Lire la plaque signalétique d’un moteur

The nameplate is the motor telling you what it is, what it wants, and what it will do — and every number you set on the job comes off it, not off a guess. The first time you are handed a motor and asked to "hook it up", the plate is where you start. Here is every field, what it means, and which ones you will actually use. The plate drawn below carries practice numbers; they are not ratings for anything real.

The fields, one at a time

Manufacturer, model, serial. The line at the top. Not exciting, until the day you need a replacement or the plate is half gone — the model number is what the supplier looks up. Photograph the plate before you install the motor; you will not be able to read it once it is bolted in a corner.

HP or kW (1). The mechanical output the motor is built to deliver continuously at its rated voltage and speed. It is the shaft power, not the electrical power drawn — the difference is the losses, which is what the efficiency field is about.

Volts (2). The supply the windings are designed for. A plate with two values, like 230 / 460, is a dual-voltage motor: the windings can be connected in parallel for the lower voltage or in series for the higher one. The connection diagram on the plate (11) tells you which leads tie together for each. Read that diagram; do not wire from memory or from another motor. A small tolerance either side of the rated voltage is normal — the maker states it.

FLA — full-load amps (3). The current the motor draws at rated horsepower, at each voltage. Notice the two values on the practice plate are in the same ratio as the two voltages: same power, half the volts, twice the amps. This is the figure the overload relay is set from, following the overload's own instructions. It is not the same as the code's own full-load current values used for sizing conductors and short-circuit protection — those live in your code book, and the book says when each is used.

Phase and Hz (4). Single- or three-phase, and the supply frequency. A 60 Hz motor on a 50 Hz supply runs slower and hotter; the frequency also sets the synchronous speed, which is where the next field comes from.

RPM. The shaft speed at full load. An induction motor's rotor always turns a little slower than the magnetic field it is chasing — that lag is called slip, and it is what makes the motor produce torque at all. A field turning at 1800 RPM with a nameplate speed of 1750 has about 50 RPM of slip at full load; unloaded it creeps closer to 1800. So a plate reading 1750 tells you "this is a four-pole, 1800-class motor at full load", and a plate reading 3450 tells you "two-pole, 3600-class".

Service factor — S.F. (5). A multiplier on the horsepower the motor can tolerate for a while without immediate damage. 1.15 means it can be pushed about fifteen percent over nameplate. It is a margin for short overloads and hot afternoons, not a rating to run at — insulation life drops fast when you live in the service factor. Whether the overload may be set into it is a code question.

Frame (6). A standard designation for the physical size: shaft height, shaft diameter, mounting-hole pattern. A 184T from any maker bolts to the same feet and lines up with the same coupling as a 184T from any other. The dimensions behind the number are in the frame standard and the maker's drawing — look them up when you are laying out a base.

Enclosure (7). How the motor keeps the world out. ODP — open drip-proof — has vents; air passes through the windings, and it belongs somewhere clean and dry. TEFC — totally enclosed, fan-cooled — is sealed, with an external fan blowing over the fins, for dust, damp and washdown areas. There are others (explosion-proof, totally enclosed non-ventilated); the letters tell you which environment the motor is meant for, and the location decides which one is acceptable.

Insulation class and ambient (8). A letter grading how much heat the winding insulation can stand, paired with the ambient temperature the ratings assume. Higher letters tolerate more heat. The letter is all you need to read here; the temperature behind each letter is in the standard. The practical lesson: a motor in a hotter room than the plate assumes has less headroom, and the plate is the only place that headroom is written down.

Duty. CONT means the motor can run at rated load indefinitely. A time rating — so many minutes — means it is built for intermittent work and must cool between runs. Putting an intermittent-duty motor on a continuous job is how you get a call-back in a week.

Efficiency and power factor (9). Efficiency is shaft power divided by electrical power, as a percent — the rest becomes heat. Power factor is the fraction of the current that does real work; the rest circulates because the windings are inductive. Both are used in the motor current formula, and both explain why a motor draws more amps than the horsepower alone suggests. The glossary has the longer version. Run the practice plate through it and the numbers close: 5 × 746 ÷ (1.732 × 230 × 0.895 × 0.84) = 12.5 A, which is the FLA printed on the low-voltage side of the plate. If a plate's own fields will not reconcile like that, you have misread a field.

Code letter (10). A class for the locked-rotor current — how hard the motor pulls in the instant it starts, before the shaft is moving. Each letter covers a band of kVA per horsepower; later letters mean a bigger inrush. What it tells you: how much starting surge the protection has to ride through without tripping. The bands are in the book, and the book uses the letter when it sizes that protection.

Design letter. A class for the torque curve — how the motor's torque behaves from standstill up to speed. The common general-purpose design gives normal starting torque with a modest inrush; other designs trade starting torque against slip for loads like conveyors and punch presses. You match the design to the load, and the maker's curves say exactly what each letter does.

Thermal protection. Some motors carry their own thermal device in the windings — the plate says so and says whether it resets itself or needs resetting by hand. An auto-reset motor that restarts on its own is a safety question on some machines, which is exactly the kind of thing the overload lesson and your book are about. NONE on the practice plate means the overload relay in the starter is the only protection.

What you actually use it for

Four jobs, in the order you do them. Match the supply: the plate voltage, phase and frequency must be what the panel provides, within the maker's tolerance. Connect for the right voltage: on a dual-voltage motor, open the box and tie the leads exactly as the plate's diagram shows for the voltage you have. Set the overload: the overload relay is set from the nameplate FLA at that voltage, following the relay's own instructions and whatever the code says about the service factor. Check rotation: bump the motor uncoupled and watch which way the shaft turns; if it is wrong, swapping two of the three line leads on a three-phase motor reverses it. Then lock it out, couple it, and run it properly — the lockout walk-through has the order.

Worked: reading the practice plate above

1. The panel gives me 460 V, three-phase, 60 Hz. The plate lists 230 / 460 — I connect for the high-voltage arrangement shown on the plate's diagram.
2. At 460 V the plate FLA is 6.25 A. That is the number the overload relay is set from, per its instructions. (Had I connected for 230 V it would be 12.5 A — same power, twice the current.)
3. RPM reads 1750. That is an 1800-class motor with about 50 RPM of slip at full load: the shaft turns about 1750 at full load, a touch faster lightly loaded.
4. S.F. 1.15 means it will tolerate a short push past 5 HP; it is not a reason to load it to 5.75 HP all day.

Why the two FLA values are exactly two-to-one

Power in is roughly the same either way — the motor does the same 5 HP of work. Volts times amps has to come out the same, so half the volts means twice the amps. On a real plate the ratio is usually exactly two-to-one for a series/parallel dual-voltage winding, which is a quick sanity check that you are reading the right pair of numbers.

Sur le chantier

Questions fréquentes

Voir aussi

• Service factor — the margin on the plate, and SFA.
• Motor control course — the control circuit that starts and protects this motor.
• The overload relay — where the FLA setting goes and what it opens.
• Contactor wiring diagram — the power contacts the motor hangs off.
• Motor start/stop station — the seal-in circuit.
• Single & three phase formulas — the motor current formula the efficiency and power factor feed.