Electric Motors And Generators/AC Motors and Generators
- See Single-phase motors and generators, and 3-phase motors and generators, and AC basics, and also this.
AC generators
edit- A very simple AC generator consists of 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 electro-magnetics.
AC motors
editAC 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
editDC 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. See also Wikipedia: "Testatika" Electrostatic generators
DC Motors
editDirect Current (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. See also Wikipedia: Car starter
Other electric motors
editUniversal
editSee Wikipedia: Universal motors
Stepper
editA stepper motor is a brushless, synchronous electric motor that can divide a full rotation into a large number of steps, for example, 200 steps.