ECM Motors and the Air balancing Technician
By: Aaron Hastings
Sales Engineer
TETCO Geothermal
NCI Certified Air balancer
NATE Certified Installer and Technician in 7 specialties
More and more air balancing techs and technicians in general are stumbling upon ECM or Electronically Commutated Motors in Air handlers and furnaces. These cannot be treated as a standard PSC motor. Let us consider what is in an ECM motor and how they are applied by OEM’s. We will then consider how to treat the motor in an air balancing or troubleshooting application.
What is an ECM motor?
Regal Beloit is the overwhelming leader in ECM motor production and design, so this article will consider it as the model. The ECM 2.3 is its premium model, the other model offerings differing mainly in what options are not included. Emerson Climate Technologies has just entered the marketplace with a very similar motor designed for use in a whole system application that ‘talks’ to the indoor AHU board, Outdoor condenser and T-stat. This is barely entering the marketplace, and from an air balancing standpoint, is nearly identical in its behavior compared to the Regal Beloit model.
The construction of the ECM motor differs from a PSC motor, the windings mounted on the shell, or case of the motor, and the permanent magnets mounted on the rotor:
Image provided courtesy of GE ECM by Regal Beloit. www.theDealerToolbox.com
This means that there are no brushes to distribute power to the windings, which are usually found on the rotor. This translates into less wear (no rubbing parts) and less friction and heat, since the motor does not have to devote energy to overcoming the friction of the brushes and the arching that is usually evident inside the motor as it works. Better efficiency and quieter operation also result. This design is possible because of the control module located at the end of the motor. In this motor control consists of a small amount of memory and a processor that it uses to distribute rectified dc current to individual windings, simulating the phase alternating function of the brushes in a PSC motor. It has complete control over the RPM, Torque, and Amp Draw of the motor, which are all variable, depending on the application.
The motor has 2 plugs on the control, a 16 pin and a 5 pin connection. The 5 pins are for power, and the 16 pins are for the ECM motor’s control. The motor is designed internally for 240V application, and so in 120V installations the 5 pin plug will have a common, hot, and neutral, and the last two pins will be jumped to internally enable a voltage doubling within the motor. The 16 pin cable uses several conductors (usually not all 16), as determined by the OEM to monitor what the system is doing so that the motor can do its part to cooperate – more on that later.
The motor is commonly used without a capacitor since none is needed. The phase shift the capacitor used to provide is done through the onboard electronics. However, significant improvements in efficiency can be realized with an inductive choke added in line with one of the power leads. This is from about 20% to nearly 50% depending on motor, choke and application. The choke looks like a chubby transformer with only 2 leads. This part is necessary on some motors to allow it to meet its UL and ETL electrical safety ratings. It basically smoothes the power it draws, removing spikes that are a result of the internal electronics. Without the spikes factored into the overall amp draw, the motor performs the same work with less power.
Benefits of the ECM Motor
1) Constant airflow - Because of the internal workings of the motor, it can deliver airflow under a variety of conditions. The Motor delivers what it is told to deliver, no more, no less, within its operating parameters as set by the OEM that programs it - even in a direct drive application. For example in our TETCO Geothermal Model ESIII 6 ton unit due for release in January, we were able to run the fan motor at 12 different static pressure points in 2nd stage cooling mode and get the rated 2400 CFM from .01” TESP up to 1.25” TESP, measured airflow varying from 2% over to 7% under the rated Airflow. This is a 1 HP motor mounted in a direct drive blower, no adjustments to the motor, just adjustments to the mounted ductwork to increase and decrease resistance. This means no over blowing at lower statics and no loss of flow at higher statics. You get what you ask for from a compact direct-drive blower.
2) Power usage - The power usage of the motor is only that which is necessary to meet the needs. In the above mentioned test, power consumption ranged from .08 amps when delivering 1200 CFM at .03” Static pressure, to just over 8 amps at 2400 CFM and 1.25” TESP. If the duct is oversized, the fan will draw less amperage to deliver the same airflow. PSC motors just run and draw what they were designed to draw, never more or less. This ability is the key of at least one OEM’s ability to beat 11 SEER in a SDHV system, reaching up to 18 SEER when mated to the right condenser.
3) Noise – ECMs are inherently quieter. They also tend to run at a lower RPM since they only run as fast as they need to and OEM’s can now use larger blowers at lower RPM’s to achieve better efficiency knowing they do not need to be tied down by a multi-tap PSC’s RPM limitations. ECM motors ramp up and down to their rated speed. This means minimized or eliminated duct popping and air whooshing when the system starts. The only opportunity for increased noise is when not installed according to ACCA standards for duct sizing. When duct is very small, the motors WILL deliver the airflow they are told to deliver. This can be noisy in a severely undersized ducting application or in an improperly applied zoning application. It will do all it can to deliver 3 zone’s worth of air through 1 zone’s duct if not properly installed.
4) Reliability – ECM motors have pre-programmed limitations that prevent them from hurting themselves through overamping, the common cause of death for nearly every other electric motor. They do fail but not nearly as often. At TETCO, we have been including ECM motors for more than 10 years on our ESII model ground-to-air heat pump and I can recall only one replacement motor sold in the last year.
ECM motors and the OEM
These benefits are achieved by adapting the motor to its application by the individual OEM. This is called characterization. This is basically a process that teaches the motor how to get along with the blower it is mounted to within the cabinet it finds itself. The duct used for this process must be large enough to accommodate very low static pressures and yet be able to generate very high pressures to provide a full spectrum of information to feed into the software provided by Regal Beloit. Basically I tell the motor, ‘when I run you at 1080 RPM, 42.6% max torque, 1.1” SP and 3.2 amps, I get 1132 CFM instead of 1200’. When repeated for a few dozen data points the motor learns how to get along with its blower. This relationship is described by 4 constants that when fed into the motor, make it able to give me whatever airflow I select.
