When it comes to fans and motors more of a good thing is not always a good thing. A recent U.S. Environmental Protection Agency (“EPA”) study stated that almost 60% of the fans within buildings today are oversized. The study went on to say that almost 10% of the fans were oversized by at least 60%. Although the magnitude of the issue may be surprising, the problem is well known to anyone involved with the design and selection of fan systems. The conservative approach, often taken when designing and purchasing a fan and motor, results in a product that exceeds the system requirements and consumes more energy than necessary.
Oversizing fan/motor systems can end up creating a host of other issues, including higher installed and operating costs, increased maintenance, and possibly a higher level of vibration and noise. It is very common for the application of safety margins to be compounded through the specification and purchase process with the accepted remedy being the addition of a VFD, “to ramp down the speed.”
The issue becomes even more complicated in HVACR applications with requirements for fan speeds well below that of the standard, 4-pole, 1,800 RPM AC induction motor.
The difficulty derives from the fact that properly sizing a motor to lower speed design requirements, for example selecting an 8-pole 900 RPM, or a 6-pole 1200 RPM AC induction motor, must be weighed against the additional cost and inferior energy efficiency associated with these machines. Hence the most common solution has been to use a lower cost, more efficient 1,800 RPM induction motor, gear it down mechanically with belts and pulleys, and then control the final desired speed range with a VFD. Of course, belts and pulleys introduce their own inefficiencies, costs, maintenance requirements, and design complexity.
This issue is increasingly faced in the design and construction of custom air handling equipment used in mission critical data center applications. Here energy efficiency is of utmost importance due to the 24/365 duty cycle and the enduser focus on lifetime operation and maintenance costs. Also, these data center applications are tending toward high volume, low static pressure requirements, best addressed by larger diameter fans run at slower speeds.
Recent advancements in permanent magnet motor technology are now offering engineers an alternative to the inherent inefficiency and high cost of 900 RPM and 1,200 RPM AC induction motors.
Permanent Magnet Motor Advantages
Permanent magnet motors are more efficient than induction motors primarily due to the magnetic field in the rotor being produced by permanent magnets rather than induced electrically. And, unlike induction motors, permanent magnet motors maintain their high efficiency over a broad operating range — ideal for variable speed applications.
Permanent magnet motors also enjoy a higher power density than AC induction motors. They are able to produce more torque for their relative physical size than a comparable AC induction motor.
The permanent magnet motor’s ability to continuously deliver high torque at low speed may also eliminate the need for gearing or other mechanical transmission devices in many applications.
Because motor losses (energy inefficiency) translate to motor heating, higher efficiency PM motors operate at lower temperatures than comparable AC induction motors, particularly at lower speeds where induction motor efficiency drops off much faster than is the case with PM motors. Lower operating temperatures translate into longer motor life. With time the exposure to higher levels of heat can degrade the insulation, ultimately shortening the life expectancy of a motor. A rule of thumb is that an increase in motor temperature of 10% decreases the insulation life by 50%. Bearing grease life, hence bearing life, is affected, as well.
Recent Permanent Motor Advancements
Permanent magnet motors have been available for decades and have been widely acknowledged to produce higher efficiencies at a wider range of speeds compared to the more ubiquitous AC induction motors. The biggest barrier to adoption of permanent magnet motor technology in highly competitive HVACR applications has been that permanent magnet motors have been cost prohibitive, often two to three times the price of an induction motor, mainly due to the expense of the rare earth magnets needed to achieve the flux necessary to produce sufficient torque.
A new, flux focusing, PM motor design utilizes a unique conical geometry to solve the cost issue by allowing the substitution of low cost, readily available ferrite magnets for the rare earth magnets previously required. This new design provides rare-earth-like permanent magnet motor performance and efficiency at a price that is more comparable to induction motors.
Besides focusing flux, this new motor geometry has other distinct advantages over both AC induction motors and conventional permanent magnet motors available today.
The compact bobbin windings on the laminated stator poles eliminate end turns, reducing copper usage and associated I2R motor losses.
The straight axial flux path also allows for the use of grain-oriented steel, which reduces iron losses.
Hence, while significantly less costly, this new permanent magnet design not only compares well with induction motors, but is also more efficient than many of the conventional, higher cost, permanent magnet designs on the market today.
Application to 900 RPM and 1,200 RPM motors
While PM motors in general and the conical design in particular produce significant efficiency gains against AC induction motors at 1,800 RPM rating point, that advantage grows dramatically at 900 RPM and 1,200 RPM.
With this new design, modifications to the stator, without changing pole count, readily converts the motor’s rated set speed to points from 1,800 RPM to the equivalent of a 900 RPM (8-pole) or 1,200 RPM (6-pole) AC induction motor. (2,400 RPM and 3,600 RPM models are also available).
The resulting lower speed permanent magnet motor possesses the same high-efficiency characteristics as the 1,800 RPM permanent magnet motor, but operates at a speed range closer to the ideal fan design point. For example, a 5 hp, 900 RPM motor of this type is rated at 93% vs. a typical 82% to 83% in an induction motor of the same rated speed and power. And that percentage distance grows as one turns the speed down from 900 RPM to the 400 to 600 RPM often required by the application.
This provides HVAC equipment engineers the opportunity to optimally size and direct drive their fans at low speeds without sacrificing (actually enhancing) motor efficiency, while, at the same time, eliminating the costs, inefficiencies, and maintenance associated with belt drives — achieving unprecedented and cost effective efficiency levels not available until now.
Variable speed applications now make up nearly one-third of new HVAC equipment designs. While turning down the speed of a fan/motor reduces overall system energy consumption, it increases the inefficiency of the induction motor, offsetting some of the gains made.
Substitution of permanent magnet (PM) motors for induction motors can eliminate that offset, but until now PM motors have been prohibitively expensive.
New, conical stator/rotor geometry motors, eliminate the cost obstacle.
The advantages in terms of both cost and energy savings are even dramatically more significant when applied to 900 RPM and 1,200 RPM models, thus opening the door for their economical application to low speed, direct drive fan applications.