Finding a replacement motor.
Nearly everything you need is stamped on the motor already. Read five figures off the nameplate, match those, and confirm three more off the drawing — this page is what each of those figures means and which of them decide whether the replacement actually fits.
Photograph the nameplate before the motor comes out
It is the single most useful thing you can do, and it takes ten seconds. A plate that is legible in situ is often unreadable once the motor is on the floor covered in coupling grease, and a corroded plate on a failed motor is a genuinely common reason a replacement gets specified wrong.
What is on the nameplate
Rated output — the kW figure
Rated output is the mechanical power the motor delivers at the shaft, not what it draws from the supply. It is the figure that has to match most closely, because it sets whether the motor can do the work at all.
Older plates and imported motors sometimes give horsepower instead. 1 hp = 0.746 kW, so a 10 hp motor is 7.5 kW. The common ratings are not evenly spaced — 0.75, 1.1, 1.5, 2.2, 3, 4, 5.5, 7.5, 11, 15, 18.5, 22 kW and upward — because they follow a preferred number series rather than round figures.
Going up a size to get an easier match is a decision with consequences, not a free upgrade. A larger motor draws more starting current, may need bigger protection and cable, and runs less efficiently at part load. Going down a size risks nuisance tripping and a shortened life.
Poles and speed
Pole count sets the speed, and speed and torque trade against each other at a given power. A 6 pole motor at the same kW as a 4 pole turns slower and delivers proportionally more torque.
| Poles | Synchronous speed at 50 Hz | Typical full-load speed on the plate |
|---|---|---|
| 2 | 3000 rpm | 2850 – 2950 |
| 4 | 1500 rpm | 1420 – 1470 |
| 6 | 1000 rpm | 940 – 980 |
| 8 | 750 rpm | 700 – 735 |
Many plates show only the full-load speed, not the pole count. The difference between the two is slip — the motor turns slightly slower than the rotating field, and that difference is what produces torque. To get the pole count, round the plate speed up to the nearest synchronous speed: 1455 rpm is a 4 pole motor, 965 rpm is a 6 pole.
Voltage, frequency and connection
Australian three-phase supply is nominally 415 V at 50 Hz. Motors are generally tolerant of about ±10% supply variation, which is why a motor plated 400 V will normally run here.
The connection matters as much as the number. Most three-phase motors carry two voltage ratings — commonly 240/415 V or 415/690 V — where the lower figure is the delta connection and the higher is star. The same motor is either, depending on how the terminal links are set. A plate reading 415 V DELTA and one reading 415 V STAR are not describing the same machine on the same supply.
Frame size — the fitment figure
The IEC frame designation is the most important thing on the plate for deciding whether a replacement physically goes in. In a designation like 132M:
- 132 is the shaft centre height in millimetres, measured from the mounting feet to the centre of the shaft.
- M is the frame length — S short, M medium, L long. A 132M is longer than a 132S and shorter than a 132L, with different foot hole spacing.
Because IEC frames are standardised, a 132M from one manufacturer mounts the same way as a 132M from another. That standardisation is what makes cross-brand replacement possible at all.
Motors built to the American NEMA standard use a different frame system — 213T, 256T and so on — which is dimensionally unrelated. A NEMA frame is never an answer to an IEC frame requirement, and BecSpec will not offer one.
Mounting arrangement
The IEC mounting code describes how the motor is held. It is frequently absent from the plate and has to be read off the motor itself, which is easy enough once you know what you are looking at.
| Code | Arrangement |
|---|---|
| B3 | Foot mounted. Bolted down through feet on the base — the most common by a wide margin. |
| B5 | Flange mounted, no feet. Large flange with clearance holes, bolted to the driven machine. |
| B14 | Face mounted, no feet. Smaller flange with tapped holes. |
| B35 | Both feet and a B5 flange. |
| V1 | Vertical, shaft down, flange mounted. |
The three you cannot match from a nameplate
These are not in registration data and generally are not on the plate either. They are the most common reason a correctly specified replacement still will not go in, so they need checking against the manufacturer's dimension drawing.
- Shaft diameter and length. Standardised by frame in principle, but variants exist, and the coupling or pulley has to fit what arrives.
- Keyway size. Follows shaft diameter conventionally, and conventionally is not always.
- Terminal box position. Top is usual, but side-mounted boxes are common, and a box on the wrong side can mean the gland does not reach the existing conduit.
Worth confirming alongside them: whether the motor will be driven by a variable speed drive. Inverter duty is a real requirement — insulation and bearing arrangements differ — and it is not recorded in the register.
Efficiency, while you are there
A replacement is the one moment when efficiency is cheap to change. Over a motor's working life the electricity usually costs several times the purchase price, so the difference between an IE2 and an IE3 machine at the same rating is worth more attention than it normally gets at the point of failure — when the pressure is to fit whatever is on the shelf.
Full-load efficiency is recorded in the register, so the finder shows it on every result and the spread at a given rating is on each rating page.
Matching it
With the five figures in hand, the finder matches them against every three-phase motor registered for supply in Australia, shows which of your requirements each result confirms, which differ and by how much, and which the register cannot answer.
You do not have to fill in every field. BecSpec only judges what you actually specify — a search on power and poles alone is a legitimate search, and it will tell you plainly that it has not checked frame or mounting.