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What is servo drive and how does it work?

What is servo drive and how does it work?

Servo drives are used for both DC and AC motors, but in the industry, these early devices were generally DC. Because for many years high current control was done through SCRs. When transistors found the ability to control high current and convert it to higher frequencies, AC servo drives became more popular. Older engines had a cooling fan attached directly to the shaft when the engine was running at low speeds.

The fan could not do the cooling job. Newer motors have separate fans that are installed to allow for the best cooling results. The constant voltage source feeds this fan so that the maximum RPM at any moment does not depend on the speed of these devices. One of the useful types of motors in the self-control system is a permanent magnetic (PM) type motor, its motor voltage can be AC or DC. This type of engine is similar to other previous types.

In both of them, you can see the housing, rotor, and stator, which are very similar to the previous motors. The main difference between the above motor and them is that a reduction in the number of gears has been created to move more loads faster from the stationary position. This type of PM motor has an encoder inside the motor housing.

This action ensures that the device accurately indicates the position or speed of the motor axis. The response speed of servo drives to the error signal is high and they quickly accelerate the load, and the main difference between these devices and ordinary motors can be seen in reading the output feedback. This means that servo drives can be called closed-loop control systems (loop close) and electric motors can be called open-loop control systems (loop open).

A feedback control system is a control system that leads to maintaining an assumed relationship between a controlled quantity and a reference quantity by comparing their functions and using the difference as a means of control.

The electrical feedback control system generally relies on electrical energy to work. The important characteristics that are usually required for such control are:

  • quick answer
  • High accuracy
  • Careless control
  • Remote operation.
servo drive

Servo drives can rotate from zero to 360 degrees. To rotate these devices, a pulse must be applied on the middle base, which must have a certain frequency (usually 50 to 100 Hz) and its positive pulse width must be between 1 and 2 milliseconds. When the width of this pulse is 1 millisecond, the motor goes completely to the left, and when it is 2 milliseconds, the motor turns completely to the right, and when it is 1.5 milliseconds, the motor is locked in the middle position.

The step-by-step repair process of servo motor!

The advantages of servo drives in industry and the reason for their increasing use in industrial automation include the following:

• Squirrel cage rotors are simple and structurally more robust compared to machine armature windings.

• Servo drives with brushes are not for commutation and do not need permanent repair and maintenance.

• There is no insulation around the armature conductor as there is in the motor, so the armature can spread the heat much better. Because the armature does not have twisted insulated windings, its diameter can be greatly reduced to reduce rotor inertia. This helps to prevent overshoots in the servo drive mechanism.

A servo drive is a two-phase induction motor, except for special aspects of its design;

The output mechanical power of a servo drive varies from 2 watts to several hundreds of watts. Motors larger than this power are very low efficiency and if they are made with optimal torque and speed specifications, they will be very problematic for use in servo drive applications. Precision servo drives are used in computers, servo drive tools, and several applications that require high precision.


Servo motors can be used in three working modes: position, speed, and torque. In the torque mode, these devices measure and control the N/m value, in the position mode, the position value, and in the speed mode, the RPM value of the motor output. Needless to say, servo drives can control all three work modes at the same time.

servo drive

Today, servo drives in industry and industrial automation, especially in multi-axis machines, including CNCs, are given special attention due to their high accuracy and controllability.

An example of servo drive performance

In the following example, the performance of a servo drive can be analyzed in setting up a packaging line:

In this system, a servo drive from a corporate brand as the main controller controls two other servo drives from the same brand with pulse commands. In this system, the main servo drive controls the system by opening the packaging roll and simultaneously sending a pulse to servo drive number two to move the product carrier conveyor and to servo drive number one to cut the roll, the exact location of which is sensed using a sensor.

The servo controls this process in position mode by holding the unit of pulses per millimeter. The servo drive calculates, measures, and controls the traveled millimeter by generating a pulse and reading its feedback. To avoid position error for servo drives used in this industrial process, a reference or zero point determination sensor has been used for each servo drive.

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