2. 2
PROBLEM STATEMENT
Environmental costs are escalating as demand for electrical power in Pakistan spirals due
to the rapid population and economic growth. The country may face energy crisis in
coming years following healthy growth of 13 per cent in electricity demand during the
last quarter. Pakistan industry is suffering directly from this energy crisis specially
Textile industry. That’s why need of the hour is to focus on potential cost effective
energy conservation as successful energy management of the industrial units can have a
considerable impact on the manufacturing cost. Ever-increasing utility costs reduce
profits, erode capital and maintenance budgets, increase product costs, and reduce
competitiveness.
Efficient use of energy enables commercial and industrial facilities to minimize
production costs, increase profits, and stay competitive. According to a research carried
out by Finance and Economics magazine of America electricity fee usually is the third or
fourth biggest cost of the enterprise and to most enterprises, the electricity fee is the
biggest cost which is out of control, but it is the only cost which can be controlled. After
the implementation of WTO, two aspects that need to be focused are quality of the
products and cut down the production cost in order to compete in the international
market.
Apparel industry is a major consumer of electricity and increased cost of electricity is
making it difficult to reduce the manufacturing cost. Apparel industry is one of the
industries that have the greatest material handling time. A study by industrial engineers
in Sweden over 30 years ago showed that operators were only spending 20% of their time
actually sewing. The rest of the operator's time was spent in handling materials and
dealing with personal fatigue issues. But after the introduction of material handling
systems such as Eton System and Switchtrack System, this time has increased to almost
40 to 60%. Thus it can be concluded that occupational coefficient of a sewing machine
(time for which sewing machine is actually sewing on the material) is very low. This
means that almost 50% of the electricity used to power the sewing machines on an
apparel production floor is wasted if sewing machines keep on running all the time.
Motors are considered as power horses of most of the industries due to their huge energy
consumption. Surprisingly, the electricity used to power a motor represents
3. 3
approximately 90 percent of its total lifetime operating costs. The combined costs of
purchasing, installing, and maintaining a motor comprise the remaining 10 percent.
Considering that a commercial building or manufacturing plant may have tens, hundreds,
or even thousands of motors operating within the facility, managing motor energy costs is
good business. Improving motor efficiency may also improve productivity, reduce
operating and maintenance costs and help improve air quality by reducing greenhouse gas
emissions.
There are two types of industrial sewing machines available in the market based on the
motor type i.e. clutch and servo. Servo motor is quite a new technology that’s why clutch
motor is the motor you will find in most of the small scale industries. This is not just
because its new technology but also it has much more price as compared to the clutch
motor. Mostly clutch motor sewing machines are used in small manufacturing units of
Pakistan which has more electricity consumption and running cost than the latest servo
motor sewing machines. Big price difference between two motors attract more and more
small scale garments manufacturers to buy the sewing machine with clutch motor
ignoring the running cost. Due to a sharp increase in the electricity rate in Pakistan,
running cost is adding comparatively much more than what it is expected to. The only
solution to this running cost problem is to buy energy efficient motors.
4. 4
OBJECTIVES:
Main objective of the study is to decide the better motor by comparing the costs and
benefits of using two types of motor. Since clutch motor is supposed to have more power
consumption thus second objective is to design and develop a power conservation device
for the clutch motor. Following is a list of the objectives:
Measurement of power consumption and cost associated with this consumption in
clutch and servo motor machines at different operations of a garment article
Comparison of power consumption and the cost of the power consumption
between two types of the motors
Determination of the better option by comparison of costs and benefits of two
types of motors
Designing and developing a device for power conservation of clutch motor
machines which can turn the clutch motor off while it is not in use
5. 5
ELECTRIC MOTORS
An electric motor converts electrical energy into mechanical energy. Electric motors
consume approximately 60 per cent of the electricity supplied to industry. This
mechanical energy is used for, for example, driving sewing machine parts, rotating a
pump impeller, fan or blower, driving a compressor, lifting materials etc. The majority of
these motors are AC induction motors, commonly referred to as the workhorses of
industry.
Its origin can be traced to machines conceived and tested by Michael Faraday, the
experimenter who formulated the fundamental concepts of electromagnetism. These
concepts basically state that if a conductor, or wire, carrying current is placed in a
magnetic field, a force will act upon it. The magnitude of this force is a function of
strength of the magnetic field, the amount of current passing through the conductor and
the orientation of the magnet and conductor. The direction in which this force will act is
dependent on the direction of current and direction of the magnetic field.
Electric motor design is based on the placement of conductors (wires) in a magnetic field.
A winding has many conductors, or turns of wire, and the contribution of each individual
turn adds to the intensity of the interaction. The force developed from a winding is
dependent on the current passing through the winding and the magnetic field strength. If
more current is passed through the winding, then more force (torque) is obtained. In
effect, two magnetic fields interacting cause movement: the magnetic field from the rotor
and the magnetic field from the stators attract each other. This becomes the basis of both
AC and DC motor design
How a motor works
The general working mechanism is the same for all motors:
An electric current is a magnetic field will experience a force.
