How does a clutch servo work

"What exactly is a servo motor?"

"You papa, where do the little babies come from?" Are you not sure how to answer that? But not only this, but also the next question can embarrass even experts: “What exactly is a servo motor?” What exactly a servo motor is is actually interpreted differently. We are trying to present some terms around the term servo motor.

The term “servo” comes from the Latin “servus”, meaning the slave1. The term "motor" from the Latin "moto", "I move"2. As unpleasant as the origin of the term “servo” is, the term “servo motor” is just as appropriate for a hardworking servant who moves something unconditionally.

If there were no further restrictions, an unregulated belt drive or a simple fan motor would also be included. However, this is not the case in general understanding.

In general, a servomotor is understood as a motor that has at least a control system or can provide feedback on its rotary or linear position. Here are some definitions from literature and the Internet:

  • Servo motor (drive technology): servo motor, servo motor. Any drive system with good controllability.3
  • A servo motor is an electric motor that is current, speed and / or position controlled. It enables preset currents, speeds and / or angles of rotation to be set4.
  • Servo motors are special electric motors that allow you to control the angular position of your motor shaft as well as the speed of rotation and acceleration. They consist of an electric motor, which is also equipped with a sensor for determining the position. The rotational position of the motor shaft determined by the sensor is continuously transmitted to control electronics, the so-called servo controller, which are usually installed outside the actual motor, which control the movement of the motor according to one or more adjustable setpoints - such as the setpoint angular position of the shaft or setpoint speed - in a control loop regulates.5

The model-making servo, which is only referred to as a "servo" and which also sets a predetermined rotational position, is excluded from the term servo motor in most cases.6

“Servo motor” does not describe a specific type of motor, rather servo motors can be of a wide variety of designs. Electro-hydraulic or electro-pneumatic are also sometimes referred to as servomotors (in contrast to the Wikipedia definition). In the following, only electric motors are treated as servomotors.

Permanent magnet synchronous motors, asynchronous motors and brushed DC motors are widely used as servomotors. But even with these engines there is a multitude of terms, some of which are used differently. Here is an overview of some frequently used terms with an explanation:

AC and DC servo motors

AC servomotors are usually referred to as motors that are operated on supply voltages that are generated directly by rectifying an alternating voltage (single-phase or three-phase). These are usually direct voltages from approx. 230 VDC to over 600 VDC. In contrast to this, servomotors that can be operated directly on a low voltage of up to approx. 48 VDC are referred to as DC servomotors7. However, this classification does not allow a statement to be made about the engine type. According to this definition, permanent magnet synchronous motors are used as both AC and DC synchronous motors. Asynchronous motors and reluctance motors are mainly used as AC servo motors; permanent or separately excited brushed DC motors rather than DC servo motors8. Even if it doesn't make sense at first glance, the market also offers stepper motors that receive position feedback with the help of an encoder and are therefore offered as DC servo motors.

BLDC and EC servomotors

BLDC (Brushless Direct Current) and EC (Electrically Commutated) servomotors are technically permanent magnet synchronous motors. These terms are often associated with rather small, block-commutated motors that are operated in the low voltage range. In the meantime, the motors known as BLDC and EC are often controlled in a field-oriented manner, which means that they are in no way inferior to motors known as "AC Servo" in terms of dynamics, noise behavior and energy efficiency.

Servomotors, servo amplifiers and servo drives

Many motors that are offered as servo motors cannot turn themselves because they do not have an integrated servo amplifier. Only servomotors with an integrated servo amplifier can independently control torque, speed or position without external components after applying a supply voltage in servo mode. This is done according to specifications via a bus connection or via digital or analog inputs.

In spite of this, it has become generally accepted that motors that require an external servo amplifier are referred to as servomotors. These still represent the largest proportion of servomotors. The combination of motor and amplifier is usually referred to as a servo drive.


Are all servomotors highly dynamic? Many “servo motor” definitions do not deal with dynamics. Nevertheless, when you think of a servo motor you think of a highly dynamic drive. Mechanical time constants, i.e. the duration from standstill to 63% of the maximum speed without load, are in the range from approx. 1 to a few milliseconds for highly dynamic servomotors. A high level of dynamism is achieved through several factors. In addition to a high flux density and a low rotor mass inertia, the servo amplifier must be matched to the motor winding so that the motor can work dynamically.

Motor feedback

For a long time, resolvers were the measure of all things in motor feedback systems for servomotors. With digitization, magnetic and optical systems that provide fully digital output signals are on the advance. It is important that the feedback system matches the servo amplifier in terms of robustness, accuracy and interface. Motors with a fully integrated feedback system and servo amplifier have the advantage that the feedback system is matched to the controller.

Now you can at least answer questions on the subject of servomotors. You can then also manage the question with the babies. In this sense: "Servus!"

As a leading manufacturer of drive technology, Dunkermotoren develops and produces high-quality linear and rotary drive systems with around 1,200 employees. The company, headquartered in Bonndorf in the Black Forest, presents itself as a global partner for solutions for everything to do with electrical drive technology. Thanks to the modular system with drive components of up to 1100 watts of continuous output power and the integration of logic and power electronics with various bus interfaces, Dunkermotoren offers its customers high flexibility. Dunkermotoren is the world market leader in the field of integrated, smart BLDC servo drives.


1) Term servo: "Servo". In: Wikipedia, The Free Encyclopedia. Processing status: March 14, 2017, 11:14 UTC. URL: (Accessed: May 26, 2017)

2) Term motor: "motor". In: Wiktionary, The Free Dictionary. Updated on May 25, 2017, 12:44 AM UTC. URL: (Accessed: May 26, 2017)

3) Wagner, Markus: Technical dictionary mechatronics / automation German-English [CD-ROM], Seligenstadt: Lehrmittel-Wagner, 2011

4) Gerke, Wolfgang: Electrical machines and actuators: An application-oriented introduction, Munich: Oldenbourg Wissenschaftsverlag GmbH, 2012, p. 10

5) "Servo motor". In: Wikipedia, The Free Encyclopedia. Processing status: March 7, 2017, 07:39 UTC. URL: (Accessed: May 26, 2017)

6) "Servomechanism". In: Wikipedia, The Free Encyclopedia. Processing status: March 22, 2017, 19:07 UTC. URL: (Accessed: May 26, 2017)

7) Tobin, Stephen M., DC Servos: Application and Design with MATLAB®, Boca Raton: CRC Press, 2010, p. 1

8) Firoozian, Riazollah: Servo Motors and Industrial Control Therory, Berlin: Springer Science & Business Media, 2008, p. 60