This is more than some manufacturers’ maximum allow-
able. The operating technician should keep an eye on voltage
unbalance because it causes the motor to overheat. Some so-
phisticated electronic systems have voltage unbalance pro-
tection as part of their features; otherwise, it is up to the
equipment operator to monitor.
Phase unbalance is caused by the electrical load on the
building being unbalanced or the power company supply
voltage being out of balance.
48.47
PHASE REVERSAL
Three-phase motors turn in the direction in which they are
wired to run. If the phases are reversed, the motor rotation
will reverse.
This can be detrimental for many compressors.
Reciprocating compressors will normally perform in either
direction because of bidirectional oil pumps. Scroll, rotary
screw, and centrifugal compressors must turn in the correct
direction. Phase protection is often part of the control pack-
age for these compressors. Any compressor with a separate
oil pump, such as the centrifugal, will not start under these
conditions because the oil pump will not pump when re-
versed. It is not uncommon for the power company to reverse