International Journal of Science, Engineering and Technology Research (IJSETR)
Volume 1, Issue 1, July 2012
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Design of 215kW DC Motor for Centrifugal Machine Used in Sugar Mill (IJSETR)
Sis Sis Soe
Abstract— In designing the DC machine with appropriate performance characteristics, it is essential that the designer is fully conversant with the typical performance of the machine that is needed to improve the design of DC motor. The field of direct current usage being very wide, DC machines is produced both as generators and motors for a large range of output, voltage, speed etc. DC motors offer many distinct economic and technological advantages in special application like rolling mills, overhead cranes, lifts, electric vehicles and electric trains. The DC motor basically works on the principle that when a conductor carry current is placed in a magnetic field, mechanical force acts on the current carrying conductor, as a result the conductor starts rotating in a direction depending upon the direction of current and the field is given by Flemings’ left hand rule. The e.m.f induced in the armature of alternates with a frequency of f, Hz depending upon the number of poles in the machine and the speed of the armature.
Index Terms—400V DC Supply, Compound DC Motor, Design Calculation, Sugar Mill
I. INTRODUCTION
DC motors comprise one of the most common types of actuator designed into electromechanical systems. They are a very straightforward and inexpensive means of creating motion or forces.
Electric motors are an essential component of our industrial society with no less than 5 billion motor, built world wide every year. The most wage two types of motor are AC motor and Dc motor. DC motors are widely used in many industrial applications such as electric vehicles, steel rolling mills, electric cranes, and robotic manipulators. The direct current motor is one of the first machines devised to convert electrical power into mechanical power. When a conductor carrying current is placed in a magnetic field, force is developed in the conductor. If a number of conductors connected in series are placed on a cylindrical rotor and current is allowed to flow through the conductors, a torque will motor, two magnetic fields can be found, i.e. the field produced by the magnets and the field produced by the current flowing through the armature conductors.
DC motors are classified such as shunt motor, separately excited motor, series motor, compound motor, permanent magnet DC (PMDC) motor and so on. AC motors are divided into two groups such as induction motor and synchronous motor. In this paper we are designed for the compound DC motor for the use of centrifugal machine at fugal stage.
II. Sugar Manufacturing Process
A sugar mill is a large factory used to produce raw sugar and other products from sugar cane. Mills are made up of a range of industrial plant such as boilers, storage and processing vessels, crushing and hammer mills and a large range of maintenance equipment. Mills operate in two distinct modes, crushing and non-crushing, both of which introduce a range of specific and general hazards to PCBUs, workers and others. In essence, a sugar mill can be broken into the following processes (see Fig. 1, for a diagram that shows the sugar milling process). Sugar manufacturing process consists of;
1. Cane handling (Preparation of Sugarcane)
2. Milling house
3. Clarification and evaporation
4. The pan stage
5. The fugal stage
6. Final sugar
7. Energy supply systems
8. Associated operations
Figure.1. Raw and Refined Sugar Process
A. Cane Handling
The main objective of cane preparation is shredder from the cane with the help of machine power before milling to the smallest cane chip and which is sending with preparation to cane milling. Thus, this smallest cane chip can be milled from milling machine one to milling machine four and get not only more juice but also good mill extraction for factory.
The main ambitions of cane preparation are
1. To increase in bulk density
2. To increase in crushing capacity
3. To improve in pol extraction
4. To improve in primary juice extraction
5. To perform proportion load on mills
6. To obtain higher efficiency of imbibitions
7. To reduce in hydraulic pressure and thus less power consumption
8. To reduce wearing of mill rollers
The followings are used for cane preparation
1. Cane feed table
2. Cane carrier
3. Cane leveler
4. Cane knives (cane cutter)
5. Magnetic iron separator
6. Cane shredder
7. Elevator
B. Milling House
The milling process essentially involves the removal of juice from sugarcane by squeezing the cane between pairs of large cylindrical rolls in a series of milling units collectively called a milling train, as show in Fig. 2. The first milling unit in the milling train is generally identified as #1 mill; the second milling unit is generally identified as #2 mill, and so on. The last milling unit is generally called the final mill. The milling units between the first and final mills are collectively known as intermediate mills.
