Rotating Magnetic field: Three-phase supply can create a rotating magnetic field, whereas a single phase supply will create pulsating field only.
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ROTATING MAGNETIC FIELD
Consider now the three phases of an ac winding carrying balanced alternating currents (phase sequence abc).
Then,
ia = im cos wt
ib = Im cos (wt – 120°)
ic = Im cos (wt – 240°)
Figure 8.13 shows the three-phase coils (concentrated) along with their magnetic axes, which are located
120° (elect) apart in space. The three mmfs can be expressed as
Fa = Fm cos wt cos q
Fb = Fm cos (wt – 120°) cos (q – 120°)
Fc = Fm cos (wt – 240°) cos (q – 240°)
The resulting mmf wave is then given by
F = Fa + Fb + Fc
= Fm[cos wt cos q + cos (wt – 120°) cos (q – 120°)
+ cos (wt – 240°) cos (q – 240°)]
Simplifying trigonometrically
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As per Eq. above, the resultant fi eld rotates at speed w = 2pf rad (elect)/s or n = 120f/P rpm, synchronous
speed, in the direction from the leading to the lagging phase axis; a-axis to be axis in Fig.
The direction of rotation can be easily reversed by changing the phase sequence of the currents (interchange two stator leads as connected to the source (three-phase)). The peak amplitude of the resultant field is
Whenever a three-phase winding, with 120°(elect) spatial phase difference between the axes of the three phases is fed, with balanced three-phase currents with a time phase difference of 120°(elect), the resultant mmf (and its associated B-wave) rotates at synchronous speed ws = 2pf radians (elect)/s (or ns = 120 f/P rpm).
The direction of rotation of the mmf wave is from the leading to the lagging phase and can be reversed by changing the phase sequence of currents.
Read More on AC machines
- Synchronous Machines- synchronous generator & synchronous Motor
- Posts on Synchronous machine
- Construction of Synchronous Machines
- Synchronous Motor Advantages, Disadvantages & Applications
- Single phase alternator- Synchronous Generator
- Synchronous Machines Notes and MCQ’s
- As the load is applied to a synchronous motor, the motor takes more armature current because
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