Gear Boxes and Generators 103
Focus: This lesson will provide a basic overview of the gearboxes and generators inside
wind turbines.
Grade Level: 9th Grade to 12th Grade
Time: 50 minutes
Generators
1. Ask Students: Is there a difference between a motor and a generator? Have
them give examples of electric motors. Ask what these motors have in common.
Have them give examples of electric generators. Ask what these have in
common.
*Reinforce that electric motors require electricity to make them run – electricity
is converted to mechanical movement (usually rotating a shaft).
*Reinforce that electric generators, generate electricity – mechanical movement
is converted to; the exact opposite of what a motor does.
*Wind turbines convert mechanical energy (rotating shaft) into electricity.
*There are 2 types of electricity: AC (alternating current) and DC (direct
current). Wind turbine generators produce the AC type.
Explain to students: A generator produces AC electricity as a result of
Faraday’s Law which basically says that a magnetic field moving across a wire
will cause electrons to flow in the wire (electrical current).
How much electrical current flows depends on two main things: 1) how strong
your magnet is and 2) also how fast the magnet moves across the wire.
Remember that a magnet has a North and South side to it, so when the North
side passes over the wire, electricity flows in one direction along the wire, when
the South side passes over the same wire, electricity reverses its direction and
flows the opposite way. If a magnet is rotating next to a wire, electricity flows
back and forth in the wire – this is AC electricity.
Author: Jim Carlin
Page 1
If the magnet rotates faster, there is more electricity produced and it switches
directions along the wire more quickly. If the magnet rotates slower, there is
less electricity produced and it switches directions along the wire more slowly.
If the magnet is not moving, no electricity will be produced.
So a generator is nothing more than a shaft rotating a magnet over coils of wire
that output AC electricity.
Explain to students: When we think of how fast things are rotating, we think
in terms of revolutions per minute (rpm). That is the number of complete
revolutions that happen in the span of one minute.
Ask students: How fast do you think the main shaft of a wind turbine normally
rotates?
(Ans: pretty slow---around 15-18 rpm)
Ask students: If a generator was attached directly to the main shaft, would it
be fast enough to generate a useable amount of electricity?
(Ans: no way)
Gears
2. Ask students to describe what they think a gear is and what its main features
are. Draw their description on the board.
Ask students: What is the purpose of a gear?
Describe to students: that a gear has 2 aspects to its movement: (rpm and
Torque)
Define rpm: (revolutions per minute) tells how fast a gear is rotating
Define Torque: tells how much rotating force is being applied by or to the gear.
It is usually expressed in units of Newton-meters (N-m).
[Torque is the amount of force it takes to twist off the lid of a peanut butter jar]
Explain that rpm and Torque are always related in the following way: as rpm of
a gear increases, its Torque decreases and vice-versa.
Ask students: Does the main shaft of a wind turbine have a high rpm or high
Torque?
(Ans: high Torque)
Explain that transfer of rotational energy occurs between gears. Show
animation of two gears: http://en.wikipedia.org/wiki/File:Gears_animation.gif.
Author: Jim Carlin
Page 2
Either point out on the animation or draw on a board, a large circle touching a
smaller circle.
Drive
Pinion



The gear with the larger diameter has greater Torque but low rpm.
It is called the “Drive Gear” and usually has many teeth.
The gear with the smaller diameter has less Torque but high rpm.
It is called the “Pinion Gear” and usually has fewer teeth.
Energy can be transferred either way but in this example, The
Drive gear transfers energy to the Pinion gear resulting in lesser
Torque but higher rpm on the Pinion gear [this can be easily seen
with the gear-set available for demo].
Explain a simplified example: in a wind turbine the main shaft is rotated by
the hub and blades – this is pretty slow. A large diameter Drive gear is attached
to the main shaft; it has high Torque but low rpm. The Drive gear transfers its
energy to the smaller diameter Pinion gear (attached to the generator shaft)
giving it a high rpm. This is good because the generator needs to rotate at a
much higher rpm (about 1200-1800rpm) to generate the proper amount of
electricity. The process of converting a low rpm to high rpm is called “stepping
up”.
Gear Ratio
3. Explain to students that two gears can be related by a “Gear Ratio”. This
ratio is simply the number of teeth on the “driven” gear divided by the number of
teeth on the “driver” gear.
Author: Jim Carlin
Page 3
Example 1: A wind turbine’s main shaft gear (driver) has 2500 teeth and the
generator shaft gear (driven) has 25 teeth. What is the gear ratio in this
situation?
(Ans: 25/2500 = 0.01)
This means that for every rotation of the generator shaft gear, the main shaft
gear rotates 0.01 revolutions. Or we can look at it as for every rotation of the
main shaft, the generator shaft rotates 100 times.
Explain to students that with the gear ratio, we can calculate other
relationships between gears (e.g. rpm, torque, gear diameter, etc.).
Example 2: Using the gear ratio from above, if the main shaft is rotating at
15rpm, what is the rotational speed of the generator shaft?
(Ans:
15rpm/0.01 = 15rpm*100 = 1500 rpm)
Example 3: If the generator gear has a Torque of 5 Newton-meters (N-m),
what is the Torque of the main shaft gear?
(Ans: 5N-m/0.01 = 5N-m * 100
= 500 N-m)
Gear Train
4. Draw for students: that there can be more than two gears in a group.
2
3
1
Show that gear 1 could be driving the others or that gear 2 could be driving; it
doesn’t matter which drives; the gear ratio will still relate any two-gear
combination (e.g. the gear ratio between gear 1 and gear 4 (if gear 1 drives) is:
gear ratio = 30/10 = 3).
*Tell students that in a wind turbine, there are gear trains in place to step up
rpm. The gear train is located inside the “gear box”. Gears are sealed in a
gearbox to keep out dirt, debris, moisture, etc. from the surrounding
environment. The gearbox will typically have an input and output shaft that
rotate at different rpm. The difference in rpm is determined by the gear train
configuration inside the box.
Author: Jim Carlin
Page 4
5. Comprehension Exercise:
a. What is the difference between a motor and a generator?
(Ans: motor converts electricity into mechanical movement; generator
converts mechanical movement into electricity.)
b. What type of electricity do wind turbine generators produce?
(Ans: AC electricity)
c. What are the two main things that determine how much electrical current
flows from a generator?
(Ans: strength of magnet and how fast the magnet moves relative to
the conductive wire.)
d. What does rpm stand for?
(Ans: revolutions per minute)
e. What does Torque mean?
(Ans: rotational force)
f. The process of converting from low rpm to high rpm is called what?
(Ans: step up)
g. Name two things you can figure out with a gear ratio?
(Ans: rpm, torque, gear diameter)
h. Why do we need a gearbox?
(Ans: to prevent dirt, moisture, etc. from damaging gears)
Author: Jim Carlin
Page 5