Speed Control Of Dc Machine Engineering Essay

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DC motors are electrical machines that consume dc electrical power and bring forth mechanical torsion. DC motors are classified harmonizing to the connexion of the field circuit with regard to the armature circuit. Traditionally dc motors were classified as shunt, series or individually excited. In add-ons it was common to see motors referred to as compound-wound. There is truly no cardinal difference between shunt, series or individually aroused District of Columbia motors, and the names merely reflect the manner in which the field and armature circuits are interconnected.
The District of Columbia motor has two separate circuits. The smaller brace of terminuss connect to the field twists which surround each pole and are usually in series, in the steady province all the input power to the field twists is dissipated as heat, none of it is connected to mechanical end product power. The chief terminuss convey the current to the coppices which make the sliding contact to the armature weaving on the rotor. The supply to the field is separate from that for the armature hence the description individually excited.
In shunt District of Columbia motors, the field circuit is connected in analogue with the armature circuit while DC series motors have the field circuit in series with the armature where both field and armature currents are indistinguishable. The coppices and commutators are troublesome for District of Columbia motors at really high velocity whereas little dc motors say up to 100s of Watts end product can run at possibly 12000 rev/min but the bulk of medium and big motors are normally designed for velocities below 3000 rev/min.
Direct current ( DC ) motors have been widely used in many industrial applications such as electric vehicles, steel turn overing Millss, electric Cranes, and robotic operators due to precise, broad, simple, and uninterrupted control features. The coveted torque-speed features could be achieved by the usage of conventional relative integral- derived function ( PID ) accountants. [ 1 ]
Dc motors are largely preferred because they are easy to utilize and command and non merely this they even deliver High get downing torsion and their characteristic public presentation is besides about additive. But when it comes to Rush control of District of Columbia motor the intent of a motor velocity accountant is to take a signal stand foring the demanded velocity, and to drive a motor at that velocity. The accountant may or may non really mensurate the velocity of the motor. If it does, it is called a Feedback Speed Controller or Closed Loop Speed Controller, if non it is called an Open Loop Speed Controller. Feedback velocity control is better, but more complicated. [ 2 ]
Speed Control of Separately excited Dc Motor
In this method, shunt-field current is maintained changeless from a separate beginning while the electromotive force applied to the armature is varied. Dc motors feature a velocity, which is relative to the counter voltage. This is equal to the applied electromotive force minus the armature circuit IR bead. At rated current, the torsion remains changeless regardless of the District of Columbia motor velocity ( since the magnetic flux is changeless ) and, hence, the District of Columbia motor has changeless torsion capableness over its velocity scope. [ 5a ]
The intent of a motor velocity accountant is to take a signal stand foring the demanded velocity, and to drive a motor at that velocity. The accountant may or may non really mensurate the velocity of the motor. If it does, it is called a Feedback Speed Controller or Closed Loop Speed Controller, if non it is called an Open Loop Speed Controller. Feedback velocity control is better, but more complicated, and may non be required for a simple automaton design. [ 4 ]
The velocity of a individually aroused District of Columbia motor could be varied from zero to rated velocity chiefly by changing armature electromotive force in the changeless torsion part. Whereas in the changeless power part, field flux should be reduced to accomplish velocity above the rated velocity. Control is obtained by weakening the shunt-field current of the District of Columbia motor to increase velocity and to cut down end product torsion for a given armature current. Since the evaluation of a District of Columbia motor is determined by heating, the maximal allowable armature current is about changeless over the velocity scope. This means that at rated current, the District of Columbia motor ‘s end product torsion varies reciprocally with velocity, and the District of Columbia motor has constant-horsepower capableness over its velocity scope.
