A motor nameplate is the only document that travels with the machine for its whole working life. Manuals get lost, purchase orders disappear into archives, and the person who specified the drive twenty years ago has usually retired. The stamped plate on the frame is what remains, and it holds enough information to order a correct replacement, size a protective device or diagnose why a motor keeps tripping.
The problem is that the plate is dense and unforgiving. Values sit next to each other without explanation, and two figures that look similar can mean completely different things. Below is what each field actually tells you, in the order most people need it.
Rated output is mechanical, not electrical
The kilowatt figure printed in the largest type is the mechanical power available at the shaft under rated conditions. It is not the power the motor draws from the supply. A 15 kW motor running at rated load and 92 percent efficiency pulls roughly 16.3 kW of electrical power, and the difference becomes heat inside the frame.
This distinction matters when sizing cables, transformers or a standby generator. Sizing them from the shaft rating alone leaves the installation short, and the shortfall grows as efficiency falls. Older plates from outside Europe may carry a horsepower figure instead. One horsepower is 0.746 kW, so a 20 hp motor is close to 15 kW but not identical, and rounding it to the nearest standard European size changes the frame.
Voltage, frequency and the connection symbol
Most three phase motors carry two voltages separated by a slash, together with the star and delta symbols. A plate reading 400 V delta and 690 V star means the windings are designed for 400 V between phases when connected in delta, and the same motor can run on a 690 V supply if the terminal links are rearranged into star.
Getting this backwards is one of the more expensive mistakes in maintenance. Connecting a 400 V delta motor in star on a 400 V supply gives each winding only 230 V, so the motor produces roughly a third of its torque and stalls under load. Connecting a 690 V star motor in delta on 400 V overloads the windings badly and the insulation fails within minutes.
Frequency is stamped alongside. A motor built for 50 Hz will run on 60 Hz, but it turns about 20 percent faster, which changes the load on a fan or pump considerably. Fan and pump torque rises with the square of speed and absorbed power with the cube, so a 20 percent speed increase can demand around 70 percent more power.
Rated current sets the protection
The current figure applies at rated voltage, rated frequency and full load. It is the number that overload relays and motor protection circuit breakers should be set from, not the cable rating and not a value calculated from the kilowatt figure.
Some plates also carry a starting current ratio, often written as a multiple of rated current. A value of seven means direct online starting will briefly draw seven times the rated current, which is what decides whether the supply can tolerate a direct start or whether a soft starter or frequency converter is needed.
Speed, poles and the slip you can read between the lines
Rated speed is the shaft speed at full load, and it quietly reveals the pole count. A four pole motor on a 50 Hz supply has a synchronous speed of 1500 revolutions per minute, so a plate reading 1455 rpm is a four pole machine with about 3 percent slip. Two pole machines sit near 2900 rpm, six pole near 960 rpm and eight pole near 720 rpm.
Slip is not a defect. An induction motor only produces torque because the rotor turns slightly slower than the rotating field. A larger gap between synchronous and rated speed usually indicates a design tuned for higher starting torque, which suits conveyors and crushers more than centrifugal loads.
Duty type is the field most often ignored
Duty is written as S1 through S9. S1 means continuous running, and the vast majority of industrial motors carry it. S2 covers short time duty with a stated period, S3 covers intermittent periodic duty with a stated cyclic duration factor, and the higher numbers describe starting, braking and speed changing cycles.
A motor plated S3 with a 40 percent duty factor cannot simply be dropped into a continuous application, even if the kilowatt figure matches. It will overheat, because its thermal design assumes cooling time between working periods. When an old motor fails early in what looks like a routine application, the duty code is worth checking before anything else.
Efficiency class, insulation class and temperature rise
European motors carry an efficiency class from IE1 to IE5. Since the current Ecodesign rules took effect, most new three phase motors between 0.75 and 1000 kW must reach at least IE3, and a large band of motors between 75 and 200 kW must reach IE4. An old plate showing IE1 or no class at all tells you the motor predates the scheme, which is useful when building a replacement case.
Insulation class appears as a letter, usually F or H, and it states the maximum winding temperature the insulation system tolerates. Class F allows 155 degrees Celsius, class H allows 180. Next to it sits the temperature rise class, often B. A motor with class F insulation used at class B rise is running with a deliberate thermal margin, which typically doubles insulation life compared with running it at the full class F limit.
Frame, mounting and ingress protection
The frame size is a standardised number such as 132 or 160 that fixes the shaft height in millimetres above the mounting feet. Combined with the mounting code, usually written as IM B3 for foot mounting or IM B5 for flange mounting, it determines whether a replacement will bolt into the existing machine without modification.
The IP rating describes protection against solids and water. IP55 is the normal industrial standard and copes with dust and low pressure jets. IP65 is dust tight, and higher ratings apply where washdown is routine. Fitting a lower rating than the original into a wet environment is a slow way to guarantee a repeat failure.
What to record before the motor leaves site
When a motor is removed for repair or replacement, photograph the plate before it goes anywhere. Plates get painted over, corroded or lost entirely in a workshop. The fields that determine whether a replacement fits are the output, voltage and connection, frequency, rated speed, frame size, mounting code, IP rating and duty type. With those nine values a supplier can identify a suitable machine even if the original manufacturer no longer exists, and stockists such as industrial electric motors held on the shelf can usually be matched to an older frame without redesigning the mounting.
The one field worth measuring rather than trusting is the shaft diameter and key size. Plates rarely state it, and on older machines the shaft may have been reworked at some point in its life. Five minutes with a vernier caliper is cheaper than a coupling that does not fit.
