Motor on a Hall Effect Switch

Difficulty level: 2 (simple, but requires the use of a soldering iron)
Kits covered: Kits #6, 8-10

This is a simple and probably the most reliable motor. You may take a look at how easy it is to assemble this motor from the kit.

In 1879 Edward Hall placed a thin layer of gold in a strong magnetic field. He connected a battery to the opposite sides of this film and measured the current flowing through it. He discovered that a small voltage appeared across this film. This voltage was proportional to the strength of magnetic field multiplied by the current. This effect bears his name.

For many years the Hall effect was not used in practical applications because the generated voltage in the gold film was extremely low. However, in the second half of the 20th century the mass production of semiconductor chips started. Chips based on the Hall effect became inexpensive and widely available.

The Hall effect IC (integrated circuit) is a very small chip which includes many transistors. It consists of a thin layer of silicon as a Hall generator (which works better than gold) and several transistor circuits: to amplify the Hall voltage to a necessary level; to trigger output voltage with its growth; and to provide stable work regardless of the power supply voltage changes. The picture below demonstrates the Hall effect IC:

The Hall effect IC is a solid state electronic device with no mechanical parts and therefore it is more reliable than a reed switch. To no surprise it is now the most widely used sensor in industrial brushless motors. Normally, however, they include a lot of other components. Stan designed a motor on a Hall effect switch with minimum parts based on the same unified mechanical design and it worked very well. Actually his design was so simple and clear that the diagrams below were used in Electrical Engineering: Concepts and Applications to explain how the brushless motors work. This text book is used by many universities in the Introduction to Electrical Engineering course (see Links).

The Hall effect IC used in Kits #6 and 8-10 (or available as a separate part) is a unipolar switch. It turns on and off when the South pole of the magnet passes by its branded side. The North pole has no effect on it, unless it approaches from the back side of the Hall IC. This Hall effect IC has a built in voltage regulator and may work in the range from 4.5 to 24 V. The Hall effect IC’s included in the kit, however, were tested extensively; and it was found that most of them start working at 3 V. This is a typical Hall effect IC shown from the branded side:

The Hall effect switch output current is not sufficient to power this motor, therefore it also requires a power transistor. You may find information on this component at How It Works: Reed Switch Motor With A Transistor.

This is how this motor works:

1. When magnet #1 gets close to the Hall IC, the sensor sends a signal to the base of the power transistor. The transistor opens, and allows a bigger collector current to flow through the electromagnet. The electromagnet pushes magnet #3 away.

   

2. When the rotor spins away, magnet #1 stops affecting the Hall IC. Since the signal to the base of the power transistor has been removed, it is turned off. This disables the electromagnet.

   

3. The rotor continues to spin due to inertia until magnet #2 moves into the working range of the Hall IC. The Hall IC sends a signal to the base of the transistor. The transistor opens, and allows a bigger collector current to flow through the electromagnet. The electromagnet pushes magnet #4 away. This process continues until the power is disconnected.

   

New! Watch Brushless Motors Demystified (How A Motor On A Hall Effect Sensor Works) video on our YouTube channel.

This motor can be built from Kits #6, 8-10. If you decide to design this motor yourself, you may order only the parts you need (Hall effect IC, PNP power transistor, magnet wire, magnets, heat sink).