How to choose between an ECM and a VFD
While each has its place in HVAC applications, electronically commutated motors and variable frequency drives have important differences in simplicity, flexibility, cost and other factors
- Understand the similarities and differences between electronically commutated motors and variable frequency drives.
- Learn important criteria to consider when choosing between ECMs and VFDs.
- Know where each of these technologies might be headed in HVAC systems.
Electronically commutated motors are getting a lot of notoriety recently. At times, these have been referred to as brushless direct current motors and EC motors. Variable frequency drives are interchangeably called variable speed drives and adjustable speed drives, which all mean the same thing.
Alternating current induction motors are everywhere and consume 45% of the world’s energy. But where can you find ECMs? Everyone reading this probably has ECMs in their homes. ECMs have been used in computer disk drives since the very early days.
In heating, ventilating and air conditioning applications, many of the newer residential and some commercial equipment manufacturers use ECMs for fractional and small integral horsepower loads. For industrial control panels, virtually all the cooling muffin fans are driven by ECMs.
Here are common questions about ECMs:
- Are ECMs more efficient?
- Are ECMs less expensive?
- Are ECMs easier to install?
- Are ECMs easier to replace?
- Do ECMs cause harmonics problems?
- Are ECM controls integrated with the motor?
- Are ECMs AC or DC powered?
- Are ECMs a new technology?
- Are ECMs the wave of the future?
The trouble is, the answers are all dependent on a variety of the bounds that frame the answer, such as size, application, system components, personnel training and many other factors. Both VFDs and ECMs each have their place.
Starting with the very basics, a motor requires magnets in the stator and magnets in the rotor. The opposing and attracting forces between the stator and rotor poles cause the rotor to turn. The trick is to change the polarity of the magnets at the proper time to keep the rotor spinning.
Brushed DC motors
Figure 1 is a brushed DC motor. When the rotor gets to the proper position, the brushes, which connect the voltage to the windings that produce the magnetism, connect with the next section of the commutator to reverse the magnets’ polarity. Thus, the motor keeps turning.
These motors are simple to produce, are relatively inexpensive and are still in use. However, brushes have some inherent problems. They wear out and require regular replacement, so science found a better way of commutation. Interestingly, the major influence for that better way came about during World War II when brushed DC motors in high-altitude aircraft failed when brushes rapidly deteriorate above 30,000 feet.
The first patent for what was then called a “commutatorless DC motor” was claimed by Harrison Brailsford in 1955. Then, things really got urgent when it was discovered that brushes lasted only minutes in the hard vacuum of space.
So now, 60 or 70 years later, we have ECMs that use electronic devices like power switching transistors to replace the mechanical commutator assembly of a DC motor. ECMs use electronics, such as Hall Effect sensors, to sense the position of the rotor for commutation of the magnets. In the HVAC world, ECMs are generally thought of as a motor and controls integrated into a single unit. The intelligence built into the controls turns the ECMs into specific-purpose devices.
Beyond that definition, ECMs are most often permanent magnet motors. The use of permanent magnets contributes to two important areas that will be covered in more detail later: higher efficiency and higher cost. It’s important to note here that VFDs can drive permanent magnet motors, too.
There are most commonly three types of ECMs: constant cubic feet per minute, constant revolutions per minute and constant torque. For HVAC applications, it’s the controls built into the ECM that determine its type. The motor is the same. These aren’t the only types of ECMs in the world; these are just the most common HVAC types.
Because the controls are integrated with the motor and because the controls are typically programmed by the original equipment manufacturer at the factory, the various types and sizes are not interchangeable. If not an exact duplicate, replacement requires at least the same type and possibly from the same manufacturer to assure correct operation.
Let’s take a look at how these things work. First, let’s look at a typical VFD powering an AC induction motor. Three-phase AC induction motors, which have been in use for more than 100 years, are fixed-speed devices when powered directly from the AC line. The motor speed is determined by its number of poles and the frequency of its supply, which here in the U.S. is 60 Hertz.
