Delta Connected Capacitors Vs Wye Connected Capacitors (On Motors and used in Capacitor Banks)
Wye connected capacitors
connected to motors are not appropriate because they are ungrounded and thus
present a potential difference on each phase of the capacitor which contributes
to the over-all deterioration of the capacitor.
As can be seen in fig.1 below,
the failure of any one cell of a Wye connected capacitor presents an unbalanced
load to the motor. This will effectively destroy the motor due to unbalanced
voltage, temperatures.
Example: If you have a 100HP
motor and an unbalance of 5%, this will cause a power loss of 25%. (100HP – 25%
= 75HP). If the motor was loaded to 85%, the motor now becomes overloaded. The
motor would run 113.5% overloaded, therefore overheated and will eventually burn
out.
Unprotected Wye connected
capacitors should not be used for Power Factor correction. The unbalance
protection required for this type of application is more costly than the
capacitors themselves and therefore not included with Wye connected capacitors.
Wye connected capacitors do not
circulate harmonics and thus are much more vulnerable to failure due to
harmonics.
Fig.1 - The total capacitance for the example above with a failed cell would be
5kVAR
Delta connected three phase
capacitors are more friendly to the electrical environment. They not only
provides capacitance to each phase but also provides no voltage unbalances
(harmonics).
If a capacitor cell should fail
for any reason in one phase, we are going from a closed delta to an open delta
configuration and in this instance the voltages on each phase stays the same
(see fig.2 below). Failure of a capacitor cell would only mean a drop in the
kVAR on the circuit. In other words, a 30kVAR delta connected three phase
capacitor with a failed cell would provide 20kVAr to the electrical system
instead of 30kVAR.
Delta connected capacitors
unlike the Wye connected capacitors are able to circulate harmonics currents
within the delta circuit. By doing so it reduces the harmonics on the electrical
system.
Fig.2 - The total capacitance for the example above with a failed cell would be
20kVAR
