8.1. Generation of Three-phase Current
In the preceding Chapter we have shown that a sinusoidal
alternating voltage is induced in a conductor loop which is rotated in a
homogeneous magnetic field. Several conductor loops which are mechanically
connected together can also be turned in a magnetic field at the same time.
Then, in each conductor loop, a sinusoidal alternating voltage is induced. An
arrangement where three conductor loops displaced by 120 to each other are used
has gained great importance in practice (Fig. 8.1.).
Fig. 8.1. Principle of the
three-phase oberhung-type alternator
1, 2, 3 - Rotatable coils
4 - North pole
In order to conduct the electrical energy generated in this
overhung-type alternator to the consumer, sliding contacts must be provided for
all three conductor loops. This disadvantage is not associated with the
inner-pole alternator. In This type, the three conductor loops are fixed in the
stator of the alternator (an alternating-current generator is also called
alternator) while the magnetic field in the interior is rotated. (Fig. 8.2.).
The relatively low electrical energy for the production of the magnetic field
must be fed to the rotor of the generator via sliding contacts.
In each of the three coils, a sinusoidal voltage is produced
when the magnetic field is rotating which - in accordance with the arrangement
of the coils - exhibits a phase shift of 120°. (Consequently, a voltage
maximum is always reached, when the magnetic pole is turned past the coil. For
the three coils, this always occurs after a rotation of the magnetic pole
through 120 °). The line diagram of these three voltages is shown in Fig.
Fig. 8.2. Principle of the
three-phase inner-pole alternator
1, 2, 3 - Fixed coils
4 - North pole
5 - South
Fig. 8.3. Line diagram of voltages
of three-phase current
The three voltages subjected to a phase-shift of 120° are
called three-phase current. In order to distinguish safely between the three
voltages and their three coils, the three phases are marked by L1, L2 and L3 and
coloured (L1: yellow; L2: green; L3: violet) according to an IEC-recommendation.
1) The connections of the starts of the coils can be designated by U,
V and W and the ends of the coils by X, Y and Z. For the phases L1 to L3,
sometimes R, S and T are used as designation.
1) IEC = International Electrotechnical
When a magnetic field is rotated, inside of three
coils which are displaced by 120° to each other, then three sinusoidal
voltages are produced which are called three-phase current. A phase shift of
120° exists between every two of the three voltages.
1. What is the difference between overhung-type
alternator and inner-pole alternator?
2. What are the advantages of the
inner-pole alternator over the overhung-type alternator?
3. How are the three
phases distinguished from each other by markings?
4. What is the phase angle
between the voltages of the three