Single Phase Synchronous Generators
Single Phase Synchronous Generators – Certain applications, usually restricted to less than 10 kVA are better served by a single-phase synchronous generator. Examples are emergency, domestic/office supply, portable power for construction tools etc. Because of simplicity of distribution wiring, these loads are better served by a single-phase arrangement. Single-phase generator stator winding can be arranged in two ways.
- A 12-lead three-phase winding connected to operate single-phase.
- Specialised single-phase wound stator.
In the first arrangement several connections are possible but all have to be derated compared to normal three-phase connection. Two connections are indicated in Fig. 8.74 and are compared to three-phase series star in terms of voltage and pu kVA rating.
In a single-phase generator, while rotor is similar to that of a three-phase, stator is wound single phase with short-pitched coils (short pitch y(= SPP) slots). Thus one third of stator slots are left unwound and winding has to be single layer. Obviously
the rating of the machine is 2/3rd that of three-phase wound generator. While the rotor has salient poles and produces a field distributed sinusoidally in space (because of shaping of pole faces), the stator which is single phase produces an oscillating field along a fixed axis. The expression for the fundamental component of stator field is as given in Eq. (5.36).
Equation (8.119) can be trigonometrically split as
when Fif is a sinusoidally distributed field rotating in positive direction of 0 (forward rotating) and cb is a backward rotating field. Both these fields rotate at synchronous speed as shown in Fig. 8.75 where fields are represented as rotating vectors Fif and frib, while the resultant field is an oscillating vector F field in space.
The forward rotating stator field locks into the rotor field and both move together producing electromagnetic torque and causing conversion of energy from mechanical to electrical. Figure 8.76(a) shows the rotor and stator fields at the instant /1 (max). The corresponding phasor diagram is drawn in Fig. 8.76(b) wherein only those phasors which are stationary w.r.t. each are indicated. Assuming Xq to be small, the terminal voltage can be related to excitation emf Ee (suffix e is used to avoid confusion that would be caused by suffix f used so far) by Xd as obtained by OC and SC test. The effects caused by frib which rotates at co w.r.t. stator and at speed 2e) w.r.t. to rotor in opposite direction is discussed below.
Backward rotating field Fib induces second harmonic current in the field winding (and also damper winding) and fundamental frequency voltage in stator winding. The second harmonic current in the field winding causes an oscillating field along the d-axis at the same frequency. By the argument presented earlier this field can be split into two rotating fields rotating at + 2w w.r.t. rotor or 3w and —w w.r.t. stator. Thus the stator winding has induced in it third harmonic and fundamental frequency voltages. The fundamental frequency voltages induced in stator due to Ft and Pib are both accounted for in Xd. The third harmonic voltages induced in stator cause third harmonic current in the line. Unlike slot harmonics these cannot be eliminated (or attenuated) by chording. But their amplitude is reduced by self-inductance of the field winding, eddy currents in rotor and second harmonic currents in the damper bars.