Intelligent (Smart) MCC

As per the present norms, MCCs (Motor Control Centers) are becoming intelligent. They are
evolving out from the traditional electromechanical circuits to programmable entities. But it
should be borne on mind that selecting an intelligent MCC is not always the most intelligent
decision To understand intelligence we first need to have a look into the conventional
electromechanical relay based technology..

Fundamentals-the conventional MCC

The role of a basic MCC is to provide a compact, modular grouping for motor/feeder control.
MCCs are one of the basic buildings of a plant automation system. They are a combination
FSU, overload protection, selector switches, push-buttons, contactors, and other electrical
distribution and control components. Historically, MCC units were electromechanical, with basic
functions that included a power switching device (power contactor), short-circuit and overload
protection, local and remote actuation, load managers and drive state indication. For higher
loads(>200 KW feeders), the technology remained basically same with higher rated components
and more robust power switching devices like ACB/SF6 breakers, load managers and protective
relays.. Traditionally these are used for higher voltages like 3.3KV/6.6KV/11KV/33KV etc. and
are called Switchgears. MCCs are traditionally used for 415V/3ph feeders.

A conventional motor starter module in MCC consist of a number of relays/contactors which

finally activates the power contactor, receives external inputs and gives output information to
control system in the form of potential free contacts. This requires extensive wiring and
hardware inside the MCC modules and extensive field wiring. It is almost impossible to
reconfigure a circuit once made, working inside the module has space-constraint, manufacturing
and installation time is higher, requires factory testing for 100% of modules to avoid human
errors in wiring. Intermodular interlocking is always avoided, leaving the job to the central
control system.

There are a number of reasons for MCCs popularity and extensive use:

MCCs provide a single source for coordination of components. Electrical distribution and
motor control equipment can be purchased as a pre-assembled, pre-tested system.
MCCs are modular and can have spare modules built-in for future use.
MCCs come in space-efficient packaging, with excellent configuration flexibility. They
also are friendly for centralized maintenance.
MCCs offer excellent fault containment and electrical component isolation.

MCCs are used in applications where there are multiple, remotely controlled loads linked to a
central control point. Industrial process applications include many load types, such as conveyors,
pumps, fans, heaters etc. These loads often require coordinated control and monitoring and can
be the source of important process or machine diagnostic information. Distributed control
systems (DCSs) or PLCs normally are used to provide this control and data acquisition
functionality.
Intelligent motor control systems
Just like the relay based logic system for process control graduated from electromagnetic relays
to programmable logic control, MCCs also started moving on the same path with a delay of 30
years.

With the advent of electronic overload relay, protective relays, load managers are available with
communication features and diagnostics.

- Avtar-1: The iMCC began with a simple idea of putting a small PLC rack with its
own controller, IO modules, PS units inside a shipping section (to avoid field wiring)
of the MCC. The section may typically consist of 18-24 feeders each in its own
module. The module now will have 2/3/4 contactors, a CT,PT (if required)+Ammeter,
current transducer (if required), overload relay and module fascia with lamps and
push buttons. Looks like the same old one? Maybe, but we have eliminated
marshalling of IOs for DCS/PLC, got rid of intra-locking relays and a lot of field
wiring and reduction in the cost of central control system by reducing the hardwired
I/Os, reducing panel space, power consumption etc.
- The section-PLC now takes all the status IOs, current inputs from transducer and
provides output to drive the K1/K2 relays. This PLC, in turn will be networked with
PLCs from other sections and soft-linked to the central control system (even by a
redundant link). So the cabling between the central control system is done away with
and reduced the cable cost. (In a typical power plant, 60% of control cables runs for
this purpose) This link can be Profibus-DP or Ethernet. In this configuration, the load
managers (if any) can be daisy-chained on MODBUS-RTU and connected to the
PLC. These systems are easily retrofitted to existing installations and do not require
"intelligent devices" to provide benefits. The intelligent programmable protective
relays can be networked (by MODBUS or IEC-61850) and connected by softlink to
the CCS for monitoring and diagnostics.

