Motion System Design

Driving for energy savings

Here we explore how intelligent drives boost efficiency — and examine an unwinder application to understand how the systems work.

Starting position

Energy is an increasingly scarce and expensive commodity that significantly impacts manufacturing costs. However, modern drive systems can reduce expenditures on electricity. Case in point: Unwinding machines prevent the wrinkling or tearing of material by regularly decelerating reels to predefined forces and speeds. In most arrangements, a brake circuit discharges the excess power. In newer systems, the energy can be saved and fed back to the power supply by employing a servomotor as a regenerative brake.

Figure 1

During the production process, paper, sheet steel, and textiles are supplied on a reel and unwound prior to processing. To prevent problems, the system's unwinding reel is decelerated to synchronize circumferential speed with machine speed, while maintaining constant web tension. The variable reel diameter dreel and the constant web tension Fweb determine reel braking torque:

Economic and complete

Mreel = Fweb × dreel/2

A wide correcting range for Mreel must be controlled or regulated through a tension controller appropriate for the roll diameter. Despite disadvantages, hydraulic or eddy-current brakes are often employed here.

The disadvantages of simply using brakes are most apparent during startup and emergency stops. To prevent tears, material cannot be accelerated at its maximum permissible tensile stress: Acceleration time is greatly restricted by the reel's starting diameter. In addition, during emergency stops (as when material tears) the reel is stopped over a finite amount of time. The reel is continuously supplied with power: P = Fweb × Vweb

where Vweb = Web speed

Figure 2

This is transformed into unusable heat by the brakes during slowdowns, and significant energy is wasted.

Control and inverter

Center unwinders are a newer design that, when equipped with regenerative systems, reduces this waste. Let us assume that we have an unwinder to process paper spools with a weight of 3.6 ton, a starting diameter of 1.5 m, and final diameter of 0.15 m. Maximum web speed is 8.3 m/sec. Web tension is measured with a load cell and is maintained at 1,000 N. Instead of brakes, motors control web tension and rapidly decelerate the reels. Then they collect the diverted energy and feed it back into power lines with high efficiency. Running 6,000 hours per year uses 50,000 kWh annually. If this energy is converted with an efficiency of 40% and costs roughly $0.14/kWh, yearly energy savings are about $2,800.

Figure 3

In addition, some permanent-magnet direct-drive motors can control field intensity for a constant power range; speed is increased as torque is decreased, so that the ever-shrinking inertia of an unwinding reel can be spun with high efficiency.

Web tension control

Now, required inverter output power is the product of torque at standstill multiplied by no-load speed when the motor is operated without field weakening. The characteristic curve of a traditional synchronous motor may suggest that it satisfies the same requirements as a motor with field-weakening control. However, a specific example proves this misleading: If the torque × no-load-speed product is 13 kVA for a given field-weakening motor, it can reach 53 kVA for a comparable traditional unit. In short, the traditional motor requires an inverter that is four times bigger and more expensive.

Figure 4

Modular servo inverter systems allow selection of both supply and inverter modules in various power ranges. Assume that we have a compact 40-kW supply module feeding a single 600-Vdc bus for four inverter modules. This is sufficient for two complete center unwinders. Some modules are also capable of feeding motor-saved energy back into power lines, while other inverter modules monitor and control motors in accordance with assigned setpoints.

Figure 5

Worth the trouble?

Where control cabinets accommodate drives for multiple unwinders, one drive-integrated PLC — plugged into an inverter — monitors and regulates all operating modes for the unwinder drives. Communication and the exchange of set points and actual values between the PLC and individual drive modules go through an integrated, synchronous communications bus to facilitate unwinder interconnection.

Where a PLC processes communication via Profibus with the machine control terminal, another PLC in a central control cabinet synchronizes the master set points for the entire system — because Profibus cannot exchange synchronous data. This PLC converts the master set point and then distributes the value to all participating PLCs in other control cabinets via a synchronized CANbus. In turn, the Profibus exchanges command and status information for the unwinders with the control terminal. Some of these PLCs are programmed utilizing programming languages in accordance with IEC 61131.

Figure 6

The exact reel diameter must be determined to hold the web tension at a predefined value. Some control systems can calculate this reel diameter. Where reels with varying starting diameters are used, it makes sense to determine reel diameter automatically using the machine master value. Once the diameter is determined, the required torque value is assigned to the web: Mreel = Fweb × dreel/2

Here, speed-related nonlinearities and reel friction are also considered; a PID controller can balance unknown factors and system-variable changes. Motor torque control is implemented through a digital speed controller with limiters. Here, a set point 0 in the controller prevents the reel from accelerating in an uncontrolled manner (during a tear or other major issue) by bringing the reel to a stop and holding it.

In addition, web tension can be reduced to a predefined value in the event of a standstill. This control function can be performed through the program with a PLC module integrated into an inverter — and can control up to four unwinders simultaneously, in some cases. In addition to tensile stress control and diameter calculations, the PLC handles:

  • Calculation of remaining material lengths and strength, and time to next loading

  • Rolling of remaining material after a reel change

  • Communication with higher-ranking controls

The value of using regenerative motor braking depends on overall unwinder efficiency. This efficiency is determined not only by web speed and tension, but also reel diameter and motor speed. Other factors are motor and inverter energy losses, and the energy requirements of control-cabinet cooling systems.

Figure 7

Efficiency for our example system exceeds 50% at the start of the process. As the reel's material radius shrinks, required motor torque (and loss-incurring motor current) decreases, and efficiency slightly increases. As speed increases, a portion of the saved energy is used for field weakening; motor core losses (and efficiency) become negative. In other words, power must be taken from the power supply to satisfy energy requirements. Then, the average efficiency for one reel becomes 48.3%. Referencing this as standard efficiency, energy saved per reel is 2.69 kWh in 0.66 hours. Assuming an annual run period of 6,000 hours, savings total 24,400 kWh, which correspond to a cost savings of more than $3,400 per unwinder per year.

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