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SYNCHRONOUS MACHINES
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.
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.
Interconnecting synchronous generator with bus must have the same
1. Voltage
2. Frequency
3. Phase sequence
4. Phase
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.
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 are the
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.
6.3 SYNCHRONOUS MOTORS
Start with Variable-Frequency Supply
EQUIVALENT CIRCUIT MODEL
6.4.1 DETERMINATION OF THE SYNCHRONOUS REACTANCE
Open-Circuit Test
Short-Circuit Test
6.4.2 PHASOR DIAGRAM
Synchronous motor