Electric Motor Selection Step by Step: From Two Lines of Machine Data to the Order

A feed mill is installing a new horizontal mixer. The machine builder's data sheet gives exactly two lines: the mixer shaft turns at 45 rpm, and with a full trough it needs about 1,600 Nm at the shaft. The maintenance manager has no motor rating, no speed and no mounting arrangement to go on. This guide starts from those two lines and follows every decision through to a complete motor specification on the purchase order, showing how each choice shapes the next.

The example is a mixer, but the same logic works for a conveyor, a pump or a fan: first what the machine needs, then the speed and power at which a motor can deliver it, then electrical and mechanical fit, and finally the environment and paperwork.

What the machine asks for

The first question is not "how many kW?" but "what torque does the machine need at what shaft speed?" Power follows from those two: P (kW) ≈ M (Nm) × n (rpm) / 9550. For the mixer that is 1,600 × 45 / 9550 ≈ 7.5 kW. That is the power the mixer shaft absorbs, not yet the power the motor must supply.

Note the character of the load at the same time. A mixer is a constant-torque load: its torque demand hardly falls when speed drops, and after a power cut it may have to restart with a full trough. Both facts will matter later for the power margin and the starting method. For pumps, fans and conveyors, our article on motor power calculation shows how to find shaft power with worked examples.

Bridging motor speed and machine speed

No induction motor runs at 45 rpm, so a gearbox, a belt drive or both must close the gap, and that is where the number of poles comes in. On a 50 Hz supply a 4-pole motor has a synchronous speed of about 1,500 rpm and a 6-pole motor about 1,000 rpm; the actual speed under load is slightly lower.

With a 4-pole motor rated around 1,460 rpm, the required ratio is roughly 1,460 / 45 ≈ 32.4. A 6-pole motor at about 970 rpm brings it down to around 21.6. The 6-pole motor is larger and usually dearer for the same power, while the 4-pole range is the most common and easiest to stock as a spare. In practice the decision is often made on the gearbox side: the motor's pole count is matched to a standard ratio in the chosen gearbox family. Ratio, output torque and gearbox type are covered in our gearbox selection guide. Here we continue with a 4-pole motor and a helical gearbox.

Choosing the power step

Gearbox losses must be added to the 7.5 kW at the mixer shaft. Helical units are generally efficient; taking 95% for simplicity gives about 7.9 kW at the motor shaft. With standard steps of 7.5 and 11 kW, a 7.5 kW motor falls short even on paper.

There is a second reason to go to 11 kW: restarting with a full trough, which can call for more than rated torque at breakaway. Rated torque of an 11 kW 4-pole motor is about 9550 × 11 / 1460 ≈ 72 Nm. Whether starting torque is sufficient is checked by comparing the starting torque ratio in the motor catalogue with the starting requirement stated by the machine builder. The link between starting torque and starting method is explained in torque design classes and starting torque.

Oversizing is the opposite mistake. Picking 15 kW "to be safe" leaves the motor permanently lightly loaded, lowering efficiency and power factor and enlarging the panel and cables for no benefit.

Efficiency class and running hours

The mixer will run two shifts, about 4,000 hours a year. For an 11 kW 4-pole motor, the IEC 60034-30-1 minimum efficiencies are 91.4% (IE3), 93.3% (IE4) and 94.6% (IE5). With the motor running at around 8 kW, a rough estimate puts the IE3 to IE4 difference at 700–1,000 kWh a year, far less striking than on a fan running round the clock. The choice is weighed against the price difference and the site's energy targets. Because HEM Motor's three-phase ranges offer IE3, IE4 and IE5 within the same frame system, the efficiency class does not change mechanical dimensions.

Electrical fit: voltage, connection, starting

The plant has a 400 V supply. According to the catalogue, on 400 V motors up to 3 kW are connected in star and motors of 4 kW and above in delta, so the 11 kW motor will run in delta. Then look at the panel. If it uses a star-delta starter, the motor must be wound to run in delta at 400 V, meaning a 400/690 V nameplate; a 230/400 V motor cannot start correctly from that panel. With a soft starter or a variable frequency drive, the winding voltage still has to suit the supply, but the connection stays fixed in the terminal box.

Because this load is heavy at start, star-delta deserves caution: in the star stage the motor develops only about one third of its torque, which may not turn a full trough. A method with adjustable starting torque, a soft starter or a drive, is the safer choice here. Wiring details are in our star-delta wiring guide.

Mechanical fit: mounting, frame and shaft

When the motor bolts directly to the gearbox, the gearbox decides the mounting. A motor on the IEC input flange of a foot-mounted helical gearbox is usually B5 flanged, with no feet of its own; the gearbox carries its weight. If the motor sits on a separate base with a coupling, a B3 foot-mounted motor is used, and B35 covers cases needing both feet and flange.

In IEC tables an 11 kW 4-pole motor is usually in frame 160, the number being the shaft centre height in millimetres. Shaft diameter, key and flange size must match the gearbox input bore exactly, which is verified by laying the two dimension drawings side by side; "frame 160 B5" alone is not enough. Terminal box position is settled at this stage too: if the mixer stands close to a wall, the cable entry should not face it.

Environment: dust, heat and cleaning

A feed mill has flour and bran dust, the mixer is indoors and the hall reaches 35 °C in summer. The standard three-phase range is designed for IP55 and ambient temperatures from −15 °C to +40 °C, so these conditions are within standard limits. Two points still belong on the order:

• Dust packed between the cooling fins and on the fan cowl reduces cooling, so a cleaning routine is part of the motor choice.
• Above 40 °C ambient or 1,000 m altitude, derating or the next power size may be needed, and this must be stated before ordering.

If combustible dust means the area is classified as hazardous, a standard motor cannot be used there. Ex motors are separately certified products, and that requirement should be discussed with the technical team.

The line that goes on the order

After these decisions the order line reads something like: 11 kW, 4-pole (≈1,500 rpm), IE3, 400/690 V (delta at 400 V), 50 Hz, B5 flange, frame 160, IP55, class F insulation, S1, terminal box on the right; gearbox input flange and bore drawing attached. A note adds that it will run on a soft starter or drive and may start with a full trough.

The information every quote request should carry is listed in what to include when requesting a motor quote, and typical incomplete orders from the field are collected in what to check before buying an electric motor.

Conclusion

Motor selection is a chain of linked decisions, not a form: the machine's torque and speed set the gearbox ratio, the ratio sets the pole count, the starting condition sets power and starting method, and the gearbox input sets the mounting. Get the two facts at the start of the chain right and the rest can be calculated. For geared applications you can look at our M-HEM foot-mounted helical gearboxes and work out your specification with our technical team.

Frequently Asked Questions

How do I choose a motor if the machine builder only gives kW?

Ask for the required shaft speed as well, because kW and speed together give torque. A motor bought on kW alone will run the machine at the wrong speed if the gearbox or pulley ratio in between does not match.

If the calculated power falls between two sizes, which should I pick?

Usually the next size up, particularly for heavy starting or shock loads. Going far above the calculated power, however, leaves the motor lightly loaded and lowers efficiency and power factor.

Should I use a 4-pole or a 6-pole motor?

For a slow machine both work with a gearbox. A 4-pole motor is more common, smaller and easier to keep as a spare, while a 6-pole motor lowers the gearbox ratio. The decision often follows the standard ratios of the gearbox range.

Who decides the mounting when the motor drives a gearbox?

The gearbox input does. With an IEC flanged input the motor is usually B5 or B14, and the flange and shaft size required are read from the gearbox dimension drawing.