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CTV-PRC007-EN
The solid-state starter controls the
starting characteristics of a motor by
controlling the current that flow to the
motor. It does so through the use of
SCRs (Silicon Controlled Rectifiers),
which are solid-state switching devices,
and an integral bypass contactor for
power control.
SCR’s
An SCR will conduct current in one
direction only when a control signal
(gate signal) is applied. Because the
solid-state starter is for use on AC
(alternating current), two SCR’s per
phase are connected in parallel,
opposing each other so that current may
flow in both directions. For three- phase
loads, a full six-SCR configuration is
used. The connection is shown in Figure
O-3.
During starting, control of current or
acceleration time is achieved by gating
the SCR on at different times within the
half-cycle. The gate pulses are originally
applied late in the half-cycle and then
gradually applied sooner in the half-
cycle. If the gate pulse is applied late in
the cycle, only a small increment of the
wave form is passed through, and the
output is low.
Unit
Options
Available options include:
•
Circuit Breaker — A standard
interrupting capacity circuit breaker is
available. The circuit breaker is
mechanically interlocked to disconnect
line power from the starter when the
starter door is open.
•
High Interrupting Capacity Circuit
Breaker — A high interrupting capacity
circuit breaker is available. This breaker
is also interlocked to disconnect line
power from the starter when the
starter door is open.
•
Circuit Breaker with Ground Fault —
Ground Fault protection is available
with either standard or high
interrupting capacity circuit breakers.
An indicating light is provided to
indicate if a ground fault has occurred.
•
Current Limiting Circuit Breaker — A
standard circuit breaker incorporating
the current limiters with fuse links is
available. A fault current in excess of
the circuit breaker capacity will blow
the fuse links and interrupt the fault
current. The circuit breaker cannot be
reset until the blown current limiters
are replaced.
•
Ground fault detection and protection
(available only with circuit breaker
options)
Unit Mounted
Starter (Cont.)
Figure O-3 — Six-SCR Configuration
Figure O-4 — Wave Forms
If the gate pulse is applied sooner in the
cycle, a greater increment of the wave
form is passed through, and the output
is increased. So, by controlling the SCR’s
output voltage, the motor’s acceleration
characteristic and current inrush can be
controlled. These forms are shown in
Figure O-4.
Integral Bypass Contactors
When the SCR’s are fully “phased on,”
the integral bypass contactors are
energized. The current flow is transferred
from the power pole to the contactors.
This reduces the energy loss associated
with the power pole, which is otherwise
about one watt per amp per phase.
When the starter is given the stop
command, the bypass contactors are de-
energized, which transfers the current
flow from the contactors back to the
power poles. Two-hundred fifty
milliseconds later, the SCR’s are turned
off, and the current flow is stopped.