Motor Controls

Specs for Servomotors 330

This class covers the basic types of servomotors and the components that can be used in a servo system.

  • Difficulty Advanced

  • Format Online

  • Number of Lessons 14

  • Language English


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Course Outline
  • Objectives
  • Position Control Systems
  • Servo Drives and Servomotors
  • Analog vs. Digital Servos
  • Resolvers and Encoders
  • AC Servomotors
  • Specifications for AC Servomotors
  • Synchronous AC Servomotors
  • DC Servomotors
  • Brushless DC Servomotors
  • Stepper Motors
  • Preventing Step Loss in Stepper Motors
  • Pros and Cons of Servomotors
  • Summary
  • Describe position control systems.
  • Describe servo drives and servomotors.
  • Distinguish between analog and digital servos.
  • Distinguish between resolvers and encoders.
  • Describe AC servomotors.
  • Describe specifications for AC servomotors.
  • Describe synchronous AC servomotors.
  • Describe DC servomotors.
  • Describe brushless DC servomotors.
  • Describe stepper motors.
  • Describe how to prevent step loss in stepper motors.
  • List the pros and cons of servomotors.
Vocabulary Term

AC motor

A type of electric motor that runs on alternating current. AC motors are more commonly used in industry than DC motors, but do not operate well at low speeds.

AC servomotor

A servomotor based on the design of conventional AC motors, with the addition of an amplifier and a feedback device.

AC synchronous servomotor

A servomotor based on the design of conventional AC synchronous motors, with the addition of an amplifier and a feedback device.


A mathematical process designed to systematically solve a problem. A complex algorithm is used to control and stablize the outputs in a closed-loop control system.


A device that increases the size or strength of a signal. Servomotors use amplifiers as part of the motor control system.

analog servo

A servo that outputs a continuously variable signal. Analog signals differ from digital signals in that small fluctuations in the analog signal are meaningful.


The part of a motor in which a current is induced by a magnetic field. The armature consists of a series of coils mounted on a shaft and rotates through the magnetic field.

automatic feedback

Feedback that is controlled by an electronic or electromechanical mechanism rather than a person. Automatic feedback is used in closed-loop control systems.


A condition that occurs in stepper motors in which there is an unwanted reversal of the motor shaft, resulting in step loss.


Sliding electrical contacts used to provide a connection between the armature and the external circuit. The brushes are stationary and are contacted by the rotating commutator.

brushless DC motor

A DC motor that uses a permanent magnet instead of brushes to run the motor.

brushless DC servomotor

A servomotor based on the design of a conventional brushless DC motor, with the addition of an amplifier and a feedback device.

closed-loop control

A type of control system that automatically changes the output based on the difference between the feedback signal to the input signal. Servomotors can be used on closed-loop control systems.


The rotating switch that contacts the brushes of a DC motor. The commutator maintains DC when the rotation of the armature switches the polarity of the conductor.

continuous current

The maximum amount of current that can be applied without overheating the motor. Continuous current is one of the specifications used for sizing AC servomotors.

continuous power output

The ability of a motor to output its full power for a sustained period of time. Continuous power output is one of the specifications used for sizing AC servomotors.

continuous torque

The amount of torque that can be provided by the motor under normal running conditions. Continuous torque is one of the specifications used for sizing AC servomotors.

control system

A manual or automatic mechanism used to manage dynamic processes by adjusting or maintaining physical variables such as temperature, speed, or flow rate.


An increment on an encoder that measures the distance a robotic arm has traveled.


A single increment of something that occurs repeatedly. AC flows in cycles of 360 electrical degrees.

DC motor

A type of motor that runs on direct current. DC motors have better speed control and higher torque than AC motors.

DC servomotor

A servomotor based on the design of a conventional DC motor, with the addition of an amplifier and a feedback device.

digital servo

A servo that outputs a series of pulses or signals that represent "on" or "off," often resulting in binary strings of 0s and 1s.

electrical degree

A unit of measurement for expressing the amount of rotation in a generator and the position in an AC sine wave.


A measuring device for motion control that is divided into a fixed number of increments called counts. As the encoder disk spins, light pulses from an LED are converted to electrical pulses.

feedback signal

The actual value detected by a sensor as a process is taking place. The feedback signal is part of a closed-loop control system.


A measurement of the number of complete AC cycles that occur in one second. Frequency is measured in Hertz (Hz).

holding torque

The amount of torque required for a motor to remain in a fixed position.

induction motor

A type of AC motor that uses electrical current to induce rotation in the coils.


Light emitting diode. A semiconductor device that emits a narrow spectrum of light in a forward direction.


A tiny central processing unit. Digital servos use a microprocessor to process input signals.

open-loop system

A type of control system that uses only an input signal to actuate an output. There is no automatic feedback to adjust the process, so adjustments must be made manually by the operator.

output shaft

The part of a motor that transmits torque.


To exceed a set value.

peak negative voltage

The lowest negative point on a wave. The peak negative voltage indicates the maximum negative voltage of the wave.

peak positive voltage

The highest positive point on a wave. The peak positive voltage indicates the maximum positive voltage of the wave.

pick and place

An application in which objects are transferred from one place to another.

position control

A type of control system designed for moving objects or machines to a known position. Servomotors are used for position control.


An analog motor control device that controls feedback through a sine wave. The level of voltage on the sine wave correlates to the position of the rotor within one revolution.


The rotating part of a motor.

service factor

A rating given to motors to indicate what applications they can be used for.

servo drive

A motor control device that outputs electrical signals to a servomotor to induce motion. The servo drive can be built in as part of the motor or it can be a separate device.

servo system

A closed-loop control system that utilizes a servomotor.


A type of motor used in applications that require precise positioning. Servomotors provide a high amount of holding torque to maintain their position accurately.

shaft speed

The rate at which the motor shaft turns. Shaft speed is measured in feet per minute (fpm) or in meters per minute (mpm).

sine wave

The most common type of AC waveform. A sine wave consists of 360 electrical degrees and is produced by rotating machines.


AC voltage that has only one sine wave.

soft start

A motor starting method that reduces stress on a motor by allowing it to accelerate gradually for a programmed period of time.

squirrel cage rotor

A type of three-phase AC rotor that is constructed by connecting metal bars together at each end. It is the most common AC rotor type.

start-stop test

A troubleshooting method in which the stepper motor is connected to a load and a fixed frequency is applied to the driver. Stop-start tests are used to diagnose the causes of step loss.


The stationary windings of a motor, usually inside an AC motor.

step loss

A phenomenon in which the stepper motor does not take a step after receiving an input pulse.

stepper motor

A type of motor that rotates in small, precise increments in response to an electronic pulse. Stepper motors can rotate forward or reverse, but they cannot move large loads.

synchronous motor

A constant-speed AC motor that does not use induction to operate. A synchronous motor needs DC excitation to operate.

target value

A preset value such as a specific temperature, speed, or flow rate that the control system is supposed to reach. The type of preset value for servomotors is position.


AC voltage that has three sine waves.


A system of adjusting the servomotor's outputs by adjusting values in the algorithm controlling the closed-loop system.


To fall short of or below a set value.


The conducting wire connected to the armature that energize the pole pieces of a motor.