Kirchoff's voltage law

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Kirchoff's voltage law can be stated in words as the sum of all voltage drops and rises in a closed loop equals zero. As the image below demonstrates, loop 1 and loop 2 are both closed loops within the circuit. The sum of all voltage drops and rises around loop 1 equals zero, and the sum of all voltage drops and rises in loop 2 must also equal zero. A closed loop can be defined as any path in which the originating point in the loop is also the ending point for the loop. No matter how the loop is defined or drawn, the sum of the voltages in the loop must be zero.

The sum of all voltages or potential differences in an electrical circuit loop is 0.

 

KVL example

V_S = 12V, V_R1 = -4V, V_R2 = -3V

V_R3 = ?

Solution:

∑V_k = V_S + V_R1 + V_R2 + V_R3 = 0

V_R3 = -V_S  - V_R1 - V_R2 = -12V+4V+3V = -5V

The voltage sign (+/-) is the direction of the potential difference.

Kirchhoff's Current Law

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In an electrical circuit, the current flows rationally as electrical quantity. As the flow of current is considered as flow of quantity, at any point in the circuit the total current enters, is exactly equal to the total current leaves the point. The point may be considered anywhere in the circuit Suppose the point is on the conductor through which the current is flowing, then the same current crosses the point which can alternatively said that the current enters at the point and same will leave the point. As we said the point may be anywhere on the circuit, so it can also be a junction point in the circuit. So total quantity of current enters at the junction point must be exactly equal to total quantity of current that leaves the junction. This is the very basic thing about flowing of current and fortunately Kirchhoff Current law says the same. The law is also known as Kirchhoff First Law and this law stated that, at any junction point in the electrical circuit, the summation of all the branch currents is zero. If we consider all the currents enter in the junction are considered as positive current, then convention of all the branch currents leaving the junction are negative. Now if we add all these positive and negative signed currents, obviously we will get result of zero

 "The algebraic sum of all currents entering and exiting a node must equal zero"

SLIP RING INDUCTION MOTOR

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 The slip ring induction motor has two distinctly separate parts, one is the stator and other is the rotor. The stator circuit is rated as same in the squirrel cage motor, but the rotor is rated in frame voltage or short circuit current.    A slip ring (in electrical engineering terms) is a method of making an electrical connection through a rotating assembly. Slip rings, also called rotary electrical interfaces, rotating electrical connectors, collectors, swivels or electrical rotary joints, are commonly found in electrical generators for AC systems and alternators and in packaging machinery, cable reels, and wind turbines.  A slip ring consists of a conductive circle or band mounted on a shaft and insulated from it. Electrical connections from the rotating part of the system, such as the rotor of a generator, are made to the ring. Fixed contacts or brushes run in contact with the ring, transferring electrical power or signals to the exterior, static part of the system.

STATOR

The stator consists of 3-ph winding forms wound 'poles' that carry the supply current to induce a magnetic field that penetrates the rotor. In a very simple motor, there would be a single projecting piece of the stator (a salient pole ) for each pole, with windings around it; in fact, to optimize the distribution of the magnetic field, the windings are distributed in many slots located around the stator, but the magnetic field still has the same number of north-south alternations. The number of 'poles' can vary between motor types but the poles are always in pairs

SLIP RING ROTOR

The slip ring induction motors usually have “Phase-Wound” rotor. This type of rotor is provided with a 3-phase, double-layer, distributed winding consisting of coils used in alternators. The rotor core is made up of steel laminations which has slots to accommodate formed 3-single phase windings. These windings are placed 120 degrees electrically apart. The rotor is wound for as many poles as the number of poles in the stator and is always 3-phase even though the stator is wound for 2-phase. These three windings are “starred” internally and other end of these three windings are brought out and connected to three insulated slip-rings mounted on the rotor shaft itself. The three terminal ends touch these three slip rings with the help of carbon brushes which are held against the rings with the help of spring assembly These three carbon brushes are further connected externally to a 3-phase star connected rheostat Thus these slip ring and external rheostat makes the slip ring induction motors possible to add external resistance to the rotor circuit, thus enabling them to have a higher resistance during starting and thus higher starting torque. 

