three phase

Three Phase Systems:

Three Phase Systems

Slide 2:

Three Phase Systems AC generation on aircraft is a three phase system In (a) the single phase generator has one rotating coil (wound many times). One cable going to the consumer units. In (b) there are two coils on the same armature at 90° to each other and there would be two cables going to the consumer units. In (c) there are three coils on the same armature at 120° to each other producing three outputs at 120° phase difference. Three supply cables would be used - one for each phase, each going to its own bus on the aircraft. The phases is usually connected together either a STAR (  ) configuration or DELTA ( ) configuration.

Slide 3:

Each winding is connected to a common point called the NEUTRAL or STAR point. The three LINES are taken from the start of each phase winding LINE Current and PHASE Current are the same , I L = I ph LINE Voltage is derived from the two phases that are connected across each pair of lines. Because the phase voltages are 120° out of phase with each other, the LINE voltage is found by multiplying the PHASE Voltage by √3, V L = √3 x V ph Star Connected Generator Three Phase Systems ( cont’d)

Slide 4:

Delta Connected Generator LINE voltage (V L ) and PHASE voltage ( V ph ) is the same (V L = V ph ) LINE current is derived from the currents that are flowing in the two windings that are connected to that particular line. As the phases are 120° out-of-phase with each other the LINE current (I L ) is found by multiplying PHASE current ( I ph ) by √3 , I L = √3 I ph Three Phase Systems ( cont’d)

Slide 5:

Three Phase Voltages As the output windings of the generator are identical, their outputs will be winding of equal amplitude. These outputs are represented both graphically and as phasors If the values above zero are called ‘+’ and those below zero are called ‘-‘, the sum of the voltages at any instant in time is always zero. Three Phase Systems ( cont’d)

Slide 6:

A Symmetrical Three-Phase System The phase voltages are of the same magnitude and of the same phase displacement. The phase currents may have differ magnitudes and phase displacements, depending upon the type and magnitude of the phase loads. Three Phase Systems ( cont’d)

Slide 7:

Balanced Three-Phase System The phase loads are equal and, therefore, the phase currents are equal in magnitude and are operating at the same phase angle. The sum of the instantaneous currents (a current at an instant in time) in a BALANCED system always equals zero. Three Phase Systems ( cont’d)

Slide 8:

The relationships Between LINE and PHASE Voltages If the instantaneous values of two phases (say, A and B) are added together, the result will be a third waveform (line AB). To do this with phasors , it is necessary to reverse one of the two (say B) and then add the two phasors . If this process is repeated in the same order for the other two pairs of phases, the result will be as shown. Three Phase Systems ( cont’d)

Slide 9:

Power in a Balanced Three-Phase System In the Star and Delta connected loads, the True Power in each phase is given by: P = Vph Iph cos Ø Watts In a symmetrical and balanced system, the True Power is given by the sum of the phase powers.: P = 3 Vph Iph cos Ø Watts It is however, usual to express True Power in terms of line values, in which case it is given by : P = √3 V L I L cos Ø Watts Three Phase Systems ( cont’d)