Dc motors offer a solution, which is good for merely obtaining velocities greater than the base velocity. A fleeting velocity decrease below the District of Columbia motor ‘s base velocity can be obtained by overexciting the field, but prolonged over excitement overheats the District of Columbia motor. Besides, magnetic impregnation in the District of Columbia motor permits merely a little decrease in velocity for a significant addition in field electromotive force. If field control is to be used, and a big velocity scope is required, the base velocity must be proportionally lower and the motor size must be larger. If velocity scope is much over 3:1, armature electromotive force control should be considered for at least portion of the scope. Very broad dynamic velocity scope can be obtained with armature electromotive force control. However, below approximately 60 % of base velocity, the motor should be de rated or used for merely short periods. [ 5b ] The velocity ( N ) of a DC motor is relative to its armature electromotive force ; the torsion ( T ) is relative to armature current, and the two measures are independent, as illustrated in Figure below.
Dc Motor features
Operation of Single-Phase Half-controlled Bridge Rectifier
A fully-controlled rectifier circuit contains merely controlled-rectifiers, whereas a semi-controlled rectifier circuit is made up of both controlled and uncontrolled rectifiers. Due to presence of rectifying tubes, A free-wheeling operation takes topographic point without leting the span end product electromotive force to go negative.A In a semi-controlled rectifier, control is effected merely for positve end product electromotive force, and no control is possible when its end product electromotive force tends to go negative since it is clamped at zero V. Here the operation of a single-phase half-controlled rectifier is explained. [ 3 ]
.
One-half controlled Bridge rectifier
In this circuit, SCRs S1 and S3 behavior during a & lt ; wt & lt ; p. During P & lt ; wt & lt ; ( p + a ) , the device in conductivity is diode D and the end product of the span is clamped at nothing. During ( p + a ) & lt ; wt & lt ; 2p, the devices in conductivity are SCRs S2 and S4. Diode D would carry on during 0 & lt ; wt & lt ; a.
Here are some consequences taken with the simulation of individual stage half-controlled span rectifier on firing angle of zero grade.
Steady-State operation of Separately Excited Dc Motor
Steady province is such a signifier for Dc motor features in which it indicates how the motor behaves when any transient effects have died off and conditions have one time once more go steady. Steady State features are normally much easier to foretell than transeunt features. Under steady province conditions the armature current I is changeless.
The equation below is the armature circuit electromotive force equation.
V = E + IR + L ( dI/dt )
Where electromotive force V is the electromotive force applied to the armature terminuss and E is the internally developed motional e.m.f. The opposition and induction of the complete armature are represented by R and L.
Under driving status, the motional e.m.f E ever opposes the applied electromotive force V, and for this ground it is known as back e.m.f. And for the current to be forced in to the motor V must be greater so E. The last spot of the above mentioned equation represents the inductive V bead due to armature self induction. This electromotive force is relative to the rate of alteration of current. So under steady province this last term will be ZERO. So we can disregard that last term for steady province operations. Then under steady province conditions the above equations becomes,
V = E + IR
So,
I = ( V-E ) /R
In shunt District of Columbia motors, the field circuit is connected in analogue with the armature circuit. It has the undermentioned tantamount circuit:
Fig. 1. Equivalent Circuit of
DC Shunt Motor
Under steady province status the clip derivative is zero presuming that the motor is non saturated. Some of import field and armature equations are as follows.
For field circuit,
The back e.m.f is given by:
The armature circuit
Now the torsion and velocity under the steady province status can be found with the undermentioned expression: The motor velocity can be easy derived:
If Ra is a little value, or when the motor is lightly loaded, i.e. Ia is little,
That is if the field current is unbroken changeless, the motor velocity depends merely on the supply electromotive force.
The developed torsion is:
The needed power is:
Experiment
In the District of Columbia motor the field twists is used to excite the field flux. And the armature current is supplied to the rotor via coppice and commutator for the mechanical work. This Interaction of field flux and armature current in the rotor produces torsion. When a individually aroused motor is excited by a field current of if and an armature current of Iowa flows in the circuit, the motor develops a back e.m.f and a torsion to equilibrate the burden torsion at a peculiar velocity. The if is independent of the ia.Each twists are supplied individually. Any alteration in the armature current has no consequence on the field current. The if is usually much less than the Iowa.
