BLDC Motor fans draw considerably lower power vs the normal AC fans, when I have checked their data.
For about 220 CMM, BLDC fans draw 28W, vs 78W by the older fans (split phase, please correct me if I'm wrong) fo a slightly higher airflow.
Can someone please explain how this efficiency is achieved?

 

For one, a BLDC uses a permanent magnet field so this is a reduction in any current required, a AC induction motor, particularly a 1ph version is not a particularly efficient motor in the production of field.
Also BLDC operates synchronously.
Max.

 

Will that be that inefficient?
Meaning,
bldc 28w gives 220 CMM. 7.86 CMM/W.

Normal AC fans, 78w gives 235 CMM. 3 CMM/W.

Even if we assume the BLDC is 100% efficient, then the AC fans (induction or split phase) are only 3/7.86=38% efficient. Suspect that it can't be that bad. I have always thought motors are pretty efficient above the 75% mark. Where am I going wrong?

 

AC induction motors represent a transformer with a shorted secondary at the point of switch on, this decreases as the slip frequency decreases, a Shaded pole fan motor is even worse than a 1ph PSC motor as far as efficiency.
Max.

 

from search ...."Efficiency. (See also Energy savings) Full load induction motor efficiency varies from about 85% to 97%,"

Where do you get these figures from for air flow ?? Are they accurate?

If they are accurate , maybe the fan and not the motor is where the difference lies , brushless can run at a wide range of speeds perhaps allowing greater efficiency in moving air .... A manufacturer who has taken the care to use brushless ,will probably take care in designing the right prop and speed to match .

 

from search ...."Efficiency. (See also Energy savings) Full load induction motor efficiency varies from about 85% to 97%,"

Where do you get these figures from for air flow ?? Are they accurate?

If they are accurate , maybe the fan and not the motor is where the difference lies , brushless can run at a wide range of speeds perhaps allowing greater efficiency in moving air .... A manufacturer who has taken the care to use brushless ,will probably take care in designing the right prop and speed to match .

That was my first doubt too. Searched so many fans.
I am giving below a sample, which is not exactly the figures given by me earlier (Cant remember the exact ones), but are similar.

https://www.amazon.com/Westinghouse...8&qid=1535957653&sr=8-2&keywords=CEILING+FANS

61 W, 3588.69 CFM, 97 CMM.

https://aeratronfans.com/ae3-specification/

50 in. (1260mm)
18.5W, 5765 CFM, 156 CMM.

A third of power, and one and a half times airflow. That is why I am confused.