
Dynamometers
Last updated: September 9, 2009.
If your bicycle is fitted with a dynamo (a small
electricity generator that drives your front and back lamps from the back wheel), you'll know it's harder to pedal when the
lights are on than when they're off. That's because the energy you
need to make the lights come on has to come from your legs. The
faster you pedal, the quicker the dynamo turns and the brighter your
lamps shine (up to a point, anyway). So how bright your lamps are
shining is a crude measurement of how fast your pedalling and
how much force you're producing with your legs. Now suppose you
wanted to measure how much power something like a car engine could
make. You could do it using a bigger version of a bicycle dynamo,
with some sort of a meter in place of the lights to give you an
accurate reading. Machines that measure force, power, or speed in
this way are called dynamometers. Let's take a closer look at how
they work!
Photo: Testing, testing! The giant driving dynamometer (top) is turning a wind turbine generator and drive train (bottom) to simulate how well they would work in extreme wind conditions. If you can't figure out what's happening here, imagine a large electric motor and generator coupled together by a sturdy metal rod—and that's pretty much what
you can see. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (US DOE/NREL).
What is a dynamometer?
A dynamometer is a device that can measure force, power, or speed—so you can
figure out how much power you need or how much you have to hand.
Measuring how much power you need
If you've got something like a giant factory machine made up of levers,
gears,
drive belts, and other bits and pieces, and you want to know how big
an engine or electric motor you'd need to make it work, you can use
a machine called a driving dynamometer.
It's essentially just an ordinary engine or an electric motor with some
appropriate measuring meters or monitors attached so you know how
much power, force, or speed is being used at any given moment.
Measuring how much power you have
If you've got an engine or motor, you can use a different kind of dynamometer to measure how much torque
(turning force), power, or speed it can
produce. Here, the dynamometer acts as a variable load that the
engine/motor drives. It works by soaking up or absorbing the power
that the engine/motor produces, so it's called an absorption
dynamometer.

Photo: This absorption (chassis) dynamometer measures the power of a truck's
diesel engine. It consists of heavy metal rollers that spin round when the truck engine's wheels turn, absorbing the power. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (US DOE/NREL).
An absorption dynamometer is a bit more sophisticated and interesting
than a driving dynamometer. If you think about it, it needs a way of soaking up and dissipating potentially a
huge amount of power, and there are all kinds of different ways to do
this. One easy way is to use electromagnetism.

Photo: An electric motor and a generator are essentially the same device working
in exactly opposite ways. An electric motor can work as a driving dynamometer; an electric generator can work as an absorption
dyanamometer.
If you wanted to test out the power of an electric motor, you
could connect its driving axle up to the axle of a generator. As the
motor spins around, it would make the generator spin too, producing
an electric current proportional to the motor's power; measure the
generator current and you have an indication of how powerful the
motor is.
A car speedometer is a different kind of dynamometer that
uses electromagnetism. A spinning metal disc connected by a cable to
the car's wheels makes a magnet rotate inside a metal cup. As the
magnet rotates, it generates eddy currents (a kind of opposing
electromagnetism) in the cup that try to slow the magnet down.
The cup starts to rotate and that makes a pointer (the speedometer needle) turn up a dial.
Not all dynamometers use electromagnetism. Some work like water turbines:
as you spin their axles, you turn a paddle inside a drum filled with water (or thick oil). That provides
the resistance and load, and the power produced by an engine, motor,
or other machine under test is dissipated by the water or oil heating
up as the paddle turns. Other kinds of dynamometers use hydraulic
pistons or friction to dissipate power.
Some dynamometers can work in either absorption or driving mode. An electric motor is a
good example: feed electricity into it and it can drive another
machine; turn its axle using another motor, engine, or machine and
it will spin in reverse, producing a measurable electric current like an electricity generator.
Instruments that work in both ways (driving and absorption) are
sometimes called universal dynamometers.

Chassis dynamometers
If you've ever set foot in a garage, you'll have noticed some metal rollers on the
ground (or on a movable stand) onto which cars and trucks can be
driven for testing their engines.
Once the vehicle is in place, brakes on the rollers are disengaged.
Now when the vehicle engine starts turning
its wheels, the rollers turn too but the vehicle remains stationary.
The rollers are actually very heavy metal drums connected up to
sophisticated electronic measuring instruments that figure out
how much torque, horsepower, or speed the engine is capable of
producing by measuring how fast the drums take to accelerate. A
device like this is called a chassis dynamometer.
It's a type of absorption dynamometer that uses the drums' heavy inertia
to soak up the car engine's power.
Photo: Testing a car with a chassis dynamometer. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (US DOE/NREL).