How to read an electric motor nameplate

What the plate is for

The nameplate is the motor's rating label, defined for IEC machines in IEC 60034-1. Every value is a rated value: the condition under which the manufacturer guarantees the motor will run continuously without overheating. The motor is not limited to those values, but loading it beyond them shortens its life.

A typical IEC plate for a three-phase motor reads:

• 3~ Mot. 4 kW, Δ/Y 230/400 V, 14.2/8.2 A
• 50 Hz, 1440 min⁻¹, cos φ 0.82
• IE3, η 86.6%, S1
• Ins. cl. F, IP55, 1.15 SF, IEC 112M

Each field is explained below.

Power, voltage, current and frequency

Rated power is the mechanical power at the shaft, not the electrical power drawn from the supply. It is given in kW in IEC countries and in horsepower (HP) in North America. One HP is about 746 W, so 4 kW is about 5.4 HP.

Rated voltage and current come in pairs. Two values separated by a slash, such as 230/400 V and 14.2/8.2 A, belong to delta and star connection respectively (see the star-delta guide). The current is always a line current, and the voltage a line-to-line voltage.

Rated frequency is the supply the motor is wound for, 50 Hz or 60 Hz. A motor can often run on the other frequency if the voltage is changed in proportion, but speed, and for fans and pumps the load, change with it.

The single-phase equivalent is simple: one voltage, 230 V for example, and one current. The shaft power is then limited by V × I × η × cos φ.

Speed, poles and slip

The plate gives the full-load speed, in rpm or min⁻¹. It is always slightly below the synchronous speed, N_s = 120 × f / p, so the plate also tells you the number of poles: the synchronous speed is the standard value just above the rated speed.

• 2 poles: 3000 rpm at 50 Hz, plate speed about 2850 to 2950
• 4 poles: 1500 rpm, plate speed about 1400 to 1480
• 6 poles: 1000 rpm, plate speed about 920 to 980
• 8 poles: 750 rpm, plate speed about 690 to 740

The rated slip follows from the plate speed:

s = (N_s − N) / N_s

A 1440 rpm motor at 50 Hz has N_s = 1500 rpm and s = 4%. More detail is in the slip guide.

Power factor and efficiency

cos φ is the power factor at rated load, typically 0.8 to 0.9 for mid-size motors. It is lower at part load because the magnetizing current stays almost constant.

Efficiency η is the ratio of shaft power to electrical input power at rated load. Under IEC 60034-30-1 the efficiency classes are IE1 (standard), IE2 (high), IE3 (premium) and IE4 (super premium). In the EU, IE3 is the minimum for most new three-phase motors from 0.75 kW upward (with higher classes required for some larger ratings).

Electrical input power follows from the plate:

P_in = √3 × V × I × cos φ

and the shaft power is P_out = η × P_in. Check the plate for consistency: η × √3 × V × I × cos φ should come out near the rated power.

Duty, insulation, protection, frame and service factor

• Duty type S1 means continuous running at rated load until thermal equilibrium. S2 is short-time duty and S3 is intermittent duty with a given duty cycle, such as S3 40%. A motor not marked S1 may overheat if it is run continuously.
• Insulation class is the highest winding temperature the insulation tolerates: class B is 130 °C, class F is 155 °C and class H is 180 °C. Most modern motors are class F with a temperature rise limited to class B, which leaves a thermal reserve.
• IP code, such as IP55, is the enclosure protection. The first digit is dust protection (5 means dust-protected) and the second is water protection (5 means water jets, 4 means splashing, 6 means powerful jets).
• Service factor (SF) is the continuous overload the motor can take. SF 1.15 means the motor may carry up to 115% of rated power continuously under the stated conditions (supply, ambient temperature), with a smaller thermal reserve. Most IEC motors have no service factor listed, which means 1.0.
• Frame size, such as IEC 112M, gives the shaft height in mm (112 mm) and the footprint length (S, M or L). It determines the mounting dimensions, so a replacement motor of the same frame bolts into the same place.

Worked example

Plate: 4 kW, 400 V (star), 8.2 A, 50 Hz, 1440 rpm, cos φ 0.82, efficiency 86.6%.

1. Poles: the synchronous speed just above 1440 rpm is 1500 rpm, so p = 120 × 50 / 1500 = 4 poles. 2. Slip: s = (1500 − 1440) / 1500 = 4%. 3. Input power: P_in = √3 × 400 × 8.2 × 0.82 ≈ 4.66 kW. The shaft power is 0.866 × 4.66 ≈ 4.0 kW, which agrees with the plate. 4. Losses: 4.66 − 4.00 ≈ 0.66 kW, released as heat. 5. Rated torque: T = 9550 × P / n = 9550 × 4 / 1440 ≈ 26.5 N·m.

The constant 9550 comes from T = P / ω with ω = 2π × n / 60, giving 60 000 / (2π) ≈ 9549 when P is in kW. Pull-out and starting torque are given as multiples of this rated torque in catalogues.

Try it in the induction motor simulators

In the three-phase induction motor simulator the six nameplate fields Rated power, Rated voltage, Rated current, Rated frequency, Rated speed and Pole count are editable. They default to 4 kW, 400 V, 8.2 A, 50 Hz, 1440 rpm and 4 poles. The 3D motor carries a nameplate showing the power, voltage, current, frequency and speed values.

• Change the Pole count to 2, 6 and 8 and watch the Synchronous speed readout: 3000, 1000 and 750 rpm. Set the Rated speed to match each pole count (for example 2850, 960 and 720 rpm), because the rated speed must stay below the synchronous speed.
• With the default Rated speed of 1440 rpm, set Mechanical load to 100% and read Slip, Efficiency and Power factor. Compare them with your hand calculation. The simulator estimates efficiency and power factor from its model, so they will not match the example above exactly.
• At 100% load compare the Motor torque readout with 9550 × P / n, using the Output power (in kW) and the Rotor speed shown in the simulator.

The single-phase induction motor simulator uses a 0.75 kW, 230 V, 5.2 A, 1400 rpm, four-pole default. Its nameplate gives 750 W at the shaft from 230 × 5.2 = 1196 VA, so efficiency × power factor can be at most 0.63.

Try it in the simulator