The Operating Principles of an Automatic Transfer Switch

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An automatic transfer switch (ATS) is a device used to reroute power in special circumstances. For example, during a natural catastrophe the public utility power may go out at a hospital and the automatic transfer switch starts the back-up generator. There are a lot of issues involved in such a transfer – not the least of which is making the decision about when it is safe to switch back to public utility power.

ATS are used to insure the continuity of power supply, although this can mean different things in different situations. In a typical home, small business or institution, continuous power may mean that a short interruption can be tolerated.

For example, if a backup generator is used to supply backup power when public utility power fails, there will be a pause while the generator starts up. In a hospital any interruption of more than a few seconds may be catastrophic.

There are several ways that ATS can insure that the interruption is very brief – including batteries to fill in the gap from the cessation of public utility power to the start of backup generator supply. Some automatic switches sense the temporary dips and spikes in public utility power that precedes failure and starts up the generator before the complete failure of public power.

Engineers typically install transfer switches to switch a load between two different sources of electrical current. Some are manual and can be activated when the user flips a switch while others, like automatic transfer switches, will switch depending on how the power source changes. When the source for electrical power fails, the automatic transfer switch may go into effect to power a building.

Automatic Starting Control Principles

An ATS can control when a backup generator depends on the voltage in the primary supply for a building. When they do this they have to also transfer the load to the backup generator. They work by blocking the backup generator from becoming a source of electrical power until the generator itself is on for temporary power.

One example of a step-by-step process an ATS may use is:

  1. When the electrical power in a building goes out, the ATS starts the backup generator. This causes the generator to ready itself to supply electrical power to the house.
  2. When the generator is ready to perform, the ATS switches the emergency power to the load.
  3. The ATS then commands the generator to shutdown when the utility power is restored. 

When power fails, the automatic transfer switch commands the generator to start. When the generator is ready to supply power, the ATS switches emergency power to the load. Once utility power is restored ATS switches to utility power and commands generator shutdown.

If your house had an ATS that controlled a backup generator, the ATS would start the generator when a power outage occurred and the backup generator would begin to supply power. Engineers generally design houses and transfer switches such that the generator remains separate from the system that distributes power throughout the building. This protects the generator from overloading. Another protective measure that engineers use is that they have "cool down" times to prevent the generator from overheating.

ATS designs sometimes allow for load shedding or changing the priority of other circuits. This allows the electricity and power to circulate in ways that are more optimal or useful for the purposes of the building. These options can come in handy to prevent generators, motor controller circuit boards and other components from overheating or overloading with electricity.

Soft loading is a method that lets the load transfer from the utility to the synchronized generators more easily, which can also minimize voltage loss during these transfers.

Make Your Own Circuit Board With an Automatic Transfer Switch

Power system and electrical engineers have the knowledge, experience and skills to create their own automatic transfer switches. Individuals without these kinds of credentials or qualifications should not attempt to create their own as they don't possess the necessary training. Still, there are ways you can make your own circuit breaker panel boards to deal with electrical signals between devices for various purposes.

It requires general equipment used in electrical engineering processes including the automatic transfer switch itself, a circuit board, an AC meter, circuit breakers, busbars, DIN rails, LED lights and soldering equipment. Do not perform these steps unless you have safety precautions to protect yourself from current.

The general steps to make your own circuit board with an automatic transfer switch are:

  1. Install a DIN rail to mount circuit breakers in a container that will be the enclosure of the automatic transfer switch. DIN rails are used when building devices and electronics that use industrial equipment like circuit boards and wires. Make sure to secure it tightly and that there is a hole to let cables pass into the container.
  2. Then you can install the neutral and ground busbars. These busbars are used as breakers, metallic strips that are used in switch equipment to let current distribute itself appropriately throughout the equipment. You may also use appropriate insulation materials to make sure that the potential between the neutral and safety grounding busbars is always zero. This is essential for breaking and making circuits between generators by detecting the power differences between them.
  3. Connect the busbars to your installation. You may use stranded wire to prevent significant voltage drop between the breakers for the automatic transfer switch and the rest of your installation. 
  4. If you want to, you can add LED indicators between the breakers and incoming power supplies. This will help you detect if a breaker is closed or not. 
  5. Add the automatic transfer switch itself and the AC meter to the installation. The transformer that alters current should be around the output of the automatic transfer switch. The AC meter should detect how much voltage the installation is using. Keep it tight and secure to prevent voltage leaks and other issues. 
  6. Test your setup for safety before implementing it. If there is any excess heat from the resistors that may cause issues such as overheating, make sure to fix that by changing resistance or using more safety precautions such as changing the setup of the circuit breakers.

