With the Pxt series, Michael Koch supplies active energy management systems for electric drives. As active connecting elements between inverters and storage units, they are characterized by a wide voltage range and independence from drive electronics manufacturers. The devices meet the requirements for higher energy efficiency, uninterrupted energy supply, load peak reduction and reduction of grid perturbations. There is now an additional plug & play feature: load peaks are intercepted at a user-defined level via the 'PxtMX' plug-in module. The 'Pxt' product family, which can also be pronounced 'P times t' (equals energy), thus achieves greater functionality.
functionality.

The system temporarily stores surplus or required electrical energy from the drive system, largely independent of the mains. As the connecting element between the drive and the storage unit, it regulates the energy balance of the drive system, whether for a single drive or for several axes in the DC link system. The objectives are manifold: from simply saving electrical energy, i.e. increasing energy efficiency, to adapting a machine to a weak power supply infrastructure.

Active energy management

The tasks are performed by two active basic devices that are connected directly to the DC link of the drive controllers, i.e. frequency inverters or servo controllers.

The active buffer module for DC intermediate circuits 'PxtFX' is designed for specially developed electrolytic capacitors. With a maximum storage voltage of 450 V(DC) and a maximum current carrying capacity of 40 A for one minute or a continuous current of 20 A, a peak power of 18 kW can be realized.

The active energy management device 'PxtRX' can handle storage voltages of up to 800 V(DC) with a current carrying capacity of up to 60 A peak current for 45 s or 30 A continuous current.

The dimensioning of the system is the responsibility of the application engineer: The power required by the drive system in the worst case of the load cycle as a product of voltage times current must be just as suitable as the available energy as a product of power times time. This is where the modular system comes into its own, because the power output is variable thanks to the parallel connection of the active devices. This is also the case for the amount of energy by adding suitable storage units. For example, a system starts with a PxtFX device with 2 kJ of storage and theoretically any number of PxtRX devices can be connected in parallel in conjunction with an equally unlimited number of double-layer capacitor modules.

While the 'PxtFX' is designed for short cycles, its bigger brother 'PxtRX' is designed for use with higher energy density storage (see Fig. 1). In combination with electrolytic capacitors and qualified double-layer capacitor modules, a modular system is available that enables application-specific solutions with outputs up to over 200 kW and energies up to over 2 MJ.

The core portfolio of active energy management devices with storage units is supplemented by peripheral products: from passive and active discharge units to mains-independent 24 V emergency power supplies and systems based on individual devices through to ready-to-connect control cabinet solutions.

A different way to reduce peak loads

The little black box makes all the difference: the 'PxtMX' module enables peak load reduction via Plug&Play. To do this, the module is placed on the active energy management device 'PxtFX'. The result is a unit with 4 kJ of energy for automatic load peak reduction for low power requirements.

© Michael Koch

Previously, communication with the drive was necessary to reduce peak loads: As a command receiver from the Drive Controller, the system provides the necessary energy precisely when the inverter actively requests it. This type of peak load reduction was therefore controlled via the drive controller, but without the direct influence of the power requirements from the grid. This is now changing with the 'PxtMX' plug-on module. In addition to load peak reduction, it provides more connectivity via a fieldbus connection.

The small black and red box is plugged and screwed onto the energy management device. It offers six digital I/Os, USB and K-bus connection, fieldbus communication and an external 24 V supply. The latter enables operation even when it is not plugged in. Status indicators via LEDs, boot loading and reset options help the user directly on site. A micro SD memory card enables firmware updates, among other things.

However, the central feature of the 'PxtMX' is the load peak reduction via Plug&Play. In conjunction with an active energy management system, the module makes it possible to reduce the power consumption on the mains side in cyclical applications to almost the level of the required average effective current without any further intervention. The overall system is therefore based directly on the current variable; the module is controlled depending on an effective current value between the supply network and the intermediate circuit of the drive. The basis for determining the effective current value is again the measurement of the input mains current of the drive system. Koch considers this to be the decisive variable, as it influences the electrical infrastructure of the entire drive system. Load peaks are decisive for the dimensioning of fuses and cables. The task is to avoid these peaks in relation to the power grid, i.e. to smooth them out. Other variables such as power or voltage are secondary to the current.

The plug-on module regulates the active energy management devices and contains its own independent computer structure for this purpose. This hardware independence enables, among other things, fine and correspondingly fast control in order to detect even short current peaks in the millisecond range and include them in the calculation of the effective current value. Algorithms are also used to calculate the target current value and the corresponding specifications for the active Pxt device(s), which further accelerate the process.