What is an Automatic Transfer Switch (ATS)?

In this blog post, we will discuss the construction features and operating principles of an ATS. An automatic transfer switch (ATS) is a self-acting, intelligent power switching device governed by dedicated control logic. The principal purpose of an ATS is to ensure the continuous delivery of electrical power from one of two power sources to a connected load circuit. An Automatic Transfer Switch (ATS) will start the emergency generator and maintain the site until commercial power is restored.

Components and theory of operation

Automatic Transfer Switches provide around-the-clock control for starting and stopping the generator along with the ability to switch from commercial and generator power. This occurs through a series of solid-state monitoring devices.

Example of a poorly maintained Automatic Transfer Switch

Controls

An Automatic Transfer Switch is one giant control. It contains several inputs and outputs which determines when to start the generator and transfer the load.

Commercial power monitoring

Even though many ATS manufacturers use various names to describe their systems, they all work in roughly the same manner. The commercial power monitoring is no exception; there are inputs to a solid-state controller, which is monitoring both the presence and quality of commercial power. The controller is looking at voltage values and frequency. There are predetermined points of where the controller will signal for the start of a generator. On some systems, you make these adjustments at external points on the controller itself. On others, you connect a laptop and make the adjustments by reprogramming the controller. Common adjustments you make are percentage of desired voltages and frequency.

Engine start control

The engine start control is usually an output of the controller itself. These can be either normally open or normally closed set of contacts. The set of contacts you use will depend on the generator you are attempting to start. In the normally open configuration, once commercial power goes out, they will close providing a path to the generator start control, and this will cause the engine to start. When using the normally closed configuration, the circuit will open causing the generator controller to start the engine. There is also a time delay feature. This feature allows for a several second delay from the time the power goes out until the generator starts. This helps eliminate unnecessary startups in the case of a small short power bump.

Generator power monitor

The generator power monitor senses generator voltage and frequency. Once voltage and frequency are within limits, the monitor sends a signal to switch the transfer switch. You can make several adjustments to this monitor including percentage of desired voltage and frequency.

Transfer control

The transfer control sends a signal to the transfer motor to transfer to emergency.

Retransfer control

The retransfer control sends a signal to the transfer motor to retransfer to commercial (normal) power. This control also has a time delay, which is based upon how long commercial power has been back up. Most times this is also adjustable.

Engine shutdown timer

This engine shutdown timer ensures that the generator has time to cool down before it shuts down. Some generators have a built-in time down cooler when they start automatically; you may be able to disable this feature.

Other sensors and controls

On older systems, there may be a time delay relay external to the controller itself. Additionally, there may be inductive relays that perform some of the actions that the solid-state components now perform. These tend to be less reliable because of the large amount of moving parts and the relatively high voltage required to initiate the relays, which can cause arcing over time.

Power supply

In order to run solid-state circuits and other devices, a lower voltage is needed to help reduce arcing on the small paths for electricity.

Transformers

The Automatic Transfer Switch uses transformers to bring the system voltage down to a usable level. Many manufacturers will use these transformers to obtain 24 volts, alternating current (VAC).

Transfer/retransfer motor

Most transfer/retransfer motors are 24 volts, direct current (VDC) motors because most single-phase AC motors can only rotate in one direction.

Rectifier

Because manufacturers use DC motor for transferring and retransferring, a rectifier must be used to convert incoming AC to DC for the operation of the motor.

Automatic Transfer Switch Operation

The operation of an Automatic Transfer Switch relies on a series of events all working together to accomplish the transfer of the load to the generator then back.

Commercial power outage

During a commercial power outage, the commercial power monitoring circuit detects a problem with the incoming commercial power. Once commercial outage drops out of the programmed parameters, it actuates a time delay; this can be from almost immediate to several seconds. At that time, the engine start control sends the appropriate signal to the generator to tell it to start. As the generator is coming up to speed, the generator power monitoring circuit looks for the proper voltage and frequency; once this is obtained, it signals the transfer control. At this point, the transfer control sends a signal to the transfer motor causing it to switch to generator power.

Commercial power returns

As the generator is connected to load the commercial power, the monitoring circuit is constantly monitoring for the return of power. Upon the return of commercial power, the monitoring circuit sends a signal to the retransfer control. The retransfer control starts a timer to ensure that commercial power is steady for the desired amount of time. Once the prescribed amount of time has passed, the retransfer control sends a signal to the motor, and the motor will retransfer the load to commercial. At this point, the engine shut down timer starts counting down. At the end of this period, it sends a signal to the generator telling it to shut down. The engine shuts off, and the ATS goes back to monitoring for power failures.

Do they require maintenance?

Automatic Transfer Switches require careful routine maintenance at regular intervals. This depends on the manufacturer, as with any equipment maintenance, testing and inspections are necessary to keeping an ATS working. Typically on a routine service you will do a functional test of the complete emergency power transfer system, a thorough visual inspection checking for loose connection and damaged components. Maintaining your ATS is an important part of ensuring it delivers the electrical power when you need it!

We at Tactical Power Generation Services, LLP are knowledgeable and more than qualified to service, maintain, and repair your Automatic Transfer Switch.