How to Read a Motor Nameplate — and Order the Right Replacement

Every line on an electric motor's plate, what it means, and which ones must match when you replace it. Written for the person standing next to a dead motor with a phone, whether that is a maintenance fitter or the owner of the machine.

The five lines that must match

Most of a nameplate is detail. Five things decide whether a replacement motor bolts on, wires up and drives the load. Get these off the old plate before anything else:

Power — kW, or HP in North America. Equal or one size up; never down.
Speed — rpm. It tells you the number of poles, and the poles must match or the pump, fan or gearbox runs at the wrong speed.
Voltage and frequency — what your supply gives, and how the motor is connected to it.
Frame — the size code (112M, 184T) that fixes the foot holes, shaft height and shaft. Same frame, same holes.
Mounting — feet, flange or both (B3, B5, B14, B35; on NEMA motors the C or D after the frame).

Power: kW or HP

The rated output is the mechanical power at the shaft, not what the motor draws from the supply. 1 HP = 0.746 kW, and the standard ratings don't line up exactly: a 7.5 kW motor is sold as a 10 HP one. Going one size up is safe — a motor only draws what the load asks for — but a smaller motor on the same load overheats, however well it starts.

Speed tells you the poles

An induction motor runs a little under its synchronous speed, which is fixed by the supply frequency and the number of poles. Read the plate's rpm and find the nearest row:

Poles	Synchronous 50 Hz	Nameplate 50 Hz	Synchronous 60 Hz	Nameplate 60 Hz
2	3000	2850–2950	3600	3450–3550
4	1500	1420–1480	1800	1720–1780
6	1000	940–980	1200	1140–1180
8	750	700–735	900	860–885

A plate that says 1440 rpm is a 4-pole motor at 50 Hz. The same motor on a 60 Hz supply turns 20% faster and needs 20% more voltage to keep the same torque — that is why a 400 V 50 Hz plate often also lists 460 V 60 Hz.

Voltage, and star or delta

A three-phase plate usually gives two voltages, such as 230/400 V Δ/Y or 400/690 V Δ/Y. The motor runs on either: connected in delta (Δ) for the lower voltage, in star (Y) for the higher one. On a 400 V supply, the first motor is wired star and the second is wired delta. The second one can also be started star-delta; the first cannot on a 400 V supply, because its delta voltage is only 230 V.

North American motors are often dual-voltage 230/460 V with nine leads, wired low or high voltage by a diagram on the plate or inside the terminal box lid. Wire it for the supply you have — a 460 V motor connected for 230 V burns out in minutes.

Current (A)

The full-load current at each voltage. Set the overload relay to this figure. Size the cable and fuses from it on an IEC installation; under the NEC, size them from the code's table instead (NEC 430.6) — both are on the motor full-load current chart, with the copper size for each.

cos φ (power factor) and efficiency

cos φ, often 0.80–0.88, is the fraction of the current that does work; the rest magnetises the motor. It drops sharply when the motor is lightly loaded, which is one reason an oversized motor is a poor bargain. η or Eff is the efficiency at full load, and the IE class (IE2, IE3, IE4) says which band that falls in. The minimum for each class and power is on the motor efficiency chart — and in the EU, a new motor from 0.75 to 1000 kW must now be at least IE3.

Frame and mounting

An IEC frame number is the shaft height in millimetres, and the letter is the foot length: a 132M has its shaft 132 mm above the feet, with medium-length feet. A NEMA frame works the same way in inches: the first two digits divided by four give the shaft height, so a 184T's shaft is 4.5 in up. Same frame, same holes and shaft, whatever the brand. Every dimension is on the IEC frame chart and the NEMA frame chart.

Mounting is a separate code: B3 is feet only, B5 a large flange with through holes, B14 a small face with tapped holes, B35 feet and a flange. A gearbox or pump usually needs the flange to match exactly: the flange size, the spigot and the shaft.

The rest of the plate

Marking	Means	Match it?
IP55, IP56	Protection against dust (first digit) and water (second)	Equal or higher
TEFC, ODP	NEMA enclosure: totally enclosed fan cooled, or open drip-proof	Equal or better
Ins. Cl. F	Insulation class. F (155 °C) is standard; H is 180 °C	Equal or higher
S1	Duty: S1 continuous. S2, S3… are short-time or intermittent ratings	Must match
SF 1.15	Service factor: may run at 115% load, hotter and shorter-lived	Match on NEMA motors
Amb. 40 °C	Highest ambient for the rated output	Match in a hot room
Code G	NEMA locked-rotor kVA per HP: how hard it pulls on starting	Matters for the starter
Design B	NEMA torque characteristic; B is the general-purpose one	Match for hard starts
6205-2Z	Bearings, drive end and non-drive end	For spares, not the order

Ordering a replacement: a checklist

Photograph the plate before anything else, and the terminal box wiring before you disconnect it.
Write down power, rpm, voltage, frequency, frame, mounting, IP and duty.
Look up the frame on the IEC or NEMA chart and measure the shaft to confirm it — a rewound or modified motor may not be standard any more.
If the old motor is IE1 or unmarked and you are in the EU, the replacement will be IE3. It can be longer, but on the same frame it bolts straight on.
Check the starter or drive still fits: the new full-load current on the current chart against the overload's range.

Worked example

A plate reads 4 kW, 1445 rpm, 230/400 V Δ/Y, 8.2/4.7 A, 50 Hz, 112M, IM B3, IP55, S1, cos φ 0.82. That is a 4-pole, 4 kW motor, wired star on a 400 V supply, drawing 4.7 A — set the overload to 4.7 A. It is a 112M foot-mounted frame, so its shaft is 112 mm up, its foot holes are 190 × 140 mm and its shaft is 28 × 60 mm. Any maker's 4 kW, 4-pole, 112M B3 IP55 motor replaces it. The same 4 kW motor as a 5 HP NEMA motor would be a 184T.

Choosing a motor for a new machine instead? See how to choose an electric motor.

Doing this for every motor on site? Keep the answers in the free motor register workbook — one line per motor, with the next service date worked out for you.