AC Motors and Energy Transformations

Describe the main features of an AC motor

The AC induction generator used in large-scale power stations has a very similar structure to an AC induction motor. However, in an AC induction generator, the rotor is an electromagnet powered by a separate DC circuit, and the stator consists of 6 coils. A source of torque is used to rotate the electromagnet at 50 revolutions per second, which causes AC electricity to be generated in the field coils.

There are three types of AC motors- standard AC motors, universal motors and AC induction motors, and they each work differently. A standard AC motor is essentially identical to an AC generator, with a stator providing a magnetic field, a rotor that current is passed through, and slip rings connecting the rotor to a circuit. In addition, an AC motor usually has a fan to keep the rotor cool, a ferromagnetic core in the rotor to strengthen the magnetic field and it runs at 50 revolutions per second, the same as the frequency of AC power oscillation (50Hz). A universal motor is similar to a DC motor. It can operate on an AC or DC supply. Power is fed in, and runs through electromagnetic stators before entering a commutator. Each brush is connected to a wire that comprises one of the field coils, and is also connected to one end of a circuit. With a DC source, the commutator switches the current and the motor operates. With an AC source, although the direction of current being fed into the commutator is varying, the same variations are fed into the field coils, with the net effect that AC oscillation is cancelled out and the motor runs.

AC induction motors are entirely different. Induction motors have a rotor that is not connected to a power source- instead changing flux is used to induce a current in the rotor. This means that there is very low friction as the rotor is not actually in contact with the rest of the motor, and it also means there is very little wear and tear. AC induction motors have a more complicated stator with several field coil pairs. There are a total of 6 field coils, and each opposite pair is fed one phase of triple-phase AC power. This sets up a rotating magnetic field inside the stator. The rotor of an induction motor is generally similar to a squirrel cage (the type that allows pets to run endlessly), with two end rings and aluminium or copper bars linking the end rings to form a cylindrical shape. This cylinder is encased in a laminated iron armature so that the magnetic field passing through the rotor cage is intensified. As the field rotates, it induces current in the bars of the squirrel cage. This creates a force in the same direction as the rotation of the magnetic field, from Lenz’s Law. The squirrel cage then rotates, ‘chasing’ the changing magnetic field.

Remember- AC motors have a stator, rotor and slip rings. They also use an iron core and usually a fan. A universal motor uses a commutator and has a magnetic field generated using field coils. AC induction motors have a stator with 3 pairs of field coils (for a total of 6), and a “squirrel cage” rotor.

Perform an investigation to demonstrate the principle of an AC induction motor

An AC induction motor relies on the principle that a moving magnetic field induces a current in the rotor with a direction that, according to Lenz’s law, causes the rotor to spin in the same direction as the magnetic field. We demonstrated this principle by using a thin aluminium disk suspended by a string from a clamp on a retort stand so that the disk was free to rotate. To demonstrate the principle of an AC induction motor, we moved a strong ceramic magnet in circles around the circumference of the disk. The induced eddy currents caused the disk to rotate in the same direction as the magnet, thereby demonstrating the principle.

Remember- An aluminium disk was suspended by a string and rotated by moving a magnet.

Gather, process and analyse information to identify some of the energy trans- fers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

Electricity is simply an easy way to transmit energy from point to point which enables energy to be collected and transmitted on a large scale. The advantage of electricity is not only that it is relatively easy to transport, but also that it is easy to convert it into other forms. In light bulbs, electrical energy is converted into light energy. In the home, it is also converted into heat in devices such as heaters and toasters, and into sound through speakers. In the industry electricity is most often converted into kinetic energy which drives machinery used in the production of goods. So generally electricity is converted into kinetic energy or electromagnetic radiation in the house and in industry.

Remember- All electrical devices convert electrical energy into other forms.