EE216 Electrical Engineering Dr. Unnikrishnan P.C. Professor, EEE

Synchronous Generator-Alternator

Stator Construction Stator is identical to the induction motor Laminated low silicon steel rings joined together Slots insulated with Mylar Example of 36 slot stator with 3 coil conductors per slot, 12 slots per phase

Stator Construction

Rotor Construction Two Types of Rotor Salient Pole Cylindrical

Rotor Construction

Salient-Pole Rotor with brushless excitation

Rotor Construction

Operation as a Synchronous Generator

Equations 𝑁 𝑠 = 120 𝑓 𝑃 𝑅𝑃𝑀 𝐸 𝑅𝑀𝑆 =2.22  𝑓 𝑧=4.44  𝑓 𝑇 𝑝ℎ Volts Synchronous Speed 𝑁 𝑠 = 120 𝑓 𝑃 𝑅𝑃𝑀 where f = supply frequency required and P = Poles Induced EMF in an alternator 𝐸 𝑅𝑀𝑆 =2.22  𝑓 𝑧=4.44  𝑓 𝑇 𝑝ℎ Volts where  = Flux per pole set up by rotor current Z = Conductor in series per phase 𝑇 𝑝ℎ =𝑇𝑢𝑟𝑛𝑠 𝑝𝑒𝑟 𝑝ℎ𝑎𝑠𝑒= 𝑧 2

Synchronous Impedance E = Induced Emf per phase V = Terminal voltage per phase 𝐼 𝑎 = Armature current per phase 𝑅 𝑎 =Armature resistance per phase 𝑋 𝑙 = Armature leakage reactance per phase 𝑋 𝑎 = Reactance per phase representing Armature reaction Induced emf per phase 𝑬=𝑽+ 𝑰 𝒂 𝑹 𝒂 +j 𝑰 𝒂 𝑿 𝒍 + 𝒋𝑰 𝒂 𝑿 𝒂 𝑿 𝒔 = (𝑿 𝒍 + 𝑿 𝒂 ) is called synchronous reactance 𝒁 𝒔 = (𝑹 𝒂 +j 𝑿 𝒔 ) is called synchronous Impedance 𝑬=𝑽+ 𝑰 𝒂 𝑹 𝒂 +j 𝑰 𝒂 (𝑿 𝒍 + 𝑿 𝒂 ) =𝑽+ 𝑰 𝒂 ( 𝑹 𝒂 +j 𝑿 𝒔 ) =𝑽+ 𝑰 𝒂 𝒁 𝒔

Phasor Diagram of an Alternator at Lagging pf load BE=VSin∅ 𝑂𝐷=𝑉𝐶𝑜𝑠∅ E = (𝑉 𝑐𝑜𝑠∅+ 𝐼 𝑎 𝑅 𝑎 ) 2 + (𝑉𝑠𝑖𝑛∅+ 𝐼 𝑎 𝑋 𝑠 ) 2 −−−−−𝐹𝑜𝑟 𝐿𝑎𝑔𝑔𝑖𝑛𝑔 𝑝𝑓 E = (𝑉 𝑐𝑜𝑠∅+ 𝐼 𝑎 𝑅 𝑎 ) 2 + (𝑉𝑠𝑖𝑛∅− 𝐼 𝑎 𝑋 𝑠 ) 2 −−−−−𝐹𝑜𝑟 𝐿𝑒𝑎𝑑𝑖𝑛𝑔 𝑝𝑓

Voltage Regulation % 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛= 𝐸 − 𝑉 𝑉 x 100 A convenient way to compare the voltage behaviour of two generators is by their voltage regulation (VR). % 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛= 𝐸 − 𝑉 𝑉 x 100 Pre-Determination of Voltage Regulation EMF Method or Synchronous Impedance Method MMF Method

EMF Method or Synchronous Impedance Method Circuit diagram for open circuit and short circuit test on alternator

EMF Method …… The Armature resistance per phase 𝑅 𝑎 Open circuit characteristics (OCC)  (𝑉 𝑜𝑐 ) 𝑝ℎ 𝑉 𝑠 𝐼 𝑓 Short Circuit Characteristics (SCC) (𝐼 𝑎 ) 𝑠𝑐 𝑉 𝑠 𝐼 𝑓 The Armature resistance per phase 𝑅 𝑎 From the Equivalent Circuit: 𝑍 𝑠 = 𝐸 𝑝ℎ 𝐼 𝑎 𝑠𝑐 = (𝑉 𝑜𝑐 ) 𝑝ℎ ( 𝐼 𝑎 ) 𝑠𝑐 𝑓𝑜𝑟 𝑠𝑎𝑚𝑒 𝐼 𝑓 𝑋 𝑠 = (𝑍 𝑠 ) 2 − (𝑅 𝑎 ) 2  𝑝ℎ Equivalent circuit on short circuit

