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When several alternators are required to run in parallel, it probably happens that their rated outputs differ. In such cases it is usual to divide the total load between them in such a way that each alternator takes the load in the same proportion of its rated load in total rated outputs.

**Load Sharing**

When several
alternators are required to run in parallel, it probably happens that their
rated outputs differ. In such cases it is usual to divide the total load
between them in such a way that each alternator takes the load in the same
proportion of its rated load in total rated outputs. The total load is not
divided equally. Alternatively, it may be desired to run one large alternator
permanently on full load, the fluctuations in load being borne by one or more
of the others. If the alternators are sharing the load equally the power
triangles are as shown in Fig

**Sharing of load when two alternators are in
parallel**

Consider
two alternators with identical speed load characteristics connected in parallel
as shown in Fig:

Let E1, E2
be the induced emf per phase,

Z1, Z2 be
the impedances per phase,

I1, I2 be
the current supplied by each machine per phase

Z be the
load impedance per phase,

V be the
terminal voltage per phase

From the
circuit we have V = E1 - I1Z1 = E2 - I2Z2 and hence, I1 = E1 - V/Z1 and I2 = E2
- V/Z2

and also
V = (I1 + I2 ) Z = IZ solving above equations

I1 =
[(E1- E2) Z + E1 Z2]/ [ Z( Z1 + Z2) + Z1Z2]

I2 =
[(E2- E1) Z + E2 Z1]/ [ Z( Z1 + Z2) + Z1Z2]

The total
current I = I1 + I2 = [E1Z2 + E2Z1] / [ Z( Z1 + Z2) + Z1Z2]

And the
circulating current or synchronizing current I_{s} = (E1 - E2) / (Z1 +
Z2)

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