A rough sine wave is an AC waveform that roughly approximates a pure sine wave. It can have a stepped envelope shape, be represented by a sequence of rectangular or trapezoidal pulses of different polarity. An approximate sine wave AC signal is present at the output of most uninterruptible power supplies (UPS) in use.
Assignment of uninterruptible power supplies
The main task of the UPS is to save the AC voltage supplying consumers in case of its sudden loss in the network. They do not perform the function of mains voltage stabilization, but are emergency sources of AC voltage with an approximated sinusoid. Uninterruptible power supplies are not intended for continuous operation of the equipment connected to them in the event of an accident on the mains voltage lines.
UPS must keep consumers running long enough to complete processes that cannot be interrupted suddenly. During operation of the UPS with an approximated sine wave, conditions will be created for a normal load shutdown. This will keep it working for future use. You can often find a reference to a UPS-uninterruptible power supply. UPS in it is an abbreviation of the English designation UPS.
Classification of sources
Depending on the requirements for the quality of the supply AC voltage, UPS-uninterruptible power supplies can be divided into several types:
- off-line sources that are emergency sources of a backup type;
- line-interactive or line-interactive sources capable of stabilizing the mains voltage with slight deviations from the nominal value, and switching to the emergency power supply mode with an approximated sine wave when it is completely lost;
- on-line, or UPS with double conversion of electrical energy, providing the connected load with stabilized mains voltage in case of significant voltage fluctuations and performing the functions of an emergency source when it is completely turned off.
An uninterruptible source of any kind includes a rechargeable battery, which is a store of electrical energy. Depending on the requirements for power quality of the devices that are the load of the UPS with an approximated sine wave,certain circuit solutions are applied.
UPS off-line
Passive UPS of this kind is the source of standby action. Its switching device, under normal mains voltage quality parameters, provides direct connection of the load to the mains. In the event of a short-term power failure at the UPS input, the switching device transfers the power supply to consumers in emergency (backup) mode. The source of electrical energy in this case is the built-in battery.
Its DC voltage is converted by the inverter into a modified AC voltage sine wave, which is supplied through the contacts of the electromagnetic relay of the switch to power the load. To recharge the battery (battery) is the voltage rectifier of the external power supply. To protect it from high-frequency interference that occurs during the operation of the pulse inverter, the UPS is provided with a filter. Its elements also protect load devices from short-term power surges.
Line interactive sources
Line-interactiv A UPS, unlike a standby source, is able to stabilize the mains voltage when the load is directly connected to it through the contacts of an electromagnetic relay. This is ensured by using an autotransformer in the input circuit. Switching of its windings is performed automatically depending on the magnitude of the mains voltage. With its increase or decrease, the alternating voltage at its outputmaintains the nominal value of 220 V. The autotransformer of the most common UPS of this type regulates in the range (150-270) V of the change in the voltage of the external supply network.
Switching of the contactor and transition to the emergency power supply mode of the load with a modified sinusoid occurs when the mains voltage changes excessively, beyond the regulation range. The regulation accuracy is determined by the number of taps (steps) of the autotransformer. In the emergency mode of supplying load devices with an approximated sinusoid AC voltage, the operation occurs in the same way as in an off-line UPS.
On-line UPS
Double conversion of electrical energy in uninterruptible power supplies of this type allows you to get an approximate sine wave of the output AC voltage, approaching in shape to a pure sine. The mains voltage converted to DC by the rectifier is then converted back to AC by the inverter, which is used to power the load.
The built-in battery is charged by the direct current of the rectifier and is put into operation in emergency mode. The accumulated DC energy is converted by the inverter into an output AC voltage that supplies the load equipment. This type of source circuit allows you to connect an additional external battery for the UPS.
When the mains voltage is restored within its normal value, switching occursload supply from the primary AC network. The electrical capacity of the battery for the UPS determines the time during which consumers can work with a sudden loss of mains voltage.
UPS specifications
The choice of UPS is made on the basis of its technical characteristics. The technical characteristics that determine the quality of fulfillment of the requirements for products of this purpose include:
- power at the output of the source with the connected load, measured in volt-amperes (VA) or watts (W);
- output voltage range with load connected, measured in volts (V);
- transfer time, which defines the time to transfer power to the load in battery standby mode, measured in milliseconds (ms);
- input mains voltage range within which the UPS is able to stabilize the output voltage without switching to battery backup mode;
- nonlinear distortion factor, which determines in percentage terms the difference in the output signal shape from a pure sinusoid;
- operating time in stand-alone battery backup mode with a connected load, determined by the electrical capacity of the internal battery;
- service life of own batteries, depending on the technology of their manufacture.
When choosing a UPS, the price of an uninterruptible power supply also plays an important role. It directly depends on the circuitry solutions used by the developer when creating the model chosen by the consumer, and ranges from two to dozensthousand rubles.
Applications
UPS uninterruptible power supplies can be used in conjunction with active load - devices that include switching power supplies.
UPS is not used to power consumers with a significant reactive inductive component (power network transformers of old models of electronic devices, motor windings, domestic heating boiler pumps). Each of the existing uninterruptible power supply schemes is designed for a certain range of consumers, taking into account its advantages and disadvantages.
Conclusion
The article gives a brief overview of the current uninterruptible power supplies with a modified sine wave. The circuit solutions used by the developers during their creation are considered. After reading the material presented, the reader, who is interested in purchasing a UPS to solve his problems, will be able to study the set of documentation for it without experiencing any difficulties. At the same time, one should not forget that a high product implies a higher price when buying it.
