DC motors are electrical machines that ingest dc electrical power and produce mechanical torque. DC motors are labeled based on the connection of the field circuit with regards to the armature circuit. Customarily dc motors were classified as shunt, series or separately excited. In improvements it was common to see motors known as compound-wound. There is very no important difference between shunt, series or separately ecstatic dc motors, and the labels simply reflect the way in which the field and armature circuits are interconnected.
The dc engine has two independent circuits. Small couple of terminals connect to the field windings which surround each pole and are usually in series, in the regular express all the type power to the field windings is dissipated as warmth, none from it is connected to mechanical outcome power. The primary terminals convey the current to the brushes which will make the slipping contact to the armature winding on the rotor. The supply to the field is independent from that for the armature hence the description separately fired up.
In shunt dc motors, the field circuit is connected in parallel with the armature circuit while DC series motors possess the field circuit in series with the armature where both field and armature currents are equivalent. The brushes and commutators are troublesome for dc motors at high acceleration whereas small dc motors say up to a huge selection of watts productivity can run at perhaps 12000 rev/min but the most medium and large motors are usually made for speeds below 3000 rev/min.
"Direct current (DC) motors have been widely used in many industrial applications such as electric vehicles, metal rolling mills, electric cranes, and robotic manipulators anticipated to precise, vast, simple, and ongoing control characteristics. The required torque-speed characteristics could be performed through conventional proportional integral- derivative (PID) controllers. "[1]
Dc motors are generally preferred because they are simple to use and control and not just this they even deliver High starting torque and their quality performance is also almost linear. However when it involves Speed control of dc engine the purpose of a motor swiftness controller is to take a transmission representing the demanded acceleration, and also to drive a motor unit at that swiftness. "The controller may or might not actually measure the speed of the motor unit. If it can, it is called a Feedback Velocity Controller or Closed Loop Quickness Controller, if not it is called an Open Loop Acceleration Controller. Feedback rate control is better, but more complicated. "[2]
Speed Control of Separately fired up Dc Motor
"In this technique, shunt-field current is managed constant from another source while the voltage applied to the armature is mixed. Dc motors include a velocity, which is proportional to the counter-top emf. This is equal to the applied voltage minus the armature circuit IR drop. At ranked current, the torque remains frequent whatever the dc motor swiftness (because the magnetic flux is constant) and, therefore, the dc electric motor has constant torque functionality over its quickness range. "[5a]
"The purpose of a motor swiftness controller is to have a signal representing the demanded swiftness, and to drive a electric motor at that speed. The controller may or might not exactly actually gauge the rate of the electric motor. If it does, it is called a Feedback Acceleration Controller or Closed Loop Rate Controller, if not it is named an Start Loop Acceleration Controller. Feedback swiftness control is way better, but more difficult, and may not be needed for a straightforward automatic robot design. "[4]
The speed of the separately excited dc motor could be mixed from zero to scored acceleration mainly by differing armature voltage in the constant torque region. Whereas in the regular vitality region, field flux should be reduced to accomplish swiftness above the scored swiftness. "Control is obtained by weakening the shunt-field current of the dc electric motor to increase speed and to reduce output torque for confirmed armature current. Because the rating of a dc motor is determined by heating, the maximum permissible armature current is around constant over the speed range. Which means that at scored current, the dc motor's productivity torque varies inversely with acceleration, and the dc engine has constant-horsepower potential over its swiftness range.
Dc motors give you a solution, which is good for only obtaining speeds greater than the base speed. A momentary quickness lowering below the dc motor's bottom part speed can be acquired by overexciting the field, but extended over excitation overheats the dc motor. Also, magnetic saturation in the dc electric motor permits only a small reduction in swiftness for a considerable upsurge in field voltage. If field control is to be used, and a large speed range is necessary, the base velocity must be proportionately lower and the motor size must be bigger. If acceleration range is much over 3:1, armature voltage control is highly recommended for at least part of the range. Very huge dynamic acceleration range can be obtained with armature voltage control. However, below about 60% of base speed, the motor should be de rated or used for only short periods. "[5b]The quickness (N) of any DC engine is proportional to its armature voltage; the torque (T) is proportional to armature current, and both quantities are impartial, as illustrated in Number below.
