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4.

4.1

2

Theory of Operation

Principle of the Space-Vector Modulation

Figure 4-1.

Typical structure of an inverter-fed induction motor.

Figure 4-1.shows the typical structure of a three-phase induction motor connected to a VSI (Volt- age Source Inverter). Since the motor is considered as a balanced load with an unconnected n e u t r a l , , , a n d . S i n c e t h e u p p e r p o w e r s w i t c h e s c a n o n l y b e O n o r O f f , a n d s i n c e t h e V n V a V b V c + + 3 = 3 / ) ( c a a b n a a n V V V V V = = 3 / ) ( a b b c n b b n V V V V V = = 3 / ) ( b lower ones are supposed to always be in the opposed state (the dead-times of the inverter legs are neglected), there are only eight possible switching states, as shown on Figure 4-2. Six of them lead to non-zero phase voltages, and two interchangeable states lead to zero phase volt- ages. When mapped in a 2D-frame fixed to the stator using a Concordia transformation [1,2], the c c a n c c n V V V V V = =

six non-zero phase voltages form the vertices of a hexagon. (See Figure 4-3.)

V V β α

=

2 / 3 2 / 3 0 2 / 1 2 / 1 1

c n b n a n V V V

Figure 4-2.

Possible switching configurations of a 3-phase inverter

Application Note

7546A–AVR–12/05

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