If the current carrying wire is bent into a loop, then the two sides of the loop,
which are at right angle to the magnetic field, will experience forces in opposite
directions.
The pair of forces creates a turning torque to rotate the coil.
6. 6
An electrical motor system includes a power supply, motor controls, the electric
motor itself, and a mechanical transmission system in a commonly used induction
motor. While in latest motors, equipped with servo control, known as servo motors
use feedback mechanism. These motor systems are often components of other
systems such as sewing machines.
SEWING MACHINE MOTORS
Sewing machines today are equipped with an electric motor responsible for all the motion
in the sewing machine. Power is supplied to the motor which is converted into motion
that is transferred to various parts of the sewing machine by means of cams and shafts to
carry out their mechanisms such as movement of the needle, movement of the feed dog.
Motors used in sewing machines are low power motors because they have to drive small
loads. Sewing machine motors are of two types:
Clutch Motor
Servo Motor
Clutch motors were commonly used in most of the sewing machines as they had less
price and good working in terms of constant speed. But after the introduction of servo
motor, the performance of clutch motor seems to be very low. Servo motors provide
excellent motion control, much high speed, and increased energy efficiency leading to
very low running cost. But still some sewing machines are equipped with clutch motors
due to a huge price difference in clutch motor and servo motor. A normal sewing
machine clutch motor price ranges from $50 to $100 while price range of servo motor is
$400 to $600. Actually the motor type used in a sewing machine depends on eth purpose
for which the sewing machine is being used. For example latest overlock sewing
machines are still equipped with clutch motor due to their same kind of operation at
constant speed. But in lockstitch and special purpose sewing machines latest servo
motors are used as their operation is more complex.
CLUTCH MOTOR
Motors equipped with a clutch or brake is usually known as clutch motor. Clutch is used
for coupling a continuously rotating shaft and a load. Uncoupling the load results in
7. 7
stopping. Sewing machines with clutch motor consist of an AC induction motor having a
clutch on the outer side of the shaft. The pulley and belt system that transfers the motion
to the sewing machine wheel is mounted on this clutch. This clutch is connected with a
paddle. When the paddle is pressed, clutch engages with the shaft allowing the belt to
transfer the motion to the wheel of sewing machine.
Clutch motor for the sewing machine is also equipped with soft starter due to the higher
frequency of start and stop in the sewing machine operation. This soft starter makes it
possible for the motor to start from the rest and reach maximum speed within little time
and fewer variations in the current and voltage applied to the motor.
AC induction motors are the most common motors used in industrial motion control
systems, as well as in main powered home appliances. Simple and rugged design, low-
cost, low maintenance and direct connection to an AC power source are the main
advantages of AC induction motors.
An induction motor has two main parts:
Construction of Clutch Motor
The stator is the outer body of the motor which houses the driven windings on an
iron core. It has two copper windings known as main and auxiliary windings.
Main winding creates a set of N, S poles. Auxiliary winding only operates during
the brief period when the motor starts up The stator core is made up of a stack of
round pre-punched laminations pressed into a frame which may be made of
aluminum or cast iron. The laminations are basically round with a round hole
inside through which the rotor is positioned. The inner surface of the stator is
made up of a number of deep slots or grooves right around the stator. It is into
these slots that the windings are positioned. The arrangement of the windings or
coils within the stator determines the number of poles that the motor has. A
standard bar magnet has two poles, generally known as North and South.
Likewise, an electromagnet also has a north and a south pole. As the induction
motor Stator is essentially like one or more electromagnets depending on the
stator windings, it also has poles in multiples of two. i.e. 2 pole, 4 pole, 6 pole etc.
The winding configuration, slot configuration and lamination steel all have an
effect on the performance of the motor. The voltage rating of the motor is
8. 8
determined by the number of turns on the stator and the power rating of the motor
is determined by the losses which comprise copper loss and iron loss, and the
ability of the motor to dissipate the heat generated by these losses. The stator
design determines the rated speed of the motor and most of the full load, full
speed characteristics.
Figure: Clutch Motor Parts
The Rotor comprises a cylinder made up of round laminations pressed onto the
motor shaft, and a number of short-circuited windings. The rotor windings are
made up of rotor bars passed through the rotor, from one end to the other, around
the surface of the rotor. The bars protrude beyond the rotor and are connected
together by a shorting ring at each end. The bars are usually made of aluminum or
copper, but sometimes made of brass. The position relative to the surface of the
rotor, shape, cross sectional area and material of the bars determine the rotor
characteristics. Essentially, the rotor windings exhibit inductance and resistance,
and these characteristics can effectively be dependant on the frequency of the
current flowing in the rotor. A bar with a large cross sectional area will exhibit a
low resistance, while a bar of a small cross sectional area will exhibit a high
resistance. Likewise a copper bar will have a low resistance compared to a brass
bar of equal proportions. Positioning the bar deeper into the rotor, increases the
amount of iron around the bar, and consequently increases the inductance
exhibited by the rotor. The impedance of the bar is made up of both resistance and
inductance, and so two bars of equal dimensions will exhibit different A.C.
impedance depending on their position relative to the surface of the rotor. A thin
bar which is inserted radialy into the rotor, with one edge near the surface of the
9. 9
rotor and the other edge towards the shaft, will effectively change in resistance as
the frequency of the current changes. This is because the A.C. impedance of the
outer portion of the bar is lower than the inner impedance at high frequencies
lifting the effective impedance of the bar relative to the impedance of the bar at
low frequencies where the impedance of both edges of the bar will be lower and
almost equal. The rotor design determines the starting characteristics. A shaft
mounted inside the rotor is used to transmit the motion produced to the part where
the motion is required usually with the help of a pulley and belt system.
