# DC vs AC efficiency



## dougingraham (Jul 26, 2011)

The efficiency of the motor/controller combination under your most common load is what is important. The difference between a typical AC and DC setup is not usually significant at your normal operating levels. The real differences are other areas.

Depending on your driving style you might see a regenerative braking recovery of 5 to 15 percent of the energy that would be wasted in friction brakes on a DC system. If all your miles are highway this doesn't matter at all. If all your miles are in stop and go traffic then this could be significant.

No idea why there would be a DC motor in an HVAC system at all.


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## rwaudio (May 22, 2008)

dougingraham said:


> No idea why there would be a DC motor in an HVAC system at all.


I agree, seems bizarre, unless referring to a brushless DC motor which may allow speed control and lower energy use (not necessarily more efficient) by reducing motor rpm.

I can't see how a real brushed DC motor would provide a measurable efficiency increase in an application like that unless the AC motors being replaced are truly horrible but I can't see that.


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## Roy Von Rogers (Mar 21, 2009)

nedram said:


> I read about DC and AC motors for EV and I am told that AC is more efficient. To make my furnace more efficient I replace the AC with DC motor. I'm confused. Which is more efficient? Why don't the HVAC and the EV guys agree?


Your a bit confused here. That supposed dc motor you claim is in your HVAC unit, is called an ECM motor (_Electronically Commutated Motor) and has no brushes.

The dc motors we talk about in here have brushes.


Roy

_


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## Sunking (Aug 10, 2009)

nedram said:


> I read about DC and AC motors for EV and I am told that AC is more efficient. To make my furnace more efficient I replace the AC with DC motor.


You wasted your time and more importantly your money. Most likely your blower motor was an AC Induction Squirrel cage AC motor, the most efficient motor there is. Or in your case *was *the most efficient. Bottom line if both cost the same, DC motors and controllers would be in the museum of DIY EV history. Only thing keeping DC motors in the game is cost. A equivalent AC system cost 2 to 3 times more than DC. However AC efficiency is high enough to justify the added expense in some circumstances. 

BLDC motors are misleading because they use a 3-phase AC Variable Frequency Drive powered by a DC Voltage. The motor itself is pure AC power.


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## PStechPaul (May 1, 2012)

Here is an explanation of why DC HVAC (blower) motors may be twice as efficient as AC, but at higher cost. Mostly it is due to having variable speed or torque control, but AC blower motors are often rather cheaply made and may have efficiency less than 70% and also low power factor so they draw more current for the same power:
http://cozycomfortplus.com/furnace-motor-ac-or-dc/

The higher efficiency may only apply to air conditioning, while for heating, any losses in the motor just produce additional heat, so they are essentially almost 100% efficient, whether AC or DC. But the variable speed and torque may be able to provide the most efficient setting for forced air depending on the load, which may vary due to the settings of the heating/cooling vents.

More arguments for DC motors in HVAC:
http://hvac-talk.com/vbb/showthread.php?168783-Furnace-DC-Motor-Cost-Savings


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