The first thing is to wire up the circuit of individually excited District of Columbia motor with DMS2 informations acquisition system as mentioned on the manual provided for experiments.
Steady State Features
After get downing the DMS2 Data acquisition system package the users parametric quantities were defined in the package manually which are Input power, Output power and Efficiency for which the values has to be recording equipment. After seting all the parametric quantities the District of Columbia motor started with the burden torsion value of 0 Nm and so the values for the parametric quantities which were introduced in the package were taken in an machine-controlled mode utilizing the F2 button on the keyboard. And so bit by bit increases the values of burden torsion from 0 – 0.5 Nm with 50 stairss with each measure entering the values of parametric quantities predefined in DMS2 Software.
From no-load to full burden the velocity falls linearly as a consequent the back e.m.f falls linearly excessively. The power losingss in the armature opposition are I2R. The power converted from electrical to mechanical is given by VI. The power required to get the better of clash and Fe losingss can be found under no-load conditions and acquire deducted from the laden status when the losingss are non taken into history. Two of import observation follow from these computations. First the velocity bead with burden is really little. This is really desirable for most applications. Since all we have to make to keep about changeless velocity is to put the appropriate armature electromotive force and maintain it changeless. Second a delicate balance between V and E is revealed. The current is in fact relative to the difference between V and E. so that rather little alterations in either E or V give rise to disproportionately big alterations in the current. Hence to avoid inordinate current difference between E and V must be limited. The no-load velocities are straight relative to the applied electromotive force, while the incline of each curve is the same, being determined by the armature opposition. The smaller the opposition the less the velocity falls with burden.
Motor Armature Voltage & A ; Current in Steady State Operation
Steady province is such a signifier for Dc motor features in which it indicates how the motor behaves when any transient effects have died off and conditions have one time once more go steady. Steady State features are normally much easier to foretell than transeunt features. Under steady province conditions the armature current I is changeless.
The 2nd portion of the experiment consists of utilizing the DMS2 system as a digital storage CRO. In this experiment the instantaneous wave forms are recorded at burden torsion values of 0.1, 0.3 and 0.4 Nm. As we increases the value of burden torsion it causes the motor velocity to diminish bit by bit.
Consequences
Steady-state features
Wave forms of motor armature electromotive force and current in steady-state operation
0.1 Nm
0.3 Nm
0.4 Nm
Discussion
For a DC motor, the velocity ( RPM ) is relative to the applied electromotive force. The current that the motor will take from the supply is relative to the burden. A motor with no burden will take really small current, a motor which is loaded and making some work will take more current. The burden on the motor appears as a torsion on the end product shaft, the more burden, the more torsion. So to seeking and sum-ups: After Puting the motor velocity by puting the supply electromotive force, as we increase the burden ( torsion ) on the motor, the current taken from the supply will increase. The motor will besides decelerate down a spot as an increase in the burden torsion. We have seen that if the burden torsion on the shaft of motor additions, the velocity falls and the armature current automatically increases until equilibrium of torsion is reached and the velocity addition becomes steady. If the armature electromotive force is at its maximal value, and we increase the mechanical burden until the current reaches its rated value, we are clearly at full- burden i.e. we are runing at the full velocity and full torsion.
Clearly if we increase the burden on the shaft still more, the current will transcend the safe value, and the motor will began to overheat. But the inquiry which this prompts is if it were non for the job of overheating, could the motor deliver more and more end product, or is there a bound?
