The term
electric machine describes a large range of
devices that impart or extract
energy from rotational motion. The two most common electric machines are the electric
motor and
generator. In principle, these two classes of machines use the same electromagnetic principles and can be considered identical electrically, so we will refer to them as simply
machines below.
Among electric machines there exist direct current (DC) machines and alternating current (AC) machines. We will briefly discuss each of these in turn.
DC Machines
DC machines generally comprise a cylindrical magnetic housing (the stator) and a central rotor. A strong magnetic field is created inside the stator through the use of permanent magnets, or a fixed wire winding. Machines that use a winding to create the stator field are called shunt wound.
The rotor is mounted within the stator and rotates there. This rotor consists of the rotating shaft (which is connected to the source or load), and the armature. The armature is a winding of wire attached to the rotor. In a DC machine, the armature is wound in such a way that it creates a strong magnetic field when DC current is passed through it. This field opposes/attracts the field of the stator and causes the rotor to spin.
Every half-revolution, the field created by the rotor must be reversed as the rotor field will be aligned with the stator field. This is accomplished through the use of a commutator and brushes. The commutator has two halves that are connected to the ends of the armature winding, and is bound to the shaft and rotates with it. The brushes are part of the stator housing and ride on the commutator as it turns. Every half revolution, the commutator reverses its configuration, and the current flows from the brushes through the commutator into the armature winding in the opposite direction.
As the brushes ride on the commutator, there is a constant source of wear from friction and electrical erosion. Brushes are usually made of a carbon composite so that they have low resistance, yet achieve low friction and long life. Commutators can also be coated with a conductive film which reduces friction. Brushes must be replaced periodically in many industrial DC machines.
AC Machines
AC Machines are often called induction machines due to the way they operate. In such a machine, a winding is made on the stator, which creates a rotating magnetic field inside when connected to an AC voltage source. A rotor with armature is placed inside, however the armature winding is closed in this case, and is not connected to the outside. There is no commutator or brushes. The rotating stator field interacts with the rotor winding and induces electric currents in the winding, creating an opposing magnetic field. This causes the rotor to spin.
Single phase AC induction motors usually make use of what is called a squirrel cage rotor, often a cylindrical piece of metal with vertical slots cut in it.
Three-phase induction motors typically employ three separate windings on the stator which create a uniform rotating field inside the motor. The rotor also has three windings which create the opposing field.
While AC machines are very useful in industrial mover and generator applications, they have deficiencies. Speed control of AC machines is more difficult than DC machines, as the rotation of the field is tied to the AC frequency. Variable frequency drives are often built into the machine feed system to allow a finer control of machine speed. AC machines are also undesirable in applications requiring precise positioning. DC machines are far more flexible in this regard.
Motors vs. Generators
Technically speaking, an electric machine could be used as a motor or generator, depending on the direction of energy flow. In reality, depending on the application and efficiency and mechanical considerations, a machine is built specifically to be either a generator or motor. However this won't stop your power professor from trying to trick you up on exams.