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Lunar roving vehicle wheel

Hub motors

by Chris Woodford. Last updated: May 10, 2015.

We're so used to the idea of cars being, well, car-shaped, that we find any other body layout extraordinary. But there's no real reason why a car has to have an engine at the front, a trunk at the back, and a passenger compartment stuck in the middle. Nor is there any good reason why the passenger zone—the most important part of a car for most of us—has to take up only half the total space. Cars look the way they do largely for historical reasons: they've always been built that way. What if we could do away with the bulky, gasoline engine entirely and devote more room to the passengers and their cargo? That's one of the exciting possibilities that opens up if you use hub motors (compact electric motors built into each wheel) instead of engines. Let's take a closer look!

Photo: One of the aluminum mesh wheels from NASA's lunar roving vehicle, an electric car used on the Moon in the early 1970s, with tires made from zinc and steel. Though not exactly hub motors as such, each wheel was nevertheless powered by its own separate 10,000 rpm electric motor. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).

What are hub motors?

Hub motor on back wheel of an electric bicycle

If you've read our main article on electric motors, you'll know the basic idea of turning stored electricity into motive power: feed an electric current through tightly coiled wire that sits between the poles of a magnet and the coil spins around making a force that can turn a wheel and drive a machine.

Most electric-powered vehicles (electric cars, electric bicycles, and wheelchairs) use onboard batteries and a single, fairly ordinary electric motor to power either two or four wheels. But some of the latest electric cars and electric bicycles work a different way. Instead of having one motor powering all the wheels using gears or chains, they build a motor directly into the hub of each wheel—so the motors and wheels are one and the same thing. That's what we mean by a hub motor.

Photo: The hub motor of an electric bike. Note the thick copper coils of wire that convert electric power from the battery into the movement that pushes you along. Picture by courtesy of Fabian Rodriguez, published on Flickr under a Creative Commons Attribution 2.0 License.

How does a hub motor differ from an ordinary motor?

The basic idea is just the same. In an ordinary motor, you have a hollow, outer, ring-shaped permanent magnet that stays static (sometimes called the stator) and an inner metallic core that rotates inside it (called the rotor). The spinning rotor has an axle running through the middle that you use to drive a machine. But what if you hold the axle firmly so it can't rotate and switch on the motor? Then the rotor and the stator have no choice but to swap roles: the normally static rotor stays still while the stator spins around it. Try it with an electric toothbrush. Instead of holding the plastic case of your toothbrush (which, broadly speaking, connects to the static part of an electric motor), try holding only the bristles and then turn on the power. It's quite tricky to do, because the brush moves so fast, but if you do it right you'll find the handle slowly rocks back and forth. This is essentially what happens in a hub motor. You connect the central, normally rotating axle to the static frame of a bicycle or the chassis of a car. When you switch on the power, the outer part of the motor rotates, becoming a wheel (or wheels) that powers the vehicle forward.

How does a brushless DC (BLDC) motor work?

Top: Photo showing the outer case of a brushless DC motor or BLDC. Bottom: Photo showing the inside, inner component parts of the same motor.

Ordinary electric motors use a mechanical device called a commutator and two contacts called carbon brushes to reverse the electric current periodically and ensure the axle keeps turning in the same direction.

Hub motors are typically brushless motors (sometimes called brushless direct current motors or BLDCs), which replace the commutator and brushes with half-a-dozen or more separate coils and an electronic circuit. The circuit switches the power on and off in the coils in turn creating forces in each one that make the motor spin. Since the brushes press against the axle of a normal motor, they introduce friction, slow it down, make a certain amount of noise, and waste energy. That's why brushless motors are often more efficient, especially at low speeds. Getting rid of the brushes also saves having to replace them every so often when friction wears them down.

Here are some photos of a typical brushless DC motor. First, look at the fully assembled motor shown in the top picture. In a normal motor, you'd expect the inner coil to rotate (it's called the rotor) and the outer magnet to stay static (that's called the stator). But in this motor, the roles are reversed: the inner part with the coils is static and the gray magnet spins around it. Now look inside and you can see exactly how it works: the electronic circuit sends power round the nine copper coils in turn, making the gray outer case (which is a magnet split into a number of sections, bent round into a circle) spin around the copper coils and circuit board (which remain static).

How does the circuit know which of the nine coils to switch on and off—and when? You can't really see in this photo, but there are several tiny magnetic field sensors (known as Hall-effect sensors) positioned between some of the coils. As the permanent magnets on the outer rotor sweep past them, the Hall-effect sensors figure out where the north and south magnetic poles of the rotor are and which coils to activate to make it keep spinning. The trouble with this is that it means the motor does need an electronic circuit to operate it, which is something you don't need for an ordinary DC motor.

Photo: A small brushless DC motor taken from a computer's floppy disk drive and seen from outside (top) and inside (bottom). Bigger versions of these images are available on our Flickr page.

