Skip to main content

SOLENOIDS

A solenoid is the term used for a heavy duty relay and like a relay acts as a switch saver. Solenoids are used to switch circuits that require a very large current to operate. An example of this type of circuit is the starter circuit. Consider the diagram below.


Since the starter circuit requires a lot of current to operate, the diagram above would be impractical. This is because the contacts of the ignition switch would need to be very large to carry the large current. Also the large cables required to carry the current would need to be very long.

By using a solenoid, small ignition switch contacts can be used and the length of cable can often be reduced. This will reduce the voltage drop in the circuit.

Voltage drop is the term used to describe a reduction of voltage between two points. e.g. between the ignition switch and the starter solenoid. Voltage drop is proportional to resistance. Long cables, small diameter cables and corroded connections all cause an increase in resistance, which creates voltage drop. To avoid voltage drop, manufacturers design circuits so that, the shortest route, is used by the largest diameter cable with the minimum number of connections. Consider the diagram below.


Like a relay a solenoid has a winding and contacts. When the ignition switch is turned to the cranking position, current flows through the winding and around the soft iron core. Unlike a relay the soft iron core is free to move. The magnetic effect of the winding pulls the iron core into the winding. This brings the moving and fixed contacts together which allows current to flow through the contacts and turn the starter motor.

When the ignition switch is moved away from the cranking position, current flow to the winding ceases. The return spring then pulls the moving contacts and iron core away from the winding. This breaks the current path to the starter so stopping the motor from turning.

Comments

Post a Comment

Popular posts from this blog

ADVANTAGES AND DISADVANTAGES OF CORONA

Corona has many advantages and disadvantages. In the correct design of a high voltage overhead line, a balance should be struck between the advantages and disadvantages. ADVANTAGES (i) Due to corona formation, the air surrounding the conductor becomes conducting and hence virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stresses between the conductors. (ii) Corona reduces the effects of transients produced by surges. DIS-ADVANTAGES (i) Corona is accompanied by a loss of energy. This affects the transmission efficiency of the line. (ii) Ozone is produced by corona and may cause corrosion of the conductor due to chemical action. (iii) The current drawn by the line due to corona is non-sinusoidal and hence non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring communication lines.
Read more

ADVANTAGES OF PER UNIT SYSTEM

PER UNIT SYSTEM The per-unit system expressed the voltages, currents, powers, impedances, and other electrical quantities basis by the equation: Quantity per unit (pu) = Actual value/ Base value of quantity ADVANTAGES OF PER UNIT SYSTEM While performing calculations, referring quantities from one side of the transformer to the other side serious errors may be committed. This can be avoided by using per unit system. Voltages, currents and impedances expressed in per unit do not change when they are referred from one side of transformer to the other side. This is a great advantage. Per unit impedances of electrical equipment of similar type usually lie within a narrow range, when the equipment ratings are used as base values. Transformer connections do not affect the per unit values. Manufacturers usually specify the impedances of machines and transformers in per unit or percent of name plate ratings. Transformers can be replaced by their equivalent series impedances. ...
1 comment
Read more

ABSOLUTE AND SECONDARY INSTRUMENTS

The various electrical instruments may, in a very broad sense, be divided into (i) Absolute Instruments (ii) Secondary Instruments. Absolute Instruments are those which give the value of the quantity to be measured, in terms of the constants of the instrument and their deflection only. No previous calibration or comparison is necessary in their case. The example of such an instrument is tangent galvanometer, which gives the value of current, in terms of the tangent of deflection produced by the current, the radius and number of turns of wire used and the horizontal component of earth’s field.  Secondary Instruments  are those, in which the value of electrical quantity to be measured can be determined from the deflection of the instruments, only when they have been pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless. It is the secondary instruments, which are most generally used in ev...
Post a Comment
Read more