| Many companies today are switching to high power Personal Computers (PC's) for industrial applications such as process control, data acquisition and computer aided manufacturing. The industrial PC market is growing at a rapid rate. The market for these products amounted to $500 million in 1994-representing a 20% growth. The trend has been towards distributed processing or the downsizing of mainframe computers to networked PC's throughout the factory. PC's are being networked together with larger computers by Local Area Networks (LANs) and Wide Area Networks (WANs). Many manufacturers also offer plug-in data acquisition and control boards for these computers.
Another area growing rapidly is the sophisticated, yet easy to use, Man Machine Interface (MMI) software (such as WonderWare InTouch™). This kind of development environment allows any company to create a Supervisory Control and Data Acquisition System (SCADA) at a small fraction of the cost of computers, controllers and software of the past decade. Using distributed computing power, these systems are able to communicate data with one another throughout the factory. Moving the PC to the factory floor offers many advantages. It gives the operator instant access to information concerning the controlling process. It allows for easy modification of the process in real time. Since there are many small computers rather than just one big system, it reduces the risk of the entire factory going down in the event of a computer problem. Other advantages include:
- Low cost, high performance hardware
- Large number of manufacturers offering compatible hardware
- Low cost software due to large installed base of hardware
- New operating systems that run on different platforms (UNIX, Windows-NT, Novell)
- Ease of networking
- Rack mount PC's and hardware which saves valuable factory floor space
However, with this trend brings new problems. Unlike mainframe computers housed in environmentally controlled rooms, PCs on the factory floor are constantly subjected to fluctuations in power caused by motors, machinery, and other equipment. Many times these are the same pieces of equipment that the PC's are controlling. The solution is a power protection system. The cost of these systems is easily justified since the data collected, the software programs, and the hardware are all vital to the process. Down time of just a few hours can run into the tens of thousands of dollars or more.
Some types of power anomalies from the power company include blackouts (no power), dropouts (very short blackout), brownouts (lower voltage than normal), surges (higher voltage than normal) and phase shifts. Some power problems are caused by the customer (you or your neighbor). These can be blackouts (overloading local sub-station), brownouts (starting a large motor), phase shifts (using low power factor loads that require the power company to switch in power factor correcting capacitors), transients, and high frequency noise (such as from arc welding). Mother Nature can sometimes be blamed for transients (lightning hitting a power line) and blackouts (wind blowing down a power line).
Factories have many large motors in process control equipment. When turned on, this equipment will pull 500% or more of their full load current until they are up to full speed. This can cause a sag in the voltage rebooting the computer . Motor starters help resolve this problem. However, now instead of a sag, the motor starter itself may be generating electrical noise in the factory causing problems with electronic equipment like your computer, causing hard disk errors and corrupted data. Conveyor belts that have to run at different speeds use variable speed motor drives. These generate noise like the motor starter. However, unlike the motor starter that stops producing noise after reaching full speed, the variable speed drive continues to produce noise while the system is running.
What can be done about these potential problems? There are different levels of protection that can be used depending on the level of power problems encountered. Power protection should be viewed as an insurance policy. Consider not only the cost of the equipment protected, but also the cost of down-time. Your computer, interface cards and peripherals may cost several thousand dollars but when this equipment is down your company may lose tens of thousands of dollars an hour. Also to be considered is the time required to reconstruct lost data from the system.
The simplest form of protection is a surge protected power strip. This only protects against transients and some of the highest frequency noise. Another type of protection is a Power Conditioner which protects against a broader range of noise and may have voltage regulation. An Uninterruptible Power System protects against most or all of the power problems, and is the solution of choice.
What is an Uninterruptible Power System (UPS)? As the name implies, this system provides an uninterruptible source of power to any properly sized piece of equipment. It does this by sensing the loss of power from the utility and switching over to an internal battery. When the utility returns, it recharges the battery. Three basic types of UPS systems are: On-Line, Line-Interactive and Stand-by.
An On-Line system has an inverter which supplies 100% of the load, 100% of the time. The utility is converted into DC and stored in bus capacitors. Then the DC is converted into regulated AC in the inverter to power the load. When there is a loss of utility, the power is supplied by the batteries with no break in the AC power flow to the load. When the utility is available, the batteries are recharged by the charger. The main advantage of on-line systems is that if there is a loss of utility there is no transfer time because the inverter is already supplying the load. Another important advantage is that the inverter is conditioning and regulating the voltage to the load. This is better than just surge suppression and filtering because the AC is precisely filtered and regulated.
One of the most popular UPS products is a line-interactive with high performance and low cost. The line interactive UPS has a separate inverter and voltage regulator circuit. The regulator uses a transformer with "taps" or extra connections to give the transformer selectable voltages that are selected by switching devices. The utility supplies the load through the tap-changer and if the input voltage drops low enough, the tap is switched to one that steps-up the voltage for regulation. The better systems also have a high voltage tap to decrease a high input voltage. When the utility is lost, the isolation switch opens and the inverter supplies the output usually through a low voltage winding on the tap-changer transformer. Charging is done through a separate charger or may be done by backfeeding the inverter.
Standby power supply (SPS) is defined as an AC backup system where the inverter is normally off and only supplies the load when the utility fails. There are different types of SPS systems, but normally they are made up of an inverter, batteries, charger and isolation switch. The inverter is connected downstream of the isolation switch in parallel with the utility. When the utility is available the power is connected to the load through the isolation switch. When the utility is lost the isolation switch opens and the inverter is energized supplying the load.
A variation of the standby system uses a ferro-resonant transformer as a regulator instead of a tap-changer. The utility supplies the ferro-resonant transformer which regulates the load voltage. When the utility is lost, the isolation switch disconnects the transformer input from the line and the inverter supplies power to the transformer for the load. When the utility is available, battery charging can be done using a separate charger or by back feeding the inverter. The advantage of this approach is that the regulator gives linear voltage compensation instead of a step-change inherent with tap-changer systems. This approach also allows the manufacturer to use a cheaper square-wave inverter that the ferro will filter to look like a sine wave.
The trend to distributed computing and placing PC's throughout the factory has created new problems. However, many forms of power protection are currently available to solve these problems. With more PC manufacturers offering space saving rack mount systems, the trend is toward rack mounting the power protection equipment. With the many choices in sizes and configurations available, no factory computer equipment should go unprotected.
Mark James is president of Asgard Engineering, a Mission Viejo, CA company which provides electronic and mechanical engineering services with emphasis on power related requirements-Phone: 714-597-9334. Jerry Zechmeister is president of Automated Power Technologies (APT), a Lake Forest, CA, manufacturer and distributor of power protection systems specializing in rack mount power products-Phone: 714-768-5965.