18 Questions To Ask When Selecting Exhaust Fans

Of the fan manufacturers that use permanent-magnet motors in their fans, most are using Permanent Magnet AC (PMAC) motors. Surprising?

Our thought is, if you are an engineer or contractor selecting an exhaust fan, why not get the most reliable, efficient and user-friendly fan product available on the market? Based on research, feedback from customers, and future expectations around energy and efficiency regulations, we decided to do just that.

To make it perfectly clear, ENERVEX is NOT using PMAC motors. We are using a BLDC (brushless) permanent-magnet motor. There IS a difference. A big difference.

Different Fans, Different Motor Technologies

Every ENEgreenconstructionRVEX exhaust fan is a highly featured, ultra-efficient innovation in motor technology crafted to include or accept Electronically Commutated Motors (ECM). We don’t believe in smoke and mirrors, just giving customers an exhaust fan product that won’t let them down.

To clear up any confusion you may have about the benefits of BLDC motors vs. PMAC motors, here are 18 questions to ask when selecting exhaust fans. Reviewing these will help you make better decisions and achieve top system performance, now and on down the road:

BLDC motors vs. Permanent Magnet AC:  18 Questions That Will Clear the Air

1What’s the difference between PMAC, Brushless AC, PMSM (Permanent Magnet Synchronous Motor) and our Domel BLDC (Brushless DC)?


PMAC, PMDC and Brushless AC are synonymous terms (in the following, we will use “PMAC” as the common term). PMAC motors typically employ permanent magnets affixed to the inside of the motor frame, rotating wound armature and commutator (brushes). They are PM machines that operate on a PWM (Pulse Width Modulation) AC drive or control similar to an induction motor, but with software to control a PM machine.

Both the PMAC and the Domel BLDC motors are Permanent Magnet Synchronous Motors, and are very similar in terms of construction. But, the BLDC motor has permanent magnets that rotate inside a fixed armature, eliminating problems associated with connecting current to a moving armature.

In a Brushless DC motor, two coils of the armature are energized at a time with equal and opposite polarities: one pushes the rotor away from it while the other attracts the rotor toward it. This increases the overall torque capacity of the motor and the motor drive determines which two coils must be energized to achieve this strategy.

2What’s the relationship between Electronically Commutated Motors (ECM) and Permanent Magnet (PM) Motors?


Both typically employ permanent magnets affixed to the inside of the motor frame, rotating wound armature and commutator (brushes). A PM can either have EC (Domel) or brushes (traditional commutation). Both EC Motors and PM Motors are often available using a built-in inverter, although this will dramatically limit the motor’s overall heat resistance. The term “ECM” does not imply that the motor has a built-in inverter.

3In terms of construction, how do Domel Brushless DC (BLDC) motors differ from PMAC and induction motors?


The major difference between PM and induction motors is in the rotor itself. In a typical induction motor, current is induced into the rotor from the field (stator) through the air gap, and conducted through aluminum bars, which are most often die-cast in the slots of the rotor laminations. This induced current in the rotor then provides the opposing magnetic field that drives the motor.

In the case of BLDC motors, the rotor itself contains permanent-magnet material, which is either surface-mounted to the rotor lamination stack or embedded within the rotor laminations. The permanent magnets provide the force that drives the rotor. In both cases, electrical power is supplied through the stator windings.

4What are the primary benefits of BLDC motors versus AC induction?


Permanent Magnet BLDC motors are inherently more efficient due to elimination of rotor conductor losses, lower resistance winding and a “flatter” efficiency curve. Due to their synchronous operation, these motors offer more precise speed control. The motors provide higher power density due to the higher magnetic flux, compared to induction machines. And, PM motors generally operate cooler, resulting in longer bearing and insulation life.

5What are some of the major differences in performance between BLDC motors and AC induction?


