Contactor
Ride-Through
Fuse
Technology
Ride-through capability is another possible solution for momentary
‘outages of control voltage. A special electronic circuitry can provide
this short-term ride-through capability to a contactor by maintaining
the control voltage at the minimum coil-holding level during
momentary sags and voltage dips. This circuitry improves the
performance of a contactor in distribution systems where power
quality is a concern.
‘There are two kinds of power fuses: standard (general purpose) and
current limiting fuses. Ratings of standard fuses depend on (1) the
normal continuous current and (2) the time it takes for the fuses to
respond to the different magnitude of overcurrent. Fusable links, the
principle elements of every fuse, can have different lengths and
thickness and can be made of different metals or alloys. The inter-
rupting medium of a fuse also influences its operating characteristics.
This characteristic of the standard fuse is an inversely proportional
time-overcurrent curve, as shown schematically in the figure
Time-current characteristic curve of a power fuse
Time in log scale
@ Inrush current of an induction motor
Normal full load current
™ Range of fuse operation
@ Operating range of device
protected by the fuse
Current in log scale
Proper selection of a fuse for an induction motor is very important.
An improperly selected fuse can permanently damage the motor and
other equipment in the system. Both starting inrush and normal load
currents have to be consideredPrinciple of the The function of a current limiting (CL) fuse is to suppress the fault
Current Limiting Fuse current before it can reach the maximum value. Since in ac power
systems the current can peak in a few milliseconds (4.16-8.66 ms
for 60 Hz and 5-10 ms for 50 Hz), the CL fuse has to react very
pically this is accomplished by
quickly to limit this peak.
designing a device that can rapidly develop a voltage higher than
the power system source voltage thus providing a reverse net
voltage. Since the fault current has an inductive nature in every
power system, the developing fault current will actually change
from rising to falling if the net voltage changes polarity as indicated
in the figure.
Current limiting fuse circuit
prospective
fault current
‘@ actual fault
L CLtuse current with
CL fuse
source
voltage
—=-
Some special devices incorporate a CL fuse, others use bypass
devices as well as electronic sensing, logicand control. These
“intelligent” devices have a group of characteristics adjustable to
more specific customer needs and can perform a variety of different
protective functions. Increased price has to be considered in the
design process in comparison to increased functionality.