Part 2: EC Indoor Blower Motors, Resistance is Futile
Part two of this three-part series on indoor blower ECMs provides information on airflow performance characteristics.
A common misconception about EC indoor blower motors is that they all maintain airflow and solve airflow issues such as undersized ductwork and/ or dirt load on the air side components. In part one of this article, I explained that there are actually two different types of EC indoor blower motors, constant airflow and constant torque.
In that article, I described constant airflow ECMs as communicated motors and constant torque ECMs as multi-tap motors. This information was used to understand the different methods of selecting the output of the motor to match the airflow requirements of the HVAC system using the information provided by the HVAC system manufacturer. In this article, I will explain the performance differences of these two motors related to how they are programmed to operate.
Constant Airflow ECMs
Part of what makes the constant-airflow ECM unique is the engineering required to operate accurately at a specific cfm in a given HVAC system. Using a software program (often called “characterizing”) from the motor manufacturer, the HVAC system engineer operates the ECM in each HVAC unit (furnace, air handler or package system) at various operating points (cfm and TESP) as required by the characterization process. (cfm = cubic feet per minute, TESP = total external static pressure).
This process develops a set of operating parameters (constants or data) that relate torque and speed to cfm and TESP. When the process is completed, this set of data is unique to that single HVAC system by size (capacity) and model. The data may be stored in the motor control or in the HVAC system control board and communicated to the motor control.
The HVAC system control board communicates a selected cfm value to the motor control related to the system demand (heat, cool, etc.). The microprocessor in the motor control is programmed with a constant-airflow algorithm (operating program or firmware). The microprocessor uses the characterization data (or constants) to plot a constant-airflow profile or operating line related to torque and speed for each communicated cfm value, from low TESP to high TESP.
The TESP range is determined by the HVAC system engineer. The motor control adjusts current (motor torque) and speed (rpm) to achieve a torque/speed relationship anywhere on the operating line dictated by the TESP.
I will use Figure 1 to pull all this together with some basic operating examples. When the ECM is given an input to operate at a particular cfm value (800 cfm in this example), the motor control increases the motor torque, which increases motor speed, until the torque/ speed relationship meets at a point along the 800-cfm operating line (the red line in Figure 1) dictated by the TESP. In this example, the TESP is low (point A). If the TESP were higher, this point would be farther to the right.
If the TESP increases or decreases while the ECM is operating, the motor control automatically adjusts its operation to keep the torque/speed relationship on the operating line. If the TESP increases, the rpm will also increase (due to the reduced load/cfm), with no change in motor torque. This action will cause the torque/speed relationship to drift off the operating line (point B). The motor control will compensate by increasing torque, which will in turn increase rpm until the torque/speed relationship gets back to the operating line (points C, D and E).
If the TESP decreases, the rpm will also decrease (due to the increased load/cfm), with no change in motor torque. This will cause the torque/speed relationship to drift off the operating line (point F). The motor control will compensate by decreasing the torque, which will in turn decrease rpm until the torque/speed relationship gets back to the operating line (points G, H and A).
Figure 1: Speed vs. torque, with constant-airflow profile
The ability of the indoor blower motor to provide constant airflow is a huge benefit to HVAC systems in which the TESP is affected by the design and construction of the ductwork, registers, and grilles. Filter sizing and the dirt load on all air-side components can also have an effect on the TESP.
Figure 2 compares a PSC motor and a constant-airflow ECM. Both motors are rated 1/2-hp, operating in a 3-ton system. TESP and cfm are plotted on this chart. The PSC motor is operating on high speed to achieve 1,200 cfm at a TESP of 0.5 in. wc. The ECM is set for 1,200 cfm. This information comes from lab testing using the same HVAC unit with each motor. It is easy to see the value of maintaining a constant airflow. System capacity and efficiency (related to heat transfer) are maintained across the entire recommended TESP range. Induction motors produce less torque and cfm when TESP increases.
The typical field measurement of power (current, or amperage) can be misleading when comparing ECM and PSC motors. Constant airflow ECMs use more power when the TESP increases (to maintain the desired cfm). PSC motors use less power when the TESP increases, but also produce less airflow (cfm). However, the equation for calculating the total power (in watts) of ECMs and PSC motors is volts multiplied by amperes multiplied by the power factor (V × A × PF). ECMs operate at a lower power factor than PSC motors. Therefore, the amperage of an ECM could be equal to or even higher than that of a PSC motor, yet the ECM could be using less total power (watts) and at the same time be providing constant airflow. Figure 3 illustrates this concept. Once again, both motors are 1/2-hp motors operating in a 3-ton system. The ECM is producing 1,250 cfm at a TESP of 0.5 in. wc, and 1,230 cfm at 0.9 in. wc. The PSC motor is producing 1,261 cfm at 0.5 in. wc, and 1,050 cfm at 0.9 in. wc. This information derives from lab testing that uses the same HVAC unit with each motor.