All conventional VFDs are of the same basic design to use an input rectifier section to convert the AC to DC. That DC power is then stored in capacitors. In Figure 2, the output section draws from that DC power and sends pulses to the motor in a method called pulse width modulation. By varying the timing and duration of the pulses, the VFD can control the speed of the motor by varying the frequency of the pulses.
Note the similarities to the VFD in the ECM in Figure 3. Most of the HVAC ECMs are three-phase permanent magnet motors.
The significant difference here is that the ECM has the motor and speed control built into a single unit. The width of the pulses here varies the average voltage the motor sees, just like a VFD. Remember, though, these ECM pulses happen rates in the tens of Hertz, where VFDs send pulses at switching frequencies in the tens of thousands of Hertz.
Now let’s look at some of those dependencies noted earlier (see Table 1). ECMs have definite advantages over fractional and small horsepower single-phase motors without speed control; however, they become cost-prohibitive in higher horsepowers. VFDs and induction motors are available to meet any HVAC application.
|Communications||Dedicated||Building automation system networks|
Table 1: These are dependencies to consider when evaluating electronically commutated motors and variable frequency drives. Courtesy: Yaskawa America Inc.
ECMs are typically programmed for a single purpose at the factory, while VFDs can be programmed at any time to do whatever job is required. Also, because of the wide range of capability built into VFDs, they are flexible enough to be programmed to not only satisfy the application required, but also to accommodate changes and be adjusted to fix problems and unexpected surprises.
VFDs can be tied into a building automation system through embedded communication networks, such as BACnet, whereas most ECMs lack that capability.
Because the controls are built in, ECMs are limited in their environments by the electronics. Because VFDs are separate, they can be located in friendlier locations than the motors they drive.
Regarding standardization, ECMs that are integrated with their application, such as fans, are typically built with a unique design for that specific purpose. AC induction motors are built to standards like NEMA (National Electrical Manufacturers Association) and IEC (International Electrotechnical Commission) and VFDs are sized to meet those standards. If you have a motor-VFD combination and need a replacement, you have a wide variety of suppliers to choose from to do the job for each of those devices. You might have a challenge, though, if your ECM fan in an array fails and that model is no longer available.
Now let’s look more closely at some other dependencies for single-phase motor types (see Table 2).
|Criteria||Shaded pole||Permanent split capacitor||ECM|
|Efficiency||15% to 25%||40% to 60%||70% to 80%|
|Speed control||Single||Single or multiple||Variable|
Table 2: Although higher cost initially, electronically commutated motors have significant advantages over conventional single-phase motors in the long run. Courtesy: Yaskawa America Inc.
First, when considering ECMs for residential HVAC applications, it’s important to decide priorities. If first cost is the driving factor, then the traditional shaded pole AC induction motor is the obvious choice. However, if thinking is more long-term, the efficiencies of the three designs are dramatically different. To calculate the payback, one needs to know the duty cycle for the application, which could translate to payback in a year or two for continuous operation or infinity if the motor only runs intermittently.
Speed control for traditional AC motors is dependent on the speed taps available. ECM speeds, on the other hand, are continuously variable based on the program built into the controls at the OEM.
The programmability of the ECM can certainly contribute to occupant comfort when properly applied, which also has value.
Motors for commercial buildings
Commonality is more of a commercial or institutional facility concern. What is most common in the facility? Does the building have VFDs of various sizes throughout the operation and have service people trained to program and maintain them? And is the supply chain in place with suppliers to provide new or replacement VFDs when needed? If so, then you might want to maintain that commonality and avoid ECMs.
On the other hand, if VFDs are foreign objects, then ECMs — although not always simple to apply — might be the better choice, provided that they are available in the size and application you need.
