dc machine - construction, working, types, emf equation & applications
The DC machine can be classified into two types namely DC motors as well as DCgenerators. Most of the DC machines are equivalent to AC machines because they include AC currents as well as AC voltages in them. The output of the DC machine is DC output because they convert AC voltage to DC voltage. The conversion of this mechanism is known as the commutator, thus these machines are also named as commutating machines. DC machine is most frequently used for a motor. The main benefits of this machine include torque regulation as well as easy speed. The applications of the DC machine is limited to trains, mills, and mines. For example, underground subway cars, as well as trolleys, may utilize DC motors. In the past, automobiles were designed with DC dynamos for charging their batteries.
A DC machine is an electromechanical energy alteration device. The working principle of a DC machine is when electric current flows through a coil within a magnetic field, and then the magnetic force generates a torque that rotates the dc motor. The DC machines are classified into two types such as DC generator as well as DC motor.
The main function of the DC generator is to convert mechanical power to DC electrical power, whereas a DC motor converts DC power to mechanical power. The AC motor is frequently used in industrial applications for altering electrical energy to mechanical energy. However, a DC motor is applicable where good speed regulation & an ample range of speeds are necessary like in electric-transaction systems.
The construction of the DC machine can be done using some of the essential parts like Yoke, Pole core & pole shoes, Pole coil & field coil, Armature core, Armature winding otherwise conductor, commutator, brushes & bearings. Some of the parts of the DC machine is discussed below.
Another name of a yoke is the frame. The main function of the yoke in the machine is to offer mechanical support intended for poles and protects the entire machine from moisture, dust, etc. The materials used in the yoke are designed with cast iron, cast steel otherwise rolled steel.
The pole of the DC machine is an electromagnet and the field winding is winding among pole. Whenever field winding is energized then the pole gives magnetic flux. The materials used for this are cast steel, cast iron otherwise pole core. It can be built with the annealed steel laminations for reducing the power drop because of the eddy currents.
Pole shoe in the DC machine is an extensive part as well as to enlarge the region of the pole. Because of this region, flux can be spread out within the air-gap as well as extra flux can be passed through the air space toward armature. The materials used to build pole shoe is cast iron otherwise cast steed, and also used annealed steel lamination to reduce the loss of power because of eddy currents.
In this, the windings are wounded in the region of pole core & named as field coil. Whenever current is supplied through field winding than it electromagnetics the poles which generate required flux. The material used for field windings is copper.
Armature core includes a huge number of slots within its edge. The armature conductor is located in these slots. It provides the low-reluctance path toward the flux generated with field winding. The materials used in this core are permeability low-reluctance materials like iron otherwise cast. The lamination is used to decrease the loss because of the eddy current.
The armature winding can be formed by interconnecting the armature conductor. Whenever an armature winding is turned with the help of prime mover then the voltage, as well as magnetic flux, gets induced within it. This winding is allied to an exterior circuit. The materials used for this winding are conducting material like copper.
The main function of the commutator in the DC machine is to collect the current from the armature conductor as well as supplies the current to the load using brushes. And also provides uni-directional torque for DC-motor. The commutator can be built with a huge number of segments in the edge form of hard drawn copper. The Segments in the commutator are protected from the thin mica layer.
Brushes in the DC machine gather the current from the commutator and supply it to the exterior load. Brushes wear with time to inspect frequently. The materials used in brushes are graphite otherwise carbon which is in rectangular form.
The excitation of the DC machine is classified into two types namely separate excitation, as well as self-excitation. In a separate excitation type of dc machine, the field coils are activated with a separate DC source. In the self-excitation type of dc machine, the flow of current throughout the field-winding is supplied with the machine. The principal kinds of DC machines are classified into four types which include the following.
In Shunt wound DC Machines, the field coils are allied in parallel through the armature. As the shunt field gets the complete o/p voltage of a generator otherwise a motor supply voltage, it is normally made of a huge number of twists of fine wire with a small field current carrying.