Constant-airflow ECMs should never be considered a solution to undersized ductwork. The best use of ECM technology is in a HVAC system with properly designed and installed components (including ductwork, filters, registers and grilles) and regular planned maintenance to keep the air-side components clean. This allows the ECM to compensate for filter loading in between regular maintenance inspections. However, if the system design or installation is poor, or if the dirt load on the air-side components becomes excessive, the TESP will operate at a higher level for longer periods of time (in some cases, for the entire life of the system). The following key points should be understood about ECM technology in relation to any given system’s TESP:
- A system’s recommended TESP can be found on the data plate and in the manufacturer’s installation manuals. This figure represents the TESP at which the system’s capacity and efficiency originally were rated.
- When the TESP increases, rpm and torque also increase to maintain the desired cfm. This means that the ECM will use more power, and the operating noise of the system may increase.
- Conversely, when the TESP decreases, the rpm and torque also decrease— again in order to maintain the cfm. This means that the ECM will use less power, and the operating noise of the system may be reduced.
- If the TESP is high for prolonged periods of time, the life expectancy of the ECM and other HVAC system components may be reduced.
- If the TESP becomes extreme (higher than the maximum recommended by the HVAC system manufacturer), airflow may decrease, and the system may suffer failures related to low airflow. There is a speed limit built into most ECMs. This limit is programmed by the HVAC system manufacturer into the motor control. Keeping in mind that the rpm will increase with TESP, the speed limit is an overtemperature protection control that limits motor torque (current) at the speed limit.
Figure 2: Comparing PSC motor and constant-airflow ECM (cfm vs. TESP)
Figure 3: Comparing PSC motor and constant-airflow ECM (watts vs. TESP)
Constant-torque ECMs
The constant-torque operating program in the motor control operates the motor at the torque value stored in the tap that is energized by the HVAC system demand input. The speed (rpm) is dictated by the TESP and is essentially irrelevant to this operating system. However, if the TESP changes, the speed and power consumption will also change as a result of maintaining motor torque.
Constant-torque ECMs and PSC induction motors produce similar airflow curves. When you compare the two motors on a graph relating cfm and TESP, you can see that both motors produce less airflow as TESP increases. However, the constant-torque ECM produces a little more airflow at higher TESP values, due to its ability to keep motor torque constant. By contrast, the induction motor produces less motor torque at the higher TESP, due to reduced load/cfm (see Figure 4).
A comparison of power consumption shows that the constant torque ECM also uses more power as the TESP increases (in order to maintain motor torque), whereas the induction motor uses less power. However, as stated earlier, due to its electrical efficiency, the ECM uses less total power than the PSC, even at higher TESP values. The typical field measurement of power— which is current, or amperage—can be misleading. The equation for calculating total power (watts) for ECM and PSC motors is voltage multiplied by current multiplied by the power factor (W = V × A × PF). ECMs operate at a lower power factor than PSC motors. Therefore, the current drawn by the constant torque ECM could be equal to or even higher than that of the PSC motor, and yet the ECM could be using less total power (watts) and providing moderate airflow improvement (see Figure 5). Note: The data in Figures 4 and 5 are from the same two 1/2-hp motors in the same HVAC system operating under lab conditions. The watts at any given TESP in Figure 5 correspond directly to the airflow (cfm) at the same TESP in Figure 4.
- The key to achieving and sustaining proper airflow with this motor is to design and maintain the TESP between the HVAC manufacturer’s recommended value and the maximum value.
- The key to maintaining the designed life expectancy of this motor is to design and maintain the TESP as close to the manufacturer’s recommended value as possible.
Constant torque ECMs were introduced to the HVAC industry primarily to provide energy savings. This is evident in their adoption into HVAC systems with the 13 SEER energy regulation in 2006 and the FER energy regulation in 2019.
- The key to achieving and sustaining proper airflow with this motor is to design and maintain the TESP at a value that achieves the desired airflow based on the manufacturer’s performance charts.
- The key to maintaining the designed life expectancy of this motor is to design and maintain the TESP as close to the manufacturer’s recommended value as possible.
HVAC systems that use either one of these motors can provide years of comfort, energy savings and reliability, if we follow the installation guidelines provided by the HVAC system manufacturer and maintain the system to meet these guidelines, throughout the life of the system.
Watch for Part 3 of this article series coming in the October 2020 issue of RSES Journal, where we will discuss the diagnostics and replacement of these motors.
Christopher Mohalley is the Training Manager for Regal Rexnord. He has applied his 25+ years of HVAC field experience, instruction and extensive product training to create a nationally-recognized ECM training program.
He serves as a NATE Technical Committee SME, is a Member of RSES and is NATE certified in all HVAC disciplines.
Interested in reading more about this topic Check out Mohalley’s book Understanding Electronically Commutated Motors (SKU 200-523x), published as a part of the RSES Sustainability Series at www.rses.org/store. Use promo code ECM2020 and get 10% off the purchase price.
Figure 4: Comparing PSC motor and constant-torque ECM (cfm vs. TESP)
Figure 5: Comparing PSC motor and constant-torque ECM (watts vs. TESP)
“Part 2: EC Indoor Blower Motors, Resistance is Futile”, by Christopher Mohalley. August 2020 feature reposted with permission from RSES Journal, www.rsesjournal.com.