Adjustable transmission ratio drive

The two AC drive systems used on extruders are the adjustable transmission ratio drive and the adjustable frequency drive. The adjustable transmission ratio drive can be either a mechanical adjustable speed drive or an electric friction clutch drive. 

Belt drives use adjustable sheaves. The axial distance between the sheaves can be varied this changes the effective pitch at which the belt contacts the sheave. This, in turn, changes the transmission ratio. The speed is usually varied by a vernier screw mechanism, which is hand cranked or activated electrically. Belt drives are used up to 100 hp. The largest speed ratio is about 10 1, and a maximum speed is typically 4000 rpm. Belt drives have a reasonable efficiency, tolerate shock leads, and provide optimum smoothness in a mechanical drive. Disadvantages are heat generation, possibility of slippage, and relatively poor speed control. In addition, belt drives are subject to wear and, thus, are maintenance-intensive belts generally have to be replaced every 2000 hours. 

Rotatory dosing pumps such as gear pumps or eccentric screw pumps which are electrically driven do not have the aforementioned drawbacks. The control occurs internally in the drive system. The pump size (in ccm/revolution), the transmission ratio, and the individually determined slip factor allow exact correlation between the speed of the drive and the resulting volume flow. The internal control loop of the electrical drive adjusts the set value. The control loop usually consists of an incremental counter, which monitors the speed within the work cycle (2-5 ms). If a difference between the actual increment and the set increment is recorded (contouring error), the power uptake of the motor is adjusted with the aim of no longer having a difference at the end of the next cycle (cycle time). Factors such as viscosity changes in the adhesive result in altered power uptake by the drive. 

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