The most obvious performance difference is that a BLDC motor rotates at the same speed as the magnetic field produced by the stator windings; i.e., it is a synchronous machine. If the field is “rotating” at 1800 rpm, the rotor turns at the same speed. An AC induction motor, on the other hand, is considered an asynchronous machine, as its rotational speed is slightly slower than the magnetic field’s “speed.” An asynchronous motor is said to have “slip” (the difference between the motor’s physical speed of, say, 1750 rpm, and its stator’s magnetic speed of 1800 rpm). It cannot produce torque without this difference in speed, as the rotor is constantly trying to “catch up” with the magnetic field.

The synchronization of BLDC motors results in improved efficiency, dynamic performance and more precise speed control – a major advantage when maintaining a draft or pressure set-point.

Other performance differences include higher efficiency and power factor in a BLDC motor. Since a permanent magnet rotor lacks conductors (rotor bars), there are no I2R losses, so with everything else equal, a BLDC motor is inherently more efficient.

A BLDC motor also maintains a constant torque over a much larger rpm range than an induction motor, which is ideal for air moving (fan) equipment. Also, the BLDC motors do not require a large “inrush” current during startup – the current draw at motor start is 1x max current as opposed to 6x-7x max current for AC induction motors.

Generally speaking, BLDC motors provide higher flux density than a comparable induction motor – meaning that more power (torque) can be produced in a given physical size, or equal torque produced in a smaller package.

6What is the useful speed range for BLDC motors compared to AC induction?


BLDC machines typically have a wider speed range than AC induction machines. However the number of poles may be different for the motors being compared and speed range is also a function of the drive being used. The higher speed is achievable due to the lower operating temperature of the BLDC motor. An induction motor would likely overheat if operated at the comparable speed of its BLDC counterpart.

7What is “Back EMF”?


Back EMF (BEMF) is the voltage generated by a rotating permanent magnet machine. As the rotor spins – with or without power applied to the stator windings – the mechanical rotation generates a voltage, i.e., becomes a generator. The resulting voltage waveform is either shaped like a sine wave (AC) or a trapezoid (DC), depending upon the configuration of the magnets on the rotor. This is the major difference between “Permanent Magnet AC” (a.k.a. “Brushless AC”) and “BLDC” (Brushless DC). The faster the rotor spins (again, with or without power), the higher BEMF voltage is generated. This BEMF is sensed by the drive, which allows it to control the speed of the motor.

8Can BLDC motors be operated without a drive?


No. All commercially available, true permanent magnet motors require electronic commutation via a variable frequency drive (VFD) to operate. For ambient temperature applications, it can be integrated inside the motor. For high-temperature applications, it must be external. ENERVEX has developed the EDrive motor controller specifically for the Domel BLDC motor.

9Why does a BLDC motor have more “poles” than an equivalent AC induction machine?


It doesn’t have to. But it helps reduce cogging torque.

10What impact does speed (input frequency) have on the efficiencies of induction and BLDC motors?


Generally speaking, a BLDC motor has a “flatter” efficiency curve than its AC induction motor counterpart, providing even more energy savings at reduced speed.

11How much additional efficiency should a user expect to obtain from a BLDC motor?


In general, BLDC motor losses (inverse of efficiency) are 15-20% lower than NEMA Premium induction motors. Since each efficiency index represents 10% fewer (or greater) losses than its neighbor, efficiency ratings will be 1-3 indices higher. However, it’s worth noting that sensor-less BLDC control efficiencies fall off at lower speeds because the drive cannot accurately sense the speed. Depending on motor size, electric utility rate and duty cycle, customers could see payback in as little as one year by using BLDC motors.

12What is “power density,” and how does it relate to BLDC motors?


Power density is simply the ratio of output power or horsepower to physical size or volume of the motor. Many factors such as material characteristics and temperature constraints may limit how much power a machine can deliver of a certain size. Different topologies and machine configurations address these limitations in various ways. For example, rare earth permanent magnets produce more flux for their physical size than the magnetic energy (and resultant torque) produced by an induction motor’s “squirrel cage rotor.” As such, a BLDC motor can have higher power density than an equally rated AC induction motor.