We can see immediately that there will be a maximal point by looking at the torque-speed curve. The mechanical end product power is the merchandise of torsion and velocity and we see that the power will be zero when either the burden torsion is zero or the velocity is zero. And it is easy to demo that the peak mechanical power occurs when the velocity is half of the no-load velocity. But familiarisation with the District of Columbia motors brings the construct of high care cost and big size of the motors as compared to initiation motors. And dc machines are non suited for high velocity operations due to the commutator and coppices and they are besides non suited for the clean or explosive environments.
Decisions
A shunt or individually aroused District of Columbia motor has a torque-speed characteristic whose velocity drops linearly with increasing burden torsion. Its velocity can be controlled by altering its field current, its armature electromotive force or its armature opposition. The graph obtained from the ensuing values between torque-armature electromotive force shows that the relationship between the torsion and armature electromotive force is about additive as the armature electromotive force increases it brings a additive alteration in torsion besides. Whereas the velocity decreases as the torsion addition in a steady mode. Efficiency of the motor increases quickly and so diminish in a rapid mode as we increase the value of torsion. The consequences show that when the machine is running at the rated conditions, the steady-state values are in good understanding.
It has been observed that under maximal power conditions the overall efficiency is merely about 50 % because an equal power is burned off as heat in the armature opposition. And merely really little motors can of all time be operated continuously in this status.
Introduction
It is non merely supremely elegant as an electromechanical energy convertor, but is besides by far the most of import, with something like one tierce of all the electricity generated being converted back to mechanical energy in initiation motors. Like the d.c motor, the initiation motor develops torsion by the interaction of axial currents on the rotor and a radial magnetic field produced by the stator. But whereas in the District of Columbia motor the current has to be fed in to the rotor by agencies of coppices and a commutator, the torsion bring forthing current in the rotor of the initiation motor are induced by electromagnetic action, therefore the name initiation motor. The stator weaving non merely hence securing the magnetic field but besides supplies the energy that is converted to mechanical end product. The absence of any skiding mechanical contacts and the consequent salvaging in footings of care is a major advantage of the initiation motor over its d.c challenger.
To understand how an initiation motor operates, we must foremost unknot the enigmas of the revolving magnetic field. The rotor will be efficaciously dragged along by the revolving field, but that it can ne’er run rather every bit fast as the field. When it is needed to command the velocity of the rotor it is best to command the velocity of the field. The mechanism of the revolving field focal point on the stator twists because they act as the beginning of flux. In this portion the presence of rotor gets neglected merely to do it easier to understand what governs the velocity of rotary motion and the magnitude of the field, which are the two factors which largely influence the motor behavior. The interaction between the rotor and the stator well justifies the external features of the motor. i.e. the fluctuation of motor torsion and stator current with velocity.
Broadly talking the motor interior decorator shapes the stator and rotor dentitions to promote every bit much as possible of the flux produced by the stator to go through right down the rotor dentition, so that before finishing its way back to the stator it is to the full linked with the rotor music directors which are located in the rotor slots. This is tight magnetic yoke between stator and rotor twists is necessary for good running public presentation. And the field which provide the yoke is of class the chief or air-gap filed. The huge bulk of the flux produced by the stator is so chief or common flux. But there is some flux which bypasses the rotor music director, associating merely with the stator twist, and known as storage escape flux.
