What to consider when choosing an uninterruptible power supply

An uninterruptible power supply is an important element when building complex systems where security is guaranteed against unforeseen power outages and other problems in the power grid. Under the cut, we ’ll tell you what criteria you need to consider when choosing a UPS.

Now the market is crammed with a variety of devices that differ in both price and quality. Understanding all this variety is incredibly difficult. If the budget is limited, then you need to approach the choice of the most responsible. Therefore, first you should answer yourself a few questions:

- How important equipment are you going to protect?
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- What is the optimal battery life in case of power failure?

In order to answer the questions posed, it is worthwhile to figure out what classes of UPS are now available, and decide on the main criteria that should be considered when choosing a UPS.

UPS Classes

Classes represented on the UPS market differ from each other in behavior in different operating modes and schematics. Allocate:

- Backup or off-line UPS (BackUp),
- Line-interactive UPS ( Line-interactive ),
- UPS with double conversion ( on-line, double-conversion ).

Off-line UPS are considered the most simple and unpretentious. In the normal mode of operation from the network, electricity is fed to the input of such an “uninterrupted power supply”, and then in transit is supplied to the main load. In the event of network problems (surges and voltage drops), the UPS automatically switches to battery operation.

The disadvantages of this scheme of work is the long-term switching of power to the batteries (from 4 to 10 milliseconds). In addition, when the UPS operates from a battery, the equipment is fed not with the usual sine of the network, but with an approximated sine.



The following class of uninterruptible power supplies Line-interactive does not have any fundamental differences from the Off-line scheme. In the event of an accident, the power also switches to the battery, and is spent on it similar (from 4 to 10 milliseconds). The output also produces an approximated sine.

However, in the UPS of this class there is a transformer at the input, due to which it is possible to compensate for the very voltage drops. It is worth emphasizing that UPS class Off-line and Line-interactive are not designed to connect critical equipment.



When connecting critical equipment, it is recommended to use double conversion UPS (On-line UPS). The operation of such uninterruptible power supplies is designed so that the incoming voltage is rectified thanks to the rectifier. After that, the inverter converts the DC voltage to AC. In this scheme, the batteries are connected to the output of the rectifier and the input of the inverter, which ensures an instantaneous transition (0 milliseconds) to battery operation.

Power

One of the most important parameters that you should pay attention to when choosing a UPS is the power of the connected equipment. Underestimation of this factor can lead to very poor results - the UPS may simply not cope with the load in the event of an accident. At the same time, the use of a UPS which capacity exceeds the load imposed on it will be ineffective.

Power applied to uninterruptible power supplies can be divided into:

- Full power is the sum of active and reactive power, as well as the deviation from the current and voltage form from the sinusoidal one.

- Active power is the energy that the load takes from the energy source to further convert other useful energy.

To determine the power of the UPS, you need to know the power factor of the connected equipment. In other words, the ratio of active power to total

As a rule, input and output power factors are indicated in the technical characteristics of the UPS. Input indicates the behavior of the UPS in relation to the electrical network. The power factor at the output is very important, because it shows the power received by the load.

To calculate the capacity of the UPS, which will be necessary to ensure the load, you need to take into account the sum of the nominal equipment consumption and the load at the start of the equipment. At this level, do not forget about the power reserve of 25%, that is, the power of the UPS should be 25% higher than the capacity of the equipment.

UPS efficiency

Determine the efficiency of an uninterruptible power supply is very important because it is the main indicator of the effectiveness of its use. Inefficient UPS operation leads to unreasonable costs.



In addition, efficiency determines how much heat is released into the environment by the UPS. This indicator is important when designing a server. For example, if a small capacity UPS is installed, it will not generate much heat. On the contrary, with a high power “uninterrupted power supply” of several tens of kilowatts, the heat release will be great. To avoid overheating of the equipment, you will have to somehow remove the heat from the room, and these are additional expenses for powerful air conditioners. The bottom line is this: the greater the efficiency of the UPS, the less heat will be generated.

As an example, we present several options for the efficient and inefficient use of UPS:

- In the first case, equipment with a capacity of 50 watts was connected to a 800-watt UPS. The UPS uses about 70 watts for self-sustainment. Calculate the efficiency of the formula and we get 42%.

- In the second case, with a load of 600 W, the efficiency of the UPS will be much higher - 89%. This option is preferred and effective.

Battery life

The battery life of the UPS is the time that the uninterruptible power supply can support the equipment in case of an emergency in the power grid. The battery life is more dependent on the condition of the batteries and the load consumed.

When, in the event of network problems, it is only important to shut down the equipment correctly in a short period of time, then you can stop your choice on a UPS with built-in batteries.

If there is a need for a much longer operating time of the equipment, then it is necessary to calculate the required discharge current of the batteries. To calculate this indicator there is a special formula:



For those who have no time or desire to mess with calculations and take into account many technical and purely physical nuances, our store has a handy tool - UPS Calculator , with which you can determine all the necessary parameters.