13What is “form factor,” and how does it relate to BLDC motors?


Form factor is a different way of looking at power density. In this context, form factor refers to the physical (primarily dimensional) properties of the motor, which may be defined as the active materials or the overall envelope size. The simplest view of this is the potential frame size reduction of BLDC versus AC induction motor, assuming the same power rating. Another way of looking at this is the ability to “down-frame” the same power rating into a physically smaller package (frame size).

14What is the motor’s Service Factor, and how does this differ from “Reserve Torque Capability”?


The Domel motor’s Service Factor (SF) is rated 1.0 on VFD power, which is in line with all other inverter-duty induction motors. SF is an often misunderstood concept. Its use is limited to NEMA motors; the IEC doesn’t recognize SF. Operating any motor beyond its rated power will result in additional heating. Heat is the enemy. Intermittent operation above rated power is most normally acceptable, so long as its components can withstand the additional thermal stress. “Reserve Torque Capability” is an expression of the motor’s ability to safely deliver increased torque, due to higher peak torque capability, and is subject to the drive’s ability to deliver increased current.

15Are all AC drives suitable for operation of BLDC motors?


No. The drive used should be designed for use with permanent magnet machines. This is included in the specification for the drive and there is often a parameter to set to tell the drive that the motor attached is a PM motor. Drives should be specifically designed for PM machines, especially when dealing with BLDC motors that require a “trapezoidal” drive.

16What is the operating temperature of the BLDC motors, how does this compare with induction, and what is the benefit?


Domel BLDC motors are more efficient than induction motors and run cooler under the same load condition. This results in longer insulation and bearing life, and reduces the amount of heat that goes into the operating environment. A general rule-of-thumb is that for every 10°C (50°F) increase in operating temperature, insulation life is reduced by half; conversely, every 10°C (50°F) reduction in temperature doubles the insulation life. Domel BLDC motors are equipped with a full Class H insulation system, but the design limits operating temperature to no more than Class B rise, providing very generous “thermal headroom” – and much longer insulation life.

17How do sound and vibration levels compare between BLDC and AC induction?


Domel motors are designed for most general-purpose applications with much lower sound and vibration than an induction motor. There is no hum or spikes with voltage frequency.

18Why should I buy Domel motors?


This innovative new product is designed to reduce costs in the following critical areas of operation:

  • Energy cost – Efficiency ratings are 1-3 indexes above NEMA Premium, providing electrical energy savings from Day One. This translates to 10-30% fewer losses than a conventional motor, and provides quick return on investment. Electricity is estimated to comprise approximately 95-97% of the total lifecycle cost of electric motors; therefore, energy savings will reduce the total investment significantly.
  • Maintenance cost and reliability – Due to its lower operating temperature and design, Domel motors offer longer bearing and insulation life. The robust construction of the motor is suitable for virtually any “mission-critical” application, providing years of trouble-free operation in harsh environments.
  • Direct drop-in replacement for existing NEMA and IEC motors – Under most circumstances, replacing existing induction motors with Domel requires no mechanical changes to the equipment. The motor is air cooled and maintenance-free with pre-lubricated and sealed ball bearings. The motor with controller is rated at 92% efficiency and able to operate as low as 50 RPM. It has integrated protection against overloading, blocking over and under voltage and overheating.
  • The motor shaft is internally isolated to eliminate the need for external shaft grounding. It is rated for outdoor installation. The included EDrive Motor Control is factory programmed by ENERVEX for optimal operation of the ventilator. 

Where to learn more

If you want to learn more about ENERVEX commercial exhaust fan products including the EDrive Motor Control, call us at 800.255.2923 or download the EC Motors whitepaper:

“Why Electronically Commutated Motors?”