Figure: Clutch Motor Constructions
Working of Clutch Motor
When supply across stator windings, each winding sets up a magnetic field. The
two stator magnetic fields are out phase by less than 90◦
. The net magnetic field
induces another magnetic field in the rotor and hence a torque is produced. When
only one stator winding is used, the motor does not rotate but rather vibrates. This
is because its magnetic field is not enough to turn the rotor. At startup, both
windings are employed. When the rotor reaches 75% of its final speed, the
auxiliary winding is disconnected (or split) from the circuit and only the main
winding remains connected to the supply. This is achieved by the centrifugal
10. 10
switch. The auxiliary windings have smaller size of wire than the main winding.
Hence, the auxiliary resistance is higher.
Starting of Clutch Motor
In order to perform useful work, the induction motor must be started from rest and
both the motor and load accelerated up to full speed. Typically, this is done by
relying on the high slip characteristics of the motor and enabling it to provide the
acceleration torque. Induction motors at rest, appear just like a short circuited
transformer, and if connected to the full supply voltage, draw a very high current
known as the "Locked Rotor Current". They also produce torque which is known
as the "Locked Rotor Torque". The Locked Rotor Torque (LRT) and the Locked
Rotor Current (LRC) are a function of the terminal voltage to the motor, and the
motor design. As the motor accelerates, both the torque and the current will tend
to alter with rotor speed if the voltage is maintained constant. The starting current
of a motor, with a fixed voltage, will drop very slowly as the motor accelerates
and will only begin to fall significantly when the motor has reached at least 80%
full speed. The actual curves for induction motors can vary considerably between
designs, but the general trend is for a high current until the motor has almost
reached full speed.
The starting torque of an induction motor starting with a fixed voltage, will drop a
little to the minimum torque known as the pull up torque as the motor accelerates,
and then rise to a maximum torque known as the breakdown or pull out torque at
almost full speed and then drop to zero at synchronous speed. The curve of start
torque against rotor speed is dependant on the terminal voltage and the
motor/rotor design. The LRT of an induction motor can vary from as low as 60%
Full Load Torque (FLT) to as high as 350% FLT. The pull-up torque can be as
low as 40% FLT and the breakdown torque can be as high as 350% FLT. Typical
LRTs for medium to large motors are in the order of 120% FLT to 280% FLT.
The power factor of the motor at start is typically 0.1 - 0.25, rising to a maximum
as the motor accelerates, and then falling again as the motor approaches full
speed.
A motor which exhibits a high starting current, i.e. 850% will generally produce a
11. 11
low starting torque, whereas a motor which exhibits a low starting current will
usually produce a high starting torque. This is the reverse of what is generally
expected.
The induction motor operates due to the torque developed by the interaction of the
stator field and the rotor field. Both of these fields are due to currents which have
resistive or in phase components and reactive or out of phase components. The
torque developed is dependant on the interaction of the in phase components and
consequently is related to the I2
R of the rotor. A low rotor resistance will result in
the current being controlled by the inductive component of the circuit, yielding a
high out of phase current and a low torque. Figures for the locked rotor current
and locked rotor torque are almost always quoted in motor data, and certainly are
readily available for induction motors. Some manufactures have been known to
include this information on the motor name plate. One additional parameter which
would be of tremendous use in data sheets for those who are engineering motor
starting applications, is the starting efficiency of the motor. If the terminal voltage
to the motor is reduced while it is starting, the current drawn by the motor will be
reduced proportionally. The torque developed by the motor is proportional to the
current squared, and so a reduction in starting voltage will result in a reduction in
starting current and a greater reduction in starting torque. If the start voltage
applied to a motor is halved, the start torque will be a quarter; likewise a start
voltage of one third will result in a start torque of one ninth.
Running of Clutch Motor
Once the motor is up to speed, it operates at low slip, at a speed determined by the
number of stator poles. The frequency of the current flowing in the rotor is very
low. Typically, the full load slip for a standard cage induction motor is less than
5%. The actual full load slip of a particular motor is dependant on the motor
design with typical full load speeds of four pole induction motor varying between
1420 and 1480 RPM at 50 Hz. The synchronous speed of a four pole machine at
50 Hz is 1500 RPM and at 60 Hz a four pole machine has a synchronous speed of
1800 RPM. The induction motor draws a magnetizing current while it is
operating. The magnetizing current is independent of the load on the machine, but
12. 12
is dependant on the design of the stator and the stator voltage. The actual
magnetizing current of an induction motor can vary from as low as 20% FLC for
large two pole machines to as high as 60% for small eight pole machines. The
tendency is for large machines and high speed machines to exhibit a low
magnetizing current, while low speed machines and small machines exhibit a high
magnetizing current. A typical medium sized four pole machine has a
magnetizing current of about 33% FLC. A low magnetizing current indicates a
low iron loss, while a high magnetizing current indicates an increase in iron loss
and a resultant reduction in operating efficiency.
