| 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.
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