In series-wound D.C. Machines, the field coils are allied in series through the armature. As series field winding gets the armature current, as well as the armature current is huge, due to this the series field winding includes few twists of wire of big cross-sectional region.
A compound machine includes both the series as well as shunt fields. The two windings are carried-out with every machine pole. The series winding of the machine includes few twists of a huge cross-sectional region, as well as the shunt windings, include several fine wire twists.
The connection of the compound machine can be done in two ways. If the shunt-field is allied in parallel by the armature only, then the machine can be named as the short shunt compound machine & if the shunt-field is allied in parallel by both the armature as well as series field, then the machine is named as the long shunt compound machine.
The DC machine e.m.f can be defined as when the armature in the dc machine rotates, the voltage can be generated within the coils. In a generator, the e.m.f of rotation can be called the generated emf, and Er=Eg. In the motor, the emf of rotation can be called as counter or back emf, and Er=Eb.
We know that the main function of a DC machine is to convert mechanical energy to electrical energy. Throughout this conversion method, the whole input power cannot be changed into output power because of the power loss in different forms. The type of loss may change from one apparatus to another. These losses will decrease the apparatus efficiency as well as the temperature will be increased. The DC machine energy losses can be classified into Electrical otherwise Copper losses, Core losses otherwise Iron losses, Mechanical losses, Brush losses, and Stray load losses.
At present, the generation of electrical energy can be done in bulk in the form of AC (an alternating current). Therefore, the utilization of DC machines like motors and generators DC generators are extremely limited because they are utilized mainly for providing excitation of tiny & middle range of alternators. In industries, DC machines are used for different processes like welding, electrolytic, etc.
Generally, the AC is generated and after that, it is changed into DC with the help of rectifiers. Therefore DC generator is suppressed through an AC supply which is rectified to use in several applications. DC motors are frequently used like variable speed drives & where changes in the severe torque occur.
The application of DC machine as a motor is used by dividing into three types like Series, Shunt &Compound whereas the application of dc machine as a generator is classified into separately excited, series, and shunt-wound generators.
Thus, this is all about DC machines. From the above information, finally, we can conclude that DC machines are dc generator & dc motor. The DC generator is mainly useful for supplying DC sources toward the DC machine in power stations. Whereas DC motor drives some devices like lathes, fans, centrifugal pumps, printing presses, electric locomotives, hoists, cranes, conveyors, rolling mills, auto-rickshaw, ice machines, etc. Here is a question for you, what is commutation in dc machine?
construction of a synchronous machine - circuit globe
Construction of a Synchronous Machine, i.e. alternator or motor consists of two main parts, namely the stator and the rotor. The stator is the stationary part of the machine. It carries the armature winding in which the voltage is generated. The output of the machine is taken from the stator. The rotor is the rotating part of the machine. The rotor produces the main field flux.
The stator core is made of silicon steel material. It is made from a number of stamps that are insulated from each other. Its function is to provide an easy path for the magnetic lines of force and accommodate the stator winding.
Slots are cut on the inner periphery of the stator core in which 3 phase or 1 phase winding is placed. Enameled copper is used as a winding material. The winding is star-connected. The winding of each phase is distributed over several slots. When the current flows in a distributed winding it produces an essentially sinusoidal space distribution of EMF.
The term salient means projecting. Thus, a salient pole rotor consists of poles projecting out from the surface of the rotor core. The end view of a typical 6 pole salient pole rotor is shown below in the figure:
Since the rotor is subjected to changing magnetic fields, it is made of steel laminations to reduce eddy current losses. Poles of identical dimensions are assembled by stacking laminations to the required length. A salient pole synchronous machine has a non-uniform air gap. The air gap is minimized under the pole centers and it is maximum in between the poles.
Pole Core and Pole Shoe: It is made of laminated steel sheet material. The Pole core provides the least reluctance path for the magnetic field and the pole shoe distributes the field over the whole periphery uniformly to produce a sinusoidal wave.
Field Winding or Exciting Winding: It is wound on the former and then placed around the pole core. DC supply is given to it through slip rings. When direct current flows through the field winding, it produces the required magnetic field.
