MMF Method of Voltage Regulation

MMF Method is also known as Ampere Turn Method. The synchronous impedance method is based on the concept of replacing the effect of armature reaction with an imaginary reactance, the Magnetomotive force (MMF). The MMF method replaces the effect of armature leakage reactance with an equivalent additional armature reaction MMF so that this MMF may be combined with the armature reaction MMF.

To calculate the voltage regulation by MMF Method, the following information is required. They are as follows:

• The resistance of the stator winding per phase.
• Open circuit characteristics at synchronous speed.
• Short circuit characteristic

Step to Draw Phasor Diagram of MMF Method

The phasor diagram at a lagging power factor is shown below:

• The armature terminal voltage per phase (V) is taken as the reference phasor along OA.
• The armature current phasor I_a is drawn lagging the phasor voltage for lagging power factor angle ϕ for which the regulation is to be calculated.
• The armature resistance drop phasor I_aR_a is drawn in phase with I_a along the line AC. Join O and C. OC represents the emf E’.

 

• Considering the open current characteristics shown above the field current I’_f corresponding to the voltage E’ is calculated.
• Draw the field current I’_f leading the voltage E’ by 90 degrees. It is assumed that on a short circuit all the excitation is opposed by the MMF of armature reaction. Thus,

From the short circuit current characteristics (SSC) shown above, determine the field current I_f2 required to circulate the rated current on short circuit. This is the field current required to overcome the synchronous reactance drop I_aX_a.

• Draw the field current I_f2 in phase in opposition to the current armature current I_a. Thus,

Determine the phasor sum of the field currents I’_f and I_f2. This gives the resultant field current I_f which would generate a voltage E₀ under no-load conditions of the alternator. The open-circuit emf E₀ corresponding to the field current if is found from the open circuit characteristics.

The regulation of the alternator is found from the relation shown below:

This is all about MMF method of voltage regulation.