File Name: armature winding and motor repair in dc and ac machines .zip
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A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source. Brushed motors were the first commercially important application of electric power to driving mechanical energy, and DC distribution systems were used for more than years to operate motors in commercial and industrial buildings.
Brushed DC motors can be varied in speed by changing the operating voltage or the strength of the magnetic field. Depending on the connections of the field to the power supply, the speed and torque characteristics of a brushed motor can be altered to provide steady speed or speed inversely proportional to the mechanical load.
Brushed motors continue to be used for electrical propulsion, cranes, paper machines and steel rolling mills. Since the brushes wear down and require replacement, brushless DC motors using power electronic devices have displaced brushed motors from many applications.
The following graphics illustrate a simple, two-pole, brushed , DC motor. When a current passes through the coil wound around a soft iron core situated inside an external magnetic field, the side of the positive pole is acted upon by an upwards force, while the other side is acted upon by a downward force.
According to Fleming's left hand rule , the forces cause a turning effect on the coil, making it rotate. To make the motor rotate in a constant direction, "direct current" commutators make the current reverse in direction every half a cycle in a two-pole motor thus causing the motor to continue to rotate in the same direction. A problem with the motor shown above is that when the plane of the coil is parallel to the magnetic field—i.
In the pictures above, this occurs when the core of the coil is horizontal—the position it is just about to reach in the second-to-last picture on the right. The motor would not be able to start in this position. However, once it was started, it would continue to rotate through this position by momentum. There is a second problem with this simple pole design.
At the zero-torque position, both commutator brushes are touching bridging both commutator plates, resulting in a short circuit. The power leads are shorted together through the commutator plates, and the coil is also short-circuited through both brushes the coil is shorted twice, once through each brush independently.
Note that this problem is independent of the non-starting problem above; even if there were a high current in the coil at this position, there would still be zero torque.
The problem here is that this short uselessly consumes power without producing any motion nor even any coil current. In a low-current battery-powered demonstration this short-circuiting is generally not considered harmful.
However, if a two-pole motor were designed to do actual work with several hundred watts of power output, this shorting could result in severe commutator overheating, brush damage, and potential welding of the brushes—if they were metallic—to the commutator. Carbon brushes, which are often used, would not weld. In any case, a short like this is very wasteful, drains batteries rapidly and, at a minimum, requires power supply components to be designed to much higher standards than would be needed just to run the motor without the shorting.
One simple solution is to put a gap between the commutator plates which is wider than the ends of the brushes. This increases the zero-torque range of angular positions but eliminates the shorting problem; if the motor is started spinning by an outside force it will continue spinning. With this modification, it can also be effectively turned off simply by stalling stopping it in a position in the zero-torque i.
This design is sometimes seen in homebuilt hobby motors, e. A clear downside of this simple solution is that the motor now coasts through a substantial arc of rotation twice per revolution and the torque is pulsed. This may work for electric fans or to keep a flywheel spinning but there are many applications, even where starting and stopping are not necessary, for which it is completely inadequate, such as driving the capstan of a tape transport, or any instance where to speed up and slow down often and quickly is a requirement.
Another disadvantage is that, since the coils have a measure of self inductance , current flowing in them cannot suddenly stop. The current attempts to jump the opening gap between the commutator segment and the brush, causing arcing. Even for fans and flywheels, the clear weaknesses remaining in this design—especially that it is not self-starting from all positions—make it impractical for working use, especially considering the better alternatives that exist. Unlike the demonstration motor above, DC motors are commonly designed with more than two poles, are able to start from any position, and do not have any position where current can flow without producing electromotive power by passing through some coil.
Many common small brushed DC motors used in toys and small consumer appliances, the simplest mass-produced DC motors to be found, have three-pole armatures. The brushes can now bridge two adjacent commutator segments without causing a short circuit. These three-pole armatures also have the advantage that current from the brushes either flows through two coils in series or through just one coil.
Starting with the current in an individual coil at half its nominal value as a result of flowing through two coils in series , it rises to its nominal value and then falls to half this value. The sequence then continues with current in the reverse direction. This results in a closer step-wise approximation to the ideal sinusoidal coil current, producing a more even torque than the two-pole motor where the current in each coil is closer to a square wave. Since current changes are half those of a comparable two-pole motor, arcing at the brushes is consequently less.
