A servo system is one that corrects system errors (position, speed, torque) by using a feedback device. Making a motor a servo motor is less related to its structure and operation, but more to whether it contains a feedback in a closed loop system.
Therefore, what types of motors can be used in the servo system? This is not a question that can be answered simply. Because there are many ways to classify motors. And manufacturers usually use different terms to describe the same type of motor. This makes explaining the classification of motors a highly confusing effort. To help you solve this confusion, the following is a guide on motor terminology and a simple explanation about the common types of motors used in servo systems.
There are three ways to classify motors-through current-DC or AC; through commutation-brushless or carbon brush; through the speed of the rotating magnetic field (rotor)-synchronous or non-synchronous.
AC or DC
The basic classification of a motor is whether it is an AC or DC motor, based on the current used. From a performance point of view, the main difference between AC and DC motors is their ability to control speed. In a DC motor, the speed is proportional to the supply voltage (given a constant load, or torque). The speed of an AC motor is determined by the applied voltage and the number of poles.
Although both AC and DC motors can be used in servo systems, AC motors can withstand higher currents and are increasingly used in the industrial servo field.
Carbon brush and brushless
When discussing DC motors, the next point of bifurcation is whether the motor uses carbon brushes for mechanical commutation, or does not use carbon brushes for electronic commutation. Carbon brush motors are generally cheaper and easier to operate. However, the brushless design is more reliable, higher efficiency, and less noise.
According to the structure of the stator, the carbon brush motor is further subdivided into: series excitation, shunt excitation, compound excitation, or permanent magnet. Although the motors used in the servo system are all brushless designs, carbon brush permanent magnet DC motors are sometimes used as servo motors to simplify design and reduce costs. The brushed DC motor used in the servo system is a permanent magnet DC motor.
The brushless DC motor uses electronic commutation to replace physical carbon brushes and inverters, typically Hall sensors or encoders. Brushless DC motors are also used in servo systems.
AC motors are usually brushless, although there are some designs-such as general-purpose motors that can run on AC or DC power sources-do have carbon brushes and are mechanically commutated. The term brushless AC motors is a bit confusing because they sometimes refer to permanent magnet AC motors or permanent magnet synchronous motors. This puts us into the next category.
Synchronous or asynchronous
Although DC motors are generally classified as carbon brushes or brushless, AC motors are more often distinguished by the speed of the rotating magnetic field-synchronous or asynchronous. Recall the DC or AC motors discussed in AC motors. The speed is determined by the frequency of the supply voltage and the number of poles. This speed refers to the synchronization speed. In a synchronous motor, the speed of the motor's rotation is consistent with the speed of the stator's rotating magnetic field. In a non-synchronous motor, the normal refers to an induction motor, and the rotation speed of the rotor is generally slower than that of the stator.
When induction motors are equipped with variable frequency drives, they can obtain the same speed control and performance as servo motors. However, they do not include feedback, so they are not real servo devices.
Brushless AC and brushless DC motors are synchronous, and servo motors are usually used for both motors. In fact, some common high-performance industrial servo motors are 3-phase, synchronous, brushless AC motors.
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