 | Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.) |
 |  | (introduction...) |
| | Introduction |
|  | 1. General information about electrical machines |
| | 1.1. Definition of terms |
| | 1.2. Types of electrical machines |
| | 1.3. Operations of electrical machines |
| | 1.4. System of rotating electrical machines (generators, motors, converters) |
| | 1.5. System of stationary electrical machines (transformers) |
| | 2. Basic principles |
| | 2.1. The magnetic field |
| | 2.1.1. Definition and presentation of the magnetic field |
| | 2.1.2. Magnets Magnetic field |
| | 2.1.3. Magnetic field of a current-carrying conductor |
| | 2.1.4. Magnetic field of a current-carrying coil |
| | 2.1.5. Magnetic fields in electrical machines |
| | 2.2. Measurable variables of the magnetic field |
| | 2.2.1. Magnetomotive force |
| | 2.2.2. Magnetic flow |
| | 2.2.3. Magnetic flow density |
| | 2.3. Force action of the magnetic field |
| | 2.3.1. Force action on cur rent-carrying conductors |
| | 2.3.2. Force action on current-carrying coils (motor principle) |
| | 2.4. Voltage generation through induction |
| | 2.4.1. General law of induction |
| | 2.4.2. Stationary induction (transformer principle) |
| | 2.4.3. Motional induction (generator principle) |
| | 3. Execution of rotating electrical machines |
| | 3.1. Size |
| | 3.2. Designs |
| | 3.2.1. Definition |
| | 3.2.2. Designation |
| | 3.3. Degree of protection |
| | 3.3.1. Definition |
| | 3.3.2. Designation |
| | 3.4. Cooling |
| | 3.4.1. Cooling category |
| | 3.4.2. Cooling category designation |
| | 3.5. Mode of operation |
| | 3.5.1. Definition |
| | 3.5.2. Operational mode designation |
| | 3.5.3. Frequent nominal cycle ratings |
| | 3.6. Heat resistance categories |
| | 3.7. Connection designations of electrical machines |
| | 3.7.1. Transformers |
| | 3.7.2. Rotating electrical machines |
| | 3.8. Rotating electrical machines in rotational sense |
| | 3.8.1. Clockwise rotation stipulation |
| | 3.8.2. Direct current machines |
| | 3.8.3. Alternating current and three-phase machines |
| | 3.9. Rating plate |
| | 4. Synchronous machines |
| | 4.1. Operating principles |
| | 4.1.1. Synchronous generator |
| | 4.1.2. Synchronous motor |
| | 4.2. Constructional assembly |
| | 4.2.1. Stator |
| | 4.2.2. Rotor |
| | 4.3. Operational behaviour |
| | 4.3.1. Synchronous generator |
| | 4.3.2. Synchronous motor |
| | 4.4. Use of synchronous machines |
| | 4.4.1. Synchronous generators |
| | 4.4.2. Synchronous motors |
| | 5. Asynchronous motors |
| | 5.1. Constructional assembly |
| | 5.2. Operating principles |
| | 5.2.1. Torque generation |
| | 5.2.2. Asynchronous principle |
| | 5.2.3. Slip |
| | 5.3. Operational behaviour |
| | 5.3.1. Start |
| | 5.3.2. Rating |
| | 5.3.3. Speed control |
| | 5.3.4. Rotational sense alteration |
| | 5.4. Circuit engineering |
| | 5.4.1. Starting connections |
| | 5.4.2. Dahlander pole-changing circuit (speed control) |
| | 5.4.3. Rotational reversing circuit |
| | 5.4.4. Braking circuits |
| | 5.5. Application |
| | 5.6. Characteristic values of squirrel cage motors |
| | 6. Direct current machines |
| | 6.1. Constructional assembly |
| | 6.2. Operating principles |
| | 6.2.1. Power generation (direct current motor) |
| | 6.2.2. Torque generation (direct current motor) |
| | 6.2.3. Armature reaction (rotor reaction) |
| | 6.2.4. Excitation |
| | 6.2.5. Value relations |
| | 6.3. Operational behaviour of direct current machines |
| | 6.3.1. Direct current generators |
| | 6.3.2. Direct current motors |
| | 6.4. Circuit engineering and operational features of customary direct current generators |
| | 6.4.1. Separate-excited direct current generator |
| | 6.4.2. Direct current shunt generator |
| | 6.5. Circuit engineering and operational features of customary direct current motors |
| | 6.5.1. Direct current motor with permanent excitation |
| | 6.5.2. Direct current series motor |
| | 6.5.3. Direct current shunt motor |
| | 7. Single-phase alternating current motors |
| | (introduction...) |
| | 7.1. Single-phase asynchronous motors (single-phase induction motors) |
| | (introduction...) |
| | 7.1.1. Assembly and operating principle |
| | 7.1.2. Operational behaviour |
| | 7.1.3. Technical data |
| | 7.2. Three-phase asynchronous motor in single-phase operation (capacitor motor) |
| | 7.2.1. Assembly and operating principle |
| | 7.2.2. Operational behaviour |
| | 7.3. Split pole motors |
| | 7.4. Single-phase commutator motors (universal motors) |
| | 7.4.1. Assembly |
| | 7.4.2. Operating principles |
| | 7.4.3. Operational behaviour |
| | 7.4.4. Technical data |
| | 8. Transformer |
| | 8.1. Transformer principle |
| | 8.1.1. Operating principle of a transformer |
| | 8.1.2. Voltage transformation |
| | 8.1.3. Current transformation |
| | 8.2. Operational behaviour of a transformer |
| | 8.2.1. Idling behaviour Idling features |
| | 8.2.2. Short-circuit behaviour |
| | 8.2.3. Loaded voltage behaviour |
| | 8.2.4. Efficiency |
| | 8.3. Three-phase transformer |
| | 8.3.1. Three-phase transformation with single-phase transformers |
| | 8.3.2. Three-phase transformers |
| | 8.3.3. Vector groups |
| | 8.3.4. Application of three-phase transformers in power supply |
| | 8.3.5. Parallel operation of transformers |
| | 8.3.6. Technical data of customary transformers |
8.1.3. Current transformation
Load behaviour of the transformer
If the transformer is output-loaded, current I2 flows into coil N2. Current I2 generates the magnetic flow F2K. According to Lenz’s Law this magnetic flow is counter-positioned to the cause (F1K).
Figure 126 - Loaded transformer
In this manner the magnet flow F1K is weakened and induction voltage U10 decreases. Given uniform rated voltage, the difference increases between the two voltages U10 and U1.
Consequently, a greater input current I1 flows whereby the magnetic flow F1K is increased. The magnetic flow F in the iron core thus remains virtually constant:
F = F1K - F2K = constant
This also applies to the output voltage of the transformer.
The input current I1 increases as the load current I2 becomes greater.
Transformation ratio
Without heeded the losses of the transformer, the following applies according to the energy conservation law:
s1 = s2U1 · I1 = U2 · I2
If we arrange the equation so that the voltage and current values appears on respective sides, then
.
The following relationships may be cited for current ratio:
Conversely the currents are proportional to the voltages or numbers of turns. A transformer converts high currents into low ones or low currents into higher ones.
Example:
A welding transformer takes up 220 (current being 10A). The output voltage is 20V. How great is the welding current?
Solution:
I2 » 110A
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