Previously it has been stated that a compartment of a MCC should be dedicated for use as a relay system or PLC. This is not strictly true for each system located in a MCC there should be a relay system or PLC. For example if the MCC housed the starters for two air handling units then there should be two relay systems or PLCs each in its own compartment co-located with it’s respective starters.
The reason for a separate relay systems or PLCs for each starter system is that it keeps each control system separate, thus if it becomes necessary to shut down a system then the operational integrity of the co-located systems within the MCC is not impaired.
By their very method of construction a relay is an isolating device, relay circuits must be designed so that when a starter is switched off there are no live feeds into the starter from the relay panel.
Generally it would be a relatively small control system that would use relays, it is preferable to use PLCs as they can be reconfigured with minimum effort should the control system require modification, which if non-electrical designers are involved in the control system design is quite likely. Another benefit of using PLCs is that the MCCs may be ordered prior to the PLC program being known which allows early purchase of the MCC.
For more complex systems it is more common to use programmable logic controllers. For maximum flexibility and availability PLCs should be of the modular type with a diverse range of power supplies, CPU, input and output modules both analogue and digital. For the interface to the starters the inputs should be configured for 24Vdc where the common line is derived from the PLC, the outputs should be isolated voltage free contacts rated at 2A AC15 at 110Vac.
The power supply should be 24Vdc and derived via dc-dc converters from the MCC battery supplies. This will ensure operational integrity should the MCC mains supply fail.
The recommended manufacturer of PLCs is Hitachi and the recommended type being the EH-150 which is a modular system and has a power pack with a 24Vdc input.
A modular system allows maximum flexibility as different CPUs, memory levels, input and output modules are available and can be configured to suit particular applications.
The application will determine the CPU to be utilised, and the amount of memory required.
Each single speed single direction starter will require an output module with four volt free relay contacts with a 110Vac rating of 2A the only module that fits this requirement is the EH-YR8B and an input module with six inputs from a common rail supplied from the module, the module that meets this requirement the EH-XD8.
If one output module of the type described above is used then there will be four spare outputs, it could be argued that two single speed single direction motors could be controlled by one output module. Similarly if an input module EH-XD16 is used then the inputs of two single speed single direction motors can be catered for. The actual configuration of the modules is a decision for the system designer.