Terms and Features to Know When Choosing Your Next HVAC Motor
Whether you're installing a new HVAC motor or replacing an old one, it's important to understand some basics. A little knowledge can go a long way in helping you choose the right motor for the job.
When choosing a motor, there are three key things to think about: application, characteristics, and type. Understanding these basics is essential to picking a safe, reliable motor that does the job right. Having the right motor can save you money, time and frustration.
Electric motors convert electrical energy into mechanical energy to do work. While you'll find motors in applications all around us, here, we will focus on HVAC (Heating, Ventilation and Air Conditioning) motors. These types of motors power furnaces, air handlers and package units.
Three Critical Factors for Getting the Right Electric Motor
Picking a motor might seem complex, but once you know what to look for, the process becomes much easier. There are three factors that play a big role in how well the motor will perform in your system.
Here’s what you’ll want to focus on:
- Application: What is the motor used for?
- Characteristics: How is the motor built? How does it work?
- Type: What kind of motor is it?
Applications: What Will The Motor Be Doing?
To understand the need for a particular motor, we often ask: What is the motor's purpose? It can also be helpful to ask questions such as:
- What is this motor supposed to do?
- What kind of equipment is it powering?
- Will it be used outdoors or in extreme temperatures?
These questions will be particularly helpful if you're replacing a motor. These questions help you figure out if the old motor failed because it wasn’t the right fit. They can also help you avoid that mistake again.
Often, a general-purpose motor will do the trick. But some HVAC and non-HVAC applications call for something more specific. For example, in hazardous locations, motors need to meet strict safety standards. These safety standards can prevent sparks or overheating. In food production facilities, motors often need to withstand frequent washdowns.
Understanding the motor’s role helps you choose one that’s not just compatible—but built to last in the conditions it will face.
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Construction and Power Characteristics
Characteristics: How Is The Motor Built?
Here are seven things to look at when checking motor construction:Enclosure
This protects the motor and helps it stay cool. Common types include Open Air Over (OAO), Totally Enclosed Air Over (TEAO), and Open Drip Proof (ODP).Frame Size
This tells you the size and shape of the motor. NEMA®* rated frame sizes define the dimensions for the entire motor. These include the shaft dimensions and location.
Thermal Protection
Heat can kill a motor. Thermal protectors keep a motor from overheating. Some reset automatically, others need a manual reset.
Bearing
Bearings help the motor spin smoothly. Motor Bearings also support the shaft, rotor, and load (like a fan blade, blower wheel or pulley). Sleeve bearings are for lighter loads; ball bearings are for heavier jobs.
Rotation
Motors spin in a certain direction—clockwise (CW) or counterclockwise (CCW). Many motors can spin both ways.
Shaft
The shaft size and shape need to match your setup. There are different styles like flat, keyed, or threaded shafts. Measure the shaft length and match it to the NEMA standard diameter. Half-inch is the NEMA standard diameter for 48 Frame motors. The NEMA standard diameter for 42 Frame motors is ⅜”.
Mounting
Mounting is how the motor is attached. Common types include belly band, welded leg, and thru bolts. Manufacturers make kits to change the mounting style of a particular replacement motor.
Characteristics: What Powers the Motor?
Now that we've looked at motor construction, let’s talk about the electrical side. These details are usually printed on the motor’s label (called the nameplate). Here are six key things to check:
Horsepower (HP) Rating
Horsepower tells you how strong the motor is—basically, how much work it can do. Horsepower is usually measured in Watts.
Service Factor (SF)
Service Factor shows how much extra power the motor can handle while being self-cooled. For example, a 1 HP motor with a 1.25 SF can handle up to 1.25 HP when needed.
Voltage Rating
Voltage 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.
The International Electrotechnical Commission considers any motor below 1,000 volts to be a low-voltage motor. Motors using between 1,000 volts and 35 kilovolts are medium-voltage motors. In most cases, low-voltage motors are the preferred option. But if more horsepower is needed for an application, medium-voltage is the way to go.
Amperage (Amps) Rating
Amperage tells you how much electricity the motor uses while working. Amps also define the strength of the motor.
Revolutions Per Minute (RPM)
Revolutions per minute measure how fast the electric motor is operating at a given time.
Capacitor
Some motors need a capacitor to start or run efficiently. If your motor uses one, it’s a good idea to replace it when you replace the motor. Capacitors are rated in Microfarads (mf, uf, mfd) and Voltage (V or VAC).
Type: What Kind of Motor Is It?
There are several types of HVAC motors. Here are five of the most common:
Shaded Pole
Simple and inexpensive, but not very efficient. Shaded pole motors do not use a capacitor and feature one or two speeds. Often found in small fans or heaters.
PSC (Permanent Split Capacitor)
More efficient than shaded pole motors. PSC motors always run with a capacitor and feature one to five speeds. These are common in home furnaces and condenser fans.
Split Phase
Split phase motors use a special switch to help them start. They’re often used in belt-driven systems and are more common in commercial setups. Split phase motors use start capacitors to increase starting torque. There are three types of split phase motors – split phase, cap start, and cap start/cap run. Split phase motors run belt drive applications and are mostly found in commercial applications.
Three Phase
Three Phase motors are powerful and efficient, usually found in commercial or industrial buildings. They don’t use capacitors and are identified by the number of leads. Three phase motors are about 89 percent efficient.
ECM (Electronically Commutated Motors)
ECMs are the most efficient single-phase motors on the market. They use a built-in electronic controller. This controller can change the motor's speed or torque. You can identify an ECM by the application and connections. ECMs are common in modern HVAC systems. There are three types: constant airflow, constant torque, and constant speed. The three basic types of ECMs used in HVAC include:
- constant airflow ECM (also known as variable speed motors),
- constant torque motors, and
- constant speed ECM.
Visit Motor Mastery University® to view courses on Genteq®, Century® and FASCO® products, as well as product specifications, instructions, and benefits of both induction and ECM HVAC/R motors.
*NEMA is the trademark of the National Electrical Manufacturers Association and is not owned or controlled by Regal Rexnord Corporation, Inc.