The resistive component of the current drawn by the motor while operating,
changes with load, being primarily load current with a small current for losses. If
the motor is operated at minimum load, i.e. open shaft, the current drawn by the
motor is primarily magnetizing current and is almost purely inductive. Being an
inductive current, the power factor is very low, typically as low as 0.1. As the
shaft load on the motor is increased, the resistive component of the current begins
to rise. The average current will noticeably begin to rise when the load current
approaches the magnetizing current in magnitude. As the load current increases,
the magnetizing current remains the same and so the power factor of the motor
will improve. The full load power factor of an induction motor can vary from 0.5
for a small low speed motor up to 0.9 for a large high speed machine.
The losses of an induction motor comprise: iron loss, copper loss, winding loss
and frictional loss. The iron loss, winding loss and frictional losses are all
essentially load independent, but the copper loss is proportional to the square of
the stator current. Typically the efficiency of an induction motor is highest at 3/4
load and varies from less than 60% for small low speed motors to greater than
92% for large high speed motors.
Soft Starters
A soft starter is another form of reduced voltage starter for A.C. induction motors.
The soft starter is similar to a primary resistance or primary reactance starter in
that it is in series with the supply to the motor. The current into the starter equals
13. 13
the current out. The soft starter employs solid state devices to control the current
flow and therefore the voltage applied to the motor.
Solid state switches
These Solid State Switches are phase controlled in a similar manner to a light
dimmer, in that they are turned on for a part of each cycle. The average voltage is
controlled by varying the conduction angle of the switches. Increasing the
conduction angle will increase the average output voltage. Controlling the average
output voltage by means of solid state switches has a number of advantages, one
of the major advantages being the vast improvement in efficiency relative to the
primary resistance starter, due to the low on state voltage of the solid state
switches. Typically, the power dissipation in the starter, during start, will be less
than 1% of the power dissipated in a primary resistance starter during start.
Another major advantage of the solid state starter is that the average voltage can
be easily altered to suit the required starting conditions. By variation of the
conduction angle, the output voltage can be increased or reduced, and this can be
achieved automatically by the control electronics. The control electronics can be
preprogrammed to provide a particular output voltage contour based on a timed
sequence (open loop), or can dynamically control the output voltage to achieve an
output profile based on measurements made of such characteristics as current and
speed (closed loop).
Open loop control
Open Loop soft starters are soft starters producing a start voltage profile which is
independent of the current drawn, or the speed of the motor. The start voltage
profiles programmed to follow a predetermined contour against time. A very basic
Timed Voltage Ramp (TVR) system operates by applying an initial voltage to the
motor, and causing this voltage to slowly ramp up to full voltage. On basic
systems, the initial start voltage is not adjustable, but the ramp time is. Commonly
the voltage ramps time is referred to as the acceleration ramp time and is
calibrated in seconds. This is not an accurate description as it does not directly
control the acceleration of the motor. A lightly loaded motor can accelerate to full
speed even with a sixty second ramp selected.
14. 14
Closed loop control
Closed Loop starters monitor an output characteristic or effect from the starting
action and dynamically modify the start voltage profile to cause the desired
response. The most common closed loop soft starter is the controlled current soft
starter where the current drawn by the motor during start is monitored and
controlled to give either a constant current, or a current ramp soft start. A much
rarer closed loop format is the constant acceleration soft start where the motor
speed is monitored by a tachogenerator or shaft encoder and the voltage is
controlled to maintain a constant rate of acceleration or a linear increase in motor
speed. The controlled current soft starters are available with varying levels of
sophistication. In the most basic systems, the soft starter is essentially a standard
soft starter with a ramp freeze option where the current on one phase is monitored
and compared to a set point. If the current exceeds the set point, the ramp is
frozen until the current drops below that set point. This system is able to both
increase and reduce the start voltage to suit the application. A constant current
starter will start initially at zero volts and rapidly increase the output voltage until
the required current is delivered to the motor, and then adjust the output voltage
while the motor is starting until either full voltage is reached, or the motor
overload protection operates. Constant current starters are ideal for high inertia
loads, or loads where the starting torque requirements do not alter.
The current ramp soft starter operates in the same manner as the constant current
soft starter except that the current is ramped from an initial start current to a
current limit setting over a period of time. The initial start current, current limit,
and the ramp time are all user adjustable settings and should be customize to suit
the application. The current ramp soft starter can be used for a number of
advantages over constant current in some applications. Another form of closed
loop starter is the torque control starter where the starter models the motor under
high slip and low slip conditions and uses this mathematical model to calculate
the shaft torque being produced by the motor. This is then used as a feed back
source with linear and square law start torque curves being used to control the
15. 15
start voltage applied to the motor. The true torque control starter is able to give
much better control of the acceleration of the motor being started.
