Now, what is Arago's disc (Fig. 2.42) that is frequently used for explaining AC motors?
The principle of Arago's disc goes that, when a disc made of copper or aluminum is sandwiched between magnets and the magnets are moved, eddy current is generated on the disc. And this eddy current causes the force that tries to move the disc in the moving direction of the magnets to be generated on the disc.
One of the familiar devices operating on this principle is the integrating wattmeter (induction electricity meter) (Fig. 2.43) installed on the leading wire at your home.
In the integrating wattmeter, a magnetic core of a coil made up of a large number of turns of a fine electric wire is installed on one side of an aluminum disc (Fig. 2.44). Even if the coil is parallel-connected to the lamp line, current consumption is very small because the electric wire is fine and long. This coil is called a voltage coil.
On the opposite side of the aluminum disc, another magnetic core of a coil made up of a small number of turns of a sufficiently thick electric wire is installed.
If you series-connect this coil to the lamp line, all current consumed at home flows via this coil.
However, the voltage drop is small since the coil wire is thick and short. This coil is called a current coil. The phase of current of these two coils is shifted by approximately 90 degrees due to the difference in inductance. So they generate a rotating magnetic field in the same way as the capacitor-run motor.
The eddy-current motor is one of the practical motors that utilize eddy current. As described in Chapter 1, a rotor primarily composed of soft steel is used for this motor. Movements of the magnetic field induce eddy currents in the rotor.
Eddy-current motors have characteristics as shown in Fig. 2.45. These motors provide the ability to control the torque by changing the applied voltage, and reduce torque pulsation, even when the motor shaft is restrained.
Because of these characteristics, eddy current motors are also called torque motors.
Due to these unique characteristics, torque motors are used in pressing mechanisms, braking systems, and winding units. However, since it is difficult for these motors to maintain rotating speed without a control mechanism, they are not appropriate for use as general power motors. Therefore, they are produced in limited volumes.