AC AND DC SERVOMOTORS

Learning objective

To understand the servo systems

This topic falls under the Aerospace & Defence among the 7 Industrial vertical

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Closed Loop Control

• Controller is the “Brain”

• Drive is the “Muscle”

• Feedback Transducer is the “Eyes”

MotionCommand

Input

MechanicalMotionOutput

Controller

FeedbackTransducer

Drive or Amplifier

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Servo Motors

Servo Motors are also called Control motors. They are used in feedback control systems as output actuators and does not use for continuous energy conversion.

They are classified as AC and DC Servo Motor.

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DC Servo Motor

• DC Servo Motors are separately excited DC motor or permanent magnet DC motors.

• The figure (a) shows the connection of Separately Excited DC Servo motor and the figure (b) shows the armature MMF and the excitation field MMF in quadrature in a DC machine.

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DC Servo Motor

• This provides a fast torque response because torque and flux are decoupled. Therefore, a small change in the armature voltage or current brings a significant shift in the position or speed of the rotor. Most of the high power servo motors are mainly DC.

The Torque-Speed Characteristics of the Motor

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AC Servo Motor

• The AC Servo Motors are divided into two types 2 and 3 Phase AC servomotors.

• Most of the AC servomotors are of the two-phase squirrel cage induction motor type.

• They are used for low power applications. The three phase squirrel cage induction motor is now utilised for the applications where high power system is required.

• The stator of the Two Phase AC Servo Motor has the two distributed windings which are displaced from each other by 90 degrees electrical.

• One winding is known as a Reference or Fixed Phase, which is supplied from a constant voltage source.

• The other one is known as Control Phase, and it is provided with a variable voltage

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AC Servo Motor

The connection diagram of the two Phase AC Servo motor

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AC Servo Motor

• The control phase is usually supplied from a servo amplifier.

• The speed and torque of the rotor are controlled by the phase difference between the control voltage and the reference phase voltage.

• By reversing the phase difference from leading to lagging or vice versa, the direction of the rotation of the rotor can be reversed.

The torque speed characteristic of the two phase AC servomotor

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Properties of Servo Systems

• high maximum torque/force allows high (de)acceleration

• high zero speed torque/force

• high bandwidth provides accurate and fast control

• robustness

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Applications of Servo Systems

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How to design good Servo Systems?

Terminology: Do not confuse “servos” with “servo motors”

DC motors (brushed or brushless) are also sometimes also referred to as “servo motors”

“So when does a motor become a servo motor? There are certain design criteria that are desired when building a

servo motor, which enable the motor to more adequately handle the demands placed on a closed loop system.

First of all, servo systems need to rapidly respond to changes in speed and position, which require high acceleration and deceleration rates.

This calls for extremely high intermittent torque.

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Intermittent Torque in Servos

Torque is related to current in the brushed servo motor.

So the designers need to keep in mind the ability of the motor to handle short bursts of very high current, which can be many times greater than the continuous current requirements.

Another key characteristic of the brushed servo motor is a high ratio of torque to inertia. This ratio is an important factor in determining motor

responsiveness.

Further, servo motors need to respond to small changes in the control signal. So the design requires reaction to small voltage variations.

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Applications of Servos

1. Large manipulators in industry frequently employ hydraulic drives, • since such drives provide a higher torque-to-weight ratio than electric

motors

2. However, because of the maintenance problems associated with pressurized oil (including leaks), hydraulic motors are not used in smaller mobile robots

3. Pneumatic drives have been used as actuators in the past but are not currently popular

4. Air is compressible, resulting in nonlinear behavior of the actuator

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Thank You

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