1. Overview of Motor Types Tutorial
2. Downloadable Glossary
3. Good explanation of DC, DC and Universal Motors
DC
Series. The current flows through the field magnet and then the armature. As the current flows through the structures in this order, it increases the strength of the magnets. A series motor can start quickly, even with a heavy load. However, such a load will decrease the motor's speed.
Shunt (parallel). One part of the current goes through the magnet while the other part passes through the armature. A fine wire is wound around the field magnet many times in order to increase the magnetism. Constructing the field magnet in this way also creates further resistance to the current. The strength of the current and the level of magnetism therefore depend on the resistance of the wire rather than the load on the motor. A shunt motor will run at an even speed regardless of the load. However, if the load is too heavy, the motor will have problems starting.
Compound. Has two field magnets connected to the armature (ie two sets of field windings) ..one in series, one in parallel. Has benefits of both the series and the shunt motor. They can start easily with a heavy load and maintain a relatively constant speed, even if the load is suddenly increased.
AC
AC motors ("squirrel cage" anyway) are easy to build and convenient to use. They do not require commutators, and most of them operate on current provided by electrical outlets in households. In an AC motor, the moving part is often called the rotor, and the stationary part is frequently known as the stator. The most common AC motors include induction motors ("squirrel cage" and "wound") and synchronous motors.
Induction Asides
A word on Wound AC: The rotor is wound with low resistance windings and by introducing a ajustable resister in the rotor circuit outside the motor, which is set high initially, you can:
1. reduce rotor current causing less stator current (less chance of burn out on start up).
2. Increase the starting torque because the rotor and stator magnetic fields are more in in phase with each other.
3. Slip speed is increased (ie the required start up (slow) speed can be accomodated).
... The external resistance contribution to the rotor resistance can be gradually reduced as the rotor accelerates up to operating speed until only the rotors own resistance remains and the motor behaves as an ordinary induction motor.
Single phase-induction motors have six sub groups. Pros and cons rather involved. (See p242 of Electrical Principles for Electrical Trades -JR Jenneson).
Induction (in general)
The rotor of a is a cylindrical iron core with slots along its length. Copper bars fit into the slots and are fastened to a thick copper ring at each end. The rotor has no direct connection to the external source of electricity.
The alternating current flows around the field coils in the stator and produces a rotating magnetic field. This field induces (creates) an electric current in the rotor, resulting in another magnetic field. The magnetic field from the rotor interacts with the magnetic field from the stator, causing the rotor to turn.
Synchronous
The stator also produces a rotating magnetic field. But the rotor receives current directly from the power source instead of relying on the magnetic field from the stator to induce an electric current. The rotor moves at a fixed speed in step with the rotating field of the stator. The speed is proportional to the frequency with which the alternating current supplied to the stator reverses.
Because this frequency is (usually) fixed, synchronous motors (like shunt and compound DC motors), maintain a consistent speed, even with a changing load. They also use less energy. These motors are ideal for clocks and telescopes, which require precise timing and smooth turning.
Universal (or "Series Motor")
Universal motors are built to operate on either DC or AC electricity. A universal motor uses a commutator, and its basic construction resembles the design of a DC series motor. On DC, it performs like a series motor. If AC is used, the magnetic poles of the armature and the field coils reverse with the frequency of the current. Universal motors are popular in many appliances because of their flexibility.
Linear Motors
A lnear electric motor is a device that generates motion along a straight path and can operate moving sidewalks and sliding gates, for example, more efficiently than can a rotating motor that requiring gears or pulleys to convert its rotary motion to linear motion.
Another use of linear electric motors is in maglev (magnetically levitated) vehicles. A maglev vehicle floats above a track, suspended in midair by magnetic force. In some maglev vehicles, magnets that generate this force are part of the motor.
There are two types of linear electric motors: (1) induction and (2) synchronous. Both types rely on a basic property of magnets--unlike magnet poles attract each other but like poles repel each other. Both kinds consist of a stator (a stationary part such as a track) and a rotor (a moving part).
Linear induction motors have a stator made of a strip of a conductor (a substance through which electric current flows easily). The rotor contains a row of electromagnets (devices that become magnets when current flows through them). When the electromagnets are switched on, current flows in the rotor, establishing a magnetic field in and around the rotor. This field, in turn, generates current in the stator. The stator current then creates a wave of magnetic force in and around the stator. The wave interacts with the rotor field, pulling the vehicle forward.
Linear synchronous motors have a stator made up of overlapping electromagnets. The rotor consists of a row of permanent magnets or electromagnets. When the stator is switched on, current in the wires sets up a wave of magnetic force in and around the stator. The wave interacts with the rotor's magnetic fields, moving the rotor along the stator.