MODULE 5
Electrical Drive Advantages of Electrical Drive
The system which is used for controlling the motion of an electrical The electric drive has very large range of torque, speed and
machine, such type of system is called an electrical drive. In other power.
words, the drive which uses the electric motor is called electrical Their working is independent of the environmental
drive. The electrical drive uses any of the prime movers like diesel or condition.
a petrol engine, gas or steam turbines, steam engines, hydraulic
The electric drives are free from pollution.
motors and electrical motors as a primary source of energy. This
prime mover supplies the mechanical energy to the drive for motion The electric drives operate on all the quadrants of speed
control. torque plane.
The block diagram of the electrical drive is shown in the figure The drive can easily be started and it does not require any
below. The electrical load like fans, pumps, trains, etc., consists the refueling.
electrical motor. The requirement of an electrical load is determined The efficiency of the drives is high because fewer losses
regarding speed and torque. The motor which suited the capabilities occur on it.
of the load is chosen for the load drive.
DC MOTOR
Parts of Electrical Drive
The main parts of the electrical drives are power modulator, motor,
controlling unit and sensing units. Their parts are explained below in
details.
Power Modulator – The power modulator regulates the output
power of the source. It controls the power from the source to the
motor in such a manner that motor transmits the speed-torque
characteristic required by the load. During the transient operations
like starting, braking and speed reversing the excessive current drawn A motor is an electrical machine which converts electrical energy
from the source. This excessive current drawn from the source may into mechanical energy.
overload it or may cause a voltage drop. Hence the power modulator A DC motor is composed of the following main parts:
restricts the source and motor current.
Field coil or stator
The power modulator converts the energy according to the
This is a non-moving part of the magnetic circuit on which a
requirement of the motor e.g. if the source is DC and an induction
winding is wound in order to produce a magnetic field. The electro-
motor is used then power modulator convert DC into AC. It also
magnet that is created has a cylindrical cavity between its poles.
selects the mode of operation of the motor, i.e., motoring or braking.
Armature or rotor
Control Unit – The control unit controls the power modulator which
operates at small voltage and power levels. The control unit also This is a cylinder of magnetic laminations that are insulated from
operates the power modulator as desired. It also generates the one another and perpendicular to the axis of the cylinder. The
commands for the protection of power modulator and motor. An armature is a moving part that rotates round its axis, and is separated
input command signal which adjusts the operating point of the drive, from the field coil by an air gap. Conductors are evenly distributed
from an input to the control unit around its outer surface.
Sensing Unit – It senses the certain drive parameter like motor Commutator and brushes
current and speed. It mainly required either for protection or for The commutator is integral with the armature. The brushes are fixed.
closed loop operation. They rub against the commutator and thus supply power to the
armature conductors.
1
Operating principle types., i.e., the single phase induction motor and the three phase
So we give DC voltage as an input to DC motor which has two types induction motors.
of windings: armature winding on Rotor(rotating part of a Working Principle of an Induction Motor
motor) and field winding on Stator(stationary part of a motor). In an induction machine the armature winding serve as both the
So DC input is actually given to both the windings which would then armature winding and field winding. When the stator windings are
carry DC currents. (Currents that are unidirectional and whose connected to an AC supply flux is produced in the air gap. The flux
average in any given time interval is non-zero) rotates at a fixed speed called synchronous speed. This rotating flux
induces voltages in the stator and rotor
Basics: A current carrying coil produces magnetic field in the space
around it. Two magnetic fields in the space interact with each other
and force or torque is acted upon each of the two windings creating
two magnetic fields.
In DC motor, the two magnetic fields result from the DC currents
given to both the armature and field windings.
A mutual torque is produced because of these two magnetic fields
from armature and field, which sets only the rotor carrying armature
windings to motion while the stator stays stationary. winding. If the rotor
This is because of the mechanical arrangement of stator, which is circuit is closed, the current flows through the rotor winding and react
fixed and inside which rotor is housed, facilitating it to freely rotate with the rotating flux and a torque is produced. In the steady state, the
under torque. rotor rotates at speed very close to synchronous speed.