Just like any other system, the lacunae started manifesting:-

- All programming has to be done at the PLC level with a laptop equipped with
development software (expensive)
- Not much of diagnostics at module level.
- Generally No common HMI at MCC level. So the operator has still to around hunting
for the module. Of course a dedicated HMI can be provided for each shipping section
which will increase the cost.
- Due to having a PLC processor in each shipment section, the system becoming
costlier and more so if redundancy is sought.

- Avtar-2: In this configuration, the IOs are distributed in each MCC module and
connected to the central control system by softlink. The control module/s sitting in
each MCC are the intelligent being (controller) which has the capability of collecting
the required information, diagnostic data and communicate to the central
communication system. This reduces a lot more internal wiring, can accommodate
 communication redundancy and can offer various communication network like
Profibus-DP, Ethernet, DeviceNet. Here overcurrent, overload, KW monitoring are
all possible. This architecture is becoming most popular due to the following
reasons:-
- Lesser hardwiring by using networking technology.

- Reduction in the cost of central control system by reducing the hardwired I/Os,
reducing panel space, power consumption etc.

- Some degree of enhanced diagnostic or protective functionality is included.

- Maximizing the value of a component in a given application. (by maximizing the

value means decreasing the total cost of ownership which includes cost of
design,manufacture, inventory management, plantwide standardization, install,
commission, operate, maintain and upgrade.).

- MCCs in this category are characterized by simplicity. They will almost always
deliver the lowest installed cost. They allow localization of failures to improve
maintenance and usually provide some additional functionality not economically
feasible with hardwired systems.

- Computational resource bandwidth requirement is low because an entire MCC (20-40

drives) represents one single node.

- Installation and commissioning time significantly reduced.

- Intelligent devices imbedded in the MCC not only provide network communications,
but extensive functionality not available on standard devices, such as network
configuration, diagnostics, extensive process information, and advanced protection
for each unit.

- Significant technical expertise is required, but the systems have excellent life-cycle
benefits in process applications where the additional information, diagnostics, and
protective features easily justify the incremental cost of the equipment.

- Integration into PLC-based control schemes is straightforward.

- Redundancy is not required at module level since there is no common fault condition.
Redundancy can however be provided at the communication cable.

The decision making

The decision making while specifying an MCC for a project is very vital. The decision maker
should be very objective rather than subjective. The cost, capability and complexity are counter-
acting considerations. So the more the merrier concept should be avoided. When one is the
user, the total cost of ownership is to be considered while for an EPC contractor the decision is
made by the user.

Specification of intelligent MCCs requires taking into consideration the variables of cost,
capability, and complexity. Therefore, it is important to realistically assess your capabilities and
objectives. Keep in mind that additional functionality usually comes with additional cost or
complexity that you may not need.

The following list of questions maybe considered by the user/consultant when selecting and
specifying intelligent MCCs.

Will I maximize my return on investment by lowering the installed cost of the MCC
equipment or by providing significant additional functionality?
Will last minute changes be likely?
Is future expansion or modification likely?
How valuable is a reduction in installation and commissioning time?
Can the equipment be integrated into my overall control and information architecture?
Does my control system or network have enough capacity to handle the data provided?
Do I have a technical staff capable of maintaining the system?
What are the cabling and connection requirements of the system?

If most of the answers to the above questions are NO, it will be more intelligent to go for a
non-intelligent MCC!

While selecting a supplier for an iMCC, the most important questions that should be asked and
confirmed are the following:-

- Does the supplier have adequate experience of manufacturing such iMCC with same
hardware/software?
- Can they arrange for complete functional testing including diagnostics (not just
wiring checks) at the factory before dispatch?
- Do they have in-house programming/configuration tools for handling such PLC or
intelligent modules?
- Are they willing to work with the central control system supplier to establish the
functionality of the iMCC?