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Science project idea

How fast a toy motor spins with one, two, three and four AA cells

The speed of a DC motor rises almost in proportion to the voltage across it. More cells in series give more speed and also a larger current.

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What it shows

The speed of a DC motor rises almost in proportion to the voltage across it. More cells in series give more speed and also a larger current.

The science

The voltage applied to a motor makes current flow in its coil, and the coil turns in the magnetic field until the voltage it generates back (back emf) almost balances the supply. Because back emf is proportional to speed, a higher supply voltage gives a proportionally higher speed with no load. The extra voltage also pushes extra current, so the cells discharge faster.

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.

  • A DC toy motor rated 6 V or more
  • A 4 x AA battery holder with four cells and a lead to tap each stage, or four single holders
  • A coloured paper flag on the shaft
  • A multimeter for current and voltage
  • A phone camera with slow motion video
  • A notebook

Safety

For this project: Cells only. Do not let the motor stall while switched on: it gets hot and the current rises sharply. Keep the flag small and light.

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 rules

Step 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.

  1. Fit a small paper flag on the motor shaft with a drop of glue or tape; make it light and even.

  2. Connect one cell (1.5 V) to the motor and record its speed by filming the flag in slow motion and counting turns per second.

  3. Measure the motor current with the multimeter on 10 A or 200 mA, in series with the cell.

  4. Repeat with two cells in series (3 V), three (4.5 V) and four (6 V), changing one cell at a time.

  5. Run each test for only a few seconds so the cells and motor do not warm.

  6. Calculate revolutions per minute for each voltage from your video count.

  7. Plot a graph of rpm against voltage and check whether the points lie on a straight line.

  8. Write your conclusion and explain why the speed at 1.5 V may be less than a quarter of that at 6 V.

What you should see

The graph of speed against voltage is close to a straight line that rises with every added cell, and the current also rises.

Viva questions

A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.

1What is back emf?

The voltage a spinning motor generates which opposes the supply.

2Why does speed rise with voltage?

A higher voltage drives more current and the motor must spin faster before back emf balances it.

3Why not stall the motor?

With no back emf the current is large and the coil overheats.

4How are the cells connected for 6 V?

Four 1.5 V cells in series.

Free ebook: Viva Questions for Science Projects

Take it further

Repeat with a small load, such as a cardboard propeller, and see how the lines change.

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.

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