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Electric Motor Terminology and Characteristics

Fundamental electric motor knowledge is a key building block to identifying the information required for proper motor replacement. Today, we will discuss the three critical elements of electric motors– application, characteristics and types, which are vital to selecting a replacement motor that will operate properly, safely and reliably.

At the most basic level, an electric motor is an electrical machine that converts electrical energy into mechanical energy to do work. For the purposes of this article, we will focus on motors used in residential and light commercial HVAC (Heating, Ventilation and Air Conditioning) including furnaces, air handlers and package units.

The Critical Three for Electric Motors

The critical three elements of electric motors are:

  • Application
  • Characteristics (Electrical and Mechanical)
  • Type of Motor

Application Considerations for Electric Motors

To understand the need for a particular motor, we must ask:

What is the motor's purpose?

It can also be helpful to ask questions such as:

  • What type of equipment did this motor come out of?
  • Is this motor exposed to outdoor conditions?
  • Are there any extreme temperature requirements?

Asking these questions about the application in which the electric motor was used can help indicate whether the motor may have failed because it was misapplied.

7 Mechanical Characteristics of Electric Motors

Before you can identify a proper replacement, it’s necessary to next consider the seven mechanical characteristics of electric motors.

The enclosure protects the internal components and may contribute to how the motor dissipates heat. Open Air Over (OAO), Totally Enclosed Air Over (TEAO), and Open Drip Proof (ODP) are the most common types of enclosures.

Frame Size
The frame size defines certain construction parameters and is often identified solely by the diameter of the motor. NEMA®* rated frame sizes define the dimensions for the entire motor including the shaft dimensions and location.

Thermal Protection
Thermal protectors protect the motor’s insulating materials from excessive heat. Excessive heat causes the overload device to open the circuit to the motor, causing it to turn off. An automatic overload will automatically reset once the motor has cooled down while a manual overload will require a manual reset with a push button.

The motor bearing allows the motor to turn freely while it supports the shaft, the rotor, and the load such as a fan blade, blower wheel or pulley. Sleeve bearings are typically used for light to medium loads while ball bearings are typically used for heavy loads and/or higher ambient temperatures.

Rotation is the direction the motor will operate and is typically listed as Counter Clockwise (CCW) or Clockwise (CW). Most replacement motors are built to operate in both directions and all three phase motors are reversible as well.

The length of the shaft must be measured and matched to the NEMA standard diameter. ½" is the NEMA standard diameter for 48 Frame motors, while ⅜” is the NEMA standard diameter for 42 Frame motors. Other special shaft features to look for include a flattened shaft, keyed shaft, double flat shaft, threaded shaft and a shaft diameter other than NEMA.

Mounting refers to the method used to secure the motor to the application. Common mounting types include belly band, welded leg, resilient, and thru bolts. It is always a good practice to ask how the motor was mounted. There are kits available to modify the mounting style of a particular replacement motor.

6 Electrical Characteristics of Electric Motors

In addition to the mechanical considerations, there are also electrical characteristics that go into selecting the proper replacement electric motor. All of these characteristics should be listed on the motor nameplate.

Horsepower (HP) Rating
This rating defines the strength of the motor by giving the actual work output of the electric motor measured in Watts.

Service Factor (SF)
The Service Factor indicates the rated percentage over nameplate horsepower that a motor can be operated at while being sufficiently self-cooled. For example, 1HP motor with a 1.25 SF can work up to 1¼HP if needed.

Voltage Rating
This indicates the rated line voltage required for proper operation. Typical residential voltages are 115 volts or 208-230 volts. Most electric motors are rated to operate within +/- 10% of their nameplate rating.

Amperage Rating
This rating also defines the strength of the motor by measuring the amount of energy consumed by the motor to perform work.

Revolutions Per Minute (RPM) Rating
Revolutions per minute measure how fast the electric motor is operating at a given time.

Capacitors are used or required by some motors to improve starting torque and electrical efficiency and are rated in Microfarads (mf, uf, mfd) and Voltage (V or VAC). The industry best practice is to replace a capacitor whenever a motor is replaced.

5 Types of Electric Motors Common to HVAC

The final element of the critical three when selecting a proper electric motor replacement is motor type. There are five types of electric motors common to HVAC:

Shaded Pole
Shaded pole motors are typically constructed with one or two speeds and do not use a capacitor. They are the least efficient motor type and are common in manufactured home furnaces, smaller ventilation units, and unit heaters.

Permanent Split Capacitor (PSC)
PSC motors are typically constructed with 1-5 speeds and always run with a capacitor. They are twice as efficient as a shaded pole motor and are common in most home furnaces and condenser fans.

Split Phase
There are three types of split phase motors – split phase, cap start, and cap start/cap run. These motors use a centrifugal switch to properly energize the start and run circuits. The start capacitors increase starting torque. These motors are commonly used for belt drive applications. They can be found in a small amount of residential applications but are primarily used for commercial applications.

Three Phase
Three phase electric motors use three legs of power supplied to the motor. They are commonly identified by the number of leads and do not use capacitors. They are approximately 89% efficient and are commonly used in commercial and industrial applications.

Electronically Commutated Motors (ECM)
ECMs are the most efficient single phase motor in the market and are built with an electronic control module and a motor. They do not use capacitors and are commonly identified by the application and connections. The three basic types of ECM used in HVAC include constant airflow (variable speed) ECM, constant torque ECM, and constant speed ECM.

*NEMA is the trademark of the National Electrical Manufacturers Association and is not owned or controlled by Regal Rexnord Corporation, Inc.

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