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A Guide to Buying the Right Electric Motor for Your Application

A vital component to any power unit is the electric motor.

That’s why it’s important to learn these basics to help you select the right electric motor for your power-unit application.


The first thing you should know is how much power an electric motor needs to operate your application (eg: 5 HP, 10 HP, 20 HP, etc). The service factor (1.15 on standard units) is a multiplier that indicates how much a motor can run above its rated horsepower for intermittent periods. Locked-rotor or start-up torque is an issue when any motor must start under load. The common locked-rotor torque is 100% of full-load torque, but we recommend that you select a motor that is designed to provide a more generous 140% or greater locked-rotor torque.

Rotary Speed (RPM)

Rotary speed or revolutions per minute (rpm) indicates how many complete revolutions the motor shaft makes in a 60-second period. Most AC motors run at fixed speeds of 3600, 1800, 1200, and 900 rpm. For example, most hydraulic pumps for industrial applications are designed to run at 1800 or 1200 rpm. These lower-speed motors are more efficient, and have fewer balance problems.

Voltage & Phases

Voltage (V) and phases are determined by the electric service that will power the motor. Most motors operate at 230 or 460 V, 3-phase. When you identify voltage for an electric motor, always cite the number of phases. Single-phase is typically associated with smaller motors used for rural or residential power, while three-phase is associated with larger motors used for urban or commercial industrial power – and is most commonly used in power unit applications.

Frame Size & Configuration

The frame size and configuration of your motor will matter when it comes to mounting it to your power-unit. The National Electrical Manufacturers Association (NEMA) has established detailed standard motor dimensions for interchangeability. The motors on most power units mount to the pumps through flanges and are supported by feet. Some mountings do not require feet. When ordering, be sure to identify feet (or base) or no feet (round body). (Many designers select direct coupling to attach the motor to a pump because this arrangement saves space).

Also, determine where the motor’s junction box is located. When looking at the motor from the shaft end, the standard location for the junction box is on the left-hand side (called F-1 mount). If you need a motor with the junction box on the right-hand side, make sure that you order an F-2 mount.

Finally, a short shaft (known as a TS frame) should be specified on all 1800 or 1200-rpm motors. This may require a modification of the basic motor. Note that most manufacturers do not stock special motors, so some extra lead time should be expected on F-2 mount. short shaft, or roundbody models.


In what kind of environment do you need a motor to operate your application? Motor enclosures protect internal parts from moisture, but still allow for cooling. Open drip-proof motors (which are cooled by the outside atmosphere) can be used where ambient conditions are fair. The more expensive alternative, totally enclosed fan-cooled (TEFC) motors, provide maximum protection even in a hazardous or aggressive environment.

Four Things to Know When Consulting Your Motor Vendor

Once you know the five basics, you can request several different documents from your motor vendor to help you finalize your selection – and provide information for the end-user if he or she will be someone else. These might include:

  1. Performance Data Sheet – lists all specifications (electrical performance) you need to know about an electric motor
  2. Dimensional Drawings – eliminate costly errors due to size differences between what your application needs and what the motor vendor provides
  3. List of Features & Benefits – explains the motor’s electrical and mechanical features, and help you to make an accurate comparison between different manufacturers’ products
  4. Installation & Maintenance Manual – provides you with all necessary care-and-feeding instructions for the motor. The information should cover: storage, transport, mounting, balancing, re-greasing the bearings, operating sound levels, and other technical information not found elsewhere.


Knowing what to ask for and what to expect from a motor vendor will help ensure that you get the right motor for your power unit. If you are selling the power units, the ability to provide your customers with all necessary intelligence about the equipment they are buying adds real value to the transaction.