SERVO MOTOR
A motor equipped with servo control system is known as servo motor. An automatic
feedback control system for mechanical motion in which the controlled or output quantity
is position, velocity, or acceleration. It consists of several devices which control or
regulate speed/position of a load.
Needle positioning is one of the features that can only be provided in a machine that has a
servo motor. A needle positioning motor is electronically controlled, and offers the ability
to provide a variety of extra functions. The main function is needle positioning. The
motor can be configured to make the machine stop with the needle either in the work, or
out of the work, which eliminates the need for the operator to manually position the
needle using the hand wheel when turning a corner or removing work. Other options
available are digital control of sewing speeds and automatic foot lift at the beginning and
end of a sewing cycle. Usually servo motor contains an AC induction motor or AC
synchronous motor. AC induction motor is discussed in the clutch motor section.
SYNCHRONOUS MOTOR
The synchronous motor is basically the same as the induction motor but with slightly
different rotor construction. The rotor construction enables this type of motor to rotate at
the same speed (in synchronization) as the stator field. There are basically two types of
synchronous motors: self excited (as the induction motor) and directly excited.
The self excited motor (may be called reluctance synchronous) includes a rotor with
notches, or teeth, on the periphery. The number of notches corresponds to the number of
poles in the stator. Oftentimes the notches or teeth are termed salient poles. These salient
poles create an easy path for the magnetic flux field, thus allowing the rotor to "lock in”
and run at the same speed as the rotating field. A directly excited motor (may be called
hysteresis synchronous, or AC permanent magnet synchronous) includes a rotor with a
cylinder of a permanent magnet alloy. The permanent magnets north and south poles, in
effect, are the salient teeth of this design, and therefore prevent slip. In both the self
16. 16
excited and directly excited types there is a "coupling" angle, i.e. the rotor lags a small
distance behind the stator field. This angle will increase with load, and if the load is
increased beyond the motor's capability, the rotor will pull out of synchronism.
The synchronous motor is generally operated in an "open loop" configuration and within
the limitations of the coupling angle (or "pull-out" torque) it will provide absolute
constant speed for a given load. Also, note that this category of motor is not self starting
and employs start windings (split-phase, capacitor start), or controls which slowly ramp
up frequency/voltage in order to start rotation.
A synchronous motor can be used in a speed control system even though a feedback
device must be added. Vector control approaches will work quite adequately with this
motor design. However, in general, the rotor is larger than that of an equivalent
servomotor and, therefore, may not provide adequate response for incrementing
applications. Other disadvantages are: While the synchronous motor may start a high
inertial load, it may not be able to accelerate the load enough to pull it into synchronism.
If this occurs, the synchronous motor operates at low frequency and at very irregular
speeds, resulting in audible noise. Also for a given horsepower, synchronous motors are
larger and more expensive than non-synchronous motors.
Working of a Servo System
A command signal which is issued from the user's interface panel comes into the
servo's "positioning controller". The positioning controller is the device which
stores information about various jobs or tasks. It has been programmed to
activate the motor/load, i.e. change speed/position.
The signal then passes into the servo control or "amplifier" section. The servo
control takes this low power level signal and increases, or amplifies the power up
to appropriate levels to actually result in movement of the servo motor/load.
These low power level signals must be amplified: Higher voltage levels are
needed to rotate the servo motor at appropriate higher speeds and higher current
levels are required to provide torque to move heavier loads.
This power is supplied to the servo control (amplifier) from the "power supply"
which simply converts AC power into the required DC level. It also supplies any
low level voltage required for operation of integrated circuits. As power is applied
17. 17
onto the servo motor, the load begins to move and speed and position changes. As
the load moves, so does some other "device" move. This other "device" is a
tachometer, resolver or encoder (providing a signal which is "sent back" to the
controller). This "feedback" signal is informing the positioning controller
whether the motor is doing the proper job.
The positioning controller looks at this feedback signal and determines if the load
is being moved properly by the servo motor; and, if not, then the controller makes
appropriate corrections. For example, assume the command signal was to drive
the load at 1000 rpm. For some reason it is actually rotating at 900 rpm. The
feedback signal will inform the controller that the speed is 900 rpm. The
controller then compares the command signal (desired speed) of 1000 rpm and the
feedback signal (actual speed) of 900 rpm and notes an error. The controller then
outputs a signal to apply more voltage onto the servo motor to increase speed until
the feedback signal equals the command signal, i.e. there is no error.
Therefore, a servo involves several devices. It is a system of devices for
controlling some item (load). The item (load) which is controlled (regulated) can
be controlled in any manner, i.e. position, direction, speed. The speed or position
is controlled in relation to a reference (command signal), as long as the proper
feedback device (error detection device) is used. The feedback and command
signals are compared, and the corrections made. Thus, the definition of a servo
system is that it consists of several devices which control or regulate
speed/position of a load.
