UPS and electricity recovery: how to cross a hedgehog with a snake?

From the physics course, we know that the electric motor can also work as a generator; this effect is used to recover electricity. If we have something massive driven by an electric motor, then when braking, the mechanical energy can be converted back into electrical energy and sent back to the system. This approach is actively used in industry and transport: it can reduce energy consumption, but is poorly compatible with uninterruptible power supplies. In a system with recovery, they should be used with great care.

When does recovery take place with a UPS?

The problem occurs with certain types of industrial load, most often it is some kind of machine tools or other devices with a mechanical drive. They are controlled by so-called frequency converters or servo drives, which in essence are also frequency converters with feedback. When the engine of such an installation stops supplying power, it can switch to the generator mode, start generating electricity during braking and supply it to the input network.

Modern industrial plants with recovery are often protected by UPS from power failures. For example, consider CNC machines used for high-precision processing of expensive blanks. The technological cycle should be completed correctly, and if the process is interrupted, it will not be restored and the workpiece will have to be disposed of. It can cost more than one million rubles if we are talking about engineering, shipbuilding and aircraft building, as well as military and space technology.

Why are UPSs incompatible with recuperation?

The frequency converter passes the generated electricity and gives it to the input. The power management system should initially assume the possibility of returning energy to the network for beneficial use. Such a system is carefully calculated and more expensive, but it allows you to reduce energy costs and avoid accidents. If several protected UPS installations operate simultaneously, the energy generated by one of them can be consumed by neighboring ones. If, however, there are problems with control and load calculation, or only one unit is operating in the system, recovery will affect the UPS. The devices built according to the classical scheme are simply not designed for this: energy passes through an inverter, which begins to play the role of a kind of booster, which leads to an increase in voltage on the DC bus. Virtually no modern UPS is able to fully cope with this problem, after the protection is triggered, it will go into bypass mode.

Where is the way out?

In order to prevent the frequency converter from exploding, through which the energy generated by the unit during regeneration goes into the system, special modules with brake resistors are mounted. They are included in the circuit at the right time, dissipate excess energy in the form of heat and in addition to industrial equipment also protects the UPS. The task, we repeat, is solved at the design stage of the technological complex: the load and the power management system must be correctly configured. You can also connect several UPSs in parallel to a small load - in this case, the recovery is “crushed” with power and it can no longer disable the uninterrupted power supply system.