If the shaft of a DC motor is turned by an external force, the motor will act like a generator and produce an Electromotive force EMF. The back EMF is the reason that the motor when free-running does not appear to have the same low electrical resistance as the wire contained in its winding.
This is the same EMF that is produced when the motor is used as a generator for example when an electrical load, such as a light bulb, is placed across the terminals of the motor and the motor shaft is driven with an external torque.
Therefore, the total voltage drop across a motor consists of the CEMF voltage drop, and the parasitic voltage drop resulting from the internal resistance of the armature's windings. The current through a motor is given by the following equation:. As an unloaded DC motor spins, it generates a backwards-flowing electromotive force that resists the current being applied to the motor. The current through the motor drops as the rotational speed increases, and a free-spinning motor has very little current.
It is only when a load is applied to the motor that slows the rotor that the current draw through the motor increases. In a dynamo, a plane through the centers of the contact areas where a pair of brushes touch the commutator and parallel to the axis of rotation of the armature is referred to as the commutating plane.
In this diagram the commutating plane is shown for just one of the brushes, assuming the other brush made contact on the other side of the commutator with radial symmetry, degrees from the brush shown.
In a real dynamo, the field is never perfectly uniform. Instead, as the rotor spins it induces field effects which drag and distort the magnetic lines of the outer non-rotating stator. The faster the rotor spins, the further the degree of field distortion. Because the dynamo operates most efficiently with the rotor field at right angles to the stator field, it is necessary to either retard or advance the brush position to put the rotor's field into the correct position to be at a right angle to the distorted field.
These field effects are reversed when the direction of spin is reversed. It is therefore difficult to build an efficient reversible commutated dynamo, since for highest field strength it is necessary to move the brushes to the opposite side of the normal neutral plane. The effect can be considered to be somewhat similar to timing advance in an internal combustion engine. Generally a dynamo that has been designed to run at a certain fixed speed will have its brushes permanently fixed to align the field for highest efficiency at that speed.
DC machines with wound stators compensate the distortion with commutating field windings and compensation windings. Brushed DC motors are constructed with wound rotors and either wound or permanent-magnet stators. The field coils have traditionally existed in four basic formats: separately excited sepex , series -wound, shunt -wound, and a combination of the latter two; compound-wound.
In a series wound motor , the field coils are connected electrically in series with the armature coils via the brushes. In a shunt wound motor, the field coils are connected in parallel, or "shunted" to the armature coils. In a separately excited sepex motor the field coils are supplied from an independent source, such as a motor-generator and the field current is unaffected by changes in the armature current.
The sepex system was sometimes used in DC traction motors to facilitate control of wheelslip. Permanent-magnet types have some performance advantages over direct-current, excited, synchronous types, and have become predominant in fractional horsepower applications. They are smaller, lighter, more efficient and reliable than other singly-fed electric machines. Originally all large industrial DC motors used wound field or rotor magnets.
Permanent magnets have traditionally only been useful on small motors because it was difficult to find a material capable of retaining a high-strength field. Only recently have advances in materials technology allowed the creation of high-intensity permanent magnets, such as neodymium magnets , allowing the development of compact, high-power motors without the extra volume of field coils and excitation means.
But as these high performance permanent magnets become more applied in electric motor or generator systems, other problems are realized see Permanent magnet synchronous generator. Traditionally, the field has been applied radially—in and away from the rotation axis of the motor. However some designs have the field flowing along the axis of the motor, with the rotor cutting the field lines as it rotates. This allows for much stronger magnetic fields, particularly if halbach arrays are employed.
This, in turn, gives power to the motor at lower speeds. However, the focused flux density cannot rise about the limited residual flux density of the permanent magnet despite high coercivity and like all electric machines, the flux density of magnetic core saturation is the design constraint.
Speed control can be achieved by variable battery tappings, variable supply voltage, resistors or electronic controls. A simulation example can be found here  and. This is commonly done with a special set of contactors direction contactors.
The effective voltage can be varied by inserting a series resistor or by an electronically controlled switching device made of thyristors , transistors , or, formerly, mercury arc rectifiers. Series-parallel control was the standard method of controlling railway traction motors before the advent of power electronics. An electric locomotive or train would typically have four motors which could be grouped in three different ways:. This provided three running speeds with minimal resistance losses.
