Print this page Print this page

Overview of Uninteruptive Power Systems (UPS)

A. Bhatia, B.E.

Course Outline

The critical applications such as computers, servers, instruments or life saving equipments require 100% redundancy in every aspect to ensure high degree of reliability and operation. Although the power supply is available from commercial power supply sources, it is subject to disturbance of various types like brownouts, voltage spikes, frequency instability and harmonic distortion, all of which can disturb the smooth functioning of the modern equipments. The UPS can protect the electrical and electronic loads from the damages caused by the utility power supply malfunction and power disturbance of electric loads in the building, etc.

This 4-hour course is a summary extract from the Technical Manual of Department of Army: TM 5-693: Uninterruptible power Supply System Selection, Installation, and Maintenance for Command, Control, Communications, Computer, Intelligence, Surveillance and Reconnaissance (C4ISR) Facilities.

The course includes a glossary and multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the reader will:

Intended Audience

This course is aimed at students, electrical engineers, facility managers, architects, H &S professionals, and energy auditors, technical and sales representatives.

Course Introduction

An UPS is a device that sits between a power supply and a device (e.g. a computer) to prevent undesired features of the power source (outages, sags, surges, bad harmonics, etc.) adversely affecting the performance of the device. It is designed to automatically provide power, without delay or transients, during any period when the normal power supply is incapable of performing acceptably. UPS provides reliable protection against short-term power outages by accessing a battery source as backup. It not only ensures a supply of power in the event of a black or brown out, many of the latest UPS systems actually "condition" the power coming into your system right from the utility source. A UPS, therefore, has the ability to eliminate surges, noise, spikes, and waveform distortions.

This course discusses some basic fundamentals of uninteruptive power systems in detail.

Course Content

The course content is in a PDF file Overview of Uninteruptive Power Systems (UPS). You need to open or download this document to study this course.

Course Summary

A UPS is ideal equipment, which provides continuous power to critical connected electrical and electronic loads even in the event of blackout or variations in voltage. It regulates the voltage to the constant range that is safe for its loads. The principle of the UPS is to keep the connecting loads continuously working without any interruption. Whenever power problems occur, the UPS still supplies power by regulating, or drawing backup power from the storage battery, converts it back to AC power and supplies to the loads. These can be broadly classified into two categories namely: Static UPS and Rotary UPS.

A static UPS consists of

1) Charger or Rectifier - takes the utility AC power and converts to DC power and then charges the batteries
2) Inverter - takes the DC power from the batteries and converts to AC power for electrical and electronic loads
3) Battery - backs up power to supply during the power problems. It can supply DC power to the inverter in case of the UPS cannot take the utility AC power.
4) Stabilizer - regulates the voltage to the constant range that is safe for the electrical and electronic loads

Not every UPS is the same. There are different designs and technologies based on the type of equipment you need to protect and the amount of power, or the load, the unit will be required to handle. Typically, with stand alone PCs offline UPS may suffice the needs. Also known as a standby UPS, this device turns on backup battery power in the event of a power failure. The next level of UPS protection is line-interactive. Its called interactive because, unlike the off-line technology, it filters a line current to the power load and converts a trickle of DC power to the battery to keep it fully charged at all times. When the power fails, this UPS doesn't need to turn on, but rather switches from the standard utility source to the backup battery. The highest level of protection is the on-line UPS. This UPS continually charges the battery source so that if the power goes down, backup is instantaneously available, with no start up or switching required.

A rotary UPS consists of a synchronous motor driving a synchronous generator with a large flywheel. During normal operation the motor drives the flywheel and the synchronous generator at constant speed proportional to the power supply frequency. When input power is momentarily lost or degrades, the flywheel supplies its stored energy to the generator and the frequency is maintained within the required tolerance for duration depending on the flywheel inertia. Rotary UPS systems are typically outdated now days due to rapid advances made in the field of electronics; the static UPS have become very popular since it has no moving parts. Rotary UPS are more costly for small capacities but become competitive with static units around 300kVA.

UPS may be needed for a variety of purposes. Before proceeding for the selection and sizing of UPS, the purpose(s) must be clearly known. The factors influencing the selection are evaluating critical loads, switchover time, acceptable delay, backup time and the criticality of the load that the UPS must bear. All critical loads shall be connected to the UPS. If you get a UPS that's too big, then you've overpaid, but your equipment can survive a longer outage. If you get a UPS that's too small, your equipment might not be protected. Therefore, you have to be a little conservative in sizing your requirements. Unfortunately, this costs money and the decision shall be best made in consultation with a professional.


Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.