Construction And Working:
It is an ordinary de generator with special provision being for connecting the neutral wire to the armature windings.
The two outers are connected to the positive and negative brushes as in a two-wire system. In this arrangement, a balance coil wound around an iron core with low resistance wire is connected between the two taps made 180 electrical degrees apart directly to the armature windings.
The neutral wire is connected to the mid-point of the balance coil so that at all times voltage between the neutral and either line wire is one-half the generator voltage.
The voltage induced in the balanced coil is alternating at frequency PN/120 and the current in this coil is very small owing to its high reactance. But due to low resistance, very little hindrance is offered to direct current flowing in from the neutral wire.
The balance coil may be mounted directly on the armature and the neutral point is brought out of the generator by means of a single slip-ring and brush, as shown in fig.
When both sides of the system are equally loaded i.e. I1 = I2 , no current flows through the neutral wire.
In case of unbalanced loading i.e., I1 ≠ I2, the difference of currents I1 and I2, i.e. I1 - I2, flows through the neutral wire.
This current divides into two parts in the balancing coil, which enters the armature and then passes through the positive and negative brushes.
In some three wire generators, the taps from the armature winding are brought out through two slip-rings and brushes and are connected to an external stationary balance coil.
In order to provide compounding even when the load on the generator is unbalanced. it is necessary that the series field, when used on the 3-wire generator, is divided with one half of the winding connected on the positive side and the other half on the negative side of the generator.
For operating 3-wire compound generators in parallel, it is necessary that two equalizer connections are used to connect all series fields in parallel because there are two series fields on each machine.
In order to obtain better distribution of current in the armature two or even three coils may be used, the former arrangement requiring four slip-rings and the latter three slip-rings, corresponding to two-phase and star-connected three-phase arrangements and used on ac circuits.
Also, the inductance is sometimes built into the armature of the machine in which case only one slip-ring is needed.
The three-wire generator is naturally somewhat more expensive than the standard type and for a particular system, this additional cost must be balanced against the costs or disadvantages incident to the use of a balancer set.