AC variable frequency drives vary in horsepower from 1 to 10,000. AC variable frequency drives are energy efficient. They only use the amount of electrical energy required for the job. In rolling mills, it is very common to use a fraction of the horsepower that is available. By having an energy efficient AC variable frequency drive, you only use the energy that is needed at the time. Of course, you can vary the torque on the drive at the control console or main control cubical. Sometimes very little torque is required for a large drive during feed up or light reduction. The main advantage of an AC variable frequency drive is that the motors are enclosed. Large motors can be water cooled. Another advantage is that you can vary the speed. The speed and torque can be automatically controlled. AC variable frequency drives can have a substantial amount of safety features and many drives can withstand overloads up to 100% for a period of time. While the motors are inexpensive, compared to old DC motors, the drive itself is more expensive, but of course, this technology is improving with cost and quality every day. One of the costs overlooked in AC variable frequency drives is regeneration. For a rolling mill to be efficient, the uncoiler tension reel’s large motors may act as a brake, and the recoiler tension reel is under substantial tension. Therefore, it is best to take the energy generated from the uncoiler to the recoiler. This saves a substantial amount of electrical energy. However, this becomes more complicated than an AC to DC drive. Another disadvantage of an AC variable frequency drive is it does not have the ability to stop quickly. Rolling mills have a tremendous amount of kinetic energy, and to stop the mill quickly, in the event of an accident or strip breakage, it is best to have a regenerative system to bring the equipment to a stop quickly. This means you must have substantial expensive resistors and electronics in the drive system.