What is Electric Motor ?

As the requirements for larger scale power generation increased, a new limitation rose: the magnetic fields available from

permanent magnets. Diverting a small amount of the power generated by the generator to an electromagnetic field coil allowed the

generator to produce substantially more power. This concept was dubbed self-excitation

The field coils are connected in series or parallel with the armature winding. When the generator first starts to turn, the small amount of remanent magnetism present in

the iron core provides a magnetic field to get it started, generating a small current in the armature. This flows through the field coils, creating a larger magnetic field

which generates a larger armature current. This "bootstrap" process continues until the magnetic field in the core levels off due to saturation and the generator reaches

a steady state power output.

Very large power station generators often utilize a separate smaller generator to excite the field coils of the larger. In the event of a severe widespread power outage

where islanding of power stations has occurred, the stations may need to perform a black start to excite the fields of their largest generators, in order to restore

customer power service.[17]

Specialized types of generator

Direct current (DC)

An important class of direct-current generators are the dynamos - these are electrical machines with commutators to produce (DC) direct current, and are self excited -

their field electromagnets are powered by the machine's own output. Other types of DC generator use a separate source of direct current to energize their field

magnets.

Homopolar generator

A homopolar generator is a DC electrical generator comprising an electrically conductive disc or cylinder rotating in a plane perpendicular to a uniform static magnetic

field. A potential difference is created between the center of the disc and the rim (or ends of the cylinder), the electrical polarity depending on the direction of rotation

and the orientation of the field.

It is also known as a unipolar generator, acyclic generator, disk dynamo, or Faraday disc. The voltage is typically low, on the order of a few volts in the case of

small demonstration models, but large research generators can produce hundreds of volts, and some systems have multiple generators in series to produce an even

larger voltage.[18] They are unusual in that they can produce tremendous electric current, some more than a million amperes, because the homopolar generator can be

made to have very low internal resistance.

MHD generator

A magnetohydrodynamic generator directly extracts electric power from moving hot gases through a magnetic field, without the use of rotating electromagnetic