Motor Nameplate: What is on it? How to Read the Data?

A motor nameplate is a small metal plate that is attached to the exterior of an electric motor. It contains important information about the motor’s specifications, including its voltage, frequency, horsepower, and other technical details. Understanding how to read the data on a motor nameplate is essential for anyone who works with electric motors, as it can provide valuable information about the motor’s capabilities and limitations. In this article, we will explain what is on a motor nameplate and how to interpret the data it contains.

What Is On A Motor Nameplate?

The specifications that are available on a motor nameplate are:

1. Voltage

Voltage rating is abbreviated V on the nameplate of a motor. It indicates the voltage at which the motor is designed to operate.

The voltage of a motor is usually determined by the supply to which it is being attached. When the motor is used at other voltages than the voltage indicated on the nameplate, its performance will be affected.

2. Frequency

The line frequency rating of a motor is abbreviated on the nameplate as CY or CYC (cycle), or Hz (hertz). A cycle is one complete wave of alternating voltage or current. Hertz is the unit of frequency and equals the number of cycles per second. In the United States, 60 cycles/second (Hz) is standard, while in other countries 50 Hz (cycles) is more common. If more than one frequency is marked on the nameplate, then other parameters that will differ at different input frequencies have to be indicated on the nameplate as well.

3. Phase

This parameter represents the number of AC power lines that supply the motor. The phase rating is listed as direct current (DC), single-phase alternating current (1ϕ AC), or three-phase alternating current (3ϕ AC).

4. Current

The current indicated on the nameplate corresponds to the rated power output together with voltage and frequency. Current may deviate from the nameplate amperes if the phases are unbalanced or if the voltage turns out to be lower than indicated. Motors that are not fully loaded draw less than the rated nameplate current. Similarly, overloaded motors draw more than the rated nameplate current.

5. Type

This will include the name and logo of the manufacturer along with catalog numbers, parts numbers, and model numbers used to identify a motor. Some manufacturers use type to define the motor as single-phase or poly-phase, single-phase or multi-speed, or by type of construction. Each manufacturer uses a unique coding system.

6. Power factor

The power factor is indicated on the nameplate as either “PF” or “P .F” or cos φ. The power factor is an expression of the ratio of active power (W) to apparent power (VA) expressed as a percentage. Numerically expressed, the power factor is equal to the cosine of the angle of lag of the input current with respect to its voltage. The motor’s nameplate provides you with the power factor for the motor at full load.

Motors are inductive loads and have power factors less than 1.0, usually between 0.5 and 0.95, depending on their rated size. A motor with a low power factor will draw more current for the same horsepower than a motor with a high power factor. The power factor of induction motors varies with load and drops significantly when the motor is operated at below 75 percent of full load.

7. kW or horsepower

kW or horsepower (HP) is an expression of the motor’s mechanical output rating – that is its ability to deliver the torque needed for the load at the rated speed. When an application calls for a horsepower / kW falling between two sizes, the larger size is chosen to provide the appropriate power to operate the load.

8. Full-load speed

Full-load speed is the speed at which rated full-load torque is delivered at rated power output. Normally, the full-load speed is given in RPM. This speed is sometimes called slip-speed or actual rotor speed. The number of poles in the motor and the frequency of the supply voltage determine the speed of an AC motor. The speed of a DC motor is determined by the amount of supply voltage and/or the amount of field current.

9. Efficiency

Efficiency is included on the nameplate of many motors. The efficiency of a motor is a measure of the effectiveness with which the motor converts electrical energy into mechanical energy. Motor efficiency varies from the nameplate value depending on the percentage of the rated load applied to the motor. Most motors operate near their maximum efficiency at rated load. Efficiency is guaranteed by the manufacturer to be within a certain tolerance band, which varies depending on the design standard, eg IEC or NEMA. Therefore, pay attention to guaranteed minimum efficiencies, when you evaluate the motor’s performance.

10. Duty

The duty cycle is listed on the motor nameplate as “Duty”, “Duty Cycle”, or “Time rating”. This parameter defines the length of time during which the motor can carry its nameplate rating safely. In many cases, the motor can do it continuously, which is indicated by an S1 or “Cont” on the nameplate. If nothing is indicated on the nameplate, the motor is designed for duty cycle S1.

