Why are DC motors more energy efficient than AC motors?



Take for example ceiling fans: the newer DC motor ones use up to 70% less electricity than conventional AC motor fans for the same size/output (although cost more to purchase).

In: Physics

To drive a motor forwards, you need to get electricity and push it through a thing, causing it to move.

AC voltage does from 230V (positive) to 230V (negative), meaning for part of the cycle, it’s literally at 0V. Only for a brief moment, since it does that whole cycle 50x a second. (Using the standards here, but it’ll be similar everywhere). Now when the voltage goes negative, it actually wants to pull the object backwards, so you do something in the motor or circuit to push it, and it kinda works, but it doesn’t do it very efficiently.

Switch to DC voltage, pinned at a set voltage, say 24V, it sits at +24V the whole time, pushing the same current through, in the same direction, consistently delivering power in a good way. Perfect.

The problem is that the electricity is delivered to your house in 230V AC, so you need a fancy bit of circuitry, a step down transformer, an AC to DC rectifier, and all that isn’t perfectly efficient. But it’s way better for the fan.

So you pay more, because there’s more components and stuff, but it’s more efficient than trying to power a fan with AC.

I hope that kind of explained it like you’re 5? Let me know if any of that needs breaking down more.

DC motors are more efficient, if you already have DC, like in your car. If you have AC, like in your house, and the fan includes a low cost “wall wart” to make DC for the fan then the efficiency of the motor is wasted in the inefficiency of the conversion.

As the motor gets bigger, the efficiency flips. Electric cars and cruise ships all use AC induction motors for higher efficiency at a variety of speeds.

AC current flips polarity 120 times per second (giving it a frequency of 60 Hertz).

So half the time the power is flowing the wrong way.

AC became the standard because the human nervous system is DC.

If a human contacts DC electricity of one polarity, muscles will lock closed. The other polarity makes them open.

Because AC changes polarity every 0.5 seconds, if someone contacts AC the most they will lock up for is half a second until the polarity flips and forces their muscles open so they can break contact.

With an AC supply, keeping something moving in one direction (like a fan motor) requires converting AC to DC so it always applying power in the same direction.

> There is a loss of 5% to 20% when AC power is converted to DC power

Source:. https://www.electricalindustry.ca/latest-news/1018-9-reasons-why-dc-may-replace-ac

On top of that:

> – DC power is significantly more energy efficient than AC power.
> – DC motors and appliances have higher efficiency and power to size characteristics.

I don’t think this is necessarily true if the AC motor is used correctly.

The problem with AC motors is that the speed the motor runs at generally has to be tied to how fast the alternating current alternates, we call this speed its synchronous speed. At its synchronous speed, the motor is generally pretty efficient, the problem is that at non synchronous speeds there are a lot of problems.

AC motors don’t really run at all at non synchronous speeds, you have to convert frequencies which is actually a big challenge in electronics, and this is where much of the waste comes from. House fans tend to like having variable control for their speeds and this is where much of the problem results.

Modern brushless DC motors actually run basically using alternating current, they just use the trick that converting DC to AC current is actually pretty easy, you just switch the current directions every other moment and smooth things out a little bit, and you can make any AC frequency you want by changing how often you switch the DC current, though the speed of the motor is still tied to the voltage supplied and the load it has to turn.

For DC house fans, you of course have to eat the efficiency penalty of converting from AC to DC, but done properly this isn’t that inefficient. Its just older motors that try to go directly from AC to other AC tend to be pretty inefficient in this regard.

AC motors in general are not inefficient, and the most efficient motors are AC, but shaded pole motors used in many fans are quite inefficient, but they’re popular because it’s a very cheap design to manufacture. The “shaded pole” is how the motor spins in a specific direction, despite only having 1 phase power input.

If you see “brushless DC” It’s actually an AC motor(3 phase), which needs extra control circuitry to drive. Basically it’s converting the AC from the wall into DC, then back into AC at whatever frequency gets the fan to spin at the desired speed, and whatever voltage makes manufacturing it cheapest.

Cheap DC motors are more efficient than cheap AC motors.

But the most efficient motors are actually AC with smart controls (see Switched Reluctance Motors). But the thing is that the frequency of the power on the motor is different from the one from the main line, so it typically involves a conversion to DC somewhere.