What to Check Before Buying an Electric Motor: Where Orders Usually Go Wrong

You usually find out that a motor order went wrong the moment the motor is unpacked next to the machine. The flange holes don't line up, the shaft is too thick for the coupling, or the motor runs fine but the star-delta starter in the panel can't be used with it. In almost every one of these cases there is nothing wrong with the motor itself. What went wrong was the information passed on with the order.

Below are the ordering mistakes we run into most often in practice, what causes each one, how it shows up on site, and how to catch it before the order is placed.

"A 7.5 kW motor" is only half a specification

Power alone doesn't describe a motor, because the same power means very different torque at different speeds. Shaft torque is roughly 9550 × kW ÷ rpm. A 5.5 kW motor delivers about 35 Nm at 1500 rpm, but about 52 Nm at 1000 rpm. If the pulley ratio or gearbox was sized around one of those, a motor with the wrong speed will either overspeed the machine or struggle with the load.

Speed follows from the number of poles. On a 50 Hz supply, a 2-pole motor runs at roughly 3000 rpm, 4-pole at 1500, 6-pole at 1000 and 8-pole at 750. The nameplate will show a figure slightly below these round numbers, because an induction motor always runs a little slower than synchronous speed under load. A nameplate reading just under 1500 rpm means a 4-pole motor.

Check the voltage line: 230/400 V and 400/690 V behave differently

The dual voltage on the nameplate tells you the winding voltage in star and in delta. On a 400 V supply, a 230/400 V motor runs in star, while a 400/690 V motor runs in delta.

This matters for star-delta starting, where the motor starts in star and then switches to delta. That only works if the motor is designed to run in delta at 400 V, in other words a 400/690 V motor. Send a 230/400 V motor to a site whose panel is built around a star-delta starter and the motor is perfectly healthy but can't be started properly with the existing panel. As a general pattern, smaller ratings tend to come with 230/400 V windings and larger ones with 400/690 V, but the nameplate is what counts. The wiring side is covered in our star-delta wiring guide.

When the motor doesn't fit the machine

Correct power and speed don't guarantee a motor will bolt on. Foot mounting (B3), large flange (B5), small face flange (B14) and combined foot-and-flange versions (B35, B34) are different mechanical versions of the same electrical motor, and vertical installations bring positions such as V1 and V5 into play.

The pair most often confused is B5 and B14. Both are flanged, but a B5 flange has clearance holes the bolts pass through, while a B14 face has tapped holes and a smaller diameter. On a motor that mounts directly onto a gearbox or pump, that difference means it simply won't go on. We compare the two in detail in B5 vs B14 flange selection.

Shaft diameter and key size follow the IEC frame number, and the frame number itself is the shaft centre height in millimetres: a frame 132 motor has its shaft axis 132 mm above the base. Knowing the frame size of the old motor tells you straight away whether foot holes and shaft will match. All mounting options are listed on the mounting types page.

Leaving out the operating environment

Standard three-phase motors are designed around IP55 protection, class F insulation and an ambient temperature between −15 °C and +40 °C. Above 1000 m altitude, or where the ambient goes beyond 40 °C, the motor can't deliver the same output with the same margin, so either a larger rating is chosen or the order is adjusted accordingly.

The same goes for enclosure protection. A food line cleaned with pressurised water, a crushing plant covered in dust or a pumping station by the sea all call for a separate check on whether IP55 is enough. Options such as IP56 and IP65, and class H insulation, exist for exactly these situations. For hot and dusty sites, see our article on insulation class and frame material.

Running on a drive? Say so up front

Three-phase motors are suitable for speed control with a frequency inverter, but knowing this at the ordering stage changes a few decisions:

• The motor is cooled by its own shaft-mounted fan. If it will run at low speed and full torque for long periods, that fan may not be enough and a separately powered (forced) fan may be needed.
• With a long cable between drive and motor, a dv/dt or sine filter should be considered.
• Larger frames may need an insulated bearing or shaft grounding against bearing currents.

Variable-torque loads such as pumps and fans behave very differently on a drive from constant-torque loads such as conveyors. We explain the difference in constant vs variable torque.

Weighing efficiency class against running hours

IE3, IE4 and IE5 are the efficiency levels defined in IEC 60034-30-1. The gap looks small on paper: for a 7.5 kW 4-pole motor the minimum efficiency is 90.4% for IE3, 92.6% for IE4 and 94.0% for IE5. Those percentages, however, show up in the electricity the motor uses over its whole life. For a standby pump that runs a few hundred hours a year, IE3 is usually enough. For a fan or compressor running three shifts, the extra cost of a higher class can pay back quickly, depending on running hours and electricity price. Choosing on purchase price alone leaves that calculation out.

Small details that still cause trouble

• Terminal box position: Will the cable come from the left or right relative to the panel? On a tight baseframe, a terminal box on the wrong side forces awkward cable bends. More in terminal box orientation.
• Winding temperature sensors: If the protection concept includes PTC thermistors or PT100 sensors, order them with the motor; adding them later is impractical.
• Frame material: Cast iron is preferred for vibrating and shock loads, while aluminium frames are offered as an alternative in selected small ratings.

Getting the information together before you order

When replacing an existing motor, the old nameplate is the most reliable source. A clear photo of it covers most of what is needed in one go: power, speed, voltage, frequency, frame and mounting. If you are replacing another manufacturer's motor, our guide to like-for-like replacement shows which dimensions must match.

For a new machine, start from the application itself; the step-by-step selection guide walks through it. Either way, an order that states power, speed or pole number, voltage and frequency, mounting arrangement, frame size or shaft dimensions, efficiency class, operating environment and whether a drive will be used leaves very little room for a wrong delivery.

In short

A wrong motor delivery rarely comes from one big error. It usually comes from a small detail that was skipped because it seemed standard. Settle speed, nameplate voltage, mounting and environment when you order, and the motor fits first time. You can browse the range on our three-phase electric motors page, or send us a photo of the old nameplate and we will help you find the right match.

Frequently Asked Questions

What if the old motor's nameplate is unreadable?

Measure the shaft diameter, the height of the shaft centre above the base, the distance between the foot holes and, if there is one, the flange diameter and hole pattern. Shaft height gives you the frame size, and together with the machine's operating speed these dimensions narrow down the equivalent motor considerably. If you are unsure of the power, measuring the current the machine draws also helps.

What happens if I fit a motor of the same power but a different speed?

Both machine speed and available torque change. A faster motor speeds the machine up and delivers less torque; a slower one does the opposite. Pulleys or a gearbox can compensate, but the drive then has to be recalculated.

Can a 230/400 V motor be used on a 400 V supply?

Yes, connected in star. It can't be started star-delta, though, so direct-on-line starting, a soft starter or a drive has to be used instead.

Can an IP55 motor be installed outdoors?

IP55 protects against dust and water jets from any direction and is enough for many outdoor applications. If the motor will be exposed directly to rain, sun or pressure washing, the installation and a higher protection class should be considered separately.