UPS and electricity recovery: how to cross a hedgehog with a snake?
From the course of physics, we know that an 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, mechanical energy can again be converted 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 recovery system, they should be used with great care.
When does recovery occur with a UPS?
The problem arises with certain types of industrial load, most often these are some machines 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 a plant is no longer supplied with power, it can switch to generator mode, start generating electricity during braking and supply it to the input network.
Modern industrial recovery plants often protect by UPS against power failures. For example, we can consider CNC machines used for precision machining of expensive workpieces. The technological cycle should complete correctly, and if the process is interrupted, it cannot be restored and the workpiece will have to be disposed of. It can cost more than one million rubles, if we talk about engineering, shipbuilding and aircraft manufacturing, as well as military and space technology.
Why are UPSs not compatible with recovery?
The frequency converter passes the generated electricity and gives it to the input. The power supply management system should initially assume the possibility of returning energy to the network for useful use. Such a system is carefully calculated and costs more, but it can reduce energy costs and avoid accidents. If several protected UPS units operate simultaneously, the energy generated by one of them can be consumed by neighboring ones. If there are problems with the management and calculation of the load, or if only one installation is operating in the system, the 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.
Where is the way out?
In order not to explode the frequency converter, through which the energy generated by the installation during recovery goes into the system, special modules with brake resistors are mounted. They are connected to the circuit at the right time, dissipate excess energy in the form of heat, and in addition to industrial equipment, they also protect the UPS. The problem, we repeat, is already being solved at the stage of designing the technological complex: the load and the energy 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” by power and it will no longer be able to disable the uninterrupted power supply system.