Achieving Energy Efficiency and Comfort with PSC Motor Replacements

By Bryan Stout, engineering manager for distribution at Genteq

When you are faced with a failed motor or a comfort complaint, you need to make some quick decisions that will resolve the customer’s issue. But the best decision isn’t always obvious. This article reviews the advantages and disadvantages of some common solutions to meet the customer’s need. A quick overview of the most popular HVAC motor technologies is a good place to start.

MOTOR TECH 101

It is important to remember that, any time the blower motor is replaced, the operation in heating and cooling must be checked to ensure that it meets the OEM’s specification with the new motor installed.  For heating, the installer should check for conformance with the specification for the temperature rise of the furnace while operating (in both first and second stage on two stage furnaces).  For cooling, the installer should measure the airflow and confirm operation within the manufacturer’s airflow specification.

Permanent Split Capacitor (PSC) motors are widely used when starting torque requirements are not too high and moderate electrical efficiency is needed.  They are typically used on fans, blowers, small pumps and gear motors. PSC motors, like most AC induction motors, have a peak efficiency that occurs at a speed point 5-10% below synchronous speed.  Synchronous speed is the speed of the rotating magnetic field from the stator windings. The speed is equal to 120 times frequency divided by the number of stator poles ((120 x 60hz)/8-pole stator winding = 900RPM).  Typically PSC motors are designed with multiple speeds, only achieving peak efficiency on the highest speed.  As lower speeds are used, such as in constant fan mode, the efficiency of the motor decreases dramatically.

Variable Speed Electronically Controlled motors include the category known as Electronically Commutated Motors (ECM).  ECM motors are capable of controlling current and frequency to the motor winding, which in turn provides torque and speed control.  Current and frequency control, along with the use of magnets in the construction of the motor, enable ECMs to operate up to the 80% efficiency range. While induction motors can be designed to run at good efficiencies at a single speed, ECM motors maintain high efficiencies across wide speed ranges. The electronic control enables OEMs to work with motor manufacturers to optimize the motor for improved efficiency as well as comfort.  With these levels of motor performance, OEMs continue to shift more system designs toward ECM technology.

REPLACEMENT MOTOR SOLUTIONS 

So what is the best solution when you are faced with a failed PSC motor? Should you contact the original equipment manufacturer and order a motor identical to the original? Perhaps you can substitute the failed motor with a stock replacement. Or maybe this situation would be a good candidate for a high-efficiency ECM replacement motor.  If the system is still under warranty, then it is recommended to follow the OEM’s guidelines for warranty replacement.

Customer dissatisfaction with comfort can also present the need to replace a blower motor in the field.  Inconsistent temperature (hot or cold spots), air quality and noise are common consumer complaints.  These issues can often be addressed through replacement with an ECM motor.

Each option must be carefully considered to meet the need of the customer.  Let’s take a closer look at each option along with the pros and cons of each.

 

PSC Motors

Going direct to the manufacturer for an identical replacement motor may seem like the simplest solution for a failed motor. You know the replacement will work as intended and will meet the manufacturer’s system configuration perfectly. Plus, since the motor is mechanically and electrically identical, installation should be clear and simple.

In many situations a stock motor can sufficiently replace a failed motor. This can be a more economical solution than using an OEM motor, as well as a possibly quicker solution since you can pick up a stock motor from your local distributor the same day, in most cases.

There are stock replacement motors that can offer slight improvements in airflow over the OEM motor.  Since stock motors may provide more speeds than that of a direct replacement, you can select a speed that provides more or less airflow to the system if needed.

Keep in mind, though, that some stock motors require mechanical modifications, and they may not match the system efficiency. You will need to ensure the proper motor speed and rotation, and you may need to install separate mounting brackets.

 

ECM Retrofit Motors

It is important to understand how ECM motors can be used to replace failed PSC motors or address comfort complaints. Consumers are continuously looking for ways to save money.  ECM motors can provide significant energy saving in addition to the comfort benefits. Here’s what you need to know.

ECM replacement motors provide a host of benefits to the homeowner over a PSC motor.  ECM motors provide a soft start feature that reduces the common “clunk” start-up noise experienced when using PSC motors.  The hum and the speed transition noises are also greatly reduced with the use of the ECM motor.  With PSC motors, the use of constant fan can be noisy and result in high cost of operation.  Constant fan will keep the air circulating resulting in cleaner air and reduced temperature differences throughout the home.  Unlike PSC motors, ECM retrofit motors operate at constant fan speeds for a much lower cost, enabling the homeowner to use this as the default.  Efficiency is a significant advantage, with an average 25% reduction in watts in operating mode and 74% fewer in constant fan mode.  

 

 

Refer to Table 2, showing a homeowner who was able to reduce the cooling speed.  The owner adopted the ECM retrofit motor and was able to reduce the airflow, following the OEM’s recommendations, resulting in quieter operation and watt reduction. 

For the contractor, the ECM motor offers additional benefits: dual voltage, rotation sensing, multi-speed and multi-horsepower.  Flexibility has another big advantage for the contractor.  As few as two motors are all that are needed to cover almost all PSC applications.  With fewer SKUs, the contractor can keep motors in the truck and avoid the trip to pick up a replacement motor.

However, there are drawbacks. The motor itself is slightly longer and sometimes requires separate mounting brackets.  Installation may take an addition 5-10 minutes with experience and 15-20 for the first installation.  The replacement cost is higher compared to that of other options.  However, contractors should check with the local utilities for available rebates.  Some areas of the country offer $50-200 to customers installing ECM retrofit motors.

While there are advantages and disadvantages to each of the options provided, it all comes down to what the customer wants and needs.  Is the customer concerned with air quality or noise?  Does the customer have an Indoor Air Quality system that they would like to better utilize with constant fan?  Does the customer want to reduce their “carbon footprint?”  Or are you dealing with a blower motor failure in the middle of winter that requires a quick repair?  In all cases – in terms of reliability, comfort, efficiency and flexibility of installation – the ECM retrofit motor provides the best solution for those needs.

 

This article ran in the December issue of RSES Journal.

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