We all know that the synchronal velocity of the initiation motor is given by Ns = 120f/P. So from this relation, it is apparent that the synchronal velocity and therefore the velocity of the initiation motor can by varied by the supply frequence. This method has its ain restrictions. The motor velocity can be reduced by cut downing the frequence, if the initiation motor happens to be the lone burden on the generators. Even so the scope over which the velocity can be varied is really less. [ 6 ]
V/f changeless Principle
Because of progresss in solid province power devices and microprocessors, variable velocity AC Induction motors powered by exchanging power convertors are going more and more popular. Switch overing power convertors offer an easy manner to modulate both the frequence and magnitude of the electromotive force and current applied to a motor. As a consequence much higher efficiency and public presentation can be achieved by these motor thrusts with less generated noises. The most common rule of this sort, is the changeless V/Hz rule which requires that the magnitude and frequence of the electromotive force applied to the stator of a motor maintain a changeless ratio. By making this, the magnitude of the magnetic field in the stator is kept at an about changeless degree throughout the operating scope. Thus, ( upper limit ) changeless torsion bring forthing capableness is maintained. When transeunt response is critical, exchanging power convertors besides allow easy control of transeunt electromotive force and current applied to the motor to accomplish faster dynamic response. The changeless V/Hz rule is considered for this application. The energy that a exchanging power convertor delivers to a motor is controlled by Pulse Width Modulated ( PWM ) signals applied to the Gatess of the power transistors. PWM signals are pulse trains with fixed frequence and magnitude and variable pulsation breadth. There is one pulsation of fixed magnitude in every PWM period. However, the breadth of the pulsations alterations from period to period harmonizing to a modulating signal. When a PWM signal is applied to the gate of a power transistor, it causes the bend on and turn off intervals of the transistor to alter from one PWM period to another PWM period harmonizing to the same modulating signal. The frequence of a PWM signal must be much higher than that of the modulating signal, the cardinal frequence, such that the energy delivered to the motor and its burden depends largely on the modulating signal. Figure 1 shows two types of PWM signals, symmetric and asymmetric edge-aligned. The pulsations of a symmetric PWM signal are ever symmetric with regard to the centre of each PWM period. The pulsations of an asymmetric edge-aligned PWM signal ever have the same side aligned with one terminal of each PWM period. Both types of PWM signals are used in this application.
Symmetric and Asymmetric PWM Signals
It has been shown that symmetric PWM signals generate less harmonics in the end product current and electromotive force. Different PWM techniques, or ways of finding the modulating signal and the switch-on/switch-off blink of an eyes from the modulating signal, exist. Popular illustrations are sinusoidal PWM, hysteresis PWM and the comparatively new infinite vector PWM. These techniques are normally used with three stage Voltage Source power inverters for the control of three-phase AC initiation motors. The infinite vector PWM technique is employed in this application.
Assume the electromotive force applied to a three stage AC Induction motor is sinusoidal and neglect the electromotive force bead across the stator resistance. Then we have, at steady province,
from which it follows that if the ratio V/ degree Fahrenheit remains changeless with the alteration of degree Fahrenheit, so A remains changeless excessively and the torsion is independent of the supply frequence. In existent execution, the ratio between the magnitude and frequence of the stator electromotive force is normally based on the rated values of these variables, or motor evaluations. However, when the frequence and hence besides the electromotive force are low, the electromotive force bead across the stator opposition can non be neglected and must be compensated. At frequences higher than the rated value, the changeless V/ degree Fahrenheit rule besides have to be violated because, to avoid insularity interrupt down, the stator electromotive force must non transcend its rated value. This rule is illustrated in Figure 2.
Voltage Versus Frequency under changeless V/ degree Fahrenheit Principle
Since the stator flux is maintained changeless, independent of the alteration in supply frequence, the torsion developed depends on the faux pas velocity merely, which is shown in Figure 3. So by modulating the faux pas velocity, the torsion and velocity of an AC Induction motor can be controlled with the changeless V/Hz rule.
Torque Versus Slip velocity of initiation motor while changeless stator flux
Both unfastened and closed-loop control of the velocity of an AC initiation motor can be implemented based on the changeless V/Hz rule. Open-loop velocity control is used when truth in velocity response is non a concern such as in HVAC ( warming, airing and air conditioning ) , fan or blower applications. In this instance, the supply frequence is determined based on the desired velocity and the premise that the motor will approximately follow its synchronal velocity. The mistake in velocity resulted from faux pas of the motor is considered acceptable. When truth in velocity response is a concern, closed-loop velocity control can be implemented with the changeless V/Hz rule through ordinance of faux pas velocity, as illustrated in Figure 4, where a PI accountant is employed to modulate the faux pas velocity of the
motor to maintain the motor velocity at its set value.
