Reference (1) B.L.Thereja; Chapter: 37 (2) Rosenblatt; Chapter: 19 Alternator Reference (1) B.L.Thereja; Chapter: 37 (2) Rosenblatt; Chapter: 19

Physical Shape of Alternator

Rotor and Stator of Alternator

Working principle In case of alternator (AC generator), armature is stationary and the field is rotating. When DC voltage is applied to the field then a constant magnetic flux is produced. At the presence of this constant magnetic flux, rotor is rotated through the prime over. When field (rotor) rotates, armature conductors are cut by the magnetic flux and hence an emf is induced in it. Since, the magnetic poles are alternately N & S, therefore an alternating emf is produced in the stator conductor.

Short-Pitch or Fractional-Pitch Winding In most alternators, opposite conductors of a coil are less than one pole pitch i.e less than spanning over 1800. This type of winding is known as short-pitch or fractional-pitch winding B. L. Thereja; Art:37.11; page:1309

Advantages & Disadvantages of short-Pitch Winding They save copper of end connection They improve the wave-form of generated emf i.e generated emf can be made to approximate to a sine wave more easily and the distorting harmonics can be reduced or totally eliminated. Due to the elimination of high frequency harmonics, eddy current and hysteresis losses are reduced, thereby increasing the efficiency.

Disadvantages The disadvantages of using short-pitch winding is that, the total voltage around the coils is somewhat reduced. Because the voltage induced in two sides of short-pitched coil are slightly out of phase. Therefore, the resultant vector sum is less than their arithmetic sum B. L. Thereja; Art:37.11; page:1309

Pitch Factor Q. What is pitch-factor? Derive the equation of pitch-factor. Ans: The pitch or coil span factor is defined as the ratio of actual coil voltage to the coil voltage for a full-pitch coil. Hence, pitch factor, KP = Vector sum of induced emf per coil/ Arithmetic sum of induced emf per coil Let, Es be the induced emf in each side of the coil. Now consider the following figures: B. L. Thereja; Art:37.11; page:1309

Which is always less than 1. Fig-1 Fig-2 If the coils are full-pitch, then the total induced emf in the coil according to Fig-1 is , E1=2ES If the coil is short-pitched by an angle, then according to Fig-2 the resultant induce emf is = = = Which is always less than 1.

Distributed Winding In each phase of an alternator if the coils are not concentrated or bunched in one slot, but are distributed in a number of slots, then such type of winding is called distributed winding. Q. What is distributed winding and distribution factor? B. L. Thereja; Art:37.12; page:1310

Distribution or Breadth or Spread or Winding factor The ratio of actual phase voltage to that which would exist for a concentrated winding is known as distribution factor. It is also known as “Breadth factor” or “Spread factor” or “Winding factor”. It is denoted by Kd So, distribution factor, Kd = e.m.f with distributed winding / e.m.f with concentrated winding B. L. Thereja; Art:37.12; page:1311

Derivation of Distribution factor Let, be the angular displacement between slots. = 1800 / No. of slots = 1800 /n Let, m= No. of slots/phase/pole = phase spread angle If be the induced emf in one slot,then total voltage induced in one polar group i.e arithmetic sum=m Now, from OAM, B.L.Thereja; Art:37.12; Page:1311-1312; Fig:37.21

Derivation of Distribution factor So, arithmetic sum= Now, from ONE, So, vector sum, So, distribution factor, Kd = When is very small then, So, Kd

Equation of induced emf Let, Z = No. of conductors or coil sides in series/phase = 2T [ where T= No. of coils or turns per phase] P = No. of poles; N = Rotor Speed in r.p.m F = Frequency of the induced emf in HZ φ = Flux per pole in wb Kd = Distribution Factor Kc = pitch or span factor K f = Form factor=1.11 B. L. Thereja; Art:37.13; page:1313

Equation of Induced e.m.f In one revolution of the rotor each stator conductor is cut by a flux of φp. So, dφ= φp & dt = s. So, average induced emf per conductor = Again, average induced emf per conductor = If there are z conductors in series/phase, then average emf/phase and R.M.S value of emf/phase= volt. If the coils are short-pitched & distributed, then actual induce emf = volt. If the alternator is star-connected then the line voltage is times the phase voltage. B. L. Thereja; Art:37.13; page:1313

Effect of Harmonics on Pitch & Distribution Factor Pitch Factor --For Fundamental --For 3rd Harmonics --For 5th Harmonics --For fundamental --For 3rd Harmonics Math: B.L. Thereja; Example:37.8,37.13 --For 5th Harmonics B. L. Thereja; Art:37.14; page:1313-1314

Alternator Characteristics = Generated Voltage = Terminal Voltage = Armature current = Armature Resistance = Armature reactance L O A d Eg Rosenblatt Art:19.9, Fig:19.13; page:381

Effect of Power factor on Generated Voltage For Lagging power Factor Rosenblatt Art:19.9, Fig:19.14; page:382 18

Generated Voltage For Unity Power Factor Rosenblatt Art:19.9, Fig:19.14; page:382 19

Generated Voltage For Leading Power Factor Rosenblatt Art:19.9, Fig:19.14; page:382 Rosenblatt; Math: Example:19.9, 19.10 20