DC MOTOR

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A  DC motor  is a mechanically commutated  electric motor  powered from  direct current  (DC). The stator is stationary in space by definition and therefore the current in the rotor is switched by the commutator  to also be stationary in space. This is how the relative angle between the stator and rotor magnetic flux is maintained near 90 degrees, which generates the maximum torque.

DC motors have a rotating armature winding (winding in which a voltage is induced) but non-rotating armature magnetic field and a static field winding (winding that produce the main magnetic flux) or permanent magnet. Different connections of the field and armature winding provide different inherent speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the voltage applied to the armature or by changing the field current.

Principle of DC Motor

This DC or  Direct Current Motor  works on the principal, when a current carrying conductor is placed in a magnetic field, it experiences a torque and has a tendency to move. This is known as motoring action. If the direction of   electric current   in the wire is reversed, the direction of rotation also reverses. When magnetic field and electric field interact they produce a mechanical force, and based on that the working principle of  dc motor  established.   The direction of rotation of a this motor is given by Fleming’s left hand rule, which states that if the index finger, middle finger and thumb of your left hand are extended mutually perpendicular to each other and if the index finger represents the direction of magnetic field, middle finger indicates the direction of   electric current then the thumb represents the direction in which force is experienced by the shaft of the  dc motor . Structurally and construction wise a  Direct Current Motor  is exactly similar to a D.C. Generator, but electrically it is just the opposite. Here we unlike a generator we supply electrical energy to the input port and derive mechanical energy from the output port

• Shunt motor

The field coil and the armature windings are connected in shunt or parallel across the power source. The armature winding consists of relatively few turns of heavy gauge wire. The voltage across two windings is the same but the armature draws considerably more current than the field coil. Torque is caused by the interaction of the current caring armature winding with the magnetic field produced by the field coil. If the DC line voltage is constant, the armature voltage and the field strength will be constant. The speed regulation is quite good; the speed is a function of armature current and is not precisely constant. As the armature rotates within the magnetic field, an EMF is induced in its wining. This EMF is in the direction opposite to the source EMF and is called the counter EMF (CEMF), which varies with rotational speed. Finally, the current flow through the armature winding is a result of the difference between source EMF and CEMF. When the load increases, the motor tends to slow down and less CEMF is induced, which in turn increases the armature current providing more torque for the increased load

Motor speed is increased by inserting resistance into the field coil circuit, which weakens the magnetic field. Therefore, the speed can be increased from “basic” or full-load, full-field value to some maximum speed set by the electrical and mechanical limitations of the motor.

Series motor

The field coil and armature windings are connected in series to the power source. The field coil is wound with a few turns of heavy gauge wire. In this motor, the magnetic field is produced by the current flowing through the armature winding; with the result that the magnetic field is weak when the motor load is light (the armature winding draws a minimum current). The magnetic field is strong when the load is heavy (the armature winding draws a maximum current). The armature voltage is nearly equal to the PS line voltage (just as in the shunt wound motor if we neglect the small drop in the series field)

Consequently , the speed of the series wound motor is entirely determined by the load current. The speed is low at heavy loads, and very high at no load. In fact, many series motors will , if operated at no load , run so fast that they destroy themselves . The high forces , associated with high speeds , cause the rotor to fly apart , often with disastrous results to people and property nearby . The torque of any DC motor depends upon the product of the armature current and the magnetic field. For the series wound motor this relationship implies that the torque will be very large for high armature currents, such as occur during start-up. The series wound motor is, therefore, well adapted to start large heavy-inertia loads, and is particularly useful as a drive motor in electric buses, trains and heavy duty traction applications. Compared to the shunt motor, the series DC motor has high starting torque and poor speed regulation.