Electra

FLEMING’s RULES

      FLEMING’s LEFT HAND RULE 

It is also known as the motor rule. This rule is followed for determining the direction of force on a current carrying conductor in a magnetic field. Here, the thumb, fore finger and middle finger at left hand are kept at right angle to each other. The fore finger represents the direction of force, middle finger represents the direction of current in the component (wire) and the thumb represents the direction of the force on the conductor.

      FLEMINGs RIGHT HAND RULE

It is also known as the generator rule. This rule is followed to determine the direction of the induced e.m.f. of a conductor that is moving in a magnetic field. The thumb, first and middle fingers at right hand are kept at right angle to each other. Here, the thumb represents the direction of motion of the conductor, the first finger represents the direction of the magnetic field and the middle finger will give the direction of induced e.m.f. in the conductor.

Know about the PRINCIPLE OF AN ALTERNATOR

           A.C. generators are generally referred as alternators.

They operate under the principle of electromagnetic induction.

 The principle is when a conductor cuts the magnetic flux, e.m.f. is induced in the conductor by the principle of Faraday’s Law of Electromagnetic Induction, this induced e.m.f. causes a current flow in the conductor when the conductor is connected to a closed circuit.

The principle is similar to the D.C. generator, but the difference in alternator is the armature will be stator while the field rotates. The armature winding is mounted on the stationary element called as stator and field winding is mounted on the rotating element called as the rotor.

Stator consists of stator frame made of cast-iron, which is the outer most layer of the alternator. Rotor is represented by alternate north and south poles that are fixed to the outer rim like a flywheel.

The magnetic poles are excited by a d.c. supply. The rotor rotates and the stator will cut the magnetic flux lines developing e.m.f. in it.

This e.m.f. will induce current in the conductor which is allowed to pass through a closed circuit. The direction of alternating e.m.f. is given by Fleming’s Right Rule.

Speed Management of a Direct Current series motor

                There are various methods to control the speed of series motor

. 1) The variation of flux by means of field diverter in which the variable resistance is connected in parallel with the field winding. If the speed has to increase the resistance is reduced which results in the reduction of main field.

2) The variation of flux by means of armature diverter gives speeds below the normal value. It is obtained by connecting the diverter across the armature.

 3) In the Tapped field method the higher speeds results by cutting out the series turns while with the full field motor runs as its minimum speed.

4) The variable resistance in series with the motor helps in controlling the speed. If the resistance increases the voltage across motor reduces. Since the voltage is proportional to speed the speed drops. T

he demerit of this method is the wastage of power as the resistance is in series.

Learn about-Starter for DC motor

                        It is necessary for the safe operation of a machine. If we connect the motor to a supply at the time of start back emf is zero and heavy current passes to the armature which is undesirable.

 So it is essential to reduce the starting current to the safe operation. Starter is used to reduce the starting current which is nothing but a resistance which cuts out from the supply after motor gathers speed and develops back emf which regulates the speed.

 Starter consists of studs. When the starter arm points to first stud it includes the entire resistance and passes the safe value of current to armature. Armature starts to accelerate and develops back emf.

 Then the arm moves to the next stud to cut out the resistances and finally connects directly to the armature. In Series motor starter is connected in series with the armature and the field.

In other types the shunt field is connected to the stud to ensure that safe operation starter is used.

Shunt Motor Versus Compound Motor

                            The field winding of the motor connected in parallel to the armature is known to be Shunt Motor.

Here the field consists of many turns of fine wire since it does not carry the entire armature current. The current path is divided to go to armature and field.

The Shunt motor is a constant speed motor because speed of the shunt motor remains practically constant with varying loads.

It is used where the constant speed operation is necessary. It is widely used in Lathe machines where one speed is maintained constant for a longer period.

The compound motor consists of both the shunt and series winding which is required for certain applications. Both the fields can be varied to operate the motor nearer to series or shunt motor.

When it operated nearer to series motor it is known as cumulative compound and when it is nearer to shunt motor it is known as differential compound.

Series motor has the disadvantage that we cannot operate it in decreasing light loads which can be overcome by this compound motor. It is also used in applications where there is a fluctuating load.

Know about CURRENT TRANSFORMERS

                         Current transformers are used to measure the large currents. In high-voltage alternating current circuits, it is impracticable to measure the current.

For measuring the current in such circuits, current transformers are connected to the circuit with low-range ammeters.

The primary coil of the current transformer is connected in series with the line. Ammeter is connected across the secondary side of the transformer. Current transformer is a step-up transformer with respect to the voltage.

 When voltage gets increased in the transformer, it is obvious that the current will get reduced. When the transformer has got a 100:10 primary to secondary current ratio, the voltage steps up by 10 times, whereas the current gets step-down by 1/10^th of the actual value.

The current transformation ratio is calculated as the ratio of the primary current to the secondary current. Now the ammeter reading is noted down.

Then the line current is calculated by the product of the current transformation ratio and the ammeter reading.