October 5, 2012

Ceiling Fans

We've come a long way from this motor.
I recently needed to help a client select a ceiling fan, and I wound up learning exciting new things about motors. (Yes, it's a motor pun. It probably won't be the last pun you'll find on this blog.)

In my occasional series about how stuff works, you'll find posts like this one. The goal is not just to satisfy idle curiosity: along with some nuts and bolts, these posts will focus on why you should care about how stuff works, what questions to ask, and how to find out more.

The Search

When I went looking for a ceiling fan for my client, I started here, at the Energy Star web site. After all, the point of a ceiling fan is to increase comfort using less energy, so efficiency is key. Among other criteria, Energy Star requires fans to have at least three speeds, to move a minimum amount of air, and to exceed minimum efficiencies at low and high speeds. Efficiency is measured in cubic feet per minute per Watt. At low speed, the minimum efficiency is 155 cfm/W, and at high speed, it's 75 cfm/W. A quick glance at the list revealed that far greater efficiencies are available.

Energy Star data for ceiling fans, sorted in Excel.
So, I downloaded the Excel version of the product list. I sorted the list by efficiency at low speed. Scanning it, I discovered a huge jump in efficiency in all three speeds. A quick glance at the model names for the models below the red line: all these models had "Eco" or "DC" in their names. This leap in efficiency turned out to be a technological leap.

The Old Way

Most of the ceiling fans on the market in the U.S. are driven by AC motors, also called induction motors. This means that they take the alternating current from the building power supply and use it to induce motion in the rotor, using mechanisms that haven't changed much since 1888 when Nicola Tesla patented the electromagnetic motor above. If you want to know more about them, www.explainthatstuff.com has a nice explanation. AC motors depend on the cycling of the power to control their speed, so speeds are commonly fixed at a few multipliers of the frequency of the power source.

The old-style DC motors, which you'll find in battery-operated power tools, use brushes to transmit a current into a coil which rotates inside a magnetic field. These motors aren't very efficient and can generate sparks, noise, electromagnetic interference, and smells. Here's how those work. When I told my husband I was looking at DC ceiling fans, it was this kind of motor that made him look at me funny, I'm sure - converting AC power to DC, only to end up with brushes and sparks? Bad idea.

The Leap

So, how could the DC ceiling fan motors be more efficient than the classic AC models? Brushless DC motors are the key. In recent years, microprocessors have become common and inexpensive enough that they can be used to control currents in motors. Someone had the bright idea to turn the brushed DC motor inside out: the permanent magnets are on the rotor, and the current runs through wires wound around the stationary part. Here's a nice clear explanation of brushless DC motors. The rotor spins freely, propelled by currents that automatically switch direction to keep the propulsion going. Smooth, quiet, and efficient.

Not only do the circuits controlling brushless DC motors give ceiling fan manufacturers greater motor efficiency, they also allow speed control to break free from the standard frequencies available in AC power. New DC ceiling fans often come with six speeds, and fan direction can be reversed by remote-control, rather than a hand-operated switch on the body of the fan. The Haiku, by Big Ass Fan Co., adds a "whoosh mode" that varies the speed of the fan during operation to simulate outdoor breezes. Other computer-on-board advantages like sleep timers become possible, independent of outside control systems.

This story has two morals: Check out ceiling fans with DC motors, and use the Energy Star web site to do much more than find a list of products.

Special thanks are due to Brian Metz, who helped me find the links to motor explanations.

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