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PARAMETERS OF AN ELECTRIC CIRCUIT.



Parameters of an electric circuit are made up of three cir­cuit components: resistance, inductance and capacitance which determine the circuit characteristics.

1. The resistance of the circuit is determined by the properties of the conductor of the circuit. It is not necessary that the circuit be made of segments of equal resistance1.

2. The amount of magnetic linkage (the number of magnetic flux tubes linking the circuit) per unit current is a mea of the inductance of the circuit:

3. If conductors are connected to metal plates, which are separated by avacuum or insulator, an electric potential will be built between the metal plates. Electrical flux tubes will extend from one plate to the other. The intensity ofthe ele­ctric field per unit potential is a measure of the capacitance2of the circuit.

The purpose of any circuit is to transfer electric energy from an electric generator battery of alternator3 to some other point of the circuit, where electric energy is converted to some other form of energy, such as heat, light or mechanical energy. It should be understood that energy is transmitted only when both electric and magnetic field exist around the сconductor. The amount of energy transfer depends on the intensities the electric and magnetic fields.

ELECTRIC MOTORS.

The electric motor is exceptionally well suited for farm jobs, as it is efficient in converting electric energy into mechanical energy, is easy to start and stop and can be con­trolled automatically. Its overload capacity makes it particu­larly important as a farm power unit since it can develop more than twice its rated power for periods of a few minutes without damage. Electric motors when properly selected and installed serve many years and require only periodic care. The electric motor has probably done more to make the farmer's standard of living equal to that of the urban resident than any other electric device including the electric lamp.

There are many different sizes of electric motors. Why are these different sizes needed or, in other words, what is dif­ferent about the various loads that causes one to select one of motor for this application and another for a different application? The answer is obtained by examining the chara­cteristics of the various loads.

Any equipment4 requiring an electric motor has moving parts. The parts represent an opposition to being moved, and in order for the motor to rotate it has to develop enough force to overcome all opposition to its rotation. The turning effort that the motor uses to overcome the opposition is called torque. The opposition is called countertorque or resisting tor­que. The amount of counter-torque depends upon the construc­tion of the equipment and upon the amount of opposition added when the machine is performing its job. Equipment with large, heavy, moving parts require- more torque than equipment with lightweight parts. A 16-ft conveyor requires more torque when it is operating at а 450 angle than it does when it is horizontal. The more torque necessary) for turning the load, the larger is the horsepower rating5of the motor re­quired for the job.

POWER TRANSFORMERS.

The power transformer is an electric machine, without mo­ving parts, for transforming alternating voltage from a higher to a lower voltage or from a lower to a higher voltage. It con­sists of two electric circuits called the primary and secondary and a steel con (which forms a magnetic circuit). Either coil transformer may be used as primary or secondary. Most transformers are designed for widely different primary and secondary voltages. The coil with the high voltage is called the high-tension winding and the one with low voltage the low-tension winding.

When a high voltage coil is the primary and the low voltage is secondary the transformer is called a step-down transformer. When the reverse connection is used, the transformer is called a step-up transformer. The ratio of the high-side voltage to the low-side voltage the ratio of transformation.

Generally speaking, transformers may he designed for any desired ratio of transformation. Common ratios are 5:1, 10:1, 20:1.

Transformer Name Plate. The name plate on the transformer gives the voltage and kva ratings of the transformer and shows how the coils are connected. It is common practice to build the lowvoltage winding in two coils and bring four lead; to the outside of the transformer case. The coils may be con­nected in series or parallel. If the coils are for 115 or 230 volts, they will supply rated kva at either voltage.

The transformer should always be connected as shown on the nameplate. Otherwise a short circuit which would damage the windings may result.

Transformer Connections. Single-phase transformers may be used for either single-phase, two-phase or three-phase connections. One single-phase transformer may he used to supply three-wire 115-230 volts or 230 460 volts

Since the secondary winding is electrically insulated from the primary, any point of the secondary may be connected to ground. In single-phase, 115-230-volt circuits it is common practice to ground the centre of the winding. If the system is a three wire, 230-volt circuit with the centre of the coil groun­ded, there is a potential of 115 volts from the ground to either wire. The transformer case should be grounded.

Notes:

1resistance - сопротивление

2capacitance - емкость

3alternator – генератор переменного тока

4equipment - оборудование

5horsepower rating – мощность в лошадиных силах

Инженерно-технологический факультет.

Вариант №1.





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