque	Conveyors, positive-displacement pumps, compressors, mixers, hoists	Needs full torque from standstill. Hard to start; check starting torque and the starter.
Constant power	Winders, lathe and mill spindles	High torque at low speed, less at high speed. Usually a drive, sized at the lowest working speed.

2. Work out the power the load absorbs

The motor's rated power is shaft output, so size it from the power the driven machine takes, not from a guess. Use whichever you know:

From torque and speed: P (kW) = T (N·m) × n (rpm) ÷ 9550. The torque calculator does the conversions.
Pump: P = flow × head × density × g ÷ pump efficiency. The pump power calculator takes m³/h or US gpm and metres or feet.
Fan: P = airflow (m³/s) × pressure (Pa) ÷ fan efficiency, in watts.
Conveyor or hoist: P = force (N) × speed (m/s), in watts, divided by the efficiency of the gearbox and chain.

Add a margin of 10–15% for wear, a dirty filter or a heavier product, then take the next standard size up. Don't add margin on margin: the pump maker's figure often includes one already. An oversized motor runs at poor power factor and lower efficiency, and costs more to start.

Standard kW	≈ HP	Typical amps, 400 V
0.75	1.0	1.7
1.1	1.5	2.4
1.5	2.0	3.3
2.2	3.0	4.6
3	4.0	6.2
4	5.4	8.1
5.5	7.4	11
7.5	10	14.8
11	15	21
15	20	28.5
18.5	25	35
22	30	41

Amps are for a 4-pole IE3 motor; the motor current chart has the full range, the US NEC table and the cable size for each.

3. Choose the speed

An induction motor's speed is set by its poles and the supply frequency: roughly 2900, 1450, 960 or 720 rpm at 50 Hz (3500, 1750, 1160, 870 at 60 Hz) for 2, 4, 6 and 8 poles. 4-pole is the most common, cheapest and quietest per kW. If the machine needs a speed in between:

a belt and pulleys — the pulley speed calculator gives the ratio;
a gearbox or geared motor for big reductions — see the gear ratio calculator; the output torque goes up by the ratio;
a variable-speed drive (VFD, inverter) when the speed must change in service.

4. Match the supply

Three-phase is the default above about 2 kW. Read the voltage of the supply and order a motor wound for it: 230/400 V Δ/Y runs star on a 400 V supply, 400/690 V Δ/Y runs delta on it; in North America, 230/460 V dual-voltage motors are the stock item. Where there is only single phase, the options are a single-phase motor (small, usually up to about 2.2 kW) or a three-phase motor on a drive with single-phase input — often the better machine, with soft starting thrown in.

5. Decide how it starts

Method	Starting current	Use it for
Direct on line (DOL)	6–8 × full load	Small motors and stiff supplies. Cheapest, full starting torque.
Star-delta	About ⅓ of DOL	Light starts (fans, unloaded pumps). Starting torque is also ⅓. Needs a motor whose delta voltage is the supply voltage, e.g. 400/690 V on a 400 V supply.
Soft starter	2–4 × full load, adjustable	Pumps (no water hammer) and conveyors that must start gently.
Variable-speed drive	About 1–1.5 × full load	Anything that needs speed control, hard starts on weak supplies, and fans and pumps that can run slower to save energy.

A motor on a drive running slowly on a constant-torque load cools less well, because its own fan slows too: below about half speed, oversize it or fit a separately powered fan. Long cables from a drive need a motor rated for inverter duty.

6. Fit it to where it lives

Enclosure: IP55 is the industrial standard; IP56 or IP66 for hose-down and outdoor; totally enclosed (TEFC) in North America. Explosive atmospheres need an ATEX or IECEx certified motor — never a standard one.
Ambient and altitude: rated output assumes 40 °C and 1000 m. Hotter or higher, the motor must be derated or one size up.
Duty: S1 is continuous running. Frequent starts and stops, or reversing, need a motor rated for that duty — say how many starts an hour when you order.
Mounting: feet (B3), flange (B5, B14) or both (B35). Coupling to a pump or gearbox usually decides it. Every size is on the IEC frame chart and the NEMA frame chart.

7. Efficiency class

In the EU a new motor from 0.75 to 1000 kW must be at least IE3, and from 75 to 200 kW IE4. Elsewhere IE3 (NEMA Premium) is still usually the right buy: over its life, a motor that runs most of the day costs many times its price in electricity. The minimum figures for each class are on the motor efficiency chart.

The order, on one line

Put it together as the supplier reads it: power, poles, voltage and frequency, frame and mounting, enclosure, efficiency, duty. For example: 5.5 kW, 2-pole, 400 V Y 50 Hz, 132S B35, IP55, IE3, S1.

Worked example: a centrifugal pump

A pump must move 30 m³/h of water against a 40 m head, and the pump curve gives 65% efficiency at that point at 2900 rpm. Absorbed power is 30 ÷ 3600 × 40 × 1000 × 9.81 ÷ 0.65 = 5.0 kW. The next standard size with margin is 5.5 kW, 2-pole to match the pump speed. On a 400 V supply it draws about 10–11 A; a soft starter avoids water hammer. The frame is 132S, and the pump's bracket wants the B35 foot-and-flange version. The order is the line above.

Replacing an existing motor rather than choosing a new one? Start from the plate instead: how to read a motor nameplate.

Once it is fitted, log it in the free motor register workbook so the next person doesn't have to read the plate again.