Dc Motor unit characteristics
Operation of Single-Phase Half-controlled Bridge Rectifier
A fully-controlled rectifier circuit has only controlled-rectifiers, whereas a semi-controlled rectifier circuit is made up of both manipulated and uncontrolled rectifiers. Because of occurrence of diodes, free-wheeling operation takes place without allowing the bridge productivity voltage to be negative. In a semi-controlled rectifier, control is effected limited to positve output voltage, no control can be done when its result voltage tends to become negative since it is clamped at zero volt. Here the operation of any single-phase half-controlled rectifier is explained. [3]
.
Half manipulated Bridge rectifier
In this circuit, SCRs S1 and S3 do throughout a < wt < p. During p < wt < (p + a), these devices in conduction is diode D and the productivity of the bridge is clamped at zero. During (p + a) < wt < 2p, the devices in conduction are SCRs S2 and S4. Diode D would conduct during 0 < wt < a.
Here are some results used with the simulation of single phase half-controlled bridge rectifier on firing viewpoint of zero degree.
Steady-State operation of Separately Excited Dc Motor
Steady state is certainly an application for Dc electric motor characteristics in which it indicates how the engine behaves when any transient effects have died away and conditions have once again become steady. Regular State characteristics are usually much better to anticipate than transient characteristics. Under continuous point out conditions the armature current I is constant.
The equation below is the armature circuit voltage formula.
V = E + IR + L(dI/dt)
Where voltage V is the voltage applied to the armature terminals and E is the internally developed motional e. m. f. The amount of resistance and inductance of the entire armature are symbolized by R and L.
Under motoring condition, the motional e. m. f E always opposes the applied voltage V, and because of this it is recognized as again e. m. f. And for the current to be forced into the electric motor V must be increased then E. The past bit of the above mentioned equation signifies the inductive volt drop anticipated to armature self inductance. This voltage is proportional to the pace of change of current. So under continuous state this previous term will be ZERO. So we can disregard that last term for steady state operations. Then under regular state conditions the aforementioned equations becomes,
V = E + IR
So,
I = (V-E)/R
In shunt dc motors, the field circuit is connected in parallel with the armature circuit. It gets the following equivalent circuit:
Fig. 1. Comparative Circuit of
DC Shunt Motor
Under steady talk about condition enough time derivative is zero let's assume that the motor is not saturated. Some important field and armature equations are as follows.
For field circuit,
The again e. m. f is given by :
The armature circuit
Now the torque and speed under the dependable state condition are available with the next formulas: The motor unit speed can be easily produced:
If Ra is a tiny value, or when the motor unit is lightly loaded, i. e. Ia is small,
That is if the field current is held constant, the electric motor speed is dependent only on the supply voltage.
The developed torque is :
The required power is :
In the dc electric motor the field windings is used to excite the field flux. And the armature current comes to the rotor via clean and commutator for the mechanical work. This Connection of field flux and armature current in the rotor produces torque. Whenever a separately excited motor unit is excited by way of a field current of if and an armature current of ia flows in the circuit, the electric motor develops a returning e. m. f and a torque to balance the load torque at a particular velocity. The if is in addition to the ia. Each windings are offered independently. Any change in the armature current has no influence on the field current. The if is generally much less than the ia.
The very first thing is to wire the circuit of individually excited dc engine with DMS2 data acquisition system as stated on the manual provided for experiments.
Steady Express Characteristics
After starting the DMS2 Data acquisition system software the users variables were defined in the program manually which can be Input power, Outcome vitality and Efficiency for which the values needs to be recorder. After adding all the guidelines the dc electric motor started with the strain torque value of 0 Nm and then your principles for the parameters which were released in the software were taken in an automatic manner using the F2 button on the keyboard. And gradually increases the values of insert torque from 0 - 0. 5 Nm with 50 steps with each step documenting the ideals of guidelines predefined in DMS2 Software.
From no-load to full weight the speed comes linearly as a consequent the back e. m. f falls linearly too. The power losses in the armature amount of resistance are I2R. The power converted from electrical to mechanised is given by VI. The power required to get over friction and flat iron losses are available under no-load conditions and get deducted from the filled condition when the loss are not considered. Two important observation follow from these computations. Firstly the swiftness drop with fill is very small. This is very desirable for some applications. Since all we must do to keep almost constant quickness is to create the appropriate armature voltage and keep it constant. Secondly a fragile balance between V and E is revealed. The existing is actually proportional to the difference between V and E. so that quite small changes in either E or V bring about disproportionately large changes in the current. Hence to avoid high current difference between E and V must be limited. The no-load speeds are immediately proportional to the applied voltage, as the slope of every curve is the same, being determined by the armature amount of resistance. Small the resistance the less the velocity falls with load.
