GENERATOR SELECTION CONTENTS - Diesel Generator

SECTION 4512
January, 1990
CONSTANT OR STEADY STATE LOADS - THREE PHASE, UNBALANCED
An unbalanced three phase load is one in which the load is not equally shared by all the three phases of a machine.
This frequently occurs in practice due to single phase loads being applied across one or rwo phases of a three phase
machine.
Only one of the formulae given in 4511 changes for single phase loading, where the constant √ 3 , is removed thus:-
kW genr. = volts x current x power factor
1,000
However in assigning values to the terms in all these formulae, it is the single phase values that must be considered.
Great care must be taken to ensure that the values selected are the correct ones. In particular the voltage value
should be given special consideration since in a 3 phase star connected machine:-
volts (line to line) = √ 3 x volts (line to neutral).
In the delta connected three phase case, single phase loads can be connected only from line to line, since there is no
neutral point.
When connecting single phase loads to a three phase a.c. generator it is advisable to distribute the loads over the
three phases as evenly as possible. Using the formulae from 4511, with the above notes, the total ‘worst case’ single
phase and two phase loadings can be calculated. To obtain the equivalent 3 phase rating for ‘kW genr’, these single
phase and two phase calculated loadings must be multiplied by 3. This will ensure that no part of the main output
winding is overloaded by carrying too much load current.
The final point about unbalanced loads is that as the amount of unbalance increase, the voltage regulation and
voltage balance between phases worsens.
For example of this type of clrculation see Section 4595.