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Published bySullivan Westrup Modified over 2 years ago

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SYNCHRONOUS MACHINES

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EXAMPLE 6.1In a factory a 3cp, 4 kV, 400 kVA synchronous machine is installed along with other induction motors. The following are the loads on the machines:Induction motors: 500 kVA at 0.8 PF lagging.Synchronous motor: 300 kVA at 1.0 PF.(a)Compute the overall power factor of the factory loads.(b)To improve'the factory power factor, the synchronous machine is overexcited (to draw leading current) without any change in its load. Without overloading the motor, to what extent can the factory power factor be improved? Find the current and power factor of the synchronous motor for this condition.

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CONSTRUCTION OF THREE PHASE SYNCHRONOUS MACHINES Synchronous machines can be broadly divided into two groups as follows: 1. High speed machines with cylindrical (or non salient pole) rotors. 2. Low speed machines with salient pole rotors.

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Interconnecting synchronous generator with bus must have the same 1. Voltage 2. Frequency 3. Phase sequence 4. Phase

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1.Voltages are not the same, but frequency and phase sequence are the The lamp voltages have equal magnitudes and therefore all the three lamps will glow with the same intensity. To make the voltages equal, the field current If must be adjusted. 2.Frequencies are not the same, but voltages and phase sequences are the same. Equal voltages will appear across the three lamps and they will glow with the same intensity. It is therefore evident that if the frequencies are different, the lamps will darken and brighten in step. To make the frequencies the same, the speed has to be adjusted until the lamps brighten and darken very slowly in step. It may be noted that as the speed of the incoming machine is adjusted, its voltages will change. Therefore, simultaneous adjustment of the field current If will also be necessary to keep the voltages the same.

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3.Phase sequences are not the same, but voltages and frequencies are the same. The voltages across the lamps are of different magnitudes and therefore the lamps will glow with different intensities. If the frequencies are slightly different, one set of phasor voltages will pass the other set of phasor voltages and the lamps will darken and brighten out of step.To make the phase sequence the same, interchange connections to two terminals; for instance, connect a to B and b to A (Fig. 6.9). 4.Phase is not the same, but voltage, frequency, and phase sequence arethe same. The two sets of phasor voltages will maintain a steady phase difference and the lamps will glow with the same intensity. To make the phase the same or the phase difference zero, the frequency of the incoming machine is slightly altered. At zero phase difference all the lamps will be dark, and if the circuit breaker is closed the incoming machine will be connected to the infinite bus. Once the synchronous machine is connected to the infinite bus, its speed cannot be changed further. However, the real power transfer from the machine to the infinite bus can be controlled by adjusting the prime mover power. The reactive power (and hence the machine power factor) can be controlled by adjusting the field current. Real and reactive power control will be discussed in detail in later sections.

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6.3 SYNCHRONOUS MOTORS

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Start with Variable Frequency Supply

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EQUIVALENT CIRCUIT MODEL

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6.4.1 DETERMINATION OF THE SYNCHRONOUS REACTANCE Open Circuit Test Short Circuit Test

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6.4.2 PHASOR DIAGRAM Synchronous motor

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