Multiple Drive Units

General practice in such a case is to use a sufficient number of drive units to reduce chain pull so that it will be within recommended capacity at all points along the entire conveyor length. Occasionally, a case will be encountered where it will prove more economical to use the next heavier system; however, this is the exception.

In a point-by-point analysis of chain pull in a conveyor system, total actual chain pull will be found to be considerably less when multiple drives are used than when a single drive is used.

The consequent reduction in stresses imposed upon horizontal turns, vertical curves, trolleys, drive sprockets and the chain itself has a beneficial effect throughout the system. Operation is more dependable and maintenance requirements are substantially reduced.

In order to successfully operate multiple drives on a single conveyor chain, certain factors must be considered at the outset.

1. Standard electric motors have slightly different speed characteristics.
2. Commercial grade drop forged conveyor chain varies slightly in length for each section, even when new. After chain has been in service, it tends to lengthen somewhat, adding to the length differential.
3. Live loads on overhead trolley conveyors generally vary continuously in weight and location as the conveyor operates. This produces continuously varying horsepower requirements for each drive in a multiple system.

Because of the combined effect of the above conditions, each drive will handle neither the same chain pull nor the same number of chain links in a given period. The solution to the synchronization problem is found in designing an element of "slip" into the drive. Multiple drives with built-in controlled slip work well together. Should on such drive in a system begin to run faster than its mates, it will automatically "slip" and decrease output speed to synchronize the chain throughout the system. If one drive begins to pull more load than any other in a system, the slip characteristic enables it to transfer part of the load to the other drive -  again producing chain feed synchronization. In a properly working multiple-drive system, the drive constantly and automatically self-regulate to the proper load speed relationship.

In constant-speed conveyor systems, it is a simple matter to provide the slip required for multiple-drive synchronization.

In systems employing variable-speed drives, solution to the problem is not simple. The same slip characteristics must be provided throughout the full useable speed range. Three different arrangements have been used successfully by Anchor to solve the variable-speed synchronization problem in a variety of installations.

AC-DC Systems

In the AC-DC system, drives are equipped with compound wound DC motors. All drives draw their current from a common generator. Speed change of the entire system is controlled by a main rheostat, which regulates the excitation current. Generator output voltage, imposed upon the motor armatures, varies accordingly.

Individual vernier rheostats and ammeters are provided to match supply characteristics for each drive.

Eddy Current Clutch

In this system, a standard AC motor is mechanically coupled to an eddy current clutch. Current for all clutches can be taken from a common excitation source. Variation of the clutch excitation current determines the amount of slip in the clutch and, therefore, regulates final output speed of the drive. Degree of slip built into each drive can be regulated through adjustments provided in the clutch.

Variable Frequency System

In the variable frequency system, conveyor speed variation is achieved through the standard AC motors which are electronically connected to a variable frequency generator or alternator. the alternator is generally driven by a separate standard AC motor through a mechanical speed changer. Speed regulation of the conveyor system is accomplished by adjusting the mechanical speed changer, which changes the output frequency of the alternator, which varies conveyor drive motor speed accordingly.

The system inherently provides means of synchronizing separate conveyors. By utilizing synchronous motors for the drives in place of squirrel cage AC motors, and by controlling frequency through a common alternator or several synchronized alternators, separate conveyors can be coordinated to operate at the same speed or at proportionate speeds. This characteristic is of particular importance where parts carried by one conveyor are to be transferred to another.

Because successful multiple drive operation involves other factors, such as drive location and take-up location, these applications should be submitted to the Anchor engineering department for specific recommendations.