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close this bookElectrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)
View the document(introduction...)
View the documentIntroduction
close this folder1. General information about electrical machines
View the document1.1. Definition of terms
View the document1.2. Types of electrical machines
View the document1.3. Operations of electrical machines
View the document1.4. System of rotating electrical machines (generators, motors, converters)
View the document1.5. System of stationary electrical machines (transformers)
close this folder2. Basic principles
close this folder2.1. The magnetic field
View the document2.1.1. Definition and presentation of the magnetic field
View the document2.1.2. Magnets Magnetic field
View the document2.1.3. Magnetic field of a current-carrying conductor
View the document2.1.4. Magnetic field of a current-carrying coil
View the document2.1.5. Magnetic fields in electrical machines
close this folder2.2. Measurable variables of the magnetic field
View the document2.2.1. Magnetomotive force
View the document2.2.2. Magnetic flow
View the document2.2.3. Magnetic flow density
close this folder2.3. Force action of the magnetic field
View the document2.3.1. Force action on cur rent-carrying conductors
View the document2.3.2. Force action on current-carrying coils (motor principle)
close this folder2.4. Voltage generation through induction
View the document2.4.1. General law of induction
View the document2.4.2. Stationary induction (transformer principle)
View the document2.4.3. Motional induction (generator principle)
close this folder3. Execution of rotating electrical machines
View the document3.1. Size
close this folder3.2. Designs
View the document3.2.1. Definition
View the document3.2.2. Designation
close this folder3.3. Degree of protection
View the document3.3.1. Definition
View the document3.3.2. Designation
close this folder3.4. Cooling
View the document3.4.1. Cooling category
View the document3.4.2. Cooling category designation
close this folder3.5. Mode of operation
View the document3.5.1. Definition
View the document3.5.2. Operational mode designation
View the document3.5.3. Frequent nominal cycle ratings
View the document3.6. Heat resistance categories
close this folder3.7. Connection designations of electrical machines
View the document3.7.1. Transformers
View the document3.7.2. Rotating electrical machines
close this folder3.8. Rotating electrical machines in rotational sense
View the document3.8.1. Clockwise rotation stipulation
View the document3.8.2. Direct current machines
View the document3.8.3. Alternating current and three-phase machines
View the document3.9. Rating plate
close this folder4. Synchronous machines
close this folder4.1. Operating principles
View the document4.1.1. Synchronous generator
View the document4.1.2. Synchronous motor
close this folder4.2. Constructional assembly
View the document4.2.1. Stator
View the document4.2.2. Rotor
close this folder4.3. Operational behaviour
View the document4.3.1. Synchronous generator
View the document4.3.2. Synchronous motor
close this folder4.4. Use of synchronous machines
View the document4.4.1. Synchronous generators
View the document4.4.2. Synchronous motors
close this folder5. Asynchronous motors
View the document5.1. Constructional assembly
close this folder5.2. Operating principles
View the document5.2.1. Torque generation
View the document5.2.2. Asynchronous principle
View the document5.2.3. Slip
close this folder5.3. Operational behaviour
View the document5.3.1. Start
View the document5.3.2. Rating
View the document5.3.3. Speed control
View the document5.3.4. Rotational sense alteration
close this folder5.4. Circuit engineering
View the document5.4.1. Starting connections
View the document5.4.2. Dahlander pole-changing circuit (speed control)
View the document5.4.3. Rotational reversing circuit
View the document5.4.4. Braking circuits
View the document5.5. Application
View the document5.6. Characteristic values of squirrel cage motors
close this folder6. Direct current machines
View the document6.1. Constructional assembly
close this folder6.2. Operating principles
View the document6.2.1. Power generation (direct current motor)
View the document6.2.2. Torque generation (direct current motor)
View the document6.2.3. Armature reaction (rotor reaction)
View the document6.2.4. Excitation
View the document6.2.5. Value relations
close this folder6.3. Operational behaviour of direct current machines
View the document6.3.1. Direct current generators
View the document6.3.2. Direct current motors
close this folder6.4. Circuit engineering and operational features of customary direct current generators
View the document6.4.1. Separate-excited direct current generator
View the document6.4.2. Direct current shunt generator
close this folder6.5. Circuit engineering and operational features of customary direct current motors
View the document6.5.1. Direct current motor with permanent excitation
View the document6.5.2. Direct current series motor
View the document6.5.3. Direct current shunt motor
close this folder7. Single-phase alternating current motors
View the document(introduction...)
close this folder7.1. Single-phase asynchronous motors (single-phase induction motors)
View the document(introduction...)
View the document7.1.1. Assembly and operating principle
View the document7.1.2. Operational behaviour
View the document7.1.3. Technical data
close this folder7.2. Three-phase asynchronous motor in single-phase operation (capacitor motor)
View the document7.2.1. Assembly and operating principle
View the document7.2.2. Operational behaviour
View the document7.3. Split pole motors
close this folder7.4. Single-phase commutator motors (universal motors)
View the document7.4.1. Assembly
View the document7.4.2. Operating principles
View the document7.4.3. Operational behaviour
View the document7.4.4. Technical data
close this folder8. Transformer
close this folder8.1. Transformer principle
View the document8.1.1. Operating principle of a transformer
View the document8.1.2. Voltage transformation
View the document8.1.3. Current transformation
close this folder8.2. Operational behaviour of a transformer
View the document8.2.1. Idling behaviour Idling features
View the document8.2.2. Short-circuit behaviour
View the document8.2.3. Loaded voltage behaviour
View the document8.2.4. Efficiency
close this folder8.3. Three-phase transformer
View the document8.3.1. Three-phase transformation with single-phase transformers
View the document8.3.2. Three-phase transformers
View the document8.3.3. Vector groups
View the document8.3.4. Application of three-phase transformers in power supply
View the document8.3.5. Parallel operation of transformers
View the document8.3.6. 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.