Steady state is such an application for Dc engine characteristics in which it indicates the way the motor unit behaves when any transient effects have died away and conditions have once more become steady. Constant State characteristics are usually much simpler to predict than transient characteristics. Under regular status conditions the armature current I is constant.
The second part of the experiment involves using the DMS2 system as an electronic storage oscilloscope. Within this experiment the instantaneous waveforms are documented at load torque prices of 0. 1, 0. 3 and 0. 4 Nm. Once we escalates the value of insert torque it causes the motor rate to decreases slowly but surely.
Results
Steady-state characteristics
Waveforms of motor unit armature voltage and current in steady-state operation
0. 1 Nm
0. 3 Nm
0. 4 Nm
Discussion
For a DC engine, the velocity (RPM) is proportional to the applied voltage. The current that the engine will need from the source is proportional to the load. A motor with no load will need hardly any current, a engine which is filled and doing some work will take more current. The strain on the motor shows up as a torque on the end result shaft, the more load, the greater torque. In like manner seeking and summaries: After Setting up the motor acceleration by setting up the source voltage, as we increase the fill (torque) on the motor unit, the current extracted from the supply will increase. The motor will also decelerate a bit as an increment in the strain torque. We have seen that if the strain torque on the shaft of motor unit increases, the speed falls and the armature current automatically enhances until equilibrium of torque is reached and the speed gain becomes continuous. In case the armature voltage reaches its maximum value, and we increase the mechanical load until the current gets to its ranked value, we are obviously at full- insert i. e. we have been operating at the entire speed and full torque.
Clearly if we improve the load on the shaft still more, the existing will go over the safe value, and the motor unit will began to overheat. However the question which this prompts is if it weren't for the problem of overheating, could the electric motor deliver more and more output, or will there be a limit?
We can see straightaway that you will see a maximum point by looking at the torque-speed curve. The mechanical output electricity is the product of torque and swiftness and we note that the energy will be zero when either the load torque is zero or the acceleration is zero. And it is simple to show that the peak mechanical electric power occurs when the speed is half of the no-load quickness. But familiarization with the dc motors brings the idea of high maintenance cost and large size of the motors when compared with induction motors. And dc machines aren't suitable for broadband operations because of the commutator and brushes and they are also not well suited for the clean or explosive conditions.
Conclusions
A shunt or separately excited dc motor unit has a torque-speed attribute whose speed drops linearly with increasing weight torque. Its acceleration can be controlled by changing its field current, its armature voltage or its armature amount of resistance. The graph from the resulting prices between torque-armature voltage implies that the relationship between your torque and armature voltage is nearly linear as the armature voltage boosts it brings a linear change in torque also. Whereas the quickness diminishes as the torque upsurge in a reliable manner. Efficiency of the electric motor increases quickly and then reduction in an instant manner as we improve the value of torque. The results show that when the machine is operating at the graded conditions, the steady-state beliefs are in good contract.
It has been detected that under maximum ability conditions the entire efficiency is only nearly 50% because the same power is burned up as heating in the armature amount of resistance. And only very small motors can ever before be operated continually in this condition.
Introduction
It isn't just supremely chic as an electromechanical energy converter, but is also the most important, with something like one third of all the electricity made being converted back again to mechanical energy in induction motors. Just like the d. c electric motor, the induction motor unit produces torque by the conversation of axial currents on the rotor and a radial magnetic field made by the stator. But whereas in the dc engine the current needs to be fed into the rotor through brushes and a commutator, the torque producing current in the rotor of the induction motor are induced by electromagnetic action, hence the name induction electric motor. The stator winding not only therefore procuring the magnetic field but also supplies the energy that is changed into mechanical result. The absence of any sliding mechanical connections and the consequent cutting down in terms of maintenance is a major advantage of the induction engine over its d. c rival.
