IM 781-2 Page 51
Table 11. Voltage to Current Converter (0-10 VDC to 4-20 mA) Specification
Signal Input
0-10 VDC
Signal Output
4-20 mA
Power Supply
24 VAC
Dimensions
3.25”H x 4.6”W x 1”D
(8.6 cm x 5.1 cm x 12.4 cm)
Hardwired Chiller Load Limiting (AO 2)
The CSM limits the loading of each chiller in the system from 100 % to 40 % by sending it a capacity limit. The CSM
generates the load limit for each chiller based on the demand limiting, soft load, or load-balancing functions. The capacity
limit is sent to every hardwired chiller via analog output (AO 1) on that chiller’s hardwire remote I/O module.
The chiller accepts load limiting (typically described as demand limiting in the chiller literature) from an external signal. A
2-10 VDC (or 4-20 mA with a converter) signal from AO 2 on the remote I/O module is wired to an analog input on the
chiller unit controller. The signal decreases linearly from 100% capacity (at 2 VDC or 4 mA) to 40% capacity (at 10 VDC
or 20 mA).
Many chillers (including McQuay MicroTech controlled chillers) only accept external demand limiting in the form of a 4-
20 mA signal. The 2-10 VDC signal from the remote I/O module must be changed to 4-20 mA with a signal converter.
This converter can be wired to the same power source that supplies 24 VAC to the module and must be spliced into the
signal wiring as shown in Figure 30. See Table 11 for voltage to current converter specifications.
Hardwired Chiller Remote I/O Module Digital Outputs
Evaporator and Condenser Pump Control (Relay 1)
The CSM enables and disables the chiller by wiring the chiller’s remote auto/stop contacts to relay #1 on the hardwired
chiller remote I/O module. See Figure 30 for details.
Evaporator and Condenser Pump Control (Relay 2 and Relay 3)
The chiller unit controller typically controls the evaporator pump and condenser pump. If the chiller unit controller is not
able to control these pumps, they can be wired to the remote I/O module and controlled by the CSM. The pump relays are
energized from an external customer provided power source which is run through the Normally Open contacts of relay 2
and relay 3 on the hardwired chiller’s remote I/O module (see Figure 30). These relay contacts are rated for 6 amps at 230
volts.
Transformer Usage
A 24 VAC transformer is required to power the hardwired chiller’s remote I/O module. This transformer can also be used
to power the other optional hardware that may be necessary to control a particular chiller. The optional hardware and
transformer usage is listed below in Table 12.
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