Electronically commutated motors (ECMs) can achieve significant energy savings in applications where fractional power is required. While NEMA Premium Efficiency motors with variable frequency drives provide the most efficient solution for driving equipment above 1 HP, induction motors are outperformed by ECMs as power ratings are reduced.
What is an electronically commutated motor?
Although ECMs are designed to run on an AC power source, it is important to note that they are actually direct current motors with permanent magnets in their rotor. Unlike conventional DC motors, which create a rotating magnetic field with a combination of brush contacts and slip rings, ECMs achieve the same effect with a voltage rectifier and electronic control circuit. As a result, friction and sparks associated with brush contacts are eliminated, which is one of the reasons why ECMs are so efficient. They also have a longer lifespan than brushed motors, as the wear associated with sparks and friction is eliminated. Compared to other common types of fractional horsepower engines, ECMs are the best choice in terms of efficiency:
- Shaded pole motors are very common and more affordable, but their efficiency is very low, reaching below 20% in some cases.
- Permanent split capacitor (PSC) motors have an average efficiency of 40%, meaning they outperform shaded pole motors. In terms of efficiency, they are an intermediate option between shaded pole motors and ECMs.
- ECM efficiency is typically over 60%, which means they consume, on average, one-third of the energy used by shaded-pole motors.
ECMs can also be manufactured with integrated speed control circuits, allowing them to operate at reduced speed without relying on an external VFD. It is also important to note that ECMs do not experience a drastic reduction in efficiency when operating below rated RPM. Fixed speed ECMs are also available for applications where speed control is not required.
Electronically commutated motors in HVAC applications
ECMs are typically the most efficient option in fractional horsepower applications, but tend to provide greater savings when used in air conditioning and refrigeration systems. Being more efficient than shaded pole and PSC motors, they also dissipate less heat, and the reduced heating effect helps AC and refrigeration systems operate more efficiently. This effect applies to all air conditioning or refrigeration components found within the conditioned space, such as air handlers and evaporators.
As an example, suppose that a cold room has an evaporator unit with five shaded pole motors, consuming 900W each. They are replaced by ECMs that consume just 300 W each.
- 600 W are saved per motor, totaling 3,000 W.
- However, this 3,000 W is also subtracted from the cooling load. If the system operates with a coefficient of performance of 3, an extra 1,000 W of electrical energy will be saved.
- In other words, this upgrade saves 3 kW in engine power and 1 kW thanks to the reduction in cooling load.
Remember that this is just a simple example and that each project requires detailed analysis to know the exact savings. However, the heat reduction benefit applies to all cases where ECM engines are used in air-conditioned or refrigerated locations.
The brushless design of ECMs makes them quieter than their less efficient counterparts, which also provides an advantage in terms of comfort. In enterprise applications, the quiet operation of ECMs helps employees focus better. ECMs are also lighter than other types of fractional horsepower engines, which makes them easier to install.
Electronically commutated motors in ventilation systems
As previously stated, shaded pole and PSC motors are inefficient. Furthermore, three-phase motors with VFDs are impractical for fractional power applications, unable to offer the efficiency that characterizes them in larger systems. Ventilation systems represent an excellent opportunity to implement MREs, for two main reasons:
- Fractional horsepower fans are common, meaning they are usually driven by shaded pole or PSC motors.
- Many fans have intermittent operation, which presents an opportunity to use ECMs running at reduced speed. For example, running a fan 80% of the time saves 20% of energy, while running it at 80% speed saves almost 50%.
ECMs are a highly recommended upgrade for furnace fans, as they can achieve much more even temperature distribution with their speed control, in addition to offering the energy savings that characterize them.
ECM Updates Payback Period
As with many energy efficiency measures, the financial benefit of an ECM upgrade changes depending on project conditions. The project payback period can only be accurately calculated after a detailed analysis by a professional energy consultant, but in general the following results can be expected:
- Replacing shaded pole motors typically produces a faster payback period than replacing PSC motors since the efficiency gain is greater. Of course there may be exceptions; Replacing a PSC motor that is used frequently can yield greater savings than upgrading a shaded pole motor that is only used moderately.
- ECMs can also be an attractive option in devices that require speed control. Other types of engines can suffer a drastic reduction in efficiency at partial speed.
- As mentioned previously, energy savings are greatest when ECMs are deployed in air-conditioned or refrigerated spaces.
The financial return on an ECM upgrade can also be improved if incentive programs exist at the project site. For example, Con Edison offers cash rebates in New York City for installing ECMs in accessible refrigerators, refrigerators, and walk-in freezers. The incentive is calculated based on annual energy savings, at a rate of US$0.16/kWh.
Conclusion
Electronically commutated motors (ECMs) can achieve significant energy savings in fractional horsepower applications, especially when replacing shaded-pole motors. However, as with any energy efficiency upgrade, professional guidance is highly recommended when defining project scope and specifications. When upgrading to ECMs, the return on investment is greater in some cases, and in others the payback period may be too long to justify the upgrade. Ideally, upgrades should focus on where the greatest return is gained from every dollar spent upfront.
As motor power increases, the combination of a NEMA Premium Efficiency motor with a VFD emerges as the best choice – a VFD with single-phase input and three-phase output can be used when three-phase power is not available.
