Kansagara Ravi J.
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What is Servomotor?
A servomotor is a rotary actuator that allows for
precise control of angular position, velocity and
acceleration. It consists of a suitable motor coupled to
a sensor for position feedback. It also requires a
relatively sophisticated controller, often a dedicated
module designed specifically for use with servomotors.
Servomotors are not a specific class of motor although
the term servomotor is often used to refer to a motor
suitable for use in a closed-loop control system.
Servo System Compatibility
An industrial servomotor is almost always part of a larger
servo system. It is important to ensure the motor is compatible
with the associated system before making a selection. This
includes compatibility with the controller, amplifier,
and programmable logic controller (PLC) components of the
system. In operation, the PLC sends position commands
specified by the user to a servo controller. These commands
are interpreted by the position controller and sent to an
amplifier which processes them for use and response by the
motor. As the motor carries out these commands, the encoder
sends position signals back to the controller, which processes
them for the next set of commands.
Speed Torque characteristics
1. The rotor of the servomotor is built with high
resistance, so that its X/R (Inductive reactance /
Resistance) ratio is small which results in linear
speed torque characteristics.(But conventional
induction motors will have high value of X/R which
results in high efficiency and non-linear speed-
torque characteristics). The Speed-torque
characteristics of normal induction motor (Curve-a)
and AC servomotor (Curve-b) are shown in figure.
2. The excitation voltage applied of two stator
windings should have a phase difference of 90°.
Speed Torque characteristics of
a) Normal induction motor
b) AC Servomotor
induction motor and AC Servomotor
The AC servomotor is basically a two phase
induction motor with some special design features.
The stator consists of two pole-pairs (A-B and C-D)
mounted on the inner periphery of the stator, such
that their axes are at an angle of 90° in space. Each
pole-pair carries a winding. One winding is called
reference winding and the other is called a control
winding. The exciting current in the winding should
have a phase displacement of 90°. The supply used to
drive the motor is single phase and so a phase
advancing capacitor is connected to one of the phase
to produce a phase difference of 90°. The stator
constructional features of AC servo motor are shown
The Drag-cup construction is employed for
very low inertia applications. In this type of
construction the rotor will be in the form of hollow
cylinder made of aluminium. The aluminium
cylinder itself acts as short circuited rotor
conductors. (Electrically both the types of rotor are
Working of Servomotor as ordinary induction
The stator winding are excited by voltages of equal rms
magnitude and 90° phase difference. These results in
exciting currents i1 and i2 that are phase displayed by
90° and have equal rms values. These current give rises
to a rotating magnetic field of constant magnitude.
The direction of rotation depends on the phase
relationship of the two currents (or voltages). The
exciting current produces a clockwise rotating
magnetic field and phase shift of 180° in i1 will produce
an anticlockwise rotating magnetic field.
The rotating magnetic field sweeps over the rotor
conductors. The rotor conductors experience a
change in flux and so voltages are induced in rotor
conductors. This voltage circulates currents in the
short circuited rotor conductors and the currents
create rotor flux. Due to the interaction of stator and
rotor flux, a mechanical force (or torque) is
developed on the rotor and so the rotor starts moving
in the same direction as that of rotating magnetic
The symbolic representation of an A.C servomotor
with control system component is shown in figure The
reference winding of A.C servomotor is excited by a
constant voltage source with frequency of 50Hz. The
speed of A.C servomotor is controlled by controlling
the control voltage. The error output of error detector
is fed to PI controller, due to the error, controller take
control action (i.e. to give control voltage) to firing
circuit. The firing circuit generates the pulse's to rotate
the motor at required speed.
It can be seen that for higher values of rotor
resistance the torque speed characteristics is linear.
If the rotor resistance is either R4 or R5, the motor
is referred to as two phase servomotor. For servo
application the motor characteristics should be
linear with negative slope (With positive damping).
It can be proved that for stable operation positive
damping is essential. Therefore ordinary two phase
induction motor with low rotor resistance is not
suitable for servo applications.
It can be seen that for higher values of rotor
resistance the torque speed characteristics is linear. If
the rotor resistance is either R4 or R5, the motor is
referred to as two phase servomotor. For servo
application the motor characteristics should be linear
with negative slope (With positive damping). It can
be proved that for stable operation positive damping
is essential. Therefore ordinary two phase induction
motor with low rotor resistance is not suitable for