Good Electrical Design Procedures
Good electrical design is more than delivering a system that works and much more than switching components on and off especially when controlled by PAC or PLC controllers. The designer needs to consider the following availability, efficiency and installation costs as a minimum in conjunction with functionality. Often circuits can be replicated on the principle of ‘it isn’t broke it don’t fix it’ however it may not be broke but could it be improved? Is always the attitude that should be taken when creating electrical designs? One important point to always consider is that the manufacturer’s data sheet this will be key, and consulting component manufacturer is often essential, to ensure that electrical designs can meet the grade for CE marking.
When designing any system it is important to consider the system lifecycle of all components and the maximum availability without detriment to the equipment. Often there is more than one way to wire a component and in many cases this will not affect the overall performance, however this does not mean components should be thrown together, when considering a standard motor circuit containing a drive, contactor and circuit breaker. It would be possible to wire the system with the contactor before or after the drive, and both cases would work however the lifecycle of the drive will be significantly increased, by fitting the contactor prior to drive and if failure of a contactor was present could even cause damage the drive. The above is only one example of component general layout and manufactures best practice should always be considered right down to the recommended fixing torque. There are many other ways which will also increase availability; one such design would be thought the reduction of relays and replacement with solid state devices, as mechanical devices only withstand a maximum number of operations. There are many components on the market that will allow for more reliable switching.
Power quality is also an important consideration and the machine directive on EMC is a good starting point as odd harmonics, not only decrease component lifecycle but cause in efficiencies and in sever cases can result in hot neutral’s where usefully power is feed directly to ground.
System inefficiencies can also be eliminated at the design stage, it is estimated that a poorly designed system can waste up to about 10% of the power consumed. Looking at the switching frequencies will determine the best switching method, often this is only a guide at design stage and it is important to evaluate all systems for energy consumption on a regular basis. Inefficiencies can also be minimised by switching of components when required, a control relay and PIR (Person in Room) may be seen as unessential component but could save thousand over the product lifecycle.
Using the same designer, panel builder and Installation Company is always a way of ensuring that the most efficient design is achieved, traditional methods have resulted in design and panel build by one contractor, PLC programming software development by another contractor and installation by another contractor. All of these different sources can be an excellent way of achieving the best rates on labour but often the design will not be the most efficient. If a system designer was in tight competition for a job he would often target the system hardware at a lower cost to get the job knowing full well that the cost of installation would be much higher. Selecting a principle contractor and allowing for them to subcontract will always result in an overall more efficient design which will best utilise all modern technologies in the full system. As the cost of copper increases and the cost of distributed control systems with remote I/O modules increase the full project should always be a major consideration.
Article written by Matthew Withe, senior PLC Programmer at AC-Sys Ltd (also published on Ezine Articles)
