Motor On A Hall Effect Switch

Difficulty level: 2 (simple, but requires the use of a soldering iron)

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.

The Hall effect IC used in Kits 6 and 8 (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 24V. The Hall effect IC's included in the kit, however, were tested extensively; and it was found that most of them start working at 3V. 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.

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).

Our experiments showed that the speed of this motor could be controlled by an extra magnet the same way the speed control unit works for the reed switch motors . The magnetic field of the additional magnet placed near the Hall effect IC interacts with the magnetic field of the magnets on the rotor. As you move this additional magnet the combined magnetic field becomes stronger or weaker depending on the orientation of the speed control magnet. It affects the time the Hall sensor sends the signal to the transistor and therefore changes the motor speed

Faraday's Law and Auto Ignition

How do you obtain 40,000 volts across a sparkplug in an automobile when you have only 12 volts DC to start with? The essential task of firing the sparkplugs to ignite a gasolene-air mixture is carried out by a process which employs Faraday's law.

The primary winding of the ignition coil is wound with a small number of turns and has a small resistance. Applying the battery to this coil causes a sizable DC current to flow. The secondary coil has a much larger number of turns and therefore acts as a step-up transformer. But instead of operating on AC voltages, this coil is designed to produce a large voltage spike when the current in the primary coil is interrupted. Since the induced secondary voltage is proportional to the rate of change of the magnetic field through it, opening a switch quickly in the primary circuit to drop the current to zero will generate a large voltage in the secondary coil according to Faraday's Law. The large voltage causes a spark across the gap of the sparkplug to ignite the fuel mixture. For many years, this interruption of the primary current was accomplished by mechanically opening a contact called the "points" in a synchronized sequence to send high voltage pulses through a rotary switch called the "distributer" to the sparkplugs. One of the drawbacks of this process was that the interruption of current in the primary coil generated an inductive back-voltage in that coil which tended to cause sparking across the points. The system was improved by placing a sizable capacitor across the contacts so that the voltage surge tended to charge the capacitor rather than cause destructive sparking across the contacts. Using the old name for capacitors, this particular capacitor was called the "condenser".

More modern ignition systems use a transistor switch instead of the points to interrupt the primary current.

The transistor switches are contained in a solid-state Ignition Control Module. Modern coil designs produce voltage pulses up in the neighborhood of 40,000 volts from the interruption of the 12 volt power supplied by the battery.

Add more annotation to coil diagram

Some modern engines have multiple ignition coils mounted directly on the sparkplugs. Instead of single voltage pulses, they may under some engine conditions produce three voltage pulses. The coil arrangement shown is on a Dodge engine.

Faraday's Law

Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. No matter how the change is produced, the voltage will be generated. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.

Further comments on these examples

Faraday's law is a fundamental relationship which comes from Maxwell's equations. It serves as a succinct summary of the ways a voltage (or emf) may be generated by a changing magnetic environment. The induced emf in a coil is equal to the negative of the rate of change of magnetic flux times the number of turns in the coil. It involves the interaction of charge with magnetic field.