Operation of Three-Phase Voltage Source
The construction of a typical three-phase electromotive force beginning power inverter is shown in Figure 6. Va, Vb and Vc are the end product electromotive forces applied to the twists of a motor. Q1 through Q6 are the six power transistors that shape the end product, which are controlled by a, a ‘ , B, B ‘ , degree Celsiuss and degree Celsius ‘ . For AC Induction motor control, when an upper transistor is switched on, i.e. , when a, B or degree Celsius is 1,
the matching lower transistor is switched off, i.e. , the matching a ‘ , B ‘ or c ‘ is 0. The on and off provinces of the upper transistors Q1, Q3 and Q5, or equivalently, the province of a, B and degree Celsius, are sufficient to measure the end product electromotive force.
Three Phase Power inverter
As shown in Figure 6, there are eight possible combinations of on and off forms for the three upper power transistors that feed the three stage power inverter. Notice that the on and off provinces of the lower power transistors are opposite to the upper 1s and so are
wholly determined one time the provinces of the upper power transistors are known. The eight combinations and the derived end product line-to-line and stage electromotive forces in footings of DC supply
electromotive force Vdc, Space Vector PWM refers to a particular exchanging sequence of the upper three power transistors of a three stage power inverter. It has been shown to bring forth less harmonic deformation in the end product electromotive forces and or currents applied to the stages of an AC motor and provides more efficient usage of supply electromotive force in comparing with direct sinusoidal transition technique.
Steady-State operation on a Squirrel-Cage Three-Phase Induction Motor
The term squirrel-cage is really a type of rotor used in initiation motor. The rotor consist of a stack of steel laminations with equally spaced slots punched around the perimeter. As with the stator laminations, the surface is coated with an oxide bed, which act as an dielectric, forestalling unwanted eddy current from fluxing in the Fe. The coop rotor is by far the most common, each rotor slot contains a solid music director saloon and all the music directors are physically and electrically joined together at each terminal of the rotor by carry oning end-rings. Cage rotors are normally cheaper to fabricate and are really robust and dependable.
The behavior of Squirrel-cage initiation motor when connected to a changeless frequence supply. This is by far the most widely used and of import manner of operation, the motor running straight connected to a changeless electromotive force brinies supply, 3-phase are the most of import to cover with in this instance.
The rotor opposition and reactance influenced the form of the torque-speed curve. For little values of faux pas, i.e. in the normal running part the lower we make the rotor opposition the steeper the incline of the torque-speed curve becomes. We can see that at the rated torque the full-load faux pas of the low opposition coop is much lower than that of the high-resistance coop. But the rotor efficiency is equal to ( 1-s ) , where s is the faux pas so, it is concluded that low opposition rotor non merely gives better velocity retention, but is besides much more efficient. There is of class a bound to how low we can do the opposition, Cu allow us to accomplish the lower opposition than aluminum. The drawbacks with a low opposition rotor is the get downing torsion gets reduced and worse till the get downing current additions. The lower get downing torsion may turn out insufficient to speed up the burden, while increased get downing current may take to unacceptable volt beads in the supply. Whereas changing the rotor opposition has little or no consequence on the value of extremum torsion.
The less attractive characteristic of initiation machines is that it is ne’er possible for all the power traversing the air-gap from the stator to be converted to mechanical end product, because some is ever lost as heat in the rotor circuit opposition. Infact it turns out that at faux pas s the entire power P traversing the air-gap ever divides so that a fraction sP is lost as heat, while the balance ( 1-s ) P is converted to utile mechanical end product. Hence, when the motor is runing in the steady province the energy transition efficiency of the motor is given by,
nr = Mechanical end product power / Rated power input to Rotor
nr = ( 1-s )
this consequence is really of import and shows us instantly why operating at little values of faux pas is desirable. With a faux pas of 5 % ( 0.05 ) for illustration, 95 % of the air-gap power is put to good usage. But if the motor was running at half of the synchronal velocity ( s = 0.5 ) , 50 % of the air-gap power would be wasted as heat in the rotor.
