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Electric Motors: How to Read the Nameplate

Nov 6, 2017

When it comes to purchasing an electric motor, it is critical to understand the specifications detailed on the motor nameplate. The information on the nameplate communicates what the motor can do and provides the information needed to choose the right electric motor for your application. Having the right motor ensures efficiency and product longevity and can translate into major cost savings for your business.

We’ve assembled some basic terms and definitions to help get your started. Understanding these concepts will allow you to ask the right questions and select the right motor for your application and industry.

Electric Motor Nameplate

The nameplate of an electric motor provides necessary information that helps you select the right AC motor for your specific application. We’ll use the following illustration of a 150 horsepower AC motor nameplate as an example. The nameplate offers specifications for the voltage and amps, speed in RPM, service factor, class insulation based on NEMA standards, motor design, and efficiency.electric motors name plate

Voltage and Amps

By design, electric motors have standard voltages and frequencies at which they operate. On the nameplate, you can see this sample motor is designed to be used on 460 VAC systems. 169.5 amps is the full-load current for this motor.

Revolutions Per Minute (RPM)

The nameplate includes the based speed given in RPM. Base speed is where the motor develops rated horsepower at rated voltage and frequency. Base speed indicates how fast a fully-loaded output shaft will turn the connected equipment when proper voltage and frequency is applied.

The sample motor has a base speed of 1185 RPM at 60 Hz. The synchronous speed of a 6-pole motor is 1200 RPM. When fully loaded, there will be 1.25% slip. If the connected equipment operates at less than full load, the output speed (RPM) will be slightly greater than what’s stated on the nameplate.

slip equation

Service Factor

When an electric motor is designed to operate at its nameplate horsepower rating, it has a service factor of 1.0, meaning it can operate at 100% of its rated horsepower. Depending on your application, you may need a motor to exceed its rated horsepower. In that case, you can say you need a motor with a service factor of 1.15. The service factor can be multiplied to the rated power, so a 1.15 service factor motor can be operated 15% higher than the motor’s horsepower mentioned on the nameplate. For example, the 150 HP motor with a 1.15 service factor can be operated at 172.5 HP. Keep in mind that any motor that continuously operates at a service factor greater than 1 will have a reduced life expectancy compared to operating at its rated horsepower. Operating at a service factor greater than one will also affect how the motor performs, such as the full load speed and current.

Class Insulation

Different operating environments have various motor temperature requirements. To meet these requirements, The National Electrical Manufacturers Association (NEMA) established four insulation classes: A, B, F, and H. Class F is the most common and Class A is hardly ever used. Before a motor is started, its windings are at ambient temperature – temperature of the surrounding air. The standard ambient temperature according to NEMA should not exceed 40° C (104° F) within a defined altitude range for all motor classes.

electric motors class insulation 800
NEMA Insulation Classes

Once the motor starts, the internal temperature rises. Each insulation class allows for a specified temperature rise. When the ambient temperature and allowed temperature rise are combined, they equal the maximum winding temperature in a motor. For example, when a motor with Class F insulation operates at a 1.0 service factor, the maximum temperature rise is 105° C. The maximum winding temperature is 40° ambient plus 105° rise, so 145° C. A point in the center of the motor’s windings where the temperature is higher is called the motor’s hot spot.

Operating the motor at the right temperature leads to efficient operation and a long life. If you operate a motor above the limits of the insulation class (155° C for Class F insulation), you reduce the motor’s life expectancy. If the operating temperature increases by 10° C for a significant amount of time, the motor’s insulation life expectancy can decrease as much as 50%.

Electric Motor Design

NEMA has established standards for electric motor construction and performance. NEMA design B motors are most common.


The efficiency of an electric motor is expressed as a percentage. It indicates how much input electrical energy is converted to output mechanical energy. You can see the nominal efficiency for this motor is 95.8%. The higher the percentage means the more efficiently the motor converts incoming electrical power to mechanical horsepower. A 150 HP motor with an efficiency rating of 96.0% consumes less energy than a 150 HP motor with a rating of 86%. Greater efficiency helps you save significantly on the cost of energy. High efficiency motors lead to lower operating temperature, longer life, and lower noise levels.

NEMA Motor Characteristics

Standard Electric Motor Designs

To match speed-torque requirements of various loads, motors are designed with certain speed-torque characteristics. NEMA has four standard motor designs: NEMA A, NEMA B, NEMA C, and NEMA D. NEMA A is not commonly used. NEMA B is the most common. Specialized applications use NEMA C and NEMA D. A motor must have the ability to develop enough torque to start, accelerate, and operate a load at rated speed. Using the sample 150 HP, 1185 RPM motor discussed previously, you can calculate torque by transposing the formula for horsepower.

electric motor plates equation

The NEMA design is most commonly used to estimate the locked rotor, or starting torque. A NEMA Design C motor will typically have a greater locked rotor torque than a NEMA design B motor.

Speed-Torque Curve for NEMA B Motor

The graph below demonstrates the relationship between speed and torque a NEMA B motor produces, from the moment it starts until it reaches full-load torque at rated speed.

electric motors speed torque 1

Starting Torque

The starting torque, also referred to as locked rotor torque, is labeled on the graph. Torque is developed when the rotor is kept at rest with rated voltage and frequency applied. This happens every time a motor starts up. When rated voltage and frequency are applied to the stator, there is a brief amount of time before the rotor turns. In this brief moment, the NEMA design B motor operates at about 150% of its full-load torque.

electric motors starting torque 1

This is a basic introduction to an electric motor nameplate, with terms and definitions. If you have any questions or are interested in learning more, don’t hesitate to contact us and we’d be happy to discuss feasibility, potential business return for your electric motor, and if WorldWide Electric is the right fit for your company.