What it shows
A DC motor is also a generator. When a second motor is turned by the first, its coil spins inside its magnets and produces a voltage that lights an LED, showing that electrical and mechanical energy can be converted in both directions.
The science
In a DC motor, current through the coil in a magnetic field produces a force that turns it. The reverse also happens: turning the coil in the field changes the magnetic flux through it, and by Faraday’s law a voltage is induced across the coil. A toy motor spun at its normal speed gives about 1 to 2 V, enough for a red LED, which needs about 1.8 V. Because the second motor loses energy to friction and heat, it never returns all the energy the first motor puts in.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Two 3 V DC toy motors
- A short piece of rubber tube or a rubber band to couple the shafts
- One red LED and a 100 Ω resistor
- A 2 x AA battery holder with cells (3 V)
- A piece of wood or cardboard as a base
- Hot glue or clamps, and a multimeter if available
Safety
For this project: An adult must use the hot glue gun. Keep hair and loose sleeves away from the spinning shafts.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Glue both motors onto the base in a line so that their shafts point at each other with a 1 cm gap.
Push a 3 cm piece of rubber tube over both shafts to couple them, or loop a rubber band over two pulleys.
Connect the first motor to the battery holder through a switch.
Connect the LED long leg through the 100 Ω resistor to one terminal of the second motor, and its short leg to the other.
Switch on: the first motor spins, drives the second, and the LED glows. If it does not, swap the LED leads.
Hold the coupling lightly to slow the shafts: the LED dims, showing the voltage depends on speed.
If you have a multimeter, measure the generated voltage at full speed and write it down.
Try driving the first motor from a single AA cell and compare the LED brightness.
What you should see
The LED connected to the driven motor glows while the shafts spin and dims as they slow, showing generation.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the second motor produce a voltage?
Its coil cuts the magnetic field as it spins, inducing a voltage by electromagnetic induction.
2Why did the LED only light one way round?
A spinning DC motor gives a voltage of fixed polarity, and an LED conducts in only one direction.
3Why can the LED never be as bright as if connected directly to the battery?
Energy is lost to friction, heat and air resistance at every conversion.
4Name a real generator that works on this principle.
A bicycle dynamo or a hydroelectric power station.
Take it further
Replace the rubber coupling with a bottle-cap pulley and belt so the generator turns faster than the driving motor.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Robotics
Motors, Gears and Motor Drivers
Make things move: DC motors, servos, steppers and how to drive them