Trane Starters Engineering Bulletin page 75

Current transformer
A device that produces a reduced current signal that is
proportional to the primary current. A current transformer
may resemble a square or round doughnut. Each motor
lead passes through the center of a current transformer
and a reduced proportional current signal is obtained at
the current transformer leads. There is no direct electrical
connection between the motor lead and the current
transformer. The voltage in the output circuit of a current
transformer will be whatever voltage is necessary to drive
the output current through the load with the voltage
limited only by the line power or saturation level of the
current transformer; therefore, the secondary of a current
transformer must never see an open circuit.
D
Differential motor protection
Means of detecting current leakage in a motor from
winding-to-winding or from winding-to-ground. Differential
motor protection requires a six lead motor. Three current
transformers, one for each motor phase, monitor the
current in and out of each of the motor windings. If, due to
leakage or some fault within the motor, the current in and
the current out are not identical in each winding, a current
will be induced in the secondary of one or more of the
current transformers. This induced current will energize a
differential relay which will open a contactor and de-
energize the motor.
Differential protection is available as part of the Enhanced
Electrical Protection Package's supplemental motor
protection option. Differential protection is not applicable
to Trane's standard, low-voltage wye-delta starters.
Differential relay
Current sensitive relay that has three coils, each
connected to one of the current transformers used for
differential protection. Energizing any one of the three
coils will operate the relay. A differential relay is furnished
with the differential protection option.
Disconnect
A device for manually isolating an electrical circuit.
Distribution fault
Transient power losses longer than 2 or 3 cycles will be
detected and cause the chiller to shut down, typically
within 6 cycles. The chiller can also shut down due to
excessive or rapid voltage sags. Shutting down the chiller
prevents the motor having power reapplied with different
phasing. Utility power outages are not considered
CTV-PRB004-EN
distribution faults—Trane refers to this type of fault as a
momentary power loss (MPL).
E
Enclosure
The sheet metal structure and access door that surrounds
the starter. The standard enclosure is NEMA Type 1,
which means general purpose indoor with a gasketed
access door. Other enclosure classifications to meet
requirements of different environments are available as
special options. See "NEMA 1, "
F
Full-load amps (FLA)
See "Amps, motor full-load amps (FLA)" ,
Fuse
A device that protects a circuit by fusing (melting) open its
current-responsive element due to an overcurrent or
short-circuit current. A fuse protects electrical
components from further damage if any one component
within that system experiences a short circuit.
Fuse - dual element
A dual element fuse is a single cartridge fuse having two
different current-responsive elements in series. A dual
element fuse is required to carry 500 percent of its
current rating for a minimum of 10 seconds. One of the
current-responsive elements will have inverse time
characteristics while the other element will handle larger
currents without intentional time delay. Since most
CenTraVacs draw inrush currents in excess of this rated
minimum ampacity during acceleration, all low-voltage
CenTraVacs with fuse protection must use dual element
fuses.
Fuse rating
NEC code allows a maximum fuse rating that is
175 percent of the RLA. To prevent blowing fuses during
motor acceleration this maximum value is allowed to
increase up to 225 percent of the RLA. CenTraVac dual
element fuses are sized at 150–195 percent of the chiller
RLA.
G
Ground fault
A leakage of line current to ground. Ground faults are
typically caused by the breakdown or cracking of wire
insulation.
Glossary
p. 78.
p. 73.
75