ECM Motor Modelling in eQuest

I am working on a project where DOAS with ECM motors are used. This is something new to me. Has anyone of you came across this?
 
The DOAS fans use EC motors and not VFDs. Apparently this fans has unloading efficiency power savings. The fan efficiency appear to be very low (around 50%-60%), but it is compensated by premium EC motors supposed to give power savings. I have no clue on how to model this.

Something bugging me (probably only me?):  “ECM Motors” is like saying “AHU Unit” or “DOAS System” or “ATM Machine” or “LCD Display…” I just don’t have room left in my brain to accommodate a second attribution to “ECM” (which an energy conservation measure!).

Nothing personal … just wanted to vent =).

Electronic Commutation (EC) motors are in many ways equivalent to VFD drives applied to AC motors (for pumps or fans) with respect to how savings are achieved relative to motors operating at constant speed:  reducing motor speeds (RPM) through affinity laws, roughly, reduces power input in a cubic fashion.  I say that first to assert:  Energy modelers should expect (and at least on occasion, inspect) that the fan system curves (power plotted against rpm or flow) for an AC motor-driven Fan + VFD & an ‘equivalent’ EC fan system should look extremely similar in shape, when overlaid.  Some consistent differences may be expected from such an overlay however, stemming from the nature of how the variable speed is achieved between the two technologies.

On paper, EC motors will eliminate (read: substantially reduce) the losses inherent to manipulating the power source within a VFD… Let’s break these losses down a bit:

VFD losses are summarily seen as internal heat gains in the space the VFD lives in, and can be categorized as variable vs. constant:

  1. A variable & proportional 2-5% (varies between manufacturers/models) of the motor load is lost at all ranges of operations.  These are called waveform losses.  In a line:  these are the losses resulting from converting/cleaning AC power into DC power, then recreating a new AC current and voltage waveform resulting in the targeted motor RPM ((AC > DC > AC).  EC motors in contrast simply have to get from AC > DC.  Losses from the simpler jump are non-zero, resulting in heat much like a simple “wall wart” cell phone charger, but not nearly so much as waveform losses in a VFD.
  2. There are also constant (unvarying) loads associated with the circuitry/controls, resistance losses, lights/screens, and whatever there may be in the way of a circulating/cooling fan for the VFD box/enclosure.  EC fan and pumps systems also feature circuit boards and can feature enough bells and whistles to be comparable to an equivalent VFD selection, though these loads are typically much lower in magnitude than the aforementioned variable waveform losses.

There are two major areas forming the basis of savings for EC motors and EC Fan/Pump systems over a AC motor + VFD equivalent selection:

  1. Eliminating/reducing the VFD losses. 
  2. DC motors by their nature can (on paper) turn WAAAY down in RPM’s while still delivering steady bhp, which means fans/pumps can be designed with extended duty points in mind to allow for stable operation at such low turndowns (more on this shortly)
  3. I said there are 2 major areas, but I’ll acknowledge here that some EC sales prose will toss belt losses into the mix as points against AC motors, to make their savings look all the better… this feels a bit below the belt (haha), but I’ll acknowledge a degree of interpretation may be needed based on context.  If there is a belt already in play for a final equivalent fan selection (say, for a cooling tower), this might be better considered as neutral ground for any EC motor substitute you’re considering.

Furthering bullet #2, you can expect in the course of a curve overlay exercise to observe some relative differences (not necessarily better) in the fan/pump impeller/housing efficiency in terms of power transfer to the fluid.  EC-driven fan and pump systems (including the moving parts and the range of duty points for the associated loop/airstream) are not necessarily superior in efficiency for all situations, but today are generally more established and a good idea for smaller, “packaged/on-the-shelf” situations*.  Specification grade fan/pump EC systems promise mechanical designs which could be better optimized for very low duty operating points.  EC motor/system manufacturers will on occasion seemingly deliberately obfuscate this matter however, and draw comparisons between “fan:motor system” efficiencies but represent it as fully due to the difference in VFD vs. EC technology alone…  Man, that hurt my head just to write – sorry for any readers!  My best takeaway advice on this matter is to not gather your model inputs from “sell sheets.”  Instead seek out your manufacturer’s “engineering literature” to clearly break down equipment performance in terms you can follow.

