Field - Kaiji (English spelling) field

A magnet used to create a magnetic field in rotating electrical machines such as electric motors and generators. There are electromagnets (wound field magnets) and permanent magnets (permanent magnetic field magnets).

Large machines use electromagnets with a winding wound around an iron core through which a direct current flows. This iron core and winding are called the field core and field winding, respectively, and the current passed through the field winding is called the field current. Passing current through the field magnet is sometimes called excitation. In DC machines, the field magnet is placed on the stator side, but in synchronous machines (generators, electric motors), it is often placed on the rotor side, which is called a rotating field structure. Changing the field current changes the strength of the magnetic field, so by controlling the field current, it is possible to control the motor's rotation speed and power factor, the generator's output voltage, and so on.

Small synchronous machines use a rotating field structure with permanent magnets arranged in the rotor. There is no loss due to the resistance of the field winding, so it is highly efficient. The magnetic field of a permanent magnet field is constant, but the field magnetic flux can be controlled by weakening the armature current and controlling the magnetic flux.

[Naoyoshi Isobe and Masayuki Morimoto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

電動機、発電機などの回転電気機械において、磁界をつくるために設ける磁石。電磁石（巻線界磁）と永久磁石（永久磁石界磁）がある。

　大型機では鉄心に巻線を巻き、これに直流電流を流した電磁石を用いる。この鉄心や巻線をそれぞれ界磁鉄心、界磁巻線といい、界磁巻線に流す電流を界磁電流という。界磁に電流を流すことを励磁とよぶことがある。界磁は直流機では固定子側に置くが、同期機（発電機、電動機）では回転子側に置くことが多く、これを回転界磁形構造という。界磁電流を変えると磁界の強さが変わるので、界磁電流を制御することにより電動機の回転数や力率、発電機の出力電圧などを制御することができる。

　小型同期機では回転子に永久磁石を配置した回転界磁構造の永久磁石界磁が使われる。界磁巻線の抵抗による損失がないので、それだけ効率が高い。永久磁石界磁の磁界は一定であるが、電機子電流を弱め磁束制御することにより界磁磁束の制御が可能である。

［磯部直吉・森本雅之］

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