As this motor runs in the unit, it compares how much torque it is delivering with the amp draw and RPM it is getting in return, the constants convert that into an airflow that it can compare to what it has been told to deliver. It will then ramp up or slow down to comply with its instructions.
After all this, I will then tell the newly characterized motor to behave as I wish under the scenarios I describe to it.
Seeing the ECM in the field
These motors are fantastically versatile and filled with options that are usually not all used. These are set up by the OEM using Regal Beloit proprietary software and programmed into the motor. These things include several profiles that allow for the same motor to run in several different modes depending on the system's operation. For example, in a heat pump, one motor may be configured to operate at full capacity on 2 stage cooling, 2/3 capacity in 1 stage cooling, around 3/4 capacity in 2 stage heating and again 1/3 less in 1 stage heating then change speeds again if the emergency electric heat comes on and again if it is emergency heat and 2 stage heat then again when the humidifier says it is too humid….You get the idea. Add on to that an OEM determined ramp up time allowing the motor to slowly come up to speed from a dead stop.
The run profile can be divided to allow to motor to run in a manner that maximizes the results for the individual unit. For example, with a 5 ton heat pump having a call for 2 stage heat, the motor ramps up from a dead stop to 800 CFM in 15 seconds, silently beginning to run then it stays there for 30 seconds, allowing the indoor coil to completely heat up. This avoids an initial rush of cold air at the beginning of the run cycle. After 30 seconds the motor can increase to the full rated speed delivering an optimized air temperature for this unit’s application. When the call for heat ends then the system ramps down to 1000 CFM for 1 minute to extract residual heat off of the coil and then slides to a dead stop. In the off cycle with the fan only mode selected the motor can gently deliver 200-300 CFM to keep the space’s air from stratifying.
This all means that the motor literally has a mind of its own.
Troubleshooting Airflow
As we go through these scenarios, please remember:
1) The motor automatically responds to restrictions in airflow, within seconds
2) The motor responds to instructions received from the thermostat.
3) The motor is indirectly making an ‘educated guess’ as to its output based on the way it was characterized. There will be some variation.
With this in mind, let’s consider how the motor will respond to the efforts of a technician in diagnosing airflow issues.
1) Opening and closing Dampers – usually when balancing a system, one recognizes that they are making it harder for the fan to deliver its flow. Basically if one takes a grille in a light commercial setting that is delivering 300 CFM and cuts it back to 100, you can expect about 175 to 190 or so will continue down the duct system to the next grille(s), the rest will ‘go back to the fan’, or will be lost as the fan slightly looses a little of it’s production at the current pulley setting due to the increase in Static Pressure. This does not happen with the ECM motor, when you push at it, it pushes back. It is common to close the first few dampers closest to the fan a little too much when making the first pass at balancing a system, knowing that as the SP increases, more air will be forced through the damper as you close the last few dampers, and the setting will go from a little too low to end up just right. This should definitely be a part of balancing an ECM system. When dampers are adjusted, PSC motor driven systems increase in SP and LOSE a little airflow, ECM systems increase in SP and then increase more to KEEP their airflow.
2) Flow hood measurements – When the air hood is placed on a central return grille it presents a restriction to flow. When measuring a grille where the measuring device (flow hood) restricts the flow, there should be a compensation made for back pressure. In this way the reading will be a better reflection of what is happening when you take the hood off the grille. With an ECM motor, seconds after the hood is placed, the motor detects a rise in TESP and adjusts. Then when the flap in the flow hood is closed for the second, comparative reading that is part of compensating for BP, the motor again sees a restriction and adjusts. This means if a BP compensated measure is taken then the reading may be the same or lower than the uncompensated reading. The measure that reflects the most accurate condition is a non-compensated reading that is taken approximately 10-15 seconds after placing the hood to allow for the motor to finish compensating for the back pressure you've introduced.
3) RPM – Because there are no external sensors, at startup the motor takes a few seconds to ‘figure out’ which way it is turning. This is evidenced by a slight ‘wiggling’ motion for 3-10 seconds at start up. This is normal, and is no cause for concern. The RPM reading, if taken will tend to bounce a little, much as a flow hood reading does in a slightly turbulent duct. This reading is a product of the environment the motor finds itself in, not the result of a hardwired setting. Regal Beloit 2.3 ECM motors can run up to 1500 RPM if the OEM that programmed it allows (1/2, 3/4, and 1 HP models - 1800 RPM for the 1/3 HP model). This does not mean that you would expect it to run that high or will ever see it run that high. RPM ceases to be a benchmark against which to measure fan performance. Our fan motor has delivered 2110 CFM at 808 RPM (.05 TESP) and 2105 CFM at 1362 RPM (1.5” TESP)
4) Target Air flow - Because of the on-the-fly adaptability of the ECM motor, the output will vary. For example, as the motor keeps 2400 CFM as it’s target and the TESP of the system increases or decreases, the output will closely track its 2400 CFM target but will vary slightly as it moves along. In our tests, output from the fan varied from 2% above to 7% below as static pressure changed and the motor automatically adjusted. This means that it may say ‘2400 CFM’ in the manual but you quite simply may not ever be able to get exactly that. In fact, it is possible that reducing the TESP around the unit may DROP the output of the unit slightly. As it slows down to keep delivering the target airflow within the program it was given, it may slide into an area of the operation programming that is more towards the -7% end instead of the +2% end of the program where it may have been before adjustments were made. This is bad news for you perfectionists out there. The motor can guarantee ‘pretty close’ over an extremely wide range, but ‘dead on’ is likely not possible.