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STUDY

Power Consumption in Clutch and Servo Motor

Sewing Machines

Mansoor Faiz Cheema

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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

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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.

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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

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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.

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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

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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

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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

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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

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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

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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 I2R 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

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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

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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.

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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

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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

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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

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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

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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

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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

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

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

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

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

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

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

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.

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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.

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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.

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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