Motor Control Centre for an Effluent Treatment Plant

This motor control centre was built to run a complete industrial effluent treatment process from one assembly — balance tank, dissolved air flotation, clarifier, sludge return and odour control. Seventeen motor circuits are driven from it, ten of them on variable speed drives with parameter units on the door, under an Allen-Bradley PLC integrated to the wider plant over Profibus and Ethernet.

Cubic modular motor control centre with door-mounted drive parameter units, selector switches and emergency stop

The completed motor control centre prior to dispatch — control and starter/VSD sections, with the incoming and outgoing column to the right.

Door-mounted Danfoss drive parameter units with hand-off-auto selector switches, each labelled to its plant item

Door-mounted parameter units and hand/off/auto selectors, each labelled to the plant item it drives.

250A

Incoming MCCB

50kA

Breaking Capacity

17

Motor Circuits

10

Variable Speed Drives

THE PROJECT

An industrial effluent treatment plant needed a single motor control centre to run the whole process line: a balance tank mixer, dissolved air flotation feed and recycle pumps, a clarifier and scum pump, return and waste activated sludge pumps, an odour control unit with its own fans and recirculation pumps, and roughing filter pumps. Three actuated valves and a set of plant instrumentation had to be driven and monitored from the same assembly.

The plant runs continuously and the process is flow-dependent, so most of the pump and fan duties needed speed control rather than fixed-speed starting. The MCC also had to feed a separate downstream control panel, leave a way spare for a future pumping station, and give operators local hand, off and auto control at every motor.

THE SOLUTION

Cubic floor-standing modular enclosure, 2880 x 2016 x 400mm, in five sections with dedicated incoming and outgoing cable chambers
250A four-pole 50kA incoming MCCB with door-interlocked rotary handle, shrouds and cable spreaders
Mains energy multimeter fed from a 250/5A current transformer, with phase failure and mains failure relays and local indication
Type 1+2+3 50kA surge protection in its own section, upstream-protected by a three-pole MCB
Outgoing MCCB ways to a downstream nanobubble control panel and a reception tank control panel, plus a way left spare for a future pumping station
Ten Danfoss VLT AQUA Drive variable speed drives from 2.2kW to 18.5kW, each with a door-mounted parameter unit and fused mains isolation
Seven direct-on-line starters with motor protection circuit breakers for the fixed-speed duties
Thermistor relays throughout, with stator over-temperature, over-pressure and low-flow protection on the duties that need them
Allen-Bradley PLC with Point I/O, Profibus segment coupling and an Ethernet switch for integration to the wider plant control system
Dual Siemens SITOP 24V DC supplies feeding individually protected control distribution, with two safety relays for the emergency stop system
Seventeen hand/off/auto selectors, three actuated valve open/close indications, zoned emergency stop indication and forced ventilation with filtered fans
Fully assembled, wired and tested at our Belfast facility before delivery

SPECIFICATION

PROJECT DETAILS

THE OUTCOME

The plant runs from a single assembly, from the balance tank through to odour control, with speed control on every flow-dependent duty in place of fixed-speed starting. Operators keep hand, off and auto control at all seventeen motors, while the PLC handles automation across the plant over Profibus and Ethernet. The spare outgoing way is in place for the planned pumping station, so bringing that into service will need no modification to the assembly.