Alternators

Problem 573: A three-phase alternator has 6 slots per pole per phase. Determine the distribution factor.

Problem 574: A 30 MVA, 15 kV three-phase alternator will have a per phase nominal impedance of ___.

Problem 575: A two-pole three phase 50 Hz Y-connected AC generator has 9 slots per pole and 6 conductors per slot. What is the flux per pole if the voltage on open circuit is 1.1 kV. Assume a coil span of unity.

Problem 576: A 6-pole, 3-phase, 60-cycle alternator has 12 slots per pole and four conductors per slot. There are 2,500,000 maxwells (0.025 weber) entering the armature from each north pole, and this flux is sinusoidally distributed along the air gap. The armature coils are all connected in series. The winding is 5/6 pitch. The winding is wye-connected. Determine the open circuit emf of the alternator.

Problem 577: A four-pole, three-phase, 50 Hz, star connected AC generator has 24 stator slots. There are 6 conductors per slot and a sinusoidally distributed flux per pole of 63 mWb. Find the terminal voltage on open circuit, if the coil span factor is unity.

Problem 579: A 1MVA, 11 kV, three-phase wye-connected synchronous generator has a synchronous reactance of 5 ohms and a negligible resistance per phase. At a certain field current the generator delivers rated load at 0.9 lagging power factor at 11 kV. For the same excitation, what is the terminal voltage at 0.9 leading power factor full-load?

Problem 580: A three-phase star connected AC generator is rated at 2.5 MVA, 11 kV, 60 Hz. The generator has a negligible resistance and a synchronous reactance 20 ohms per phase. To what value will the terminal voltage rise when full load at 80% pf lagging is switched-off?

Problem 581: A 1200 kVA, 6600 volt, 3-phase Y connected alternator has an effective resistance of 0.40 ohm and a reactance of 6 ohms per phase. It delivers full load current at 0.80 lagging power factor at rated voltage. What will be the terminal voltage for the same excitation and load current, if the power factor is 0.80 leading?

Problem 582: A 2500 kVA, 6600 V synchronous turbo alternator is operating at full load and 0.8 pf lagging. This machine has a synchronous reactance per phase is 10.4 ohms and a negligible resistance. Calculate the percentage voltage regulation?

Problem 583: A 50-kVA, 550-volt single-phase alternator draws a field current 10 A at rated load. With the same field current, the open circuit emf and short-circuit current are 300 V and 150 A. The ohmic resistance of the armature between terminals is 0.18 ohm. The ratio of effective to ohmic resistance maybe taken as 1.5. Determine the regulation at 0.8 power factor lagging.

Problem 584: A generator is rated 100 MW, 13.8 kV and 90% power factor. The effective resistance is 1.5 times the ohmic resistance. The ohmic resistance is obtained by connecting two terminals to a DC source. The current and voltage are 87.6 A and 6 V respectively. What is the effective resistance per phase?

Problem 585: To get the armature resistance of a 100 MW, 13.8 kV and 90% power factor generator, two terminals are connected to a DC source. The measured current and voltage are 87.6 amperes and 6 volts respectively. What is the DC resistance per phase?

Problem 586: A 2 MVA, 2.3-kV three-phase Y-connected alternator operates at rated kVA at a power factor of 80%. The dc armature winding resistance between terminals is 0.08 \(\Omega\). The field takes 70 A at 125 V from an exciter equipment. Friction and windage loss is 20 kW, iron losses 36 kW and stray load losses are 2.0 kW. Calculate the efficiency of the alternator. Assume the effective armature-winding resistance is 120% of the DC resistance.

Problem 587: Determine the efficiency of a 1500 kVA, 2300 V, 3-phase, Y-connected alternator, which operates at rated output with a power factor of 80%. The DC armature resistance at 70 °C between terminals is 0.08 ohm. The field takes 70 A at 120 V from the exciter equipment. Friction and windage losses are 15 kW, iron loss is 35 kW and stray load losses is 1.5 kW. Assume the effective armature winding resistance is 1.5 times the DC value.

Problem 588: A 2,500 kVA, three-phase, 60 Hz, 6.6 kV wye-connected alternator has a field resistance of 0.45 \(\Omega\) and an armature resistance of 0.05 \(\Omega\) per phase. The field current at full load 0.85 pf, is 200 A. The stray power losses amount to 82 kW. Calculate the efficiency of the alternator at full load, 0.85 pf lagging.

Problem 589: A 6.6 kV, Y-connected, 3-phase alternator has a synchronous reactance of 6 \(\Omega\) per phase and a negligible resistance. At a certain field current the armature is known to induce 5.8 kV per phase. Calculate the maximum power that can be developed by the alternator at this excitation.

Problem 591: A three-phase, 11-kV wye connected synchronous alternator has a synchronous reactance of 8 ohms per phase but negligible resistance. If the excitation is such that the open circuit voltage is 14 kV, determine the power factor at the maximum output.

Problem 592: A three-phase AC generator is supplying power to a load of 3200 kW at 2300 volts and a power factor of 60%. Assume that the loss of the line, the generator armature and the load is equal to 10% of the load, what would be the savings in watts if the power factor were raised to 80%?

Problem 594: A 3-phase 50 MVA, 13.8 kV, 3600 rpm, 60 Hz grid connected generator with a synchronous reactance of 3.8 ohms per phase operating with a lagging pf of 0.88 at rated voltage and current. If the load decreases such that its prime mover power is decreased by 25% and the rotor current decreases by the same percentage, what will be the new pf?

Problem 595: A 3-phase 50 MVA, 13.8 kV, 3600 rpm, 60 Hz grid connected generator with a synchronous reactance of 3.8 ohms per phase operating with a lagging pf of 0.88 at rated voltage and current. If the machine is to provide a lagging reactive power of 40 MVAR and still operating at its rated current, what percent change in the rotor current is required?