Open Loop/Closed Loop Control
Systems that assume motion has taken place (or is in the process of taking place)
are termed "open loop". An open loop drive is one in which the signal goes "in
one direction only" from the control to the motor. There is no signal returning
from the motor/load to inform the control that action/motion has occurred.
If a signal is returned to provide information that motion has occurred, then the
system is described as having a signal which goes in "two directions": The
command signal goes out (to move the motor), and a signal is returned (the
feedback) to the control to inform the control of what has occurred. The
18. 18
information flows back, or returns. This is an example of a "closed loop" drive.
The return signal (feedback signal) provides the means to monitor the process for
correctness.
Compensation
In order for the machine to produce good, accurate parts, it must operate in two
distinct modes: transient and steady state.
The first mode of operation, the transient state (may also be termed dynamic
response state), occurs when the input command changes. This causes the
motor/load to accelerate/decelerate i.e. change speed. During this time period,
there is an associated 1) time required for the motor/load to reach a final
speed/position (rise time) , 2) time required for the motor/load to settle and 3) a
certain amount of overshoot which is acceptable. The second mode of operation,
steady state, occurs when the motor/load has reached final speed, i.e. continuous
operation. During this time, there is an associated following accuracy (how
accurate the machine is performing). This is typically called steady state error.
The machine must be capable of operating in these two distinct modes in order to
handle the variety of operations required for machine performance. And in order
that the machine will perform without excessive overshoot, settle within adequate
time periods, and have minimum steady state error, the servo must be adjusted.
Types of Controls
The control of a motor will employ some type of power semiconductor. These
devices regulate the amount of power being applied onto the motor, and moving
the load.
One type of semiconductor is the SCR (silicon controller rectifier) which will be
connected to the AC line voltage. This type of device is usually employed where
large amounts of power must be regulated, motor inductance is relatively high and
accuracy in speed is not critical (such as constant speed devices for fans, blowers,
conveyor belts).
If smoother speed is desired, an electronic network may be introduced. By
inserting a "lag" network, the response of the control is slowed so that a large
instant power pulse will not suddenly be applied. Filtering action of the lag
19. 19
network gives the motor a sluggish response to a sudden change in load or speed
command changes. This sluggish response is not important in applications with
steady loads or extremely large inertia. But for wide range, high performance
systems, in which rapid response is important, it becomes extremely desirable to
minimize sluggish reaction since a rapid change to speed commands are desirable.
Transistors may also be employed to regulate the amount of power applied onto a
motor. With this device, there are several "techniques", or design methodology,
used to turn transistors "on" and "off". The technique or mode of operation may
be linear, pulse width modulated (PWM) or pulse frequency modulated (PFM).
The linear mode uses transistors which are activated, or turned on, all the time
supplying the appropriate amount of power required. If the transistor is turned on
half way, then half of the power goes to the motor. If the transistor is turned fully
on, then all of the power goes to the motor and it operates harder/faster. Thus
better speed stability and control is obtained.
Another technique is termed pulse width modulation (PWM). With PWM
techniques, power is regulated by applying pulses of variable width, i.e. by
changing or modulating the pulse widths of the power. In comparison with the
SCR control (which applies large pulses of power), the PWM technique applies
narrow, discrete (when necessary) power pulses. This technique has the advantage
in that the power loss in the transistor is small, i.e. the transistor is either fully
"on" or fully "off" and, therefore, the transistor has reduced power dissipation.
This approach allows for smaller package sizes.
The final technique used to turn transistors "on" and "off" is termed pulse
frequency modulation (PFM). With PFM, the power is regulated by applying
pulses of variable frequency, i.e. be changing or modulating the timing of the
pulses.
Types of Feedback Devices
Servos use feedback signals for stabilization, speed and position information.
This information may come from a variety of devices such as the analog
tachometer, the digital tachometer (optical encoder) or from a resolver.
20. 20
Analog Tachometers
Tachometers resemble miniature motors. But the tachometer is not used for a
power delivering device. Instead, the shaft is turned by some mechanical means
and a voltage is developed at the terminals (a motor in reverse!). The faster the
shaft is turned, the larger the magnitude of voltage developed (i.e. the amplitude
of the tach signal is directly proportional to speed). They can be used to provide
speed information to a meter (for visual speed readings) or provide velocity
feedback (for stabilization purposes).
Digital Tachometers
A digital tachometer, often termed an optical encoder or simply encoder, is a
mechanical-to-electrical conversion device. The encoder's shaft is rotated and an
output signal results which is proportional to distance (i.e. angle) the shaft is
rotated through. The output signal may be square waves, or sinusoidal waves, or
provide an absolute position. Thus encoders are classified into two basic types:
Absolute Encoder
The absolute encoder provides a specific address for each shaft position
throughout 360 degrees. This type of encoder employs either contact (brush) or
non-contact schemes of sensing position. The contact scheme incorporates a brush
assembly to make direct electrical contact with the electrically conductive paths
of the coded disk to read address information. The non-contact scheme utilizes
photoelectric detection to sense position of the coded disk.
