3 - 4 RTHC-IOM-1C
Minimum Circuit Ampacity (MCA)
The MCA is equal to 1.25 x the compressor RLA (on
nameplate).
Maximum Fuse/Circuit Breaker Size
The maximum fuse/circuit breaker size is equal to
2.25 x the compressor RLA in accordance with UL
1995, para. 36.15.
The recommended dual element (RDE) fuse size is
equal to 1.75 x RLA in accordance with NEC Table
430-152.
For recommended field connection lug sizes (RTHC
starters) see
Table 13
.
Note: Lug sizes are dependent on starter type
Application Of Solid-state Starters
The starter is a pre-wired platform consisting of a 3-
pole bypass contactor mounted in parallel with three
triacs (each composed of a pair of back to back
Silicon Controlled Rectifiers (SCRs)), and two gating/
logic/controller PC boards. The SCRs control the
inrush currents to the motor until it is up to speed,
through cycle by cycle “switching” of the voltage
applied to the motor winding. Such a reduced voltage
starting method reduces, significantly, the high inrush
currents normally associated with an across the line
starting method. This in turn reduces the voltage
dips that would otherwise be experienced by the
power system during startup, as well as limiting
starting torque and motor stator heat up rates and
their accompanying motor stresses.
The current is controlled according to a factory preset
ramp time and current limit settings on the starter.
Once the motor is fully up to speed, the contactor
closes, which bypasses the SCRs. The bypassing of
the SCRs eliminates the inefficiencies and heating
that would result if they carried current continuously.
The contactor, however, will experience greatly
reduced contact wear and increased life, since it only
has to carry running currents and neither has to
make or break the inductive motor load.
In the unlikely event that the motor does not come up
to speed within the factory Maximum Acceleration
time, the contactor will be pulled in by the UCP2
starter module and the motor will be put directly
across the line for full starting torque availability.
However, if the associated transient to the power
system is unacceptable, UCP2 can be programmed
to instead abort the start upon failure to accelerate.
Refer to Section 4 for UCP2 settings and
descriptions.
On RTHC chillers, solid-state starters are connected
“inside the delta” of the motor connection. This
means that, whereas most solid-state starters may
be applied “in the line connection”, on the RTHC
chiller each SCR pair is connected in series with a
motor winding coil. When applied in this fashion the
solid-state starter provides reduced inrush starting
with smaller, more economical controls, which will
see about 57% of the line current. The connections
between the line, the main circuit breaker, the solid-
state starter, and the compressor motor terminals is
illustrated in
Figure 18
.
Starters are selected and control settings are made
for a given compressor motor maximum amp rating,
voltage and frequency. Selected starters are from
the Eaton Eazy-Start EA product family. In general
the nominal ratings were reduced about 10% to allow
operation in ambients up to 122°F. These control
settings should not be reduced for the same model
chiller, which happens to have a lower nameplate
marking because of different application conditions.
All chillers must be capable of starting under the
worst-case starting conditions (typically 90° F
entering the chiller). Under these conditions the
starter must provide the necessary torque to start the
motor. For a given motor design, regardless of the
chiller’s nameplate marking, the required starting
current is a prescribed value. The solid-state starter
selected for each chiller has taken this into account,
and control settings have been prescribed so the
chiller starts reliably.
If the starter control settings are lowered, chances
are that the motor may not come up to full speed in
the available time, and the motor will then
momentarily draw full locked rotor current when the
Table 13
Recommended Field Connection Starter Lug Sizes
Starter/Control
Panel
Connection
RLA Range
Lug Size L1 - L3
(each phase)
Terminals only
000-760 (2) #4-500 MCM
761-935 (4) #4/0-500 MCM
Main Circuit
Breaker or Non-
fused
Disconnect
Switch
000-185 (1) #4-350 MCM
186-296 (2) 2/0-250 MCM
297-444 (2) 3/0-350 MCM
445-592 (2) #1-500 MCM
593-888 (4) 4/0-500 MCM
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