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Regenerative system technology provides a concrete solution to reduce operating costs of electrical equipment, especially when applied to cranes.
To understand better how a regenerative system works, let's start with the concept of "electric braking".

Electric braking applied to a hoisting motor - Principle

A hoisting motor, controlled by a frequency converter, operates with an intermittent periodic duty-cycle. It accelerates from standstill to full speed and, as a consequence, it has to be decelerated (by electric braking) in a certain range of time.
In normal applications, electrical power used by the motor is regenerated during the deceleration phase. Hence, the motor decelerates by applying negative torque to the motor shaft.
Because frequency converters are able to deliver power only from AC to DC bus, they need to "store" this regenerated energy by charging DC bus capacitors.

This storage capacity is very limited and, to avoid running into an over-voltage fault (due to the overcharging DC bus), energy should be quickly led to a resistor through a braking chopper, where it is dissipated as heat.

 

Active Front End (ACF) system overview

A regenerative system would solve such a waste of energy and it can be the perfect solution, provided that the upstream grid network is capable of receiving that recovered energy.
Danieli Centro Cranes have a reliable solution to employ the regenerative systems on its process cranes.
This system is usually comprised of:

• Active Interface Module: A line-side power component. It mainly contains a clean power filter with basic radio interference suppression, a line reactor and other devices needed for complete system functionality (e.g., pre-charging circuit for Active Line Module).
• Active Line Module: This device connects the power supply network to the drive line-up. It generates a DC voltage used to power the connected Motor Modules.
• Motor Modules: Also known as the "frequency converters", these units receive power supply from the DC bus, consequently controlling the motors.
• Control unit: To supervise all the technological functions, the system needs to be managed by a control unit. This control unit connects all the devices to the crane PLC, performing all the safety functions (e.g. safe torque off...). The number of control units depends on the architecture of the system and it changes from crane to crane.

 

Danieli Centro Cranes Concept

Danieli Centro Cranes' philosophy starts from the fact that all cranes involved in the process productivity are directly involved in the plant's performance.
Because of this, process cranes (e.g. Teeming cranes, Charging cranes…) need to perform at an increasingly high level of proficiency and reliability to avoid any kind of production downtime (a stop to production = money lost!).
Thanks to its years of experience in the field, Danieli have created a standard redundant design merging together the high technology of regenerative units with the reliability of a double active interface that merges together the AFE redundancy with a double DC bus.
Motor modules are evenly distributed, depending on the power, in two different DC buses.
In case one AFE stops working, these DC buses can be linked together (through a combination of manual switches) to ensure the continuity of service. Moreover, a failsafe PLC is always used to manage the system, with a dedicated HMI installed inside electrical equipment.

It helps maintenance people to understand what has happened and what actions need to be taken.

The manual switching satisfies both highly trained and less-experienced professionals, ensuring a simple and fast response in case of necessity. This is the most appreciated feedback from customers using the Danieli regenerative system.

Economic improvements

Thanks to remote assistance, included with the supply of all Danieli cranes, an estimation of the cost saved by adopting such AFE system, based on an application case, has been provided for reference in terms of generated energy.
 

Item	Absorbed Energy (kWh)	Generated Energy (kWh)	Balance (kWh)
Main hoist - Drive 1	9954	3243	6711
Main hoist - Drive 2	9981	3237	6743
Aux hoist - Drive 1	1871	485	1385
Aux hoist - Drive 12	0,5	0,1	0,4
Bridge - Drive 1	1805	582	1223
Bridge - Drive 12	3615	350	3266
Main trolley - Drive 1	991	87	903
Main trolley - Drive 2	109	8	101
Aux hoist 2	330	76	254
Aux hoist 3	219	15	204
Aux trolley	30	1	29
Total:	28906	8884	20819

In this study case we see that the savings on the order of the 30% of the absorbed energy have been measured.
Indeed, while providing a regenerative system with an installed power of at least 500kW is giving outstanding savings in terms of energy and OpEx (operating expense) reduction, the capital expenditure is minimized from such technological configuration as well.