I want address a related issue related to this topic of turndown, since I’m havin’ a ball:  It’s very easy to over-state turndown savings potential for a number of reasons.  I’ll try to cover a couple common ones from my experience.

To re-iterate:  EC motor driven fan/pump systems commonly offer, relative to equivalent VFD systems, a relatively deeper range of stable operation (i.e. minimum turndown).  Some of the “technology comparisons” I’ve seen in this regard are a bit overblown in the energy savings claims. VFD technology has evolved over time, and the most economical VFD’s you can find today typically utilize pulse width modulation (PWM), which is more precise, efficient, and can produce better turndowns than preceding VFD technologies.  Before considering the serviced motor and whatever it’s attached to, VFD’s on paper hit a turndown floor in the ballpark 10:1 or 20:1 on their spec sheets (3-6Hz), whereas EC systems can approach close to zero.  

HOWEVER: reviewing any such performance data demands considering the context of an application next – will you or CAN you ever operate at such low RPM/flow? 

I would for example recommend a conversation with your HVAC engineer of record** about how comfortable you might be with VAV system having a minimum airflow turndown into the single digits (%), when considering the implications on diffuser airflow distribution patterns.  Would the resulting air velocity at the air diffusers be sufficient to avoid “dumping” in cooling mode, and ensure sufficient mixing during heating mode?  

When it comes to brand new VAV systems, I would be nervous to presume/project a system turndown below ~50% before knowing the person selecting the terminal boxes and diffusers has fully thought out these implications (even then, I’d expect a floor in the ballpark of ~25-30%).  When retrofitting CV systems with a VAV conversion, my comfort “floor” rises to ~70-75% before similar conversations can occur.  “How low we can go” in such circumstances is very much dependent upon the design/state of the ductwork (leakage?), diffusers, and what degree of turndown starts to generate comfort problems in practice.

Other things to consider with regard to turndown:

  • Generally, a given fan (or pump) can only slow so much before you run into unstable operations at the impeller, which can cause vibrations, noise, chaos/destruction, and tanks efficiency to boot. 
  • Lower system airflows can, before hitting other floors around comfort and mechanical stability, run into problems with coil freezing for DX systems.
  • What are we (reasonably) assuming for duct leakage?  10 years after substantial completion of the TAB?  Assuming we are maintaining requisite duct pressures, is leakage a substantial fraction of what we’re delivering at the minimum turndown point?
  • Not sure if this happens practically-speaking airside, but a pump impeller dealing with high pressure and subject to low flow & turndowns can end up in a state producing higher delivered torque (bhp) than would be seen in regular duty points with higher RPM’s, resulting in motor burn out

… Oh, and in response to the rest of the original query: A fan efficiency of ~60% doesn’t sound “low,” to my ears.  I regularly use a rule of thumb of 65% for fan efficiency (in isolation of motor/belt losses) to estimate fan power based on design flow/TSP, short of going deeper and pulling a fan/fan-system curves or taking actual field measurements.  If, short of a full selection, you want to get into the right ballpark for bhp knowing fan type and some loose sizing parameters, I personally find Greenheck’s browser-based selection suite pretty helpful: https://www.greenheck.com/resources/software/ecaps.  More recommended reading would include tabbing the first (huge) pictorial table of the Fan chapter in the ASHRAE Handbook Systems & Applications.  Note that this table used to be “better” circa 2008 and earlier (imho).  More current versions have “normalized” all the y-axes illustrating relative performance between the various fan technologies, which I think removes a degree of relative/comparative understanding.

As per my usual disclaimers – I hope this is helpful for a few readers, and if I’ve made any gross errors or mis-statements I welcome corrections/discussion! 

~Mr. Fahrenheit***

* I understand Europe is leading the world right now in this area, and is pushing their fan/motor manufacturers to supply all on-the-shelf motors 30hp and below as EC sometime in the next decade… writing looks like it’s on the wall for other continents’ HVAC  industries to benefit from the pending growing pains – thanks in advance!!!

** …even if that is yourself – talking to yourself is a perfectly normal level of crazy in this profession

*** you’re welcome

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