**Subject :**Electrical Machines II (AC Machines)

**Unit :**Alternators

## Procedure for potier method

**Procedural Steps for Potier Method:**

1. Suppose we are given V-the terminal voltage/phase.

2. We will be given or else we can calculate armature leakage reactance XL and hence can calculate IX_{L}.

3. Adding IX_{L} (and IR_{a} if given) vectorially to V, we get voltage E.

4. We will next find from N-L curve, field excitation for voltage E. Let it be i_{f1}.

5. Further, field current i_{f2} necessary for balancing armature reaction is found from Potier triangle.

6. Combine i_{f1} and i_{f2} vectorially (as in A.T. method) to get i* _{f}*.

7. Read from N-L curve, the e.m.f. corresponding to i* _{f}*. This gives us E

_{0}. Hence, regulation can be found.

**Example: **

An 11-kV, 1000-kVA, 3-phase, Y-connected alternator has a resistance of 2 Ω per phase. The open-circuit and full-load zero power factor characteristics are given below. Find the voltage regulation of the alternator for full load current at 0.8 p.f. lagging by Potier method.

**Solution:**

*Fig: 1 Fig: 2*

The O.C.C. and full-load zero p.f. curve for phase voltage are drawn in Fig 1. The corresponding phase voltages are:

BD = leakage reactance drop IX_{L} = 1000 V − by measurement, AD = 30 A — field current required to overcome demagnetising effect of armature reaction on full-load. From fig 2 we know that,

As seen from O.C.C., field current required for 7,080 V is 108 A. Vector OD (Fig 2) represents 108 A and is drawn ⊥ to OC. DF represents 30 A and is drawn parallel to OI or at (90° 36° 52′) = 126° 52′ with OD. Total field current is OF.