|Electrical Machines - Basic vocational knowledge (Institut für Berufliche Entwicklung, 144 p.)|
|5. Asynchronous motors|
|5.4. Circuit engineering|
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)
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)
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.