And what about harmonics? The same current harmonics result from the current pulses that occur when the input rectifier section charges the DC bus in both VFDs and ECMs. Harmonics for both ECMs and VFDs without any means of mitigation is on the order of 80% to 90%. Most HVAC-specific VFDs, though, have a built-in DC link choke that cuts that level to 30% to 35%. Some ECMs are including harmonics mitigation, which adds to their cost.
Whether those harmonics are a problem is not determined by either VFDs or ECMs, but instead by the magnitude of the harmonics relative to the entire system in which they reside. IEEE 519 is clear that the limits set in the recommended practice apply to the system, not individual components in that system. Therefore, neither ECMs nor VFDs is compliant nor noncompliant to IEEE 519.
So how do you know? You can hire a consultant to do an exact evaluation of harmonics for your facility or you can get a good approximation from one of several free harmonics estimation software programs available from VFD suppliers.
ECMs are touted as being more energy efficient than the VFD-AC motor combination and in some cases, they are. Their efficiency gains come primarily from their use of permanent magnets in the motor, which enables them to avoid the losses associated with the electromagnetism of the stator in a traditional AC motor and by being specifically designed for a single purpose.
VFDs, on the other hand, can control not only AC induction motors, but also permanent magnet and synchronous reluctance motors if efficiency is more important than first cost.
Both ECMs and VFDs gain efficiency over fixed-speed motors from the characteristics of affinity laws, which state that the power consumed for centrifugal loads varies with the cube of the speed. That means that if the speed is cut in half, the power is only one-eighth (0.5 x 0.5 x 0.5).
Here is a high-level overview of the typical capabilities of VFDs and ECMs. VFDs are self-contained motor speed control devices that cover the complete range of horsepowers in most facilities. And they have capabilities far beyond simple speed control. Those, combined with decades of HVAC industry expertise, enable VFD suppliers to be trusted advisers to solve application problems as they arise. Any VFD supplier can provide that information. And, HVAC-specific VFDs have building automation networks built-in for integration to the BAS for smart building operation.
ECMs are typically designed for a single purpose and for HVAC applications usually integrated with the mechanical portion, such as fans. Their control is usually via an analog or digital signal to maintain a specific speed, torque or airflow.
Assessing VFDs versus ECMs
Figure 5 shows a means of deciding between VFDs and ECMs. Remember, where those red triangles fit on these sliding scales is influenced by all those dependencies mentioned. There is no “one size fits all” for every situation. These listed here, though, are criteria that should be considered in all evaluations.
Recent trends for VFDs include new models with higher operating temperature limits to open up more challenging application environments. In addition, programming has been expanded beyond the VFD keypad and PC programming software to include mobile devices, such as smartphones and tablets, which every service technician carries these days. Those smart devices even have the ability to provide programming power to the VFD without the need for main three-phase power. Additionally, VFD programs can be stored and retrieved via free cloud service from anywhere with an internet connection.
|Fractional to 1,000-plus horsepower||Fractional to ~10 horsepower|
|Industry expertise to solve problems||Usually integrated to application|
|Network communications||Limited communications|
Table 3: Variable frequency drives and electronically commutated motors typically have significant differences in design and application. Courtesy: Yaskawa America Inc.
The primary limiting factor for ECMs to be practical above 10 to 15 horsepower is the cost of their permanent magnets. There is research going on in a wide variety of industries, including electric vehicles and wind turbine generation, to find better magnets at lower costs and less dependence on the world’s sources of rare earth mines. So far, the magnets with better properties than the ferrites, such as neodymium or samarium-cobalt come at 10, 15 or 100 times the cost. With better, lower-cost magnets, ECMs could expand their range of horsepowers. In addition, standardized designs may relieve the need for exact replacements.
One very interesting application for ECMs and VFDs both is DC system supplies, as has been implemented in some data centers. Data centers are now are estimated to consume as much power globally as the entire United Kingdom. Some are avoiding harmonics from ECMs and VFDs by converting incoming AC power at the entry point to DC with appropriate harmonics mitigation and feeding the entire system with 380 volts DC to eliminate the need for the input rectifier section of all the speed control devices.