Although many of us focus on either the mechanical or the electrical aspects of a motion system, it’s important to have a basic understanding of both. Case in point: engineers, designers, and users who are involved with linear drives and actuators typically have some familiarity with servo and stepper technology. Similarly, for those involved in designing, specifying or maintaining conveyor systems, knowledge of their motor-drive systems is important. One drive technology that is gaining prominence in conveying applications is the variable frequency drive, thanks primarily to the energy savings that they offer.
The terms “variable frequency drive” (VFD) and “variable speed drive” (VSD) are often used interchangeably, although there is a distinction between the two. A VFD controls AC motor speed by varying the frequency of the voltage supplied to the motor, while a VSD can control the speed of either an AC or a DC motor, and can do so via mechanical, electrical, or hydraulic means.
Variable frequency drives convert AC supply voltage to DC voltage through a rectifier. The DC power is stored in a DC bus (sometimes referred to as a DC voltage intermediate circuit), which also provides filtering and smoothing of the current. An inverter then converts the DC voltage back to AC with the necessary frequency and voltage, making it suitable for the motor’s required operation.
The key performance benefit of a variable frequency, or variable speed, drive is just that—the ability to vary the speed of the motor, and thus the conveyor, to match the changing needs of the process. For example, baking and drying applications may require different speeds depending on the type or quantity of product being processed. Similarly, a conveyor that works in conjunction with other equipment, such as robotic workstations or other conveyors, may require position synchronization with the other components, which is achieved by varying the speed.
The speed of an AC motor is directly proportional to the applied frequency, as shown by the equation:
N = speed (rpm)
f = frequency (Hz)
p = number of motor poles
Another advantage that VFDs offer to conveyor systems is the ability to perform a soft start. VFDs can produce high starting torque at very low current, and they provide precise torque control. Limiting the inrush current during startup extends motor life, and limiting torque reduces wear on mechanical components. This is especially pronounced in applications that require frequent starts and stops.
A soft start is a method of gradually increasing the motor current in order to avoid the high current draw (which can be up to six times the motor full load amp current) that typically accompanies startup.
Both of these features—the ability to control speed to meet the process requirements and the reduced inrush current during startup—allow variable frequency drives to reduce energy consumption and provide long-term, sustained cost savings in many conveyor applications.
Feature image credit: Lenze