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Conveyor systems form the circulatory system of today's manufacturing, warehouse and logistics operations. They transport raw materials, work-in-process and finished goods at a speed that needs to be precisely controlled. Too fast and products will collide and tumble or fall; too slow and throughput will decrease and bottlenecks will develop. Servo motors are commonly employed to provide the precise speed control required for these conveyor systems. Unfortunately, servo motors and drives are extremely sensitive to variations in the input voltage supplied to the drive. Voltage sags will cause motor speed to droop, and voltage spikes will result in over-voltage faults and sudden stoppages. Servo motor voltage regulators are specifically designed to provide the smooth, stable power required. This article explains how they provide precise speed control.
Maintaining Constant RPM Under Load Changes
The load on a conveyor belt will vary considerably over its course. A heavily loaded section of a conveyor belt will draw more current than a light loaded section. If the voltage dips at a moment of heavy load, the servo drive will not be able to supply the power needed and will be unable to regulate speed. The belt will slow and a backup or jam may occur.
The quick-acting SCR or relay tap-switching employed by servo motor voltage regulators will hold output voltage within ±3% of the set voltage even when line voltage drops or rises by ±20% or more. The response time of 20–40 ms is far shorter than the time constant of the conveyor mechanism. While the line voltage sags, the voltage regulator will see it through and supply the drive with a stable voltage source. The drive will be able to devote its attentions to speed control without worrying about fluctuations at the input power. The conveyor speed will remain constant.
Eliminating Speed Error from Line Voltage Sags
Most industrial sites have large, heavy machinery, such as HVAC systems, welders or compressors—that cause momentary voltage sags (dips) of 10-30%. These sags will be seen by the servo drive and, due to the responsiveness of the motor's speed loop, will immediately cause a drop in speed. The servo drive will attempt to correct the speed by drawing more current, but until the voltage sag passes the motor will continue to run slowly. The result is a noticeable "hiccup" in material flow.
When a servo motor voltage regulator is installed on the input of a servo drive it becomes a buffer. Stored energy either in the control transformer of the regulator or within a capacitor bank will provide the power needed for short voltage sags of up to 100 ms. During longer sag events the regulator will react and change taps in order to boost the voltage to nominal. The servo drive will never "see" the sag, so the motor speed will never deviate from the desired value. This will be especially beneficial for high speed sorting lines or indexing conveyors where precision placement is critical.
Protecting Servo Drives from Over-Voltage Trips
Just as dips can be harmful, over-voltage conditions can be just as debilitating. An over-voltage fault will occur when power regenerated from an over-loaded motor during deceleration is supplied back to the DC bus. If the line voltage is also high at this time the DC bus voltage could reach the drive's over-voltage trip point. In this case the drive will trip out, stopping the conveyor abruptly and potentially causing product damage.
Servo motor voltage regulators incorporate over-voltage protection which will clip any line voltage spikes or incoming regeneration. These devices are especially useful for applications with frequent acceleration and deceleration, such as palletizing or indexing conveyors, where the risk of over-voltage or regenerative trips is high. The regulator significantly reduces that risk. The regulator will also limit the stress that the rectifier and DC bus capacitor will undergo on the drive. The result will be extended drive life, and consistent conveyor operation without the nuisance of voltage related drive fault tripping.
Enabling Precise Acceleration and Deceleration Profiles
In many applications, such as filling stations or precision assembly operations, the servo motor requires exact acceleration and deceleration to safely position a load without product shifting. These precise moves are made by control of the servo motor torque. When line voltage varies, the drive will also vary current in a futile attempt to regulate speed. This will manifest as an inconsistent torque, making for unsteady acceleration that can cause product misalignment or displacement.
The stiff, low impedance voltage source of a servo motor voltage regulator will ensure the servo drive maintains a consistent torque output regardless of voltage variation. The power factor of these regulators, combined with their ability to provide steady power regardless of the magnitude of incoming voltage sags and swings, will result in smooth—controlled torque. As an example, consider multiple conveyor systems all starting up on a single circuit at the same time. Without a voltage regulator, each servo drive will experience a voltage sag as each system begins to draw current from the circuit. With a regulator each system will draw the needed current, but from a stiff voltage source. Precise application of torque for consistent placement of an assembly or filling of a container will become achievable.
Conveyor systems are an integral part of modern industry. Servo motors, and the controls that make them perform—provide unparalleled control over motion. Servo motor voltage regulators allow these precise servo drives to operate consistently even in less than ideal input power conditions. By eliminating voltage sags and over-voltage spikes, and providing a stable source of voltage regardless of the load conditions of the conveyor system, servo motor voltage regulators allow you to ensure that the speed on the conveyor is always just right. With over 20 years of experience, 50,000m² of manufacturing space, an in-house development capability of core components and having been an official contributor to the drafting of the national industry standards we manufacture servo voltage regulators that give you perfectly controlled conveyor speed. Contact us today.