Countercurrent braking

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In drive technology, countercurrent braking is the term used to describe the method of braking an electric motor to a standstill by reversing the polarity of the energy supply . Countercurrent braking is the easiest way to shut down an induction motor.

method

With this method, two motor leads are swapped during operation. As a result, the direction of rotation of the motor is reversed and the motor tries to run up in the opposite direction. This brakes the shaft of the motor. As soon as the motor has come to a standstill, it must be disconnected from the mains. If the motor is not switched off after it has come to a standstill, it starts up again in the opposite direction of rotation. The motor absorbs both mechanical and electrical power , which is converted into heat in the motor. For this reason, countercurrent braking is the most unfavorable method in terms of energy.

Application in three-phase asynchronous motors

In the case of three-phase asynchronous motors , instead of counter-current braking, one also speaks of counter-rotating field braking. If countercurrent braking is used to brake three-phase asynchronous motors, this is done by interchanging two outer conductors of the stator winding . This gives the rotating field in the stator an opposite direction of rotation and the asynchronous motor is braked. The motor is braked with a high braking torque. However, the current and torque surges that occur are even greater than when the motor is switched on directly . These values ​​are reached immediately after switching to braking mode. The slip of the motor is almost twice as large at the moment of switching than when the shaft is stationary. This method is well suited for three-phase motors with lower power. It can be used with slip ring motors and motors with squirrel cage rotors . Parasitic currents arise between the rotor bars of the squirrel cage, particularly in the case of motors with a lower output. These so-called iron cross-currents close through the iron core of the rotor. This increases the torque in the counter-current braking area. In addition, the heat loss is distributed over the entire runner. Countercurrent braking is used for hoists to set down a load at constant speed.

restrictions

The use of counter-current braking is often only possible to a limited extent with three-phase asynchronous motors. Due to the mechanical shock load, it comes with backlash transmission links