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الجمعة، 13 نوفمبر 2020

DC Machines

 

1. Interdiction.

 In this research we will taking about DC machines especial dc separately excited motors, and we will talk about their construction, speed torque, how to controlling their speed and application , DC machines are generators that convert mechanical energy to dc electric energy and motors that convert dc electric energy to mechanical energy their have the same construction . Most dc machines are like ac machines in that they have ac voltages and currents within their in, dc machines have a dc output only because a mechanism exists that converts the internal ac voltages to dc voltages at their tenninals. Since this mechanism is called a commutator, dc machinery is also known as commutating machinery.

2. DC separately excited motor Construction.

                                                                                                                                                            

Any DC machine consists of two main components: Stator and Rotor. The stator is that the stationary part whereas the rotor is that the rotating part. Stator of DC machine consists of Yoke, field coil, Interpoles, Compensating Winding, Brushes, and End Cover. Rotor consists of armature core, armature winding, commutator, and shaft. A labeled diagram of the DC machine is shown within the fig.1, let’s speak about every component very well.        
Yoke: Yoke is that the core of the stator. It provides a path for the pole flux Ø and carries half it. Aside from this, it provides mechanical support to the full machine. The Yoke of the DC machine isn't laminated because it carries stationary flux and hence there's no eddy current. Iron core is employed for the development of Yoke for little DC machine whereas Steel is employed for giant DC machine

Field Poles: Field pole consists of pole core and pole shoe. The pole core is formed from the cast steel. The pole shoe of the DC machine is laminated and stuck to the pole core. These Filed Poles are welded or bolted to the Yoke.
Field Winding or Exciting Winding: The pole is worked up by a winding wound round the pole core. This winding is termed the sector Winding or Exciting Winding and made up of copper. The quantity of turns and cross-sectional of field coil depends on the 


figure .1



                  


kind of DC machine as below: 

• large number of turns of small cross-sectional area is employed for DC Shunt machine.
• For DC Series machine, atiny low number of turns of enormous cross-sectional area is employed.
• Both series and shunt field coil is applied for DC Compound machine.

Interpoles: Interpoles are fixed to the Yoke in between the main poles of DC machine. The interpole winding is made of copper and consists of few turns of thick wire. This winding is connected in series with the armature winding.

Brushes: Brushes are housed in the brush holder and connected to the end cover. It is made up of Carbon for small DC machine. For large DC machine, electro graphite is used to make brushes.  A spring keeps the brushes pressed on the commutator surface, they have a high conductivity to reduce electrical losses and a low coefficient of friction to reduce excessive wear.

Commutator: It is a cylindrical structure is typically made of copper bars insulated by a mica-type material. The copper bars are made sufficiently thick to permit normal wear over the lifetime of the motor, the mica insulation between commutator segments is harder than the commutator material itself, so as a machine ages, it is often necessary to undercut the commutator insulation to ensure that it does not stick up above the level of the copper bars.

Armature Core: It is a magnetic core made of laminated silicon steel of thickness 0.30 to 0.50 mm to minimize the iron losses. The main purpose of armature core is to house the armature conductor in its slot and provide low reluctance path to magnetic flux Ø/2 as shown in the labeled diagram of DC machine.

Armature Winding: Armature winding is made from copper. It consists of large number of insulated coils having one or more than one turns. Theses coils are placed in the armature core slots and connected appropriately in series and parallel depending on the type of winding. There are basically two types of winding: Lap Winding and Wave Winding.

Compensating Winding: Theses windings are placed in the slots cut in the pole faces of DC machine. Compensating winding is also connected in series with the armature winding.

Shaft: Shaft of DC Motor is coupled to the load to transfer mechanical power. 

Winding Insulation: the most critical part of a dc motor's design is the insulation of its windings. I f the insulation of the motor windings breaks down, the motor shorts out. The repair of a machine with shorted insulation is quite expensive, if it is even possible. To prevent the insulation of the machine windings from breaking down as a result of overheating, it is necessary to limit the temperature of the windings. This can be partially done by providing a cooling air circulation over them, but ultimately the maximum winding temperature limits the maximum power that can be supplied continuously by the machine.

The different of dc separately motor from other type of motor whose field circuit is supplied from a separate constant-voltage power supply as shown in fig.2 it show the equivalent circuit of it.

 

 

 

figure.2


3. DC motor characteristics.

Generally, three characteristic curves are considered for DC motors which are, (1) Developed Torque versus armature current (2) Speed versus armature current (3) Terminal characteristics (Speed versus developed torque) these characteristics are determined by following two relations. V. DC motor characteristics            𝐸𝐴 = 𝐾∅𝜔𝑚          ,          𝑇𝑑𝑒𝑣 = 𝐾∅𝐼.

Let’s talk about speed –torque, this characteristic is also called as mechanical characteristic, when the load increases, the output torque required to drive the load will increase. Hence, the motor speed will slow down. Consequently the internal generated voltage drops (𝐸𝐴 = 𝐾∅𝜔𝑚 ↓), increasing the armature current in motor 𝐼𝐴 = (𝑉𝑠𝐸𝐴↓)/𝑅𝐴. As the armature current increases, the developed torque increase (𝑇𝑑𝑒𝑣 = 𝐾∅𝐼𝐴 ↑) and finally the developed torque will be equal the load torque at a lower mechanical speed of rotation 𝜔𝑚. As flux Φ is assumed constant, the speed decreases with developed torque increase. But practically, due to armature reaction, Φ decreases with increase in armature current, and hence the speed decrease slightly. Thus, at heavy loads, the motor speed is almost constant.

                              

figure.3

   𝜔𝑚 = (𝑉𝑠𝑇𝑑𝑒𝑣 𝑅𝐴/𝐾∅)/ 𝐾∅, in fig.3 it Torque-speed characteristic of the motor with armature reaction present.

It is important to realize that, in order for the speed of the motor to vary linearly with torque, the other terms in this expression must be constant as the load changes. The terminal voltage supplied by the dc power source is assumed to be constant- if it is not constant, then the voltage variations will affect the shape of the torque- speed curve.

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