How do Automatic Transfer Switches Work With Multiple Generators?

ATS setups can use multiple generators to protect electrical operations that occur simultaneously in areas that are far apart from one another. These systems use multiple ATS setups to act as though there were a single ATS with a single generator. This lets the ATS systems act with multiple generators for purposes of, for example, different buildings or different types of architectural designs.

Each ATS needs a controller to make sure the power transfers safely and effectively between the utility sources and the generators. They need to be tested in both directions and distribute power accordingly. They need to make sure they take into account even the miniscule time differences between powering different buildings or different generators. For some operations, even milliseconds without power can harm the purposes of different building designs.

What Types of Automatic Transfer Switches are There?

In addition to soft loaded ATS designs, there are open transition, closed transition and static transfer switch designs for different purposes of transfer switches. Open transfer switches, including ATS ones, or break-before-make transfer switches operate by ceasing contact with one source of power and creating contact with another. This prevents unwanted backfeeding, the flow of electrical current in an unwanted direction, as well as using power from two sources that compete against one another.

By contrast, closed transfer switches or make-before-break switches transfer power without causing any sort of interruption. This is especially useful for buildings and electrical equipment that rely on their power in such a way that even an interruption for a fraction of a second can be harmful. Unlike open transfer switches, closed power switches find ways to load power to make sure the generator can and does supply power before breaking the connection with one source of power to the other.

These types of switches are more complex than open ones, and they need to monitor power flow during transition and divert power – using bypass capacitors – to prevent back flow.

Engineers refer to different sources of power as synchronized when the voltage difference between them are less than 5% or have frequency differences of less than 0.2 Hz. Isochronous governors control this shift in power. The closed switches ensure that these transfers of power can occur under these circumstances and sometimes within times of less than 100 milliseconds. These switches will turn into open transfer switches if the closed transfer isn't possible.

Finally, static transfer switches use semiconductors like Silicon-controlled rectifiers to transfer loads between sources. These setups use the energy of the motion of electrons in these semiconductors to allow the transfer to happen almost instantaneously. They're very reliable and perform independently of the sources of power available, but they must be tested to protect the load from interruptions in power frequency.

Motor Starter Role in ATS

When determining the size of ATS and the automatic starting control principles that need to be used, engineers take into account different types of current. A motor starter and the purpose it has in the system governs the inrush current, the amount of current that the circuit uses to energize an AC-powered device the first time you apply current to it.

Homemade Automatic Transfer Switch Circuits

Homes use ATS as part of their emergency system through these methods. Engineers and architects design them to make sure they're dependable, adaptable, efficient, effective and not susceptible to damage. They routinely test the ways they transfer loads in homes to make sure they operate appropriately.

ATS designs vary from use over a few circuits to an entire home when used in house architecture. Two circuit breakers can work together simultaneously to ensure the switch happens without voltage or power loss. Automatic transfers perform this switch, and, after they have restored power, they use a "cool down" process to prevent overheating.

Companies like Generac generally offer 100 amp or 200 amp ATS systems. They can cost upwards of $600.

Generator Automatic Transfer Switch Installation

Power stations use enclosed circuit breakers just like houses do for their needs. Research or equipment that rely on continuous power use Automatic Transfer Switches in more complicated arrangements to meet their unique needs. The generator automatic switch installation process needs to use these arrangements the meet the individual needs of households and buildings.

Electrical engineers can create these designs for the facilities themselves and create control rooms for their different purposes such as in hospitals or data centers. These could also be used in emergency lights that point individuals to exits when necessary, hazardous ventilation to remove toxic chemicals from rooms and even alarms when monitoring facilities for fires.

The way these automatic switch designs work can involve alarms that signal power less. This commands the automatic transfer switches to start up the backup generators, and, after detecting that they've started, the setups distribute power across the building when designing the generator automatic transfer switch installation.

Some ATS manufacturers include APC, Dell, Cummins Power Generation, General Electric and Western Telematic. These companies work to offer transfer switch products for different uses while supporting and maintaining them after installation.

References

About the Author

S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.

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