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EC010504(EE) Electric Drives & Control Dr. Unnikrishnan P.C. Professor, EEE Module II Transformer √ Three Phase Induction Motor √ Single Phase Induction Motor √ Alternator Synchronous Motor Synchronous Generator-Alternator Synchronous Motors • Constant-speed machine • Propulsion for “Queen Elizabeth 2” – 44 MW – 10 kV – 60 Hz – 50 pole – 144 r/min Queen Elizabeth 2is an ocean liner built for the Cunard Line which was operated by Cunard as both a transatlantic liner and a cruise ship from 1969 to 2008. She was designed for the transatlantic service from her home port of Southampton, UK, to New York Synchronous Motors ……… • Construction – Stator identical to that of a three-phase induction motor – now called the “armature” – Energize from a three-phase supply and develop the rotating magnetic field – Rotor has a DC voltage applied (excitation) – Rotor could be a permanent-magnet type Synchronous Motors (continued) • Operation – Magnetic field of the rotor “locks” with the rotating magnetic field – rotor turns at synchronous speed 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 Synchronous Speed 𝑁𝑠 = 120 𝑓 𝑃 𝑅𝑃𝑀 where f = supply frequency required and P = Poles Induced EMF in an alternator 𝐸𝑅𝑀𝑆 = 2.22 𝑓 𝑧 = 4.44 𝑓 𝑇𝑝ℎ Volts where = Flux per pole set up by rotor current Z = Conductor in series per phase 𝑧 𝑇𝑝ℎ = 𝑇𝑢𝑟𝑛𝑠 𝑝𝑒𝑟 𝑝ℎ𝑎𝑠𝑒 = 2 Synchronous Impedance • • • • • • E = Induced Emf per phase V = Terminal voltage per phase 𝐼𝑎 = Armature current per phase 𝑅𝑎 = Armature resistance per phase 𝑋𝑙 = Armature leakage reactance per phase 𝑋𝑎 = Reactance per phase representing Armature reaction Induced emf per phase 𝑬 = 𝑽 + 𝑰𝒂 𝑹𝒂 +j𝑰𝒂 𝑿𝒍 +𝒋𝑰𝒂 𝑿𝒂 𝑬 = 𝑽 + 𝑰𝒂 𝑹𝒂 +j𝑰𝒂 (𝑿𝒍 +𝑿𝒂 ) = 𝑽 + 𝑰𝒂 (𝑹𝒂 +j𝑿𝒔 ) = 𝑽 + 𝑰𝒂 𝒁𝒔 𝑿𝒔 = (𝑿𝒍 +𝑿𝒂 ) is called synchronous reactance 𝒁𝒔 = (𝑹𝒂 +j𝑿𝒔 ) is called synchronous Impedance Phasor Diagram of an Alternator at Lagging pf load BE=VCos∅ 𝑂𝐷 = 𝑉𝑆𝑖𝑛∅ E = (𝑉 𝑐𝑜𝑠∅ + 𝐼𝑎 𝑅𝑎 )2 +(𝑉𝑠𝑖𝑛∅ + 𝐼𝑎 𝑋𝑠 )2 −−−− −𝐹𝑜𝑟 𝐿𝑎𝑔𝑔𝑖𝑛𝑔 𝑝𝑓 E = (𝑉 𝑐𝑜𝑠∅ + 𝐼𝑎 𝑅𝑎 )2 +(𝑉𝑠𝑖𝑛∅ − 𝐼𝑎 𝑋𝑠 )2 −−−− −𝐹𝑜𝑟 𝐿𝑒𝑎𝑑𝑖𝑛𝑔 𝑝𝑓 Voltage Regulation 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 1. EMF Method or