Incremental Encoder
The incremental encoder provides either pulses or a sinusoidal output signal as it
is rotated throughout 360 degrees. Thus distance data is obtained by counting this
information. The disk is manufactured with opaque lines. A light source passes a
beam through the transparent segments onto a photo sensor which outputs a
sinusoidal waveform. Electronic processing can be used to transform this signal
into a square pulse train.
Resolvers
Resolvers look similar to small motors that is, one end has terminal wires, and the
other end has a mounting flange and a shaft extension. Internally, a "signal"
21. 21
winding rotor revolves inside a fixed stator. This represents a type of transformer:
When one winding is excited with a signal, through transformer action the second
winding is excited. As the first winding is moved (the rotor), the output of the
second winding changes (the stator). This change is directly proportional to the
angle which the rotor has been moved through.
COMPARISON OF POWER CONSUMPTION
Average annual power consumption for the two sewing machines, one having clutch
motor and the other one having servo motor, is calculated in the table 3.5. There is a clear
difference between the power consumption values of the both motors. The average and
total values of annual power consumption of the whole sample reveals that power
consumption for clutch motor is almost double as compared to the servo motor.
Table: Comparison of Annual Power Consumption of Each Motor
S. No. Clutch Motor Servo Motor
1 3857 1918
2 3965 1984
3 4078 2040
4 3788 1891
5 3902 1955
6 3835 1918
7 3798 1899
8 3662 1833
9 3931 1965
10 3817 1910
22. 22
0
1000
2000
3000
4000
5000
1 2 3 4 5 6 7 8 9 10
KWH
Graph 3.1 Comparison of Annual Power Consumption
Clutch Motor
Servo Motor
3863
1874
Graph: Average Annual Power Consumption (KWH)
Servo Motor
Clutch Motor
Power consumption is product of the actual power and the operating time. In the data
collection sheets actual power of the two motor types is almost the same, little more than
half horsepower which is also in close agreement with the nameplate power. Actual
power for new motors remains the same as stated on the nameplate but as they get old
actual power keeps on increasing than that of nameplate power. This results in an
increased power factor (ratio of actual power and nameplate power). Power factor is also
considered as a measure of the efficiency of the motor. Motors that have power factor
close to unity are considered as more efficient motors. Power factor for small motors is
quite less than that of the big motors. This is because of the fact that the total losses
inside the motor which include iron losses, copper losses, winding losses and frictional
losses tend to decrease as the size of the motor increases.
23. 23
Figure: Motor Losses
This means that actual power has little effect on the power consumption of the motors
thus decreasing the actual power may decrease the power consumption in motors but this
would be relatively small.
Operating time is the only other factor on which power consumption of the motor
depends. In the data collection sheets there is a major difference between the operating
times of the clutch motor as that of the servo motor. And the difference between the
power consumption of two motors is also due to this difference of operating time between
the clutch and servo motor. Operating time of the clutch motor is quite close to the shift
time excluding the time for different types of allowances. This is due to the fact that once
turned on clutch motor keeps on working for the whole shift until and unless the sewing
machine is turned off. While in case of servo motors when there is no load on the sewing
machine or in simple words the operator releases the paddle, motor is turned off. And as
soon as the operator presses the paddle again motor is turned on irrespective of the time
for which the paddle is in released state. This is the reason behind the difference in
operating time for the clutch and servo motor. This means that changing the time for
which motor operates make a significant difference in the power consumption of the
motors.
COMPARISON OF ANNUAL COSTS
Comparison of the annual costs of power consumption shows that each one of the clutch
motor has an additional annual operating cost of almost Rs 14,000. This means that in
case of clutch motor a company has to pay extra Rs 1000 for each clutch motor installed
in its production facility in its electricity bill.
24. 24
Table: Comparison of Annual Operating Costs
0
5000
10000
15000
20000
25000
30000
1 2 3 4 5 6 7 8 9 10
Rs
Graph: Comparison of Annual Operating Cost
Clutch Motor
Servo Motor
270430
131147
Graph: Annual Operating Costs for Ten Motors (Rs)
Servo Motors
Clutch Motors
S. No. Clutch Motor Servo Motor
1 3857 1918
2 3965 1984
3 4078 2040
4 3788 1891
5 3902 1955
6 3835 1918
7 3798 1899
8 3662 1833
9 3931 1965
10 3817 1910
25. 25
TOTAL LIFECYCLE OPERATING COSTS
Average lifecycle of sewing machine motor is almost ten to fifteen years under standard
working conditions for the clutch motor and even higher for the servo motor almost
fifteen to twenty years. Here a ten years lifecycle is supposed for both the motors to
calculate the total running cost of the motors.
A clutch motor along with the energy conservation device will have total life cycle
operating cost equal to that of a servo motor as such a device will allow clutch motor to
operate for the same duration as that of servo motor. Thus there is no need of observing
the operating time of clutch motor after the installation of the energy conservation device
and the lifecycle operating cost will also be the same as that of servo motor.
Installation & Maintenance Costs
Installation cost is the purchase price of the sewing machine and the price of
necessary attachments like table, power cables etc. plus any extra amount that
needs to be paid for the shipment and installation of a sewing machine.
