DC SERVOMOTOR

Servomotors are considered as special type of motor in the field of electrical machines.

Servomotors are also called control motors as they are involved in control system . The motor

gets its name from the word servo-mechanism.

There are two fundamental characteristics of any servo motor. These are:

The motor output torque is proportional to the voltage applied to it ( i.e control voltage

developed by amplifier in response to an error signal ) .

The instantaneous polarity of control voltage governs the direction of torque developed

by servomotors.

Types of DC Servomotor

There are various types of dc servomotors which are series motors, split series motors, shunt control motor and permanent magnet shunt motor. We will now discuss these types of dc

servomotors in brief.

Series motors: The series motor has a high starting torque and draws large current

.Speed regulation of this kind of motor is poor . Reversal can be obtained by reversing the

polarity of field voltage with split series field winding (i.e one winding for direction of

rotation). This method reduces motor efficiency to some extent .

Split series motors: Split series motor are the dc series motor with split-field rated with

some fractional kilowatt . This type of motor can operate as a separately excited field-

controlled motor. The armature is supplied with a constant current source. Split series

motor has a typical torque-speed curve . This curve denotes high stall torque and a rapid

reduction in torque with increase in speed. This results in good damping.

Shunt control motor : DC shunt type servomotor is not different from any other dc

shunt motor . It has two separate windings field windings placed on stator and armature winding placed on the rotor of the machine . Both windings are connected to a dc supply

source. In a conventional dc shunt motor , the two windings are connected inparallel

across the dc supply . In case of a servomotor , the windings are supplied with separate dc

source.

Permanent magnet shunt motor: Permanent magnet shunt motor is a fixed excitation

motor where the field is actually supplied by a permanent magnet . Performane is similar

to armature controlled fixed field motor that we are going to know in the next section.

Working principle of DC servomotor

In DC operation, servomotors are usually responds to error signal abruptly and accelerate the

load quickly. A DC servo motor is actually an assembly of four separate components, namely:

A DC motor

A gear assembly

A position-sensing device

A control circuit.

In case of field controlled dc motor, the field is excited by the amplified error signal mentioned

earlier . The armature winding is energized from a constant current source. Torque developed is

proportional to field current (Ifl) upto saturation level . This method is applied in small

servomotors . It has longer time constant owing to highly inductive field circuit so dynamic

response is slower than armature controlled dc motor.

But in armature controlled dc motor, the motor armature is energized by amplified error signal

and field is supplied from a constant current source . High field flux density also increases torque

sensitivity of motor (torque proportional to Ia) . Here dynamic response is faster because it has shorter time constant of the resistive circuit.

As you can see in the diagram, the output signal is sampled and thereafter the actuating device

rectifies the errors. After that, the servomotor gear assembly is used for controlling the position.

Applications

DC servomotors finds its applications in various domain. Some of them are given below :

For very high voltage power systems, dc motors are preferred because they operate

more efficiently than comparable ac servomotor.

It also prevents power wastage, cooling process is easier.

DC servomotors are widely used in robots, toy cars and other position controlled devices.

Widely used in radars, computers, robots, machine tools tracking system, process

controllers etc .

Hopefully you understood the concept of servomechanism as well as construction and working

principle of DC servomotor.