What are the advantages of hub motors?

It depends whether you're talking about an electric bicycle or an electric car. Adding a hub motor and batteries to a bicycle is a mixture of pro and con: you increase the bicycle's weight quite considerably but, in return, you get a pleasant and effortless ride whenever you don't feel like pedaling. Where electric cars are concerned, the benefits are more obvious. The weight of the metal in a typical car (including the engine, gearbox, and chassis) is perhaps 10 times the weight of its occupants, which is one reason why cars are so very inefficient. Swap the heavy engine and gearbox for hub motors and batteries and you have a lighter car that uses energy far more efficiently. Getting rid of the engine compartment also frees up a huge amount of space for passengers and their luggage—you can just stow the batteries behind the back seat!

Artist's impression of the lunar roving vehicle from 1969

Vehicles powered by hub motors are a whole lot simpler (mechanically less complex) than normal ones. Suppose you want to reverse. Instead of using elaborate arrangements of gears, all you have to do is reverse the electric current. The motor spins backward and back you go! What about four wheel drive? That's quite an expensive option on a lot of vehicles—you need more gears and complicated driveshafts—but it's very easy to sort out with hub motors. If you have a hub motor in each of a car's four wheels, you get four-wheel drive automatically. In theory, it's easy enough to make the four motors turn at slightly different speeds (to help with cornering and steering) or torque (to move you through muddy or uneven terrain).

Photo: An artist's impression of the lunar roving vehicle sketched out in 1969. The emphasis was on making a fold-up vehicle light enough to take to the Moon. Electric power was not only a practical choice: with no air in space to power an internal combustion engine, it was the only real option. Photo by courtesy of NASA Marshall Space Flight Center (NASA-MSFC).

What are the problems with hub motors?

Handling

Hub motors are bigger, bulkier, and heavier than ordinary wheels and change the handling of an electric car or bike: they increase the unsprung mass (the mass not supported by the suspension), giving more shock and vibration, poorer handling, and a bumpier ride.

Torque

Arrangement of brushless motor, circuit board, gearbox, and wire connections in a typical electric bicycle hub motor.

Another problem is delivering just the right amount of torque (turning force). A gasoline engine works best turning over quickly (making lots of revolutions per minute), no matter what speed you're actually doing on the road. You use a gearbox to convert the engine's high revs into high torque (and low speed) or high speed (and low torque) depending on whether you're starting off from a standstill, racing along the freeway, driving slowly uphill, or whatever. Hub motors have to be able to produce any combination of speed and torque without a gearbox; they usually work by "direct drive." But here's the snag: they sit inside the hub, at the very center of a relatively large bike wheel. If you turn the center of a wheel, its diameter works as a lever, multiplying the speed at the rim but reducing the torque by the same amount (see our article on how wheels work for an explanation). To get enough torque, you need quite a powerful motor—but not so powerful that it accelerates you too quickly and jerkily or snaps your spokes!

Hub motors typically achieve more torque by increasing the hub size quite significantly (a bigger stator and rotor make more torque than smaller ones); you can see from the electric bike photo up above that the powered hub in an electric bike is considerably bigger than the unpowered hub in an ordinary bike. Some hub motors boost their torque with internal gearboxes (typically an arrangement of planetary (epicyclic) gears in between the stator and the rotor), but since that adds weight, cost, mechanical complexity, and potential unreliability, many do not. Bigger torque brings an added problem: you need to be sure the rest of your wheel is strong enough to cope with the twisting forces a hub motor can deliver, particularly if you're converting something like an ordinary bicycle wheel into a powered wheel. The spokes on an electric bike are shorter and leave the hub at a tighter angle, which can stress them further. Suppose you mount an electric motor on the hub of a basic bike and switch on the power. Since you weigh quite a lot and there's plenty of friction between the tire and the ground, the motor could simply bend the spokes instead of moving you along the ground! So an electric bicycle typically needs stronger wheels (perhaps with stronger and more elastic spokes, different positioning of spoke holes, a thicker rim, or some other fix) than an ordinary one.

Artwork: Using internal gears to increase torque in an electric bike hub motor. In this design, you can see the brushless motor on the left, with its coils (red) and the magnets (blue) that spin around them. The motor powers the main bike axle (light blue) through one or more gears (yellow) and is controlled by an electronic circuit board (green). All these components are picked inside the hub (the outer limits of which are shown by the largest blue circle) and you can clearly see where the spokes attach to the rim. Artwork from US Patent 6,321,863: Hub motor for a wheeled vehicle by Chandu R. Vanjani, Mac Brushless Motor Company, 27 November 2001, courtesy of US Patent and Trademark Office (with colors added to the original for clarity).

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Woodford, Chris. (2008) Hub motors. Retrieved from http://www.explainthatstuff.com/hubmotors.html. [Accessed (Insert date here)]

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