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E L E CT R I C AL CONNE CT I ON
W I NT E R 2 01 5
A voltage unbalance is likely to cause
current unbalance in three-phase motors
of at least six to 10 times the voltage
unbalance. This is particularly so for lightly
loaded motors.
Voltage unbalance is generally caused
by the uneven connection of single-phase
loads, either in the installation or out on the
grid. Other causes include blown capacitor
fuses on power factor correction equipment
or photovoltaic inverters on one phase.
Voltage unbalance causes increased
three-phase motor losses, reduced
motor efficiency and increased motor
running costs. Increased motor losses
result in increased heating and loss of
motor insulation life. Effective torque
and speed may be reduced and motor
noise increased.
If voltage unbalance is too large, then it
will be important to trip a motor to protect
it from the increased heating. However,
many take the risk and don’t use any of the
protection options.
Many motor protection relays are set to
operate when the current unbalance reaches
30%. For lightly loaded motors, this may be
a nuisance in some rural areas. Bypassing or
cancelling this protection may be an option,
but it negates the motor warranty.
Motors can be de-rated but this may be
practical only if the motor is oversized. If
current unbalance protection is causing
nuisance trips, it may be appropriate to
downsize an oversized motor and this will
reduce the amount of current unbalance.
Of course, the best solution is to fix the
cause of the voltage unbalance.
Generally, if voltage unbalance is greater
than 1%, three-phase motors will need
to be de-rated. The saving grace here
is that a large percentage of motors are
oversized for the task and may not cause
noticeable problems.
EQUIPMENT
Overvoltage, undervoltage and voltage
unbalance are generally managed by the
same protection equipment – on a whole
installation, on a single piece of equipment
or at the socket outlet.
For the whole installation, a protection
relay can be used with a contactor in
the incoming supply cables (installed
complying with Rules).
For three-phase equipment, phase
reversal and protection for loss of phase
may be included, and for generators there
is protection for under frequency and
over frequency.
Some manufacturers supply a single-
pole relay that attaches to the side of
their miniature circuit breakers (MCBs)
and operates the MCB for undervoltage
conditions, so no contactor is needed.
For individual pieces of hard-wired
electrical equipment, the same options
can be used as for a whole installation.
For single-phase, plug-in equipment,
you could use socket outlets with inbuilt
undervoltage protection, but these
do not generally include overvoltage
protection. Years ago, powerboards
were available some with undervoltage
protection but they seem to have
dropped out of the market.
The more expensive motors are likely
to have a specialist motor protection
relay that will guard against various
issues including phase fail, phase reversal,
undervoltage and overvoltage, and voltage
and current unbalance.
If surge (I hate the term) protection is
needed, then it should be installed on
the incoming supply and at the sensitive
electronic equipment. Surge diverters
help but surge filters are much better. This
is also a topic for another day.
CONCLUSION
The power quality of the incoming
supply may not be perfect, so protection
may be required to ensure safety and
protect property.
This kind of protection may disrupt
production in plants and factories, but
safety should always be the overriding issue.
Problems with the incoming supply
may include sustained undervoltage,
overvoltage and voltage unbalance – and
much faster problems such as impulsive
transient events caused by lightning or
load switching (what the Wiring Rules calls
‘overvoltage’).
The Rules are silent on protection
options for sustained overvoltage and
voltage unbalance, other than with
generic clauses about the design and
safety of installations. Undervoltage
protection, according to the Rules, is a risk
management decision.
It will pay to discuss protection options
with clients, or problems could and you
may be held accountable.
The next issue of
Electrical Connection
deals with protection against the more
widely known instances of overcurrent,
short circuit and earth leakage.
> Power Logic
www.powerlogic.com.auThe protection of installations,
circuits and electrical equipment
is crucial for the safety of
clients, their customers,
families and friends.