OCC & SCC of an Alternator

EMF Method …… No load induced e.m.f. per phase, Eph can be determined from the equation 𝐸 𝑝ℎ = (𝑉 𝑐𝑜𝑠∅+ 𝐼 𝑎 𝑅 𝑎 ) 2 + (𝑉𝑠𝑖𝑛∅  𝐼 𝑎 𝑋 𝑠 ) 2 where     Vph = Phase value of rated voltage Ia = Phase value of current depending on the load condition cosΦ = p.f. of load % 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛= 𝐸 𝑝ℎ − 𝑉 𝑝ℎ 𝑉 𝑝ℎ x 100

EMF Method Advantages & Limitations Advantage:  synchronous impedance Zs for any load condition can be calculated. Hence regulation of the alternator at any load condition and load power factor can be determined. Limitation: This method gives large values of synchronous reactance. This leads to high values of % regulation than the actual. Hence this method is called Pessimistic method. 

MMF Method or Ampere Turn Method The effect of armature leakage reactance by an equivalent additional m.m.f so that this m.m.f may be combined with the armature reaction m.m.f. An alternator requires m.m.f. which is product of field current and turns of field winding-two components 1. An m.m.f. necessary to induce the rated terminal voltage on open circuit. 2. An m.m.f. equal and opposite to that of armature reaction m.m.f. The number of turns in the field winding is not known normally, so the m.m.f. is calculated in terms of the field current itself.

MMF Method …………. The field m.m.f. required to induce the rated terminal voltage on open circuit can be obtained from o.c.c. This is denoted as 𝐹 𝑂 . In s.c. test, field m.m.f. is necessary to overcome drop across armature resistance and leakage reactance and also to overcome effect of armature reaction But drop across armature resistance and leakage reactance is very small and can be neglected. So in s.c. test, field m.m.f. circulates full load current to balance the armature reaction effect. Ampere-turns required to circulate full load current can be obtained from s.c.c. Denoted as  𝐹 𝐴𝑅 .

MMF Method ………….

MMF Method …………. At full load, the total field m.m.f. is the vector sum of its two components  𝐹 𝑂  and 𝐹 𝐴𝑅 denoted by 𝐹 𝑅 This depends on the power factor of the load which alternator is resultant field m.m.f. is denoted as FR. Let us consider the various power factors and the resultant 𝐹 𝑅 .

MMF Method …………. Let us consider the various power factors and the resultant 𝐹 𝑅 . Zero lagging p.f. : The armature reaction is completely de-magnetizing. Hence the resultant 𝐹 𝑅 is the algebraic sum of 𝐹 𝑂 and 𝐹 𝐴𝑅

MMF Method …………. Zero leading p.f. : The armature reaction is completely magnetizing.Hence the resultant 𝐹 𝑅 is the algebraic difference of 𝐹 𝑂 and 𝐹 𝐴𝑅

MMF Method …………. Unity p.f. : The armature reaction is completely cross- magnetizing (Distorting). Resultant 𝐹 𝑅 is the vector sum of 𝐹 𝑂 and 𝐹 𝐴𝑅

MMF Method …………. Lagging p.f. : The component 𝐹 𝑂  is at right angles to 𝑉 𝑝ℎ while 𝐹 𝐴𝑅  is in phase with the current ( 𝐼 𝑎 ) 𝑝ℎ . 𝐹 𝑅  is the vector sum of 𝐹 𝑂  and 𝐹 𝐴𝑅 (𝐹 𝑅 ) 2 = 𝐹 𝑂 + 𝐹 𝐴𝑅 𝑠𝑖𝑛∅ 2 + 𝐹 𝐴𝑅 𝑐𝑜𝑠∅ 2

MMF Method …………. Leading p.f. : The component 𝐹 𝑂  is at right angles to 𝑉 𝑝ℎ while 𝐹 𝐴𝑅  is in phase with the current ( 𝐼 𝑎 ) 𝑝ℎ . 𝐹 𝑅  is the vector sum of 𝐹 𝑂  and 𝐹 𝐴𝑅 (𝐹 𝑅 ) 2 = 𝐹 𝑂 − 𝐹 𝐴𝑅 𝑠𝑖𝑛∅ 2 + 𝐹 𝐴𝑅 𝑐𝑜𝑠∅ 2

MMF Method …………. Once 𝐹 𝑅  is known, obtain corresponding voltage which is induced e.m.f. 𝐸 𝑝ℎ , required to get rated terminal voltage 𝑉 𝑝ℎ is obtained from o.c.c.

Operation as a Synchronous Motor