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Evaluating advancements in motor control centres

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Complete view of a Profibus motor control centre
Fig 7: Profibus based communication capable motor control centre
By Syed Q. Raza, P.Eng.

Motor control centres (MCC) are enclosed electrical assemblies, divided into sections with a common power bus, with motor starters, feeder breakers and panel boards. They are used to control the starting and stopping of different process equipment such as pumps, blowers and ventilation fans. There are various types of starters, depending on the application, including direct-on-line (DOL) starters, soft starters, variable frequency drives (VFD), etc. These operate the loads based on start and stop commands they receive and send status feedback. This requires two-way communication between the command initiating controller and the MCC.

One example would be a simple duplex sump pump system. Based on float switches inside the sump, the controller initiates start/stop commands to the pump(s). The starter in the MCC responds with the status signal such as run/fault. Based on the status signal, the controller can start the standby pump if the duty pump fails to start.

The above example requires four command signals from the controller to the starters (two for each pump) and four status signals from the starters back to the controller (two for each pump). A more complicated process would require many commands and status signals between a controller and the MCC.

Now consider a MCC with 30+ starters and the quantity of signals required to be communicated between a controller and MCC. This could be overwhelming based on the process requirements and various types of signal to be transmitted between the MCC and the controller.

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old motor control centre
Fig 1: Old motor control centre at a wastewater treatment plant at the end of its useful life.
First generation motor control centres

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The first generation of motor control centres comprised traditional electro-mechanical components for direct-on-line starters, star-delta starters, auto-transformer starters, etc. Control logic was implemented by use of relay-based circuits, either mounted inside a control section of the MCC, or inside a separate control panel enclosure.

looking inside a first generation motor control centre
Fig 2: Inside view of a first generation MCC bucket showing electromechanical components and terminal for hardwiring the control wires.

Control and status communications were done via hard-wired components. Dedicated wiring was required for each individual status or command signal to be transmitted.

This resulted in large starters with limited status and control options. Control and status input/output (I/O) were generally limited to start, stop, available, run and fault. This type of MCC can still be found in old facilities, but they are being replaced by modern types.

Second generation motor control centres

The second generation of motor control centres can be primarily differentiated by the use of solid-state devices, including electronic overload relays, variable frequency drives, and soft starters. Built-in or externally mounted programmable logic controllers (PLCs) are used to control logic. Connection between the PLCs and the starters is still by traditional hard-wiring.

These second generation MCCs provide a higher level of control and monitoring. However, the size of the MCC bucket and controlling PLC is still considerable as it is directly proportional to the number of I/O required for each starter. This is based in turn on operational control requirements. This is illustrated in Fig. 3. These MCCs are still being used and installed in facilities requiring less sophisticated automation and controls.

Illustration of PLC requirements for motor starters
Fig 3: Illustrating the increase in PLC hardware requirements, as number of starter inputs increase.
Looking at a Profibus cable.
A single Profibus cable (purple) is daisy chained between starters.
Third generation motor control centres

The third generation of motor control centres was marked by the introduction of Fieldbus (DeviceNet, Modbus, Profibus, etc.) and later with industrial Ethernet. These provide the possibility of device level communication between the PLC and MCC devices, resulting in advanced monitoring, control and diagnostics. The PLC and the MCC devices are connected via a single communication link (Fieldbus or Ethernet), which is used to transmit all control and status signals via a single cable.

This drastically reduces the hardware and wiring requirements for the MCC and the controlling PLC. These MCCs provide a multitude of monitoring points, which can be used for complex process control as well as predictive maintenance.

Figure 4 illustrates that a single communication card is required to control all the starters of the motor control centre, drastically reducing the hardware and wiring requirements.

With third generation MCCS, it is also very easy to select and add any desired status signal by simply configuring the software program. In the case of a second generation MCC, it would require additional wiring in the MCC and additional hardware in the PLC panel. Thus, these modern MCCs can save costly change orders during construction and also from potential project delays.

Profibus cable illustration
Fig 4: Illustrating that a single Profibus cable is required to communicate with an entire motor control centre.

However, there is still demand in water and wastewater facilities for hardwiring-based second generation MCCs. Their less sophisticated processes do not require the MCC to send much status feedback. The other reason is that operational staff is generally familiar with the operation and troubleshooting of hard-wiring based components.

They would require additional training to operate and maintain Fieldbus-based MCCs. A reasonable tradeoff in some facilities is to use a hybrid model, i.e., traditional hardwiring for the standard DOL starters, along with Fieldbus communication for sophisticated devices like VFDs and soft starters.

However, communication capable motor control centres are quickly taking over the market owing to less wiring requirements, ease of installation, flexibility in selecting the required signals, and overall reduction in system cost.

Syed Q. Raza, P.Eng. is a Senior Electrical Engineer and Associate at R.V. Anderson Associates Ltd.

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