After passing through a pair of rolls and expressing juice, the remaining sugarcane material is known as bagasse. Only the first milling unit in the milling train processes prepared cane. The remaining milling units process bagasse. After being processed by a mill, bagasse typically consists of 30% to 50% of fiber, 45% to 60% of water and a diminishing quantity of brix as subsequent milling units process the bagasse. Although prepared cane and bagasse are defined as different materials, bagasse as a general term to collectively refer to both prepared cane and bagasse.
Figure.2. Schematic diagram of milling train
To aid in the extraction of juice from the much drier bagasse, water or diluted juice is added to the bagasse before it enters the milling unit in a process called imbibition. The water or juice added to the bagasse is called imbibitions process, called compound imbibitions, is shown in Figure 3. Imbibition water is added to the bagasse entering the final milling unit at a rate that is typically 200% to 300% of the rate of fiber passing through the milling train. The juice expressed from the final milling unit is used as imbibition juice for the second last milling unit. The juice from the second last milling unit is then used as imbibitions juice for the third last milling unit. This process continues back to the second milling unit.
After first passing through a juice screen to remove most of the fiber in the juice, the juice from the first and second milling units, called mixed juice, is taken away for processing into sugar. The fiber removed in the juice screen, called cush, is returned to the milling train, usually before the second milling unit. The bagasse from the final milling unit is taken away for further processing, typically for burning in the boiler furnace.
C. Fugal Stage
A fugal is a large electric centrifuge which spins up to 1200 revolutions per minute dependent on its function and stage of operation. There are two types of centrifuge in use within sugar mills, high grade centrifuges (usually batch, but sometimes continuous) and low grade centrifuges which are continuous. Continuous fugal maintain a constant flow of product through them while batch fugal fill, operate and then discharge the final product.
The fugal stage removes the remaining liquid product which surrounds the crystal, washes the crystal and delivers it into the final sugar system through a series of conveyors and a drier. The material removed during the centrifuge process is known as molasses and has a range of uses including sale as stock feed, fermentation for distillery production and as a component of cattle licks.
D. Electric Motor Used in Sugar Mill
At sugar mill, electric motors are used for cane feed table, cane carrier, cane leveler, cane knives, magnetic iron separator, cane shredder, cane elevator, mill tandem and centrifugal machine. The electric motors used for sugar mill are wound-rotor induction motor with slip ring, squirrel-cage induction motor and DC motors such as separately excited motor, shunt motor and series. Especially, compound DC motors are used for centrifugal machine at fugal stage.
III. Compound DC Motor
Compound motors are a combination of series and shunt motors. A compound motor has two field windings, one is a series with the armature winding, and which is therefore made of thick wire of few turns. The second winding is called the shunt field winding and is joined in parallel with the armature and is made of thin wire of many turns (Figure.3). The series field winding is wound above the shunt field winding on the same pole shoe. In a compound motor the major portion of the flux is produced by the shunt field winding.
Figure.3.Connections of a compound motor
There are two types of compound motors according to the manner in which their field windings produce field flux;
1. Differential Compound Motor, and
2. Commulative (or addative ) Compound motor
IV. Design Theory
As compared to rotating machine, the design of DC motor is simpler, because of complex inter relations between magnetic and electric circuit. The aim, in designing the DC machine is to obtain the complete dimensions of various parts of the machine as lute below, to furnish these data to the manufacturer.
1. Main dimensions of the armature structure
2. Design details of the armature winding
3. Main dimension of the field system
4. Design details of the field winding
5. Design details of the commutator and brushes
6. Design details of inter poles and its winding
- Performance characteristics i.e. iron losses copper losses, mechanical losses, efficiency and maximum efficiency
In order to determine the above design information, for the DC motor, designer needs the following.