Experiment
All the computation made in the experiment were performed in an machine-controlled mode by agencies of computing machine driven DMS2 informations acquisition system. The lone procedure after puting up the whole circuit was to increase the value of burden torsion with the torsion boss in about 50 stairss in an ascending order.
After the increase of burden torsion, hitting upon the cardinal F2 over the keyboard gives every clip a new twine of values in the tabular array. First the load-test has been performed on the squirrel-cage initiation motor for the torque-speed curve over fixed values of frequences and after that in the 2nd experiment DMS2 has been used as a digital CRO to mensurate the motor current and electromotive force in the steady province operation over different frequences with changeless burden torsion of 0.2 Nm and so at a frequence of 40Hz with burden torsion of 0.4 Nm and 0.6Nm.
The no-load trial of an initiation motor measures the rotational losingss of the motor and provides information about its magnetisation current. The motor is allowed to whirl freely. The lone burden on the motor is the clash and the windage losingss, so all Pconv in this motor is consumed by mechanical losingss, and the faux pas of the motor is really little, perchance every bit little as 0.001 or less. With its really little faux pas the opposition matching to its power converted, R ( 1-s ) /s, is much larger than the opposition matching to the rotor Cu losingss R, and much larger than the rotor reactance X.
In this motor at no-load conditions the input power measured by the metres must be the losingss in the motor. The rotor Cu losingss are negligible because the current I2 is highly little because of big burden opposition so they may be neglected. The stator Cu losingss are given by
Pcl = 3I2R
So the input power peers
Pin = Pcl + Pcore
Pin = 3I2R + Prot
Where Prot is the rotational losingss of the motor.
Consequences
20 HZ
30 HZ
40HZ
50 HZ
75 HZ
100 HZ
Wave forms of motor electromotive force and current in steady-state operation
AT 0.2 Nm
20Hz
30Hz
40Hz
50Hz
75Hz
100Hz
NOW AT 40HZ
0.4 Nm
0.6 Nm
Decisions
Having established earlier that at any given faux pas, the air-gap flux denseness is relative to the applied electromotive force and the induced current in the rotor is relative to the flux denseness. The torsion, which depends on the merchandise of the flux and the rotor current, hence depends on the square of applied electromotive force. That ‘s why a relatively modest autumn in the electromotive force will ensue in a much larger decrease in torsion capableness, with inauspicious effects which may non be evident to the unwary until excessively late.
Having explored the torque-speed curve in the graph of initiation motor it has been found out that the torque-speed curve for the normal motoring part where the velocity lies between zero and merely below synchronal. If the synchronal velocity increases more than the synchronal velocity or go negative the torsion besides becomes negative. It is furthermore concerned to the faux pas, instead than the velocity. When the faux pas is positive the torsion is positive and frailty versa. The torsion therefore ever acts so as to press the rotor to run at zero faux pas, i.e. at the synchronal velocity. If the rotor is tempted to run faster than the field it will be slowed down, whilst if it is running at a lower place synchronal velocity it will be urged to speed up forwards. In peculiar, we note that for faux pass greater than 1, i.e. when motor is running rearward in the opposite way to the field the torsion will stay positive so that if the rotor is unrestrained it will foremost decelerate down and so alter way and accelerate in the way of field
Discussion
While making the burden trial on the squirrel-cage initiation motor one most outstanding thing which was taken in to account was that when the torsion was acquiring incremented manually with every measure so there comes a phase when increasing torsion reduces the velocity of the machine. We were asked to take the readings until the burden torsion becomes 1.2 Nm but we were unable to take any farther readings after 0.94 Nm because after this value of burden torsion the velocity was acquiring decremented continuously.

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