Modern industrial activity requires flexibility and efficiency to ensure responsiveness to market demand, but that is not the case for the power drive systems traditionally applied to scrap shredders. Danieli Automation and Danieli Centro Recycling have jointly created Smart Power Technology to realise energy savings, improved productivity, better use of wear components and reduced impact on the MV distribution network.
The graph illustrates the total power demanded by a 4000 hp motor during normal production with traditional Liquid Resistance Starter (LRS).
Analysing the power requirements, we observe the following effects:
- Many intervals at no load or reduced load (red boxes).
- Average power is low compared to the motor nominal power, with a real motor-service factor limited to 65%.
- Repetitive, uncontrolled power peaks above the motor maximum overload.
- Slow response of the LRS to tune and adapt the rotor resistance to load conditions.
- When using the LRS, there is considerable energy wasted in the heat, quantifiable up to 10%.
Power quality measurement of the LRS solution reveals other effects on the distribution network. For example, the line RMS value in load condition is characterised by rapid transients (swells and dips), due to rapid shredder load variation. Also, the voltage fluctuation influences other motors/ loads – with additional transients in network current and impact on power quality in the upstream distribution system.
A MV/LV inverter with dedicated firmware, in combination with an adaptive control strategy, ensures better productivity without network disturbances. In addition, the Plug&Shred solution makes it simple to convert an existing, fixed-speed direct on-line 3-phase asynchronous motor to a state-of-the-art variable speed drive system.
What does this mean?
- No motor derating for voltage insulation
- No motor derating for power and thermal performances
- No modification required to adapt the motor to the drive
- Easily applicable to existing motors without the need for an expensive output filter
- Eliminates the need for special VFD rated cables
- No Neutral Shift
- Motors do not require surge protection
- Limited Power peak demands
A variable-speed drive for a shredder motor limits the power peaks and provides instant control of the power requested during production. Figure 3 (10-msec sample) indicates power control for a shredder driven by 3000-hp motor.
During production, the motor never exceeds 110% of nominal power, independent of operational and overload conditions. Drive correction is immediate, responding in a few milliseconds (compared to several seconds typical of a LRS response). Power limit can be set in the HMI, according to the energy supply contract and contractual power limit available at different time slots, avoiding power excesses penalty.
- Dynamic motor torque utilization: Motor utilisation will be thermally optimised, using variable torque limits function of the windings temperature.
- Motor control accuracy and response time: Drive controls and manages motor torque in real-time, with high accuracy and immediate response, avoiding mechanical stress for the cardan shaft in case of overload.
- Energy savings: Variable-speed drive energy savings average 8-10%, or up to 15-20% depending on operational decisions.
- Reduced consumption of the wear parts: Adapting the motor speed, power and torque limits to the different scrap density, the wear of the consumable components can be reduced up to 10-15%.
- Production enhancement: The “Adaptive Auto Pilot” in combination with an inverter manages the motor in overload condition, for higher average power.
- Reduced electrical maintenance: Using an inverter reduces maintenance requirements for electrodes, electrolytic solution, and mechanical alignment, and there is no LRS cooling system.
- Reduced impact on the network: Network disturbances due to the shredder will be corrected thanks to real-time power control, smoothing the effect of sudden load variations, sources of swells and voltage dips, hence flicker disturbances.
- Higher input power factor: A MV/ LV drive ensures a higher line-power factor in the full range of speed, not requiring a power factor compensator.
- Input harmonic suppression: Plug&Shred drive meets harmonic regulator’s guidelines IEEE-519, without the use of harmonic filters.
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