11. Insulation class

Insulation class (INSUL CLASS) is an expression of the standard classification of the thermal tolerance of the motor winding. Insulation class is a letter designation such as “B” or “F”, depending on the winding’s ability to survive a given operating temperature for a given life. The farther in the alphabet, the better the performance. For instance, a class “F” insulation has a longer nominal life at a given operating temperature than a class “B”.

Motor insulation prevents windings from shorting to each other or the frame of the motor. The type of insulation used in a motor depends on the operating temperature the motor will experience. As the heat in a motor increases beyond the temperature rating of the insulation, the life of the insulation and of the motor is shortened.

12. Ambient temperature

The ambient temperature rating of a motor is abbreviated AMD or DEG on the nameplate of a motor. Ambient temperature is the temperature of the air surrounding the motor. In general, the maximum ambient temperature for motors is 40° C or 104° F unless the motor is specifically designed for a different temperature and indicates so on its nameplate.

Motors operating at or near rated full load will have reduced life if operated at ambient temperatures above their ratings. If the ambient temperature is over 104° F, a higher-horsepower motor or a special motor designed for operation at higher ambient temperatures must be used.

13. Temperature rise

A motor’s permissible temperature rise is abbreviated Deg.C/Rise on the nameplate of the motor. This indicates the amount of the motor winding temperature will increase above the ambient temperature because of the heat from the current drawn by the motor at full load. It can also be thought of as the amount by which a motor operating under rated conditions is hotter than its surrounding temperature.

14. Service factor

The service factor (abbreviated SF on the nameplate) is a multiplier that is applied to the motor’s normal power rating to indicate an increase in power output (or overload capacity) that the motor is capable of providing under certain conditions. For example, a 10-hp motor with a service factor of 1.25 safely develops 125 percent of rated power or 12.5 hp. Generally, electric motor service factors indicate that a motor can:

• Handle a known overload that is occasional.
• Provide a factor of safety where the environment or service condition is not well defined, especially for general-purpose electric motors.
• Operate at a cooler-than-normal temperature at rated load, thus lengthening insulation life.

Common values of service factors are 1.0, 1.15, and 1.25. When the nameplate does not list a service factor, a service factor of 1.00 is assumed. In some cases, the running current at service factor loading is also indicated on the nameplate as service factor amperes (SFA).

15. Motor enclosure

The selection of a motor enclosure depends on the ambient temperature and surrounding conditions. The two general classifications of motor enclosures are open and enclosed. An open motor has ventilating openings, which permit the passage of external air over and around the motor windings. An enclosed motor is constructed to prevent the free exchange of air between the inside and outside of the frame, but not sufficiently enclosed to be termed airtight.

16. Frame size

Refers to a set of physical dimensions of motors as established by NEMA and IEC. Frame sizes include physical size, construction, dimensions, and certain other physical characteristics of a motor. When you are changing a motor, selecting the same frame size regardless of the manufacturer ensures the mounting mechanism and hole positions will match.  Dimensionally, NEMA standards are expressed in English units and IEC standards are expressed in metric units. NEMA and IEC standards both use letter codes to indicate specific mechanical dimensions, plus number codes for general frame size.

17. Thermal protection

Thermal protection, when marked on the motor nameplate, indicates that the motor was designed and manufactured with its built-in thermal protection device. Several types of protective devices can be built into the motor and used to sense excessive (overload) temperature rise and/or current flow. These devices disconnect the motor from its power source if they sense the overload to prevent damage to the insulation of the motor windings.

The primary types of thermal overload protectors include automatic and manual reset devices that sense either current or temperature. With automatic-reset devices, after the motor cools, this electrical circuit–interrupting device automatically restores power to the motor. With manual reset devices, the electrical circuit–interrupting device has an external button located on the motor enclosure that must be manually pressed to restore power to the motor. Manual reset protection should be provided where automatic restart of the motor after it cools down could cause personal injury should the motor start unexpectedly. Some low-cost motors have no internal thermal protection and rely on external protection between the motor and the electrical power supply for safety. 

18. Connection diagrams

Connection diagrams can be found on the nameplate of some motors, or the diagram may be located inside the motor conduit box or on a special connection plate. The diagram will indicate the specific connections for dual-voltage motors. Some motors can operate in either direction, depending on how the connections to the motor are made, and this information may also be given on the nameplate.

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