A common method of coiling and uncoiling processed steel web is center winding. Steel coilers are powered by electric variable speed drives and motors. Large motors are usually selected for coiling/uncoiling applications because of the high torque and speed demands placed on the motor. This blog explores a new technology for the steel coiling process that allows fluid power and drive system engineers to:
- Reduce motor size requirements
- Lower installation costs
- Improve coiling process efficiency
- Minimize hydraulic losses
- Operate at lower noise levels and temperatures
Torque and speed demands
A fine balance between torque and speed is required throughout the coiling and uncoiling process. The coiler motor’s speed and torque change inversely proportional of one and other due to the tension created by the buildup of material. As the roll builds during coiling, its circumference increases, consuming more material with each rotation. To keep up, the coiler must slow down with each added layer. The buildup of material causes the coiling arm to increase. To maintain tension, the coiler must increase torque proportional to the coil diameter. This process requires maximum torque from the motor when the coil is full, and maximum speed when at the core.
How the coiling and uncoiling process works
The coiler buildup is the ratio of full coil original diameter to core/spindle original diameter.
Example: if a 72” roll has a core/spindle diameter of 24”, it has a 3:1 buildup ratio for coiling and 3:1 builddown ratio for uncoiling.
To maintain its tension, the coiler motor has to produce sufficient torque to pull the material at the necessary process tension. An electric motor transmits any remaining torque to the spindle.
To calculate the required torque, multiply the web tension by the coil radius. The required motor torque will increase proportionally to the coil buildup.
In order to maintain tension, a coil needs to match its surface speed to the incoming web speed at all times during the process. To calculate the spindle speed, divide the web speed by the coil circumference.
During buildup, web tension and line speed are constant; therefore, the coiling power requirements remain constant and equal to the web horsepower
The motor must reduce speed inversely proportional to torque during the buildup process. As the motor slows down during buildup, its output power reduces. As a result, once the coil reaches its full diameter, the motor is only running at one third of the speed. The maximum horsepower also drops by one third.
A larger motor is needed to meet the required maximum torque and speed requirements.
Reducing horsepower requirements
Drive system engineers can reduce the required horsepower through the use of different types of electric motors. For example, by using an eight-pole 900 RPM base speed motor, its size can drop to approximately 137 HP.
There are currently many DC electric motors in use. However, they are inefficient and no longer manufactured. They are still used in many steel plants because of how difficult they are to upgrade or replace.
Hydraulic systems are known for their power density and delivery. Electric motors and variable speed drives are great for their programmability and responsiveness. The idea of combining these two technologies has been floated around by fluid power and drive system engineers for years now. Today, concerns about the higher costs of electricity and the CO2 footprint have prompted a reevaluation of these technologies.
Drive controlled pumps
Advancements in new variable frequency drive (VFD) control algorithms, faster programmable VFDs, and more efficient hydraulic pumps allow fluid power and drive system engineers to successfully implement a new technology called drive controlled pumps (DCP).
"DCP hydraulic systems are less complex and more efficient than traditional hydraulic systems. They operate at a much lower noise level and temperature, resulting in quieter and cooler surroundings."
— Rashid Aidun, application engineer, Parker Hannifin
DCP controlled coiling
While traditional methods of coiling use larger motors to meet the maximum torque and speed demands, DCP uses a variable ratio hydraulic transmission to keep the motor size closer to the web horsepower.
To use DCP, the mechanical gearbox is replaced with a hydraulic pump and motor with the same displacement ratio. This produces the same speed reduction. Also, a hydraulic motor with a variable volume that offsets the buildup can be selected. A variable ratio allows the motor to run at full speed while maintaining the constant horsepower requirement.
When the coiler runs at its minimum diameter, the hydraulic motor is set to its minimum displacement, allowing the hydraulic motor to run at its maximum speed. Essentially, when the pump runs at a smaller displacement, it produces a lower torque and a higher speed, making the process more efficient.
DCP technology combines the best features of electrical and hydraulic systems to optimize the coiling and uncoiling process resulting in:
- Smaller motor requirement
- Reduced electric and drive system and installation costs
- Low heat generation
- Improved operating efficiency
- Minimal hydraulic losses
- Fewer, less complicated hydraulic valves
- Lower noise levels
This blog was contributed by Rashid Aidun, application engineer, Parker Hannifin.