A 3-phase generator's stator has three identical windings. The three windings are placed at 120°to each other . When the magnet is rotated at constant speed, the voltages induced in the three windings have the same effective values, but the peaks occur at different times.
3 极相组The number of phase group’s slots A lap winding consists of a set of phase groups evenly distributed around the stator circumference. The number of groups is given by the equation Groups= poles×phases q＝Q/(2pm)
iC
t
iC
iB
Z X Y
C B
Im
A
Y
N
Z
合成磁场方向：
t 0
C
B
向下
S
X
同理分析，可得 其它电流角度下 的磁场方向：
iA
iB
iC
t
Im
n0
A Y
60
Z
n0
Y
A Z
n0
Y C
A Z B X
N
B
X
CS
C
N
X
B
N
t 60
t 120
t 180

Through electromagnetic induction, the time changing and reversing (alternating in direction polyphase currents) rotating magnetic field induces a time changing and reversing (alternating in direction)current in the conductors in the rotor.
右图是一台三相鼠笼型异步 电动机的外形图。 下面是它主要部件的拆分图。


An AC motor is an electric motor that is driven by an alternating current. An AC motor consists of two basic parts:
Application


25W small motors from CD player, toy and CD/DVD drive reader 250W motor from a washing machine
(Ⅰ) Operating principle of 3phase synchronous motor


An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field; An inside rotor attached to the output shaft that is given a torque by the rotating field.
交流电机电枢绕组基本概念 Basic conceptions of AC windings 1 电角度和机械角度： Electric angle and mechanical angle 电机整个转子表面所占的空间几何角度360度，叫机械角度。 一对主磁极表面所占的空间距离用空间角度表示，并定为 360度，叫空间电角度。
6.1.1 导体电动势
The fundamental electromotive force in conductor 设电机的基波气隙磁密：
b B sin
导体产生的电动势：
e b lv
n w 2 p 60
交流电机电枢绕组
The WINDINGS of AC ELECTRICAL MACHINE
的电动势与磁通势
交流电机的工作原理
Basic principles of AC electrical machine

共同点
同步 synchronous 异步 Asynchronous

当定子绕组通上 三相交流电，产生的 旋转磁场切割转子导 体产生感应电动势和 感应电流,在旋转磁场 的作用下产生电磁转 矩,使转子转动.


Where a polyphase electrical supply is available, the three-phase (or polyphase) AC induction motor is commonly used, especially for higher-powered motors. The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor.
旋转磁场的旋转方向
旋转方向：取决于三相电流的相序。
iA
iB
iC
t
iA
iC
iB
t
Im
Im
n0
n0
改变电机的旋转方向：换接其中两相。
旋转磁场的转速大小
一个电流周期，旋转磁场（即定子磁场）在空间 转过360°。则同步转速（即旋转磁场的速度）为：
Im
i A i B iC
n0 60 f (转/分）
t
n0
Y
C
N
A
60
A
NLeabharlann ZB XY
CS
Z B
Y C
N
A
Z B X
S
S
X
电动机转速和旋转磁场同步转速的关系
电动机转速（即转子转速）:
n
电机转子转动方向与磁场旋转的方向一致，
但
n n0
异步电动机
提示：如果
n n0
转子与旋转磁场间没有相对运动 无转子电动势（转子导体不切割磁力线） 无转子电流 无转距
(Ⅱ) Operating principle of 3phase asynchronous motor


If we connected a 3-phase source to the stator winding, alternating currents will flow in the windings. The current will have the same value but will be displaced in time by an angle of 120°. These currents produce a rotating field. According to Faraday’s Law and Lorenz force (左右手定 则 )on a conductor, a voltage will be induced in the rotor winding. The induced voltage immediately produces a current. Because the current-carrying conductor lies in the rotating field, it experiences a mechanical force. The force always acts in a direction to drag the conductor along with the rotating field.


This sets up a time changing and counterbalancing moving electromagnetic field that causes the rotor to turn in the direction the field is rotating. The rotor always moves (rotates) slightly behind the phase peak of the primary magnetic field of the stator and is thus always moving slower than the rotating magnetic field produced by the polyphase electrical supply.
旋转磁场的产生rotating magnetic field
异步机中,旋转磁场代替了旋转磁极 (•)电流出
A
n0
Z B
i A I m sin t
iB I m sin t 120
Y
iC I m sin t 240
C
iA
iB
iC
t
Im
X
()电流入
iA
A
iA
iB

There are two types of AC motors, depending on the type of rotor used:


The synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency. The magnetic field on the rotor is either due to current transported with slip rings or a permanent magnet. The induction motor, which turns slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current.