AC MOTOR BRUSHLESS MOTOR
Definition: The motor that converts the alternating current into
mechanical power by using an electromagnetic induction
phenomenon is called an AC motor. This motor is driven by an
alternating current. The stator and the rotor are the two most
important parts of the AC motors. The stator is the stationary part of
the motor, and the rotor is the rotating part of the motor. The AC
motor may be single phase or three phase.
Synchronous Motor
The motor that converts the AC electrical power into mechanical
power and is operated only at the synchronous speed is known as a
synchronous motor.
Working Principle of a Synchronous Motor Stator windings of a BLDC motor are connected to a control circuit
(an integrated switching circuit). The control circuit energizes proper
When supply is given to synchronous motor, a revolving field is set
winding at proper time, in a pattern which rotates around the stator.
up. This field tries to drag the rotor with it, but could not do so
The rotor magnet tries to align with the energized electromagnet of
because of rotor inertia. Hence, no starting torque is produced. Thus,
the stator, and as soon as it aligns, the next electromagnet is
inherently synchronous motor is not a self-starting the motor.
energized. Thus the rotor keeps running.
SERVOMOTOR
A servomotor is a rotary actuator or linear actuator that allows for
precise control of angular or linear 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.
Induction Motor or Asynchronous Motor Servomotors are not a specific class of motor although the
term servomotor is often used to refer to a motor suitable for use in
The machine which converts the AC electric power into mechanical
a closed-loop control system.
power by using an electromagnetic induction phenomenon in called
an induction motor. The induction motor is mainly classified into two Servomotors are used in applications such as robotics, CNC
machinery or automated manufacturing.
2
A servomotor is a closed-loop servomechanism that uses position Wave Generator
feedback to control its motion and final position. The input to its The Wave Generator is an assembly of a thin-raced ball
control is a signal (either analogue or digital) representing the bearing fitted onto the periphery of an elliptical cam. The
position commanded for the output shaft. inner ring of the bearing is fixed around the cam causing
The motor is paired with some type of encoder to provide position the outer ring of the bearing to conform to the same
and speed feedback. In the simplest case, only the position is elliptical shape of the assembly. The Wave Generator is
measured. The measured position of the output is compared to the usually attached to the input shaft.
command position, the external input to the controller. If the output Flexspline
position differs from that required, an error signal is generated which
The Flexspline is a thin-walled steel cup with gear teeth
then causes the motor to rotate in either direction, as needed to bring
machined into the outer surface near the open end of the
the output shaft to the appropriate position. As the positions
cup. The bottom of the Flexspline (cup bottom) is called
approach, the error signal reduces to zero and the motor stops.
the diaphragm. The diaphragm is usually fitted to the
output shaft.
STEPPER MOTOR
A stepper motor or step motor or stepping motor is a brushless DC Circular Spline
electric motor that divides a full rotation into a number of equal steps. The Circular Spline is a rigid steel ring with teeth on the
The motor's position can then be commanded to move and hold at inside diameter. The Circular Spline has two teeth more
one of these steps without any position sensor for feedback (an open- than the Flexspline. The Circular Spline is usually fixed to
loop controller), as long as the motor is carefully sized to the a casing.
application in respect to torque and speed.
Brushed DC motors rotate continuously when DC voltage is applied Force resistors
to their terminals. The stepper motor is known by its property to A force-sensing resistor is a material whose resistance changes when
convert a train of input pulses (typically square wave pulses) into a a force, pressure or mechanical stress is applied.
precisely defined increment in the shaft position. Each pulse moves
Force-sensing resistors consist of a conductive polymer, which
the shaft through a fixed angle.
