Methods of finding Voltage Regulation in Synchronous Generator or Alternator– Direct loading method, EMF method or Synchronous Impedance method, MMF method or Ampere turns method, ASA modified MMF method, ZPF method or Potier Triangle method

The voltage Regulation of an Alternator can be determined by different methods. For small Generators, it can be determined by direct loading, but for large Generators, it cannot be determined by direct loading and must instead typically be Predetermined using various techniques. The various techniques for Anticipating Alternator Regulation are listed below.

1.  Direct loading method

2. EMF method or Synchronous impedance method

3. MMF method or Ampere turns method

4. ASA modified MMF method

5. ZPF method or Potier triangle method

Only Non-salient pole machines can use any of the Aforementioned methods other than direct loading. Since the alternators are produced in large capacities, direct loading of the alternators is not used to Establish Regulation. For the Predetermination of regulation, other techniques may be used. As a result, the following sections will discuss alternative ways to determine regulations.

1. EMF method

The synchronous impedance method is another name for this technique. Here, it is assumed that the magnetic circuit is unsaturated. This technique, which is known as the “emf method,” replaces the MMFs (fluxes) produced by the rotor and stator with their equivalent emfs.

To predetermine the regulation by this method the following information are to be determined. Armature resistance /phase of the alternator, open circuit and short circuit characteristics of the alternator.

Determination of synchronous impedance Zs

In a SC test, the Stator’s terminals are shorted out, and the short circuit current flows against the Synchronous Impedance of the stator. It is possible to calculate this Impedance from the sc and oc Characteristics.

Synchronous Impedance is the ratio of open circuit voltage to short circuit current at a specific field current or at a field current that circulates the rated current.

Synchronous Impedance Zs = (open circuit voltage per phase)/(short circuit current per phase) for same If

Hence Zs = (Voc) / (Isc) for same If

From Fig: 1.16 synchronous impedance Zs = V/Isc

Armature resistance Ra of the stator can be measured using Voltmeter – Ammeter method. Using Synchronous Impedance and armature resistance Synchronous reactance and hence regulation can be calculated as follows using emf method.

where Vt = phase voltage per phase = Vph , Ia = load current per phase

In the above expression in second term + sign is for lagging power factor and – sign is for leading power factor.

% Regulation = [ Eg – Vt ] / Vt

where 

Eg = no-load induced emf /phase, 

Vt = rated terminal voltage/phase

The synchronous impedance method is simple but only provides approximations. This method is known as a pessimistic method because it produces a value for regulation that is higher (worse) than the actual value. Fig. 1.18 displays the entire phasor diagram for the emf method.

2. MMF method

The amp-turns method is another name for this technique. The MMF method replaces all of the emfs generated by the rotor and stator with their equivalent MMFs (fluxes). The magnetic circuit is presumed to be unsaturated in this method as well. This method substitutes equivalent mmfs for both reactance drops. The entire phasor diagram for the mmf method is shown in Fig. 1.19. The mmf method uses OC and SC characteristics to determine regulation, just like the emf method. The specifics are displayed in Fig. 1.19. The details can be used to calculate the regulation at various power factors.

From the phasor diagram it can be seen that the mmf required to produce the emf E1= (V + IRa) is FR1.In large machines resistance drop may neglected. The mmf required to overcome the reactance drops is (Fa+Fal) as shown in phasor diagram. The mmf (Fa+Fal) can be found from SC characteristic as under SC condition both reactance drops will be present.

Following procedure can be used for determination of regulation by mmf method.

Because of the assumption of unsaturated magnetic circuit the regulation computed by this method will be less than the actual and hence this method of regulation is called optimistic method.

3. ASA Modified MMF Method

When calculating regulation, the ASA or modified mmf method takes the saturation effect into account. The total mmf F calculated using the mmf method is based on the erroneous assumption that the magnetic circuit is unsaturated. The magnetic circuit must be increased by a certain amount FF2 to account for the partial saturation, which can be calculated from the occ, scc, and air gap lines as explained below using Fig. 1.20 I and (ii).

If1 is the field current required to induce the rated voltage on open circuit. Draw If2 with length equal to field current required to circulate rated current during short circuit condition at an angle (90+ ) from If1. The resultant of If1 and If2 gives If (OF2 in figure). Extend OF2 upto F so that F2F accounts for the additional field current required for accounting the effect of partial saturation of magnetic circuit. F2F is found for voltage E (refer to phasor diagram of mmf method) as shown in Fig: 1.20. Project total field current OF to the field current axis and find corresponding voltage E0 using OCC. Hence regulation can found by ASA method which is more realistic.

4. Zero Power Factor (ZPF) method or Potier Triangle Method

Resistance voltage drop IaRa, armature leakage reactance drop IaXL, and armature reaction reactance are actually emf and mmf quantities, respectively, during the alternator’s operation. OCC, SCC, and ZPF test information and characteristics are needed to determine the regulation of the alternator using this method. As previously explained by AS, OCC and SCC are determined after conducting OC and SC tests. When conducting a ZPF test, a ZPF load is connected to the alternator, and the alternator is excited so that it runs at its rated speed while supplying the rated current at the rated voltage.

To plot ZPF characteristics only two points are required. One point is corresponding to the zero voltage and rated current that can be obtained from scc and the other at rated voltage and rated current under zpf load. This zero power factor curve appears like OCC but shifted by a factor IaXL vertically and horizontally by armature reaction mmf as shown below in Fig: 1.21. Following are the steps to draw ZPF characteristics.

Using appropriate tests, plot OCC and SCC. Create an air gap line. Perform a ZPF test at rated voltage while fully loaded, then fix point B. The length of the line BH should be equal to the field current necessary to produce the full load current during a short circuit. To cut the OCC, draw HD parallel to the air gap line. Draw DE parallel to the voltage axis or perpendicular to HB. Now, DE stands for the IXL voltage drop, and BE is the field current needed to cancel out the effect of the armature reaction.

The resultant field current is given by OG. Mark this length on field current axis. From OCC find the corresponding E0. Find the regulation.