To know how an induction engine operates, we must first unravel the mysteries of the rotating magnetic field. The rotor will be effectively dragged along by the rotating field, but so it can never run quite as fast as the field. When it is needed to control the rate of the rotor it is best to control the acceleration of the field. The mechanism of the rotating field give attention to the stator windings because they become the foundation of flux. Within this part the existence of rotor gets neglected merely to make it simpler to understand what governs the velocity of rotation and the magnitude of the field, which are the two factors which largely influence the motor unit behaviour. The discussion between your rotor and the stator well justifies the external characteristics of the engine. i. e. the variant of motor torque and stator current with speed.
Broadly speaking the motor designer patterns the stator and rotor pearly whites to encourage whenever you can of the flux produced by the stator to pass down the rotor tooth, so that before completing its path back to the stator it is totally associated with the rotor conductors which can be positioned in the rotor slots. This is restricted magnetic coupling between stator and rotor windings is necessary for good working performance. Plus the field which supply the coupling is of course the main or air-gap registered. The vast majority of the flux produced by the stator is indeed main or shared flux. But there is certainly some flux which bypasses the rotor conductor, linking only with the stator winding, and known as storage leakage flux.
"Everybody knows that the synchronous swiftness of the induction motor unit is distributed by Ns = 120f/P. So out of this connection, it is evident that the synchronous swiftness and therefore the rate of the induction engine can by mixed by the supply frequency. This method has its limitations. The motor unit speed can be reduced by lowering the occurrence, if the induction engine is actually the only load on the generators. Even then your range over that your swiftness can be mixed is very less. "[6]
V/f continuous Principle
Because of advances in solid condition power devices and microprocessors, variable velocity AC Induction motors powered by switching ability converters are becoming increasingly more popular. Switching power converters offer an easy way to regulate both the rate of recurrence and magnitude of the voltage and current put on a motor. As a result much higher efficiency and performance can be achieved by these engine drives with less generated noises. The most frequent principle of this kind, is the constant V/Hz principle which requires that the magnitude and consistency of the voltage put on the stator of a motor maintain a continuous ratio. By doing this, the magnitude of the magnetic field in the stator is kept at an around regular level throughout the operating range. Thus, (maximum) frequent torque producing ability is looked after. When transient response is crucial, switching power converters also allow easy control of transient voltage and current applied to the motor to accomplish faster dynamic response. The regular V/Hz principle is considered for this software. The energy that a switching ability converter provides to a electric motor is controlled by Pulse Width Modulated (PWM) alerts applied to the gates of the power transistors. PWM signals are pulse trains with fixed regularity and magnitude and variable pulse width. There is certainly one pulse of permanent magnitude in every PWM period. However, the width of the pulses changes from period to period matching to a modulating sign. Whenever a PWM signal is put on the gate of a electric power transistor, it causes the turn on and turn off intervals of the transistor to improve from one PWM period to another PWM period according to the same modulating sign. The frequency of your PWM signal must be much higher than that of the modulating indication, the fundamental consistency, such that the delivered to the motor and its own load depends typically on the modulating indication. Number 1 shows two types of PWM signals, symmetric and asymmetric edge-aligned. The pulses of your symmetric PWM indication are always symmetric with respect to the center of each PWM period. The pulses of an asymmetric edge-aligned PWM indication will have the same side aligned with one end of each PWM period. Both types of PWM signals are used in this program.
Symmetric and Asymmetric PWM Signals
It has been shown that symmetric PWM impulses generate less harmonics in the outcome current and voltage. Different PWM techniques, or ways of deciding the modulating sign and the switch-on/switch-off instants from the modulating sign, exist. Popular samples are sinusoidal PWM, hysteresis PWM and the relatively new space vector PWM. These techniques are commonly used with three period Voltage Source ability inverters for the control of three-phase AC induction motors. The space vector PWM approach is employed in this request.
Assume the voltage put on a three period AC Induction motor unit is sinusoidal and disregard the voltage drop over the stator resistor. Then we've, at steady express,
from which it employs that if the ratio V/ f remains frequent with the change of f, a remains continuous too and the torque is in addition to the supply consistency. In actual execution, the ratio between your magnitude and occurrence of the stator voltage is usually predicated on the rated ideals of these variables, or motor scores. However, when the consistency and therefore also the voltage are low, the voltage drop across the stator resistance can't be neglected and must be compensated. At frequencies higher than the ranked value, the frequent V/ f concept also have to be violated because, to avoid insulation break down, the stator voltage should never exceed its scored value. This process is illustrated in Amount 2.
