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Tutorial: Mechanic – electrician Topic: Basics of electrical engineering the 2nd. year Three-phase current

Prepared by: Škarka Miloš

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Three-phase system is a system of three harmonic voltages of the same frequency and amplitude, which have different, but same phase shift towards each other

Three phase has in comparison with one-phase these advantages:: simultaneous presence of two levels of voltage:

Phase 3x230v

Comprehensive 3x400v

Uncomplicated commencement of the torque is the main advantage allowing function and simple construction of rotating electric machines

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Alternator rotor Turboalternator stator in assembly

Machine set – turbine with alternator

Basic parts of the alternator are immovable stator and a rotor rotating on the shaft. Synchronous alternator has three-phase winding on the stator and so called rousing winding on the rotor.

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Three-phase generator of the alternating current

If the turbine or other drive rotates the rotor and there is a direct current passing through its rousing winding, the rotating magnetic field commences. This causes (induces) three-phase alternating voltage in the three-phase stator winding. It is directly proportional to revs and rousing of the rotor.

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All coils are the same and they are located near the same magnet. The course of the voltage will be the same on all coils, but it will differ in the shift in time, the courses on each coil will be delayed. The difference between coils is one third of the rev (120°), the magnet will need one third of the period T to turn from one coil to another, courses will so be delayed for one third of the period.

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Sinus course of voltage or intensity1) Maximum value2) Top voltage3) Effective value4) Period

Effective value of alternating voltage (Uef) is equal to the value of direct voltage, which would give the same average performance, if attached to resistance strain

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Phase and comprehensive voltage

Null conductor

Phase conductor

Phase conductor

Phase conductor

The voltage induced on individual coils has sine course. The voltage between the phase and the null conductor is called phase voltage - Effective value of the phase voltage is 230 V. The voltage between two phase conductors equals the sum of voltages induced on two coils and it is called comprehensive voltage.

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• There is a course of voltage on the first coil drawn on the picture. Draw the course of voltage on remaining coils into the picture

• Describe main parts of alternator

• Explain commencement of the voltage on coils

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• Put down the amount of voltage between individual conductors

• Explain the term phase and comprehensive voltage

• Describe which voltages do numbers 1-4 determine.

• Explain the term effective value of alternating voltage

• Amplitude of alternating voltage measured by the oscilloscope is 360V, what is the effective value?

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Základy elektrotechniky 2, L.VOŽENÍLEK- F. STIBŮREK www.wikipedia.cz