Features
and Benefits |
Unit Options-Unit Mounted
Starter |
Operation
The unit mounted starter
is a star (wye) delta, closed transition, reduced
voltage starter. When starting and during acceleration, the motor is connected in
its wye configuration. Because of this arrangement the
voltage applied to the motor windings is reduced to one divided by the square root of three or 0.58 times line voltage. This
reduction in winding voltage results in a reduction in
inrush current. The inrush current is 0.33 times the
full voltage locked rotor current rating of the motor. The accelerating torque of the motor is also reduced to 0.33 times the
full voltage torque rating. This is sufficient to fully
accelerate the compressor motor. The unit control
panel monitors motor current during operation via current transformers located in
the starter enclosure. When during acceleration the
line current drops to approximately 0.85 times rated load current, transition is initiated. The closed transition feature provides for a continuous motor current flow during
transition by placing resistors in the circuit
momentarily. This prevents buildup of damaging torques
to the system during this period. With the completion of transition, the motor windings are connected in the delta configuration
with full line voltage.
Three precision current transformers
monitor phase current. Contactor position and various voltage signals provide
extensive interlocking between the starter and the
microcomputer in the CenTraVac ® control panel. All
logic and subsequent instruction originate in the unit control panel. Protection against the following starter defects is provided:
- High motor current (starting and
running)
- Improper starter circuitry
- Excessive accelerating time
- Incomplete starting sequence
- Loss of phase
- Phase amperage unbalance
- Phase reversal
- Distribution fault
Features
The Trane CenTraVac Unit Mounted
Starter includes the following standard features:
- NEMA 1 enclosure, designed to accommodate padlock
- 3 KVA control power transformer with
120V secondary
- Fused 120V control circuit
- 3-phase incoming line terminals
- 6 output load terminals factory-
connected to the motor
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)
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.
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.
Figure O-3 — Six-SCR
Configuration
Figure O-4 — Wave Forms Features and Benefits
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