Synchronous Impedance Method 2. MMF Method EMF Method or Synchronous Impedance Method Circuit diagram for open circuit and short circuit test on alternator EMF Method …… • Open circuit characteristics (OCC) (𝑉𝑜𝑐 )𝑝ℎ 𝑉𝑠 𝐼𝑓 • Short Circuit Characteristics (SCC) (𝐼𝑎 )𝑠𝑐 𝑉𝑠 𝐼𝑓 • The Armature resistance per phase 𝑅𝑎 From the Equivalent Circuit: 𝑍𝑠 = 𝑋𝑠 = Equivalent circuit on short circuit 𝐸𝑝ℎ 𝐼𝑎 𝑠𝑐 = (𝑉𝑜𝑐 )𝑝ℎ (𝐼𝑎 )𝑠𝑐 𝑓𝑜𝑟 𝑠𝑎𝑚𝑒 𝐼 𝑓 (𝑍𝑠 )2 − (𝑅𝑎 )2 𝑝ℎ OCC & SCC of an Alternator EMF Method …… No load induced e.m.f. per phase, Eph can be determined from the equation 𝐸𝑝ℎ = (𝑉 𝑐𝑜𝑠∅ + 𝐼𝑎 𝑅𝑎 )2 +(𝑉𝑠𝑖𝑛∅ 𝐼𝑎 𝑋𝑠 )2 where Vph = Phase value of rated voltage Ia = Phase value of current depending on the load condition cosΦ = p.f. of load % 𝑅𝑒𝑔𝑢𝑙𝑎𝑡𝑖𝑜𝑛 = 𝐸𝑝ℎ − 𝑉𝑝ℎ 𝑉𝑝ℎ 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 𝐹𝑂 . • 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 𝐹𝐴𝑅 . MMF Method …………. MMF Method …………. • At full load, the total field m.m.f. is the vector sum of its two components 𝐹𝑂 and 𝐹𝐴𝑅 denoted by 𝐹𝑅 • 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 𝐹𝑅 . MMF Method …………. Let us consider the various power factors and the resultant 𝐹𝑅 . Zero lagging p.f. : The armature reaction is completely demagnetizing. Hence the resultant 𝐹𝑅 is the algebraic sum of 𝐹𝑂 and 𝐹𝐴𝑅 MMF Method …………. Zero leading p.f. : The armature reaction is completely magnetizing. Hence the resultant 𝐹𝑅 is the algebraic difference of 𝐹𝑂 and 𝐹𝐴𝑅 MMF Method …………. Unity p.f. : The armature reaction is completely crossmagnetizing (Distorting). Resultant 𝐹𝑅 is the vector sum of 𝐹𝑂 and 𝐹𝐴𝑅 MMF Method …………. Lagging p.f. : The component 𝐹𝑂 is at right angles to 𝑉𝑝ℎ while 𝐹𝐴𝑅 is in phase with the current (𝐼𝑎 )𝑝ℎ . 𝐹𝑅 is the vector sum of 𝐹𝑂 and 𝐹𝐴𝑅 (𝐹𝑅 )2 = 𝐹𝑂 + 𝐹𝐴𝑅 𝑠𝑖𝑛∅ 2 + 𝐹𝐴𝑅 𝑐𝑜𝑠∅ 2 MMF Method …………. Leading p.f. : The component 𝐹𝑂 is at right angles to 𝑉𝑝ℎ while 𝐹𝐴𝑅 is in phase with the current (𝐼𝑎 )𝑝ℎ . 𝐹𝑅 is the vector sum of 𝐹𝑂 and 𝐹𝐴𝑅 (𝐹𝑅 )2 = 𝐹𝑂 − 𝐹𝐴𝑅 𝑠𝑖𝑛∅ 2 + 𝐹𝐴𝑅 𝑐𝑜𝑠∅ 2 MMF Method …………. Once 𝐹𝑅 is known, obtain corresponding voltage which is induced e.m.f. 𝐸𝑝ℎ , required to get rated terminal voltage 𝑉𝑝ℎ is obtained from o.c.c. Operation as a Synchronous Motor