Wikibooks: A-level Physics (Advancing Physics)/Motors

• Format: Book
• Language: English
• Subjects: South Pole; North Pole; South pole
• Online Access: https://en.wikibooks.org/wiki/A-level_Physics_(Advancing_Physics)/Motors

Just as a moving magnetic field induces current in conductors a changing current in a magnetic field induces motion. When this motion is used to ensure that the current keeps changing relative to the magnetic field the motion will continue and so we have an electric motor. There are several types of electric motor. =Simple DC Motor= When a coil is placed inside a stationary magnetic field and a direct current is run through the coil the coil tries to align itself with the field since it becomes an electromagnet. This would be useless as a motor since it would always move to the same position when you turned it on and then stop. If however we use a split ring commutator which changes the direction of the current every half rotation then the coil would try to align itself in the opposite direction every half rotation. This means that once the rotor starts to move it continues to move. This is a DC electric motor. The permanent magnets can be replaced with electromagnets as well. The main advantage of this type of motor is that the commutator works regardless of the frequency of rotation. =Three phase Motor= The three phase power produced by a three phase generator may be used to power a motor. Each phase of power is connected to one of three coils. This creates a magnetic field which rotates once for each cycle of the power. If a permanent magnet is placed in the middle at any given time its north pole will be attracted to a south pole in one of the coils and will be repelled by a north pole in one of the coils. The converse would be true for its south pole. This means that the rotating magnetic field drags the magnet around with it causing the magnet to rotate with the same frequency as the magnetic field. The disadvantage of this type of motor is that it goes at one frequency only the frequency of the current. The permanent magnet can be replaced with a coil with direct current in it. This creates a magnetic field the advantage being that there is no need for a permanent magnet which is expensive and heavy. The ...