What is UPS? -Working & Types of UPS Explained

This article explains the UPS, its application, historical background, types and working principle of UPS with their schematic diagram.


A UPS is a device which provides an uninterruptable power supply so as to maintain the continuity of supply in case of power outage. UPS stands for Uninterruptable Power Supply.

Requirement of UPS:

There are several applications where even a temporary power failure can cause a great deal of public inconvenience leading to large economic losses. Examples of such applications are major computer installations, process control in chemical plant, safety monitors, general communication systems, hospital intensive care units (ICUs) etc. For such critical loads, it is of paramount importance to provide an uninterruptable power supply. Here, comes the importance of UPS. Application of UPS system caters to such critical loads.


UPS system used earlier were based on an arrangement of DC Motor, Alternator and DG Set. This arrangement is shown below.

Historical background of UPS system

This arrangement consists of DC motor driven alternator, the shaft of which is also connected to diesel engine. The three phase main supply, after rectification, charges a DC battery bank and feeds the DC motor as well. The uninterruptable power supply needed is driven from the alternator output terminals as shown in the figure.

When main supply fails, the diesel engine is rub to take over the loads. Since, the starting of diesel engine takes around 10 to 20 seconds, battery bank provides the required power supply to the load during these 10 to 20 seconds with the help of DC motor and fly-wheel. In this way, no-break power supply is extended to the critical loads.

However, this arrangement of UPS system is not used now a day. Static UPS system are more popular these days up to few kVA ratings.

Types of UPS:

The static UPS are of two types:

  • Short-break UPS
  • No-break UPS

In short-break UPS, the load gets disconnected from the power source for a short duration of the order of 4 to 5 ms. For this period, no supply is available to the load.

In no-break UPS, load gets continuous uninterrupted power supply from the power source. There is no any interruption in power supply in no-break UPS. Such UPS are mostly used for large computer installation. In computer installation, a break of power supply of the order of 4 to 5ms is not tolerable at all and hence no-break UPS is the right choice for such applications.

Working of UPS:

The working principle of short-break and no-break UPS is discussed along with schematic diagram in the following section.

Short-Break UPS and its Working:

In short-break UPS, the load gets disconnected from the power source for a short duration of the order of 4 to 5 ms. This type of UPS is suitable for applications where short interruption of the order of 4-5 ms is tolerable.

A simple schematic diagram of short-break UPS is shown in figure below.

Working principle of Short-break UPS explained in detail

A careful observation of the schematic reveals the technology behind this UPS system. As evident, main AC supply is rectified to DC. This DC output from the rectifier charges the batteries and is also converted to AC by an inverter. After passing through the filter, AC can be delivered to the load in case normally OFF contacts are closed.

Under normal operating condition, normally ON contacts are closed and normally OFF contacts are open. Therefore, the power supply to load is extended via main AC supply under normal condition. However, in case of power outage, normally OFF contacts become closed & normally ON contacts become open. This changeover takes time of 4-5 ms. Hence, during this period, load is neither connected to the main AC source (main AC is not available due to power outage) nor the inverter output. Once normally OFF contact becomes closed, load received power from the inverter and filter.

A momentary interruption in the supply to the load can be observed in case lamps and fluorescent tubes are a part of load. When normally-ON switch is opened and normally OFF switch is turned ON, lamps will have a transient dip in their illumination whereas fluorescent tubes will be OFF momentarily and the get turned on again.

When the main AC supply is resumed, critical loads get connected to the main AC supply source through normally ON switch. Again, momentarily interruption in illumination is observed. This arrangement of short-break UPS is also known as stand-by power supply.

No-break UPS and its Working:

In no-break UPS, load gets continuous uninterrupted power supply from the power source. There is no any interruption in power supply in this uninterruptible power supply system. Simple schematic diagram of no-break UPS is shown below.

Working principle of No-break UPS explained in detail

In this system, main AC supply is rectified and the rectifier delivers power to maintain the required charge on the batteries.  Rectifier also supplies power to the inverter continuously which in turn extend power to the load through filter and normally ON switch. Thus, the load is connected to the inverter all the time. This simply means that, power supply to the load is extended from battery power. Though battery receives power from main AC supply but in the event of power outage, the battery will deliver power to the load without interruption as per its rated capacity.

Rated capacity of batter is given in terms of AH (Ampere Hour). If this is 20AH, this means battery can deliver 2 A of current for 10 hours or 4 A of current for 5 hours or 10 A of current for 2 hours.

So, you might think when main AC supply is connected to the load through normally OFF switch? Let us discuss this requirement.

In case inverter failure is detected, the load is switched ON to the main AC supply by turning ON the normally OFF switch and opening the normally ON switch. The transfer of load from inverter to main AC supply takes 4-5 ms as compared to 40-50 ms for a mechanical contactor. Tis is the reason, main AC supply is connected to load through normally OFF switch.

After inverter fault is cleared, uninterruptible power supply is again restored to the load through normally ON switch. The batteries are now recharged from the main AC supply by adjusting the charge at maximum charge rate so that batteries are charged to their full capacity in the shortest possible time.

Advantage of No-break UPS:

Following are the main advantages of no-break UPS:

  • The inverter can be used to condition the supply delivered to load.
  • Load is always protected from the transient that may occur in the main AC supply. Hence, life of connected load is enhanced.
  • Inverter output frequency can be maintained at the desired value.

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