Uninterruptible Power Supplies

An uninterruptible power supply, or UPS, is a device that provides continuous, acceptable power to its loads regardless of the input power supplied. UPS systems come in many types and sizes.

A UPS is used to provide clean, conditioned continuous power to critical electronic equipment. It can protect the electronic equipment from most conditions experienced on the power system. The UPS will provide power to the load even in the event of a total outage. The UPS supplied power will last only as long as the systems battery bank will allow. These systems are most commonly used to protect computer systems from momentary outages such as those experienced during stormy conditions.

The way a UPS works depends upon what type of unit it is. There are four basic types of UP S on the market. They are: ferroresonant, line interactive, double conversion, and rotary units. While each type has unique features, each will contain the following:

  • Rectifier or Charging Unit - takes the utility AC power and converts it to DC. Also charges the batteries.
  • Inverter - takes the DC from the rectifier or batteries and converts it to AC for use in the computer system.
  • Battery Bank - supplies DC power for the inverter in the event of unacceptable AC input.

The rotary UPS system uses a motor generator (MG) set in conjunction with a static ups. The MG is used to provide total isolation from the external supply source. Under normal conditions the MG would supply 100% of the power for the load. In the event of an outage the MG would continue to spin for a very short period due to inertia. This would allow enough time for the inverter to come on line and supply power to the motor generator. The rotary UPS offers the highest level of protection, but is the most costly unit as well.

A ferroresonant UPS utilizes a ferroresonant transformer to condition and regulate the incoming power. When there is an outage, the unit will utilize the stored energy in the ferroresonant transformer to supply power until the inverter is brought on line by the battery bank. In this unit the inverter only comes on in the event of an outage. Ferroresonant units are used almost exclusively on single phase applications.

The line interactive UPS uses a bi-directional inverter, battery bank, power conditioner and an onboard computer. The bi-directional inverter runs continuously. The inverter supplies AC power to the power conditioner, and charges the batteries at the same time. The amount of load the inverter handles is controlled by the on board computer. In the event of an outage the inverter is supplied by the battery bank and handles all of the load.

Double conversion UPS systems use a rectifier charger, battery bank, and inverter. In this unit the inverter is continuously operating and supplies 100% of the load. If utility power is available the inverter power is supplied by the rectifier charger. In the event of an outage the inverter is supplied from the battery bank, with no interruption to the load.