How to Identify the Correct PSC Replacement Motor
As an HVAC technician, you will need to replace failed Permanent Split Capacitor (PSC) motors. PSC motors were the largest motor segment until a few years ago. Regulations and energy efficiency concerns are steering consumers to electronically commutated motors. However, Contractors in the field can expect to continue working on PSC motors for another 10 to 15 years. When you need to find a suitable PSC replacement, following a few steps can make the process simple.
First, gather information about the motor. You can find this information in a few different places:
- The motor nameplate
- From measuring the motor’s physical dimensions
- Asking questions about the motor and how it was used
As you ask these questions, keep in mind the “Critical Three” elements of electric motors: application, characteristics and types. This will help ensure you are selecting a replacement PSC motor that will be safe, reliable, and efficient.
Use a worksheet to write down your findings. The worksheet will make sure you don’t miss any critical information.
The Motor Nameplate
You can find a typical PSC motor nameplate on the side of the motor shell. Nameplates are not always legible and finding the information can be intimidating. The nameplate is important because it lists important product information like manufacturer, model number, and catalog number.
What is the Motor’s Purpose?
To understand a motor's application and purpose, ask several questions, such as:
- What type of application did this motor come out of?
- Is this motor exposed to outdoor conditions?
- Are there any extreme temperature requirements?
One of the reasons for motor failure is misapplication. It is possible the motor you are replacing may have been misapplied. It’s important to think about the application to prevent another failure in such a case.
Motor Nameplate Measurements
The motor nameplate contains details about electrical characteristics like:
- RPM (Speeds)
- Horsepower (HP)
- Service Factor (if one exists)
- Frequency (Hz)
The motor nameplate also includes mechanical characteristics:
- Enclosure (typically partially listed, but you can also find it by looking at the motor itself)
- Frame Size
- Thermal Protection (typically listed as “Thermally Protected”, which means the protection is automatic)
- Bearing Type (not always listed)
- Rotation (not always listed)
- Insulation Class
- Ambient Rating
Usually, you won't find shaft dimensions listed on the motor nameplate. Instead, you will need to measure the shaft diameter and shaft length. A shaft diameter template is another option to simplify measuring the shaft dimensions.
The motor Mount is not usually listed on the nameplate either. If there is no obvious mounting type, you will need to ask about how the motor was mounted.
While the Type of Motor is not listed on the nameplate, the wiring diagram will show what type of motor it is.
PSC Motor Replacement Options
Once you've gathered information from the nameplate, taken measurements and asked questions about application, you can look at correct replacement options. And by using a worksheet to record that information, you will have a reference to go back to.