Ian Heathcock believes this is where the integration of smart motor controls (MCCs) into a process automation system can play a vital role in energy conservation strategy.
The escalating price of fossil fuels marked by global competition is driving process industry decision makers to rethink the way their businesses will perform in the future. In order to stay competitive, manufacturers will have to develop strategies to conserve energy and limit usage of depleting fossil fuels.
However, the challenge in achieving this goal is to do so without disrupting production.
Energy consumption is the second largest operating expense in a plant after raw materials, the integration of smart motor controls to monitor energy usage can remove wasteful expenditure, prevent unplanned downtime and improve overall operational efficiency.
Distributed control systems (DCS) were designed to provide regulatory control for the process control industry. They were based upon proprietary components such as operating systems, networks, hardware and configuration tools. The standard DCS was mainly designed for communication within system boundaries. Programmable logic controllers (PLCs), on the other hand, were designed for high-speed control of discrete devices such as motors, pumps and drives.
Traditionally, PLCs controlled the electrical infrastructure while regulatory control was left to the DCS. Since it is not economical to hardwire all of the operating data from individual motor buckets to the PLC, nor to pass this data to the plant's DCS only minimal motor performance information was available to optimise energy usage. There was no mechanism to convey essential real-time power consumption data to the process operation or an existing MES set-up. It was therefore impossible to measure energy consumption for separate processes or units. This means companies cannot really take advantage of partial shut-down to improve energy efficiency, nor take up utility incentives to reduce energy consumption at peak periods.
DCS today can provide optimum control for regulatory applications and also integrate and control high-speed discrete devices such as MCCs, drives, soft starters and power metering devices. Such a system enables dynamic monitoring of motor performance leading to better energy management and operational excellence.
The integration of motor management data directly into the DCS system enables real-time monitoring of motors that can potentially detect motor problems before they occur.
Predictive measures can now be performed to prevent motor failure and damage. With motors consuming over 60 per cent of the energy of a process plant, monitoring these key components will aid the drive to efficiency and cost reduction.
Significant improvements have been made to increase motor efficiencies over recent times. Electric motors consume 10 to 25 times their purchase price in electricity each year so even a small increase in motor efficiency can provide real payback in terms of motor operation costs. Variable speed drives are becoming popular as an approach to reducing energy consumption. By regulating the speed of a drive in order to directly control flow rate, up to a 50 per cent energy reduction can be achieved in fluid control applications.
Motor control systems have a prominent role in industrial processes. Such systems are often housed in a Motor Control Centre (MCC) which contains control and monitoring devices. Controls and devices such as relays, variable speed drives and soft starters are capable of generating valuable motor data back to the control system in a clear and easy-to-read format. Each combination motor control unit is referred to as a bucket and each can be utilised in varying dimensions depending on the size of the motor and is part of the MCC where power to the individual motor is controlled. A motor designed to deliver higher horsepower would require a larger size bucket which means a larger contactor and breaker. This is where the similarity between the conventional bucket and intelligent bucket ceases. In a conventional bucket an electrical overload switch and a combination of relays would control the power to the motor via a contactor. The relays are controlled by the outputs from the PLC and the feedback is provided via auxiliary contacts to the digital inputs on the PLC.
An intelligent motor management system can be added to the bucket to make it 'smart'. The system controls and protects the motor and acts as an overload switch. The relay has a built-in current and potential transformer to help measure line current and voltage. It can then send all the information on the motor's operating condition to a DCS via a digital fieldbus. This approach enables all the data about the motor to be transferred into the DCS in a cost-effective manner and avoids the cost of hardwiring. As an example, a single cable(fieldbus) can be used to transfer data that would have required 6-12 separate wire pairs per conventional MCC. Additionally, integrating power monitoring devices in this manner to the DCS gives plant engineers a central location where all information related to production data and to the power system is recorded for analysis and improvement.
The five most important factors that affect the motor performance in any process control system are:
- Power quality - monitoring the quality of the incoming power is very important to maximising the lifespan of the motor. Inconsistencies in the voltage can be caused by harmonics introduced by variable frequency drives used around the plant. A small voltage distortion can cause large current distortion which in turn will lead to excessive motor currents and potential damage to the windings. Excess voltage to the motor causes it to waste energy and operate inefficiently. Data logged in the DCS can help identify this situation.
- Motor Operation - A DCS that can monitor, log and notify operation personnel when conditions require attention can assist improvement goals. Predictive maintenance can be aided by measuring all of the line currents in the motor. Motors typically draw six to 10 times their rated current when started leading to potential heating of windings, so a predictive maintenance strategy based upon limiting the number of motor starts will reduce degradation.
- Motor condition - Tracking, trending and alarming motor operating conditions in a DCS is a valuable tool for predictive maintenance. A motor that draws excessively large current during start-up indicates load-related problems. This results in inefficient energy usage and can cause unnecessary burden on the power system every time it starts. Analysing trends in the DCS aids the identification of these types of problems.
- Load and power consideration - Load, percentage of load, horsepower demand, kilowatt usage and power factor are important factors relating to a motor's long-term performance. Load fluctuations might indicate a potential process-related problem. Motors continue to operate within nameplate ratings while being forced to carry load demands above their capacity. These excessive load demands cause the windings to run above a safe level. Motors required to operate above nameplate horsepower ratings also suffer from greater torque demands which can inflict stress on the motor's rotor.
- Operating efficiency - To reduce the plant's total cost of ownership each facility should frequently monitor power usage and the operating efficiency of the motors. Motors are often oversized or undersized due to incorrect initial design. Oversized motors have higher initial costs and are usually more costly to repair or operate. Undersized motors perform poorly and suffer from higher energy losses. The dynamic motor data logged in the DCS can help assist with troubleshooting, repair and replacement of motors. Integration of the smart MCC in a DCS allows the data of the motor to be captured and trended for early detection and prevention of costly repairs. It also proves valuable data which can pinpoint and help correct problems with the sizing of the motor and its dynamic response to changing process conditions.
Understanding where energy is used in the manufacturing process is a key first step in the development of strategies to minimise energy consumption. Distributed control systems that are capable of integrating process and power system information can be a valuable resource to the conservation of energy in process manufacturing. Energy management systems do not replace human involvement in the decision-making process, but should be considered as a real-time tool to keep energy consumption in check. Saving energy in manufacturing has two significant paybacks: a reduction in green house gasses emission and the lowering of the energy cost for the plant operators. Technology, such as the use of Smart MCCs exists to make process control systems more efficient. By adopting smarter technologies, manufacturers can not only save energy and reduce their impact on the environment, but also become more competitive economically.
Ian Heathcock is with Siemens Industry Automation, Frimley, Surrey, UK. www.siemens.co.uk.