Hub motors
by Chris Woodford. Last updated: June 9, 2011.
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?
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 motor work?
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 have half-a-dozen or more separate coils
and replace the commutator and brushes with 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, 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.
Photo: A small brushless DC motor taken from a computer's floppy disk drive and seen from outside (top) and inside (bottom). First, look at the outside: In a normal motor, you'd expect the inner axle to rotate and the gray outer part to stay static. But in this motor, the "axle" is static and the gray bit 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 permanent magnet bent round into a circle) spin around the copper coils and circuit board (which remain static).
Hi-res versions of these images are available from 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!
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?
The most obvious problem is with 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 power into high speed or high
force (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—which is quite a tall order, if
you think about it, because most electric motors are designed to
rotate at very high speeds all the time. So you need quite a special
motor that can potentially provide both high torque at low speed (for
driving off from a standstill or climbing a hill) and low torque at high speed (for racing down the straight).
You also need to be sure the rest of your vehicle is strong enough to cope with the forces your
high-torque motor can deliver. Suppose you mount an electric motor on the hub of
an ordinary bicycle and switch on the power. Unless the wheel is
really strong, since you weigh quite a lot, the motor is just as likely to
bend, twist, and snap the spokes as move you along the ground! So an electric
bicycle typically needs much stronger wheels than an ordinary one.
Further reading
On this website
On other sites
- Electric Vehicle Technology Explained by James Larminie and John Lowry. John Wiley & Sons, 2003. Surveys and compares all kinds of electric vehicle technologies, including electric motors and battery power, fuel cells, and hydrogen supply issues.
- Electric Motors and Drives by Austin Hughes. Elsevier, 2006. Compares the different types of electric motors and the circuits needed for driving them.
