What is an ECM Motor?
Several of us were discussing ECM motors in the office the other day. Yeah, we have weird water cooler conversations like that. The general consensus was that although we were familiar with the concept behind them – they are motors that regulate their own speed without the help of an external VFD or other device – none of us really understood the magic behind what makes an ECM Motor actually work. Without a VFD or external sensors to monitor pressure, temperature, or anything else, how the heck does the motor know when to speed up and when to slow down?
What is an ECM Motor?
ECM stands for an “electronically commutated motor” which basically means a motor that uses electronic controls to vary its speed. There are three types of ECM motors: constant cfm, constant rpm, and constant torque. Since cfm, rpm and torque are all related, the basic prinicples are the same. But for ease of discussion, I’m going focus on a Constant Torque ECM motor.
A Constant Torque ECM is made up of two parts, the motor and ECM Microprocessor, that are both housed in one shell. The microprocessor is the “brains” of the motor. It holds the logic that controls the motor. The logic is a math equation or algorithm that figures out the ideal airflow for each specific piece of HVAC equipment out there and uses a formula to maintain that airflow using a calculation of the precise relationship between motor speed and torque.
It should be noted that the ECM motor programming is specific to each model of HVAC equipment, so programming is ONLY done at the factory, not in the field. When ordering a replacement ECM motor, techs must know the specs of the model in which the motor will be installed for it to work properly.
How does an ECM Motor work?
Once the settings are programmed into microprocessor in the factory and the control board dip switches are set in the field, the motor torque and airflow (CFM) should remain steady*. What WILL change is the speed of the motor (RPM). Depending on system conditions, the motor will need to spin faster or slower in order to keep a steady torque and airflow. This was where me and the folks in my office really got stuck. We couldn’t figure out what could initiate a change in torque? It turns out it’s static pressure in the system.
When the load or demand on the system increases (like it’s really hot outside, for example), a higher static pressure is present. Higher cooling demand increases condensation on the evaporator coil, reducing air flow (hence higher static pressure). Resistance to air flow can also be caused by a clogged filter or dirty coil, which will also increase static pressure. This higher pressure increases the torque on the motor. An increased torque basically means that it requires more “muscle” to turn a motor. Higher pressure essentially creates additional resistance on the blades of the fan motor, which is why it needs more “muscle” or torque to turn the motor.
When the microprocessor senses increased torque, it automatically increases the speed of the motor. A faster motor creates more airflow to make sure that CFM stays steady despite the resistance in the system from conditions like clogged filter or a hard-working evaporator. More airflow also reduces static pressure, which reduces torque. At the same time, increased airflow also provides the additional oomph of airflow that the system needs to provide additional cooling or heating capacity during high demand times for the system.
* Ideal airflow will be different in heating and cooling modes. The control board dipswitch settings (usually set in the field) need to be set up properly for proper seasonal operation.
How does an ECM Motor save energy?
The energy savings come into play when demand and static pressure decreases. When it is not needed at full speed, the motor can slow down, which uses far less energy. A motor running at full speed uses nearly 8 times the energy of a motor running at half speed. So any time you can slow it down, even a little, saves you big bucks. And since you don’t NEED the motor to run at full speed all the time, it’s a no-brainer energy savings technique!
I found an article from our friends at York that gave a “real-world” example of how ECMs work. Here is that excerpt:
“Let’s say you have a 3 ton air conditioner. So we need 400 CFM per ton or 1200 CFM to work properly. You use the programming board and set BOTH the cool and adjust profiles for as close to 1200 CFM as you can. (Always use the manufacturer’s tables for setting up an ECM motor). Now, on a call for cooling, the motor turns on and is going to try to maintain your programmed CFM. Here is where added efficiency comes in because we are maintaining ideal CFM across a range of operating conditions. Of course, as an air conditioner runs, the evaporator is going to get “wet” since we are removing latent heat and humidity. When the coil gets wet, the static pressure of the system goes up. As the static goes up, the motor senses a change in torque (like the car going uphill) and starts to increase the RPM of the motor in order to maintain the CFM of the system. Same thing occurs as a filter gets dirty, return static increases and the motor revs up the RPM to maintain the CFM. Now, as the latent heat decreases and there is less humidity or water on the coil, or someone changes the dirty filter, there is less static, a reduction in torque, so the RPM’s decrease, all the time maintaining CFM.”
“The same thing occurs in heating mode. You have programmed the motor to maintain a desired “temperature rise” for the system. The motor will deliver that CFM to maintain that rise. But, again, if the filter is getting dirty, the motor’s RPM will increase, to maintain the CFM and temperature rise.”
Another good resource is The ECM Textbook.
Are ECM Motors the Way to Go?
Good question. There is an obvious appeal to using a motor that regulates it’s own speed versus a motor plus VFD – it’s less expensive to buy one instead of two items, it takes up less room and it’s more or less pre-programmed for the unit that you have. However, there is also a down-side to an “all-inclusive” motor. If a VFD breaks, you can simply bypass it and the motor will still run. If the logic, electronics or mechanical parts fail on the motor, you have down-time. And, if you remember, these motors need to be programmed BY THE FACTORY. So it’s not likely that they’re waiting on the shelf at your local supplier. That is a big concern if you have critical equipment or even just grumpy tenants. Nobody wants to wait days for their AC to work again and waiting for heat in the dead of winter isn’t even an option. Though there are tremendous benefits to energy efficiency and greater controls, the simple fact is that the more complicated you make a mechanical item, the more opportunity there is for things to go wrong. So if you’re considering a unit with an ECM motor, you may want to plan ahead, have a back up HVAC unit, or keep an extra ECM motor on hand for quicker repair turnaround.
There is a huge amount of potential energy savings to be had from controlling motor speed. But how you choose to do that should be carefully considered. Make sure your HVAC contractor understands your business and can help you to decide the best options for you.