 | 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 |
5.4.4. Braking circuits
Counter-current braking
Mode of operation
Braking by means of counter-current is the simplest way to attain standstill of an asynchronous drive resp. the deceleration of pull-through loads, for instance in pumping stations. Two stator leads are interchanged to this end during motor operation. This changes the rotational direction of the rotating field. The rotor, which is braked, thus runs counter to the rotational direction of the rotating field. This connection can be used both for squirrel cage and slip ring motors. No additional devices are required.
The braking effect during counter-current braking bases on the altered rotational field direction. The motor tries to accelerate in the other rotational direction.
The motor must be disconnected in good time from the mains so that it does not again accelerate in the new rotational field direction. This is mainly made automatically.
Counter-current operation induces pronounced braking reaction. The current impulse on switching over is considerable greater than starting through direct connection. The motor is generally braked in star connection in order to avoid too great a current.
Figure 75 - Counter-current braking
(main circuit)
Figure 76 - Counter-current braking
(control circuit)
Circuitry description
Protection K1 switches on the three-phase motor. During switching off K2 connects the mains via two series resistors with two interchanged external conductors. The counter field brakes the rotors.
K2 falls off during motor stillstand.
Actuating S2 switches protection K1 which holds itself in the current path 2 through a closer. K2 is locked by the K1 opener in current path 5 whilst the closer in current path 3 switches the locking relay K3. Switching off by means of S1 the K1 opener closes current path 5. K2 is excited. Given standstill (n = 0) the closer of the automatic brake controller interrupts the F3 current path 5. K3 and K2 drop out.
Direct current braking
Mode of operation
During this braking procedure the machine is disconnected from the mains and the stator winding is excited through direct current. Connection to the direct current source ensues acc. to the circuit depicted in Figure 77.
The stator establishes a constant magnetic field. Induction currents are yielded in the rotor winding which is either short-circuited or connected by means of rotor resistors. These induction currents give rise to a braking torque which facilitates impulse-free braking.
The asynchronous machine with direct current braking behaves in the same manner as an external pole synchronous generator.
Direct current braking is suitable for stopping all categories of asynchronous machine drives. The dissipated heat converted through rotor circuit braking is much less than during counter-braking. The minimal exciting power and the admirably controlled speed of slip ring motors are further advantages of this circuitry.
Figure 77 - Direct current braking
(main circuit)
Figure 78 - Direct current braking
(control circuit)
Circuitry description
K1 switches on the three-phase motor. On switching off K2 connects direct voltage to the stator winding. K2 drops out after commensurate braking.
Actuating S2 switches protection K1 which holds itself via a closer in current path 2. The K1 closer in current path 3 switches on the auxiliary contactor K3 (release delay). K1 openers in current path 5 serve to lock K2. K1 drops out when S1 switches off. Its opener locks current path 5 (braking ensues through K2) whilst its closer in current path 3 switches K3 off with delay.
The closer of K3 in the current path 5 opens with delay whereby K2 drops off.
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