Installation cost of Juki DDL 8300 with clutch motor = 36,000
Installation cost of Juki DDL 8700 with servo motor = 71,000
Installation cost of Energy conservation device for clutch motor = 1,000
Installation cost of clutch motor with energy conservation device = 37,000
Maintenance cost is negligible for both clutch and servo motor as compared to
installation and running costs. Sewing machine motors are not required to drive
heavy loads for long periods of time. That is why usually sewing machine motors
do not fail or burn over their entire lifecycle. Only decrease in their performance
occurs with the passage of time. After installing an energy conservation device for
clutch motor operating time of the clutch motor will be reduced which will also
result in and increase in the lifecycle of the motor.
COMPARISON OF INSTALLATION & LIFECYCLE OPERTING COSTS
Sewing machines having clutch motors have almost half installation cost as that of one
with servo motor. But the lifecycle operating cost of clutch motors is more than double
26. 26
than that of servo motors. Also there is a relatively large difference of 75% between the
installation cost and operating cost of clutch motors as compared to 19% of the servo
motors. This comparison shows that a large investment is needed for the sewing
machines with servo motors but this large investment will pay for whole lifecycle of the
sewing machines. But in case of clutch motors sewing machines small investment is
needed but this small investment will cut down your profits for the whole lifecycle of
sewing machines.
Clutch motors having an energy conservation device have the same lifecycle operating
cost as that of the servo motors and half as compared to clutch motors with no such
device but installation cost of such motors is almost half as that of the servo motors. This
shows that such a device can save the same amount of electricity as a servo motor can do
but the installation cost needs to be paid for such a sewing machines will be half of the
sewing machines with servo motors.
27. 27
DISCUSSION
Clutch motor is thought to be an ideal solution for the conditions where continuous
recurring operation is required due to its rugged and hard deign and construction. This is
the reason even the latest overlock machines have clutch motor as the driving component
just because it has to do the same operation at the same speed when the paddle is pressed.
While servo motor is best suited for sophisticated and precise operations where position
accuracy and velocity control is of the prime importance. And for this reason latest
lockstitch and special purpose machines have a servo motor. During the start of the
research work, a naive technician gave us a simple but quite right comment on the
difference between clutch and servo motor. He said “clutch motor is mechanical motor
while servo motor is electronic motor”. In fact the major and most visible difference
between the clutch and servo motor is that clutch motor is designed for the mechanical
control while servo motor is specifically designed for the electronic control. Now a days
sewing is not just the single line stitching but complex combination of different stitches
and seams. Especially in fashion articles some stitches are too complex to be done by a
sewing machine. But in large scale manufacturing production is considered as important
as that of the quality. That is why technology up gradation is becoming essential for any
company that wants to compete internationally. Servo motor is a prime example of
always changing technology.
Clutch motor is proving to be a necessary evil of the industry due to its amazingly low
cost and high performance in certain conditions. But as world is shifting towards more
and more environment friendly culture by conserving energy and natural resources, the
era of clutch motor is near to end. As technology will advance the price difference
between the clutch and servo motor will keep on decreasing and then there will be no
excuse for using a power hungry clutch motor over a smart working servo motor.
28. 28
CONCLUSIONS
Cost comparison of clutch and servo motor sewing machines shows that purchase price
of servo motor sewing machine is almost double than that of the one with clutch motor
but running cost of the servo motor sewing machine is almost half than that of one with
clutch motor over their life cycle. This results show that if a facility that has ten sewing
machines with clutch motor will have to pay a total sum of Rs 3,064,000 in ten years.
While a facility that has ten sewing machines with servo motor will have to pay a total
sum of 2,063,000 in the same ten years. So if it is all about the cost then servo motor is
undoubtedly the better option.
A critical analysis of cost and benefits show that servo motor sewing machine is quite a
better option as compared to the clutch motor sewing machine if costs and benefits are
considered over the life cycle of both the sewing machines. Benefits offered by servo
motor are more profitable for a company than that of the clutch motor.
Energy conservation device for clutch motor can do the same energy savings as a servo
motor can do but other features offered by servo motor are almost impossible to be
incorporated in clutch motor due to its design limitations. But the price of making such a
device locally is quite small, in our case almost Rs.1000. That’s why a device with such
low cost having considerable cost reduction needs not to be compared with the state of
the art servo motor with a huge price tag.
29. 29
RECOMMENDATIONS
A sewing machine with servo motor should always be preferred over the clutch motor
except where continuous recurring operation is required like in overlock sewing machine.
Sewing machines that have clutch motor should be replaced by servo motor wherever
possible as performance and features of servo motor sewing machine are unmatched with
those of a clutch motor sewing machine
Energy conservation device can make a difference by saving noticeable amount of
electricity which is also the need of the hour due to current electricity crisis in the
country. So a company that has ten or more than ten clutch motor sewing machines
should make use of such technology to reduce its operating costs and increase profits.
Energy conservation is not an overnight job. Today every individual and every
organization has an equal role to play. Someone has rightly said that a little effort by
many can make a better difference than a huge effort by a few. So every one has to come
forward to join hands to make this world a better place for living
