Many times we have wondered if we should use a servo drive or a frequency inverter for a certain application. The answer is always the same: it depends on the application. And since not all applications can be analyzed, what we will do in this first article is to describe both types of control, the differences between open and closed loop control, as well as the main elements associated with the controllers (motor and encoder) and in The following article will compare the main features of both control systems. With this information we can choose the solution that best fits our application.
El servo drive o servo drive It is the control element of the servomotor. Said control can be supplied integrated with the motor or separated from it. The first is the case of small servos used in aeromodelling o robotics that normally supply a small torque and have their operation limited by a mechanical stop to a maximum range of 180 or 270 degrees. As we have said, the control can also be found separately from the motor. It is the usual case in industry, where the servomotors must supply a higher torque. The most commonly used servomotors today are permanent magnet motors (permanent magnet synchronous motors PMSMs or brushless AC motors BLDC) which, due to their constructive characteristics, have very fast responses to control signals and little inertia. The main manufacturers SIEMENS, ABB, OMRON, SCHNEIDER, etc., have combinations of servo drives and servo motors that cover a wide range of powers.
El Variable frequency drive, or the acronym AFD It is the control element of the asynchronous motors, usually squirrel cage motors. Depending on the control algorithm used by the drive, we can distinguish between scalar flux control (the V/f ratio remains constant) and vector flux control. This second control algorithm is much more complex but also much more precise and is capable of correcting its output based on the load applied to the motor. In frequency inverters, the control is not usually integrated into the motor, although there are exceptions such as the SIMOGEAR series of SIEMENS gearmotors that integrate the SINAMICS G110M frequency inverter into the motor. Asynchronous motors have a higher inertia and a lower peak current than servomotors.
open and closed loop
Open-loop control systems only take into account the input command, while closed-loop systems also take into account the effect produced by the control on the motor; Therefore, there is a feedback. In open loop control systems we cannot be sure that the desired speed or position target has been achieved since we have no information on the change in motor speed or position. However, in closed loop control systems and thanks to the use of encoders o resolvers the control knows the position of the motor at all times and can make the necessary adjustments to achieve the desired position or speed.
In the following article we will see that the characteristics of servo drives are more suitable for one type of application while those of frequency variators are better suited for other types of applications.
The key is in the very definition of a system,
The use of servos or not is defined by the need to control the position of a moving element or to have high dynamics in an operation.
Speed and positioning lead you to the use of the servo obligatorily together with synchronous motors.
Asynchronous squirrel-cage motors have their physical and thermal limitations that define them for continuous motion applications without great demand in the number of starts and stops. Positioning is very difficult with them.
It is best to consult a specialist who performs the kinematic calculations of the movement or application. Even using servo systems, a miscalculated application can lead to failure.
In addition to the specialists indicated in the article, there are other experienced ones such as Lenze, Bonfiglioli B&R etc… servos are the F1 of motors and in this, as in the automobile, the big manufacturers do not always have all the solutions. (This year yes…).