Voltage Versus Consistency under regular V/ f Principle
Since the stator flux is maintained constant, in addition to the change in resource occurrence, the torque developed is determined by the slip rate only, which is shown in Figure 3. So by regulating the slide speed, the torque and rate associated with an AC Induction electric motor can be manipulated with the constant V/Hz concept.
Torque Versus Slip velocity of induction engine while frequent stator flux
Both wide open and closed-loop control of the swiftness of the AC induction engine can be executed based on the regular V/Hz rule. Open-loop quickness control is used when correctness in rate response is not a concern such as in HVAC (warming, ventilation and air-con), fan or blower applications. In cases like this, the supply regularity is determined according to the desired speed and the assumption that the motor will approximately follow its synchronous swiftness. The error in velocity resulted from slide of the motor is considered acceptable. When accuracy and reliability in quickness response is a problem, closed-loop velocity control can be put in place with the constant V/Hz rule through regulation of slip speed, as illustrated in Figure 4, where a PI controller is employed to regulate the slip quickness of the
motor to keep carefully the motor speed at its set in place value.
Operation of Three-Phase Voltage Source
The structure of a typical three-phase voltage source vitality inverter is shown in Physique 6. Va, Vb and Vc will be the output voltages put on the windings of your engine. Q1 through Q6 are the six ability transistors that condition the output, which can be controlled by way of a, a', b, b', c and c'. For AC Induction motor unit control, when an top transistor is switched on, i. e. , whenever a, b or c is 1,
the corresponding lower transistor is switched off, i. e. , the equivalent a', b' or c' is 0. The on / off states of the top transistors Q1, Q3 and Q5, or equivalently, the state of a, b and c, are sufficient to judge the end result voltage.
Three Phase Vitality inverter
As shown in Amount 6, there are eight possible combinations of on and off habits for the three upper vitality transistors that feed the three phase power inverter. Notice that the on and off states of the lower power transistors are reverse to top of the ones therefore are
completely determined once the states of the top ability transistors are known. The eight combinations and the produced outcome line-to-line and period voltages in terms of DC supply
voltage Vdc, Space Vector PWM identifies a special turning sequence of the upper three ability transistors of your three phase electricity inverter. It has been shown to make less harmonic distortion in the output voltages and or currents put on the phases of your AC motor and better use of resource voltage in comparison with direct sinusoidal modulation strategy.
Steady-State operation on a Squirrel-Cage Three-Phase Induction Motor
The term squirrel-cage is actually a type of rotor found in induction motor unit. The rotor consist of a stack of metallic laminations with uniformly spaced slot machines punched about the circumference. As with the stator laminations, the surface is covered with an oxide covering, which act as an insulator, avoiding unwanted eddy current from flowing in the flat iron. The cage rotor is by far the most common, each rotor slot contains a solid conductor bar and all the conductors are actually and electrically signed up with alongside one another at each end of the rotor by conducting end-rings. Cage rotors are usually cheaper to create and are very robust and reliable.
The behaviour of Squirrel-cage induction motor when linked to a regular frequency supply. That is by far the most widely used and important function of procedure, the motor running directly connected to a constant voltage mains supply, 3-period are the main to handled in cases like this.
The rotor resistance and reactance influenced the form of the torque-speed curve. For small beliefs of slide, i. e. in the normal running region the lower we make the rotor resistance the steeper the slope of the torque-speed curve becomes. We can see that at the ranked torque the full-load slide of the low resistance cage is a lot lower than that of the high-resistance cage. However the rotor efficiency is equal to (1-s), where s is the slip so, it is concluded that low resistance rotor not only gives better speed keeping, but is also a lot more efficient. There may be of course a limit to how low we can make the level of resistance, copper allow us to attain the lower amount of resistance than aluminium. The downsides with a low amount of resistance rotor is the starting torque gets reduced and worse till the starting current boosts. The low starting torque may verify insufficient to speed up the load, while increased starting current can lead to unacceptable volt drops in the source. Whereas altering the rotor amount of resistance has little or no effect on the worthiness of optimum torque.
The less attractive feature of induction machines is that it is never easy for all the power crossing the air-gap from the stator to be converted to mechanical productivity, because some is obviously lost as warmth in the rotor circuit level of resistance. Infact as it happens that at slide s the full total vitality P crossing the air-gap always divides so that a fraction sP is lost as temperature, as the remainder(1-s)P is converted to useful mechanical productivity. Hence, when the motor unit is operating in the stable state the power alteration efficiency of the motor is distributed by,
nr = Mechanised output electricity / Rated ability input to Rotor
nr = (1-s)
this result is vital and shows us immediately why functioning at small prices of slide is desirable. Having a slide of 5%(0. 05) for example, 95% of the air-gap vitality is put to good use. If the motor was working at one half of the synchronous velocity (s = 0. 5), 50% of the air-gap vitality would be misused as heat in the rotor.
Experiment
All the calculation made in the test were performed in an automated manner by means of computer motivated DMS2 data acquisition system. The only process after setting up the complete circuit was to increase the value of load torque with the torque knob in about 50 steps in an ascending order.
After the increment of insert torque, hitting after the key F2 within the keyboard gives each time a fresh string of worth in the table. Firstly the load-test has been performed on the squirrel-cage induction motor unit for the torque-speed curve over fixed prices of frequencies and after that in the next test DMS2 has been used as a digital oscilloscope to gauge the motor current and voltage in the stable state procedure over different frequencies with constant insert torque of 0. 2 Nm and then at a frequency of 40Hz with fill torque of 0. 4 Nm and 0. 6Nm.
The no-load test of induction motor measures the rotational deficits of the motor and information about its magnetization current. The motor unit is allowed to spin freely. The only fill on the motor is the friction and the windage deficits, so all Pconv in this motor unit is used by mechanical losses, and the slide of the electric motor is really small, possibly no more than 0. 001 or less. Using its very small slip the amount of resistance corresponding to its electric power converted, R(1-s)/s, is much larger than the resistance corresponding to the rotor copper losses R, and far larger than the rotor reactance X.
In this motor unit at no-load conditions the insight power assessed by the meters must equal the loss in the motor. The rotor copper loss are negligible because the current I2 is extremely small because of large insert resistance so they might be neglected. The stator copper losses are given by
Pcl = 3I2R
So the input power equals
Pin = Pcl + Pcore
Pin = 3I2R + Prot
Where Prot is the rotational deficits of the engine.
Results
20 HZ
30 HZ
40HZ
50 HZ
75 HZ
100 HZ
Waveforms of electric motor voltage and current in steady-state operation
AT 0. 2 Nm
20Hz
30Hz
40Hz
50Hz
75Hz
100Hz
NOW AT 40HZ
0. 6 Nm
Conclusions
Having established previously that at any given slide, the air-gap flux denseness is proportional to the applied voltage and the induced current in the rotor is proportional to the flux denseness. The torque, which depends on the product of the flux and the rotor current, therefore depends on the square of applied voltage. That is why a comparatively modest land in the voltage will result in a much larger decrease in torque functionality, with adverse effects which might not exactly be noticeable to the unwary until too past due.
Having explored the torque-speed curve in the graph of induction electric motor it's been found out that the torque-speed curve for the normal motoring region where the speed is between zero and just below synchronous. When the synchronous speed increases more than the synchronous acceleration or become negative the torque also becomes negative. It really is moreover worried to the slip, as opposed to the speed. Once the slide is positive the torque is positive and vice versa. The torque therefore always operates in order to urge the rotor to perform at zero slide, i. e. at the synchronous acceleration. In case the rotor is tempted to perform faster than the field it'll be slowed up, whilst if it is working below synchronous swiftness it'll be urged to accelerate forwards. In particular, we note that for slips higher than 1, i. e. when engine is operating backward in the contrary course to the field the torque will stay positive so that if the rotor is unrestrained it will first decelerate and then change direction and accelerate in the direction of field
Discussion
While doing the strain test on the squirrel-cage induction electric motor one most prominent thing which was taken in to account was that whenever the torque was getting incremented physically with every step then there comes a stage when increasing torque reduces the rate of the device. We were asked to have the readings until the weight torque becomes 1. 2 Nm but we were not able to take any more readings after 0. 94 Nm because after this value of fill torque the quickness was getting decremented continuously.