For starting and acceleration, additional control was provided by resistances. This system has been superseded by electronic control systems. The speed of a DC motor can be increased by field weakening. Reducing the field strength is done by inserting resistance in series with a shunt field, or inserting resistances around a series-connected field winding, to reduce current in the field winding. When the field is weakened, the back-emf reduces, so a larger current flows through the armature winding and this increases the speed.
Field weakening is not used on its own but in combination with other methods, such as series-parallel control. In a circuit known as a chopper , the average voltage applied to the motor is varied by switching the supply voltage very rapidly. As the "on" to "off" ratio is varied to alter the average applied voltage, the speed of the motor varies.
The percentage "on" time multiplied by the supply voltage gives the average voltage applied to the motor. During the "off" time, the armature's inductance causes the current to continue through a diode called a "flyback diode", in parallel with the motor. The rapid switching wastes less energy than series resistors. This method is also called pulse-width modulation PWM and is often controlled by a microprocessor. An output filter is sometimes installed to smooth the average voltage applied to the motor and reduce motor noise.
Since the series-wound DC motor develops its highest torque at low speed, it is often used in traction applications such as electric locomotives , and trams. Another application is starter motors for petrol and small diesel engines.
Series motors must never be used in applications where the drive can fail such as belt drives.
A brushed DC electric motor is an internally commutated electric motor designed to be run from a direct current power source. Brushed motors were the first commercially important application of electric power to driving mechanical energy, and DC distribution systems were used for more than years to operate motors in commercial and industrial buildings. Brushed DC motors can be varied in speed by changing the operating voltage or the strength of the magnetic field. Depending on the connections of the field to the power supply, the speed and torque characteristics of a brushed motor can be altered to provide steady speed or speed inversely proportional to the mechanical load. Brushed motors continue to be used for electrical propulsion, cranes, paper machines and steel rolling mills.
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Armature windings : a practical analysis of armature windings for direct-current and alternating current machines, including rules and diagrams for reconnecting induction motor armatures. Armature winding; a practical analysis of armature windings for direct-current and alternating current machines, including rules and diagrams for reconnecting induction motor armatures. Modern armature construction, winding and repair. Armature winding; a practical manual on the construction, winding and repairing of A. Generators and motors; a practical treatise on the construction, winding, and repairing of electric generators and motors.
Спокойно подняв пистолет, он выстрелил. Пуля задела Беккера в бок, когда он уже почти обогнул угол здания. Он почувствовал это лишь после того, как сделал пять или шесть шагов. Сначала это напомнило сокращение мышцы чуть повыше бедра, затем появилось ощущение чего-то влажного и липкого. Увидев кровь, Беккер понял, что ранен.
Он знал: все уверены, что он ушел. В шуме, доносившемся из-под пола шифровалки, в его голове звучал девиз лаборатории систем безопасности: Действуй, объясняться будешь. В мире высоких ставок, в котором от компьютерной безопасности зависело слишком многое, минуты зачастую означали спасение системы или ее гибель. Трудно было найти время для предварительного обоснования защитных мер. Сотрудникам службы безопасности платили за их техническое мастерство… а также за чутье.
Консьерж покачал головой: - Невозможно. Быть может, вы оставите… - Всего на одну минуту. Она в столовой. Консьерж снова покачал головой: - Ресторан закрылся полчаса. Полагаю, Росио и ее гость ушли на вечернюю прогулку. Если вы оставите для нее записку, она получит ее прямо с утра.
Нашлось 75 песет никелевыми монетками, сдача от поездки в такси, - достаточно для двух местных звонков. Он вежливо улыбнулся озабоченной медсестре и вошел в будку.
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Именно так, черт возьми. Я был там, внизу. Резервное питание подает слишком мало фреона.
Читайте медленно и очень внимательно. Беккер кивнул и поднес кольцо ближе к глазам. Затем начал читать надпись вслух: - Q… U… 1…S… пробел… С, Джабба и Сьюзан в один голос воскликнули: - Пробел? - Джабба перестал печатать.
Директор метнул на нее настороженный взгляд, но Мидж уже бежала к аппарату. Она решила включить громкую связь. - Слушаю, Джабба. Металлический голос Джаббы заполнил комнату: - Мидж, я в главном банке данных.
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