1. Detailed specification of the DC motor
2. Limiting value of performance characteristics like iron losses, copper losses, efficiency
3. Design equation based pm which design procedure is to be initiated
4. Information regarding availability of material for various parts
The above information needed to carry out the design has been discussed in subsequent articles. Hence the design of DC motor is carried out based on given specifications, using available materials economically and to achieve lower cost, reduced size and better operating performance. Important specifications needed to initiate the design are given below (input design data). Type of field excitation; rated output power; rated output voltage; speed; type of enclosure; type of duty (short time, inter mitten, continuous); field excitation voltage; maximum temperature rise. After that we can draw good design and we can construct efficiency good DC motor for our load.
V. Design Equation Of DC Motor
Design equation expresses the relationship between the output of the machine and the main dimension of the armature in terms of specific magnetic and electric loadings. It is essential to define the terms specific magnetic and electric loadings before the derivation of output equation. For the specific magnetic loading, equation (1) is used and for the specific electric loading, equation (2) is used. For number of conductors, equation (3) is used and derivation of output equation, equation (4) is used. For gross length of armature and armature resistance, equations (5) and (6) are used. Equations (7) to (21) are used for resistance of winding, flux frequency reversal, speed, electromagnetic torque and efficiency of DC motor.
A. Design of Armature Winding
(1)
Where,
Bav = Average gap flux density, Tesla
P= Number of poles
Ø= gap flux per pole, Wb
D = diameter of armature, m
L = gross length of armature, m
(2)
Where,
q = specific electric loading
Ia = armature current
(3)
Where,
E = supply voltage, V
N = number of speed,r.p.m
Z = number of conductors
A = number of parallel paths
(4)
Where,
Pa = output power
(5)
Where,
C0 = output coefficient=0.164Bavq×10-3
(6)
Where,
Kp = ratio of pole arc and pole pitch
(7)
Where,
ρ = resistivity of material, ohm-mm2/m
Lc = length of conductors, m
ac = cross sectional area of conductor, mm2
(8)
Where,
f = flux frequency reversal, Hz
(9)
Where,
Eb= induced voltage
K = constant
φ = flux per pole of motor, wb
(10)
Where,
S= number of slots
Di=D-2hs-2dc (11)
Where,
Di=internal diameter of armature
hs=depth of slots
dc=depth of armature core
(12)
Where,
θ =temperature-rise of armature
P′=total armature iron and copper losses per
sq.cm of the cooling surface, w/cm2
vA=peripheral speed of armature, m/sec
B. Design of field Winding
(13)
Where,
Lmtf = mean length of the turns of shunt exciting
coil
Tsh = total turn of shunt field winding
Ash = cross sectional area of conductor
(14)
Where,
Tsh = total number of turn
ATf = total ampere turn of the exciting coil
Ish = shunt field current
(15)
Where,
Ap = gross area of pole
Lp = axial length of the pole
(16)
Where,
ATse= ampere turn of series winding
Ise = armature current, Ia
(17)
Where,
θ =temperature rise of field system
Pf=losses of field system per unit cooling
surface, W/cm2
vA=peripheral speed of armature in m/sec
C. Design of Commutator and Brushes
(18)
Where,
θ =temperature rise of commutator
vc=peripheral speed of commutator
(19)
Where,
Ib′=current per brush
(20)
Where,
Nb=number of brush per spindle
Ab′=total brush area per spindle
Ab=area of brush
(21)
Where,
η = efficiency
VI. Design Results
Design results can get as shown in table (2) by using input table (1) and specific magnetic loading equation (1), specific electric loading equation (2) and output power equation (4) and equations (5) to (21).To calculate the proper size with input data rating, gap flux density, core flux density and specific electric loading are assumed. Note that every flux density values never exceed 2.2 Tesla. One DC motor can get good efficiency by making starting torque may be rise to higher value. The main important fact is to reduce m motor losses.
TABLE I