In this type of rotor, there are no projected poles, but the poles are formed by the current flowing through the rotor exciting winding. Cylindrical rotors are made from solid forgings of high-grade nickel chrome-molybdenum steel. It has a comparatively small diameter and long axial length.
They are useful in high-speed machines. The cylindrical rotor type alternator has two or four poles on the rotor. Such a construction provides greater mechanical strength and permits more accurate dynamic balancing. The smooth rotor of the machine makes fewer windage losses and the operation is less noisy because of the uniform air gap.
They are driven by steam or gas turbines. Cylindrical synchronous rotor synchronous generators are called turbo-alternators and turbo generators. The machines are built in a number of ratings from 10 MVA to over 1500 MVA. The biggest size used in India has a rating of 500 MVA installed in the super thermal power plant.
construction and types of synchronous machine | electrical concepts
Synchronous Machine is electromechanical equipment in which the speed of rotating part i.e., Rotor, is equal to the speed of rotation of magnetic flux. The speed of rotation of magnetic flux is called synchronous speed. That is why these types of equipment is called synchronous machine. This article outlines the constructional detail and type of synchronous machine.
Synchronous Motor and Synchronous Generator are together refereed as Synchronous machine. Their basic construction is also same. Therefore, construction of synchronous machines discussed in the post applies well for synchronous motor and generator.
Stator Core It is the magnetic core which is slotted to accommodate armature winding. It comprises of set of slotted steel laminations pressed into the cylindrical space inside the outer frame. Stator core is made of laminated sheet of 0.5 mm thick CRGO to reduce eddy current losses. Refer the figure below for better understanding and insight into the construction of synchronous machine.
Set of insulated windings are placed inside the slot of stator core as shown in figure above. In case of three phase generators, three sets of windings are required, one for each phase connected in star. The cross-sectional area of these windings must be enough to carry rated current of the machine. Figure below illustrate the simplified construction of synchronous machine.
In case of Synchronous Generators, it is not possible to punch all the laminations in cylindrical form. Therefore, laminations are stacked in segments. A number of segments are assembled together to form cylindrical stator core. Stacking of segments to make cylindrical core of synchronous generator is shown in figure below:
Rotor is that part of synchronous machine which can rotate. It carries the field winding. From construction point of view, there are two types of rotor: Salient Pole Rotor and Cylindrical Pole Rotor. Cylindrical pole rotor is also known as round rotor or non-salient pole rotor. To know the difference between the Cylindrical Pole and Salient Pole Rotor, please read Difference between Cylindrical and Salient Pole Rotor.
Hydro-generator rotors are salient pole type. It is made of silicon steel lamination of 0.5 to 0.8 mm thickness to reduce eddy current losses and hence heating. The field winding or DC winding are concentrically wound on the salient pole rotor. This type is only suitable for low speed machine. It is not favorable for high speed machine as due to high centrifugal force on the salient pole, the pole faces may damage. The basic construction of rotor of synchronous machine is shown below.
In case of turbo alternator the rotors are manufactured form solid steel forging. The rotor is slotted to accommodate the field winding. Normally two third of the rotor periphery is slotted to accommodate the winding and the remaining one third unslotted portion acts as the pole. Rectangular slots with tapering teeth are milled in the rotor. Generally rectangular aluminum or copper strips are employed for filed windings. To get feel of rotor manufacturing by solid steel forging, refer the figure below. Figure below shows the rotor construction.
The field windings and the overhangs of the field windings are secured in place by steel retaining rings to protect against high centrifugal forces. Hard composition insulation materials are used in the slots which can withstand high forces, stresses and temperatures.
Damper windings are provided in the pole faces of salient pole alternators. Damper windings are the copper or aluminium bars housed in the slots of the pole faces. The ends of the damper bars are short circuited at the ends by short circuiting rings similar to end rings as in the case of squirrel cage rotors. These damper windings provide mechanical balance and damping effect. It also reduces the effect of over-voltages and damp out hunting in case of synchronous generator. In case of synchronous motors, damper winding act as rotor bars and help in self starting of the motor.