changes resistance in a predictable manner following application of
Stepper motors effectively have multiple "toothed" electromagnets force to its surface. They are normally supplied as a polymer sheet
arranged around a central gear-shaped piece of iron. The or ink that can be applied by screen printing. The sensing film
electromagnets are energized by an external driver circuit or a micro consists of both electrically conducting and non-conducting particles
controller. To make the motor shaft turn, first, one electromagnet is suspended in matrix. The particles are sub-micrometer sizes, and are
given power, which magnetically attracts the gear's teeth. When the formulated to reduce the temperature dependence, improve
gear's teeth are aligned to the first electromagnet, they are slightly mechanical properties and increase surface durability. Applying a
offset from the next electromagnet. This means that when the next force to the surface of the sensing film causes particles to touch the
electromagnet is turned on and the first is turned off, the gear rotates conducting electrodes, changing the resistance of the film. As with all
slightly to align with the next one. From there the process is repeated. resistive based sensors, force-sensing resistors require a relatively
Each of those rotations is called a "step", with an integer number of simple interface and can operate satisfactorily in moderately hostile
steps making a full rotation. In that way, the motor can be turned by a environments.
precise angle.
The circular arrangement of electromagnets is divided into groups,
Tactile sensors
each group called a phase, and there is an equal number of
electromagnets per group. The number of groups is chosen by the
designer of the stepper motor. The electromagnets of each group are
interleaved with the electromagnets of other groups to form a uniform
pattern of arrangement.
Harmonic drive
Basically, the HarmonicDrive -developed to take advantage of the
elastic dynamics of metal- is typically made up of only three
components: a wave generator, a flexspline and a circular spline
(Depending on its shape, the HarmonicDrive is sometimes made up
of four components; but even this four-component Harmonic Drive is
based on the same principle of motion). One can grasp the Harmonic
Drive's unique mechanisms from the way its teeth mate with one In general, tactile sensors are used to sense the contact of fingertips
another. of a robot with an object. They are also used in manufacturing of
‘touch display’ screens of visual display units (VDUs) of CNC
machine tools. Figure shows the construction of piezo-electric
3
polyvinylidene fluoride (PVDF) based tactile sensor. It has two
PVDF layers separated by a soft film which transmits the vibrations.
An alternating current is applied to lower PVDF layer which
generates vibrations due to reverse piezoelectric effect. These
vibrations are transmitted to the upper PVDF layer via soft film.
These vibrations cause alternating voltage across the upper PVDF
layer. When some pressure is applied on the upper PVDF layer the
vibrations gets affected and the output voltage changes. This triggers
a switch or an action in robots or touch displays.
Range finders: ultrasonic and light based range finders
A rangefinder is a device that measures distance from the observer to
a target, in a process called ranging.
Ultrasonic rangefinder measuring sensor is based on the
propagation velocity of ultrasound in the air is known, the ultrasonic
wave encounters an obstacle reflected back characteristics measured.
Ultrasonic transmitter to transmit ultrasonic waves in one direction,
and the launch timing, ultrasonic wave propagation in the air, the
obstacles on the way to return immediately to the ultrasonic receiver
receives the reflected wave immediately interrupt to stop the clock,
and then according to the transmitting and receiving the time
difference to calculate the actual distance of the emission point to the
obstacle. By constantly detecting the echoes reflected by the
obstacles encountered after generating wave transmitting which
measured emitting an ultrasonic wave and receives the echo of the
time difference T, and then calculate the distance. Ultrasonic
rangefinder measuring sensor by the ultrasonic wave generating
circuit, and ultrasonic reception amplifying circuit, the counting and
display circuit
Laser (light) rangefinder measuring sensors a sensor for accurate
measurement of the distance to the target using a laser. Laser
rangefinder while at work to the target injection a bunch very fine
laser, received by the photovoltaic element and the laser beam
reflected by the target, the timer time from the emission to the
reception of the laser beam was measured to calculate the distance to
the target from the observer. The laser rangefinder is currently the
most widely used distance sensor, laser range finder measuring
sensor and can be classified as two kinds of handheld laser range
finder measuring sensor and telescope laser range finder measuring
sensor.
