Electric Motors And Generators

Introduction:

A wire in a magnetic field

* Electric motors and generators are closely related.
* Generators are based on the fact that when a wire, or a coil, is moved into and out of a magnetic field then a voltage will be generated between the ends of that wire or coil. The magnitude of that voltage is proportional (within a limit) to the strength of the magnetic field, and also the number of turns of the coil, while the direction of that voltage depends on the polarity of the magnetic field (North pole or South pole), and which leads of the voltmeter are connected to which end of the wire or coil.
* The magnetic field can be either the field surrounding a permanent magnet, or it can be a magnetic field created by the flow of a current in a coil.
* A coil is a long wire that is wound around and around, many turns. The number of turns is important, because in a generator the voltage produced, or in a motor the force/torque produced, is proportional (up to a limit) to the number of turns of the coil.
* Motors are based on the fact that if a wire, or a coil, is located in a magnetic field, and carries a current, then a force or torque is applied to that wire, moving that wire. The magnitude of that force/torque is proportional (up to a limit) to the magnitude of the current, the strength of the magnetic field, and the number of turns of the coil, while the direction of that force depends not only on the direction of the current in that wire or coil, but also on the polarity of the magnetic field. See also more about motors here.
* The current into a motor is high during starting, but as the motor increases its speed that current decreases, because, in fact, the motor acts also as a generator whose voltage opposes the supplied voltage, so that the net input voltage, and therefore the current flow too, are reduced.
* As the load on the motor is increased, the motor slows down somewhat, and therefore the opposing voltage is reduced, so that the net input voltage, and therefore the input current too, are increased.
* See also The motor effect

Magnetism :

• See Magnetic field and Magnetic phenomena and Physics Study Guide/Magnetism and Permanent magnets and Current's magnetic effects and Electromagnetism and Magnetism's units.
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Current flow

* See Current and Kirchoff's Current Law (KCL) and Current (Physics) and Relationships: voltage, current and resistance.

AC generators

• A very simple AC generator consists to a permanent magnet that rotates inside a coil in such a way that the N-pole and S-pole alternate as seen from the coil. An analog voltmeter (or rather a millivoltmeter?) that has its zero at the middle of the scale is connected to the ends of the coil. As the magnet is rotated the voltmeter moves first one way, then the other way. The speed of rotation determines the number of "cycles per second", called Hertz(Hz). A rotation speed of 3000 revolutions per minute(RPM) produces 50 Hz, and 3600 RPM produce 60 Hz.
• The rotating permanent magnet can be replaced by another coil that is fed by DC and acts as an electromagnet. Doubling the number of coils will double the number of, what is called "the poles", and then only half the rotation speed is required for a given output frequency.
• See also Wikipedia: Alternator

AC generator works on the principle of Faraday's laws electromagnetics.

AC motors

AC motors are generally divided into two categories, induction and synchronous motors. The most common AC motor is the "Squirrel cage motor", a type of induction motor. These have only one or more coils within which a special kind of mechanical rotor is free to rotate. There is no electrical connection to the rotor from the outside. The general formula to determine the synchronous speed of an induction motor is

For induction motors, this is a theoretical speed, even though it will never be obtained. The motor will always run slower than synchronous speed with a slip of S. If a motor were to be operated at full synchronous speed, the relative speed of the rotor to the stator would be 0, making it impossible to induce a voltage (Faraday's law) in the rotor windings. This in turn would make the flow of current impossible. Without current no magnetic field can be generated.

Most AC motors require a starter, or method of limiting the inrush current to a reasonable level. Types of motor starting include reactive (capacitor start and inductive start), and electronic (frequency drives and soft start drives).The reactive start method is usually used on fractional horsepower motors, and the electronic method is usually reserved for larger motors (cost of the drives is the main reason for this). Connecting these motors to computers, PLC's (programmable logic controllers), and interfacing with automation systems, is becoming more prevalent.

DC Generators

DC generators are basically AC generators whose output voltage is switched the other way round at the proper moment, so that the direction of the voltage is always in a single direction. But the magnitude of the voltage keeps changing, just as it does in an AC generator, and it can be said that the output of a DC generator is DC plus a "superimposed" AC voltage, called "ripple". Connecting a capacitor across the output terminals reduces that ripple.

DC Motors

DC motors have a "Commutator" that switches the part of the coil that is closest to the poles at the time, more or less similar to the legendary "donkey" that tries to catch the carrots, but never succeeds. See the very simplified commutator shown in blue. Usually a commutator has many "segments", as many as there are taps on the coil. Starting a DC motor requires often an external resistor or rheostat to limit the current. The value, in Ohms, of that resistor is reduced in steps as the speed of the motor increases, until finally that resistor is removed from the circuit as the motor reaches close to its final speed.