3. Construction of stator
• 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.
4. Construction of stator
• 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 in figure.
7. Working of AC Servomotor
• 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.
8. Working of AC Servomotor
• 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.
9. Working of AC Servomotor
• 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 field.
10.
11.
12.
13. Cntd….
• 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
14. ADVANTAGES & LIMITATIONS OF
ELECTRIC ACTUATORS
ADVANTAGES
• Wide spread availability of power
supply.
• The basic dive element in an electric
motor is usually lighter than that for
fluid power.
• High power conversion efficiency.
• No pollution of working environment
• The accuracy and repeatability of
electric power driven robots are
normally better than fluid power
robots in relation to cost.
• Easily maintained and repaired.
• The drive system is well suited to
electronic control.
LIMITATIONS
• Electric actuators often require
some sort of mechanical
transmission system this
increases the unwanted
movement, additional power and
may complicate control.
• Due to increased complexity of
the transmission system
additional cost is incurred for
their procurement and
maintenance.
• Electric motors are not
intrinsically safe. They cannot
therefore be used in for example
explosive atmospheres.
15. Comparison of actuating systems
Hydraulic
+ Good for large robots
and heavy payload
+Highest Power/Weight
Ratio
+Stiff system, High
accuracy, better response
+No reduction gear
needed
+Can work in wide range
of speeds without
difficulty
+Can be left in position
without any damage
Electric
+ Good for all size of
Robots
+Better control, good for
high precision robots
+Higher Compliance that
Hydraulics
+Reduction gears used
reduce inertia on the
motor
+does not leak, good for
clean room
+Reliable, low
maintenance
Pneumatic
+ Many components are
usually off-the-shelf
+Reliable components.
+No leaks or sparks
+Inexpensive and simple
+Low pressure compared
to hydraulics
+ Good for on-off
applications and for pick
and place
16. Comparison of actuating systems
Hydraulic
- May leak. Not fit for clean
room application
-Requires pump, reservoir,
motor, hoses etc.
-Can be expensive and noisy,
requires maintenance.
-Viscosity of oil changes with
temperature
-Very susceptible to dirt and
other foreign material in oil
-Low compliance
-High torque, High pressure,
large inertia on the actuator.
Electric
+Can be spark-free. Good for
explosive environment.
-Low stiffness
-Needs reduction gears,
increased backlash, cost,
weight, etc.
-Motor needs braking device
when not powered.
Otherwise, the arm will fail.
-
-
-
Pneumatic
+Complaint systems.
-Noisy systems.
- Require air pressure, filter,
etc.
-Difficult to control their
linear position
-Deform under load
constantly
-Very low stiffness. Inaccurate
response.
-Lowest power to weight ratio
