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EE Engineering The fundamental principle of operation of AC machines is the generation of a rotating magnetic field, which causes the rotor to turn at a speed that depends on the speed of rotation of the magnetic field.
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Ref: https://electrical-engineering-portal.com/rotating-magnetic-field-ac-machines The fundamental principle of operation of AC machines is the generation of a rotating magnetic field , which causes the rotor to turn at a speed that depends on the speed of rotation of the magnetic field. Rotating Magnetic Field in AC machines We’ll now explain how a rotating magnetic field can be generated in the stator and air gap of an AC machine by means of alternating currents.
Figure 1 – Two-pole three- phase stator Consider the stator shown in Figure 1, which supports windings a-a′, b-b′ and c-c′. The coils are geometrically spaced 120◦ apart, and a three-phase voltage is applied to the coils. The currents generated by a three-phase source are also spaced by 120◦, as illustrated in Figure 2 below.
Figure 3 – Flux distribution in a three-phase stator winding as a function of angle of rotation Thus, the flux in a three-phase machine rotates in space according to the vector diagram of Figure 4, and the flux is constant in amplitude. A stationary observer on the machine’s stator would see a sinusoidally varying flux distribution , as shown in Figure 3. Figure 4 – Rotating flux in a three-phase machine Since the resultant flux of Figure 3 is generated by the currents of Figure 2, the speed of rotation of the flux must be related to the frequency of the sinusoidal phase currents. In the case of the stator of Figure 1, the number of magnetic poles resulting from the winding configuration is 2. However, it is also possible to configure the windings so that they have more poles. For example, Figure 5 depicts a simplified view of a four-pole stator.
Figure 5 – Four-pole stator In general, the speed of the rotating magnetic field is determined by the frequency of the excitation current f and by the number of poles present in the stator p according to where ns (or ωs) is usually called the synchronous speed. Now, the structure of the windings in the preceding discussion is the same whether the AC machine is a motor or a generator. The distinction between the two depends on the direction of power flow. In a generator, the electromagnetic torque is a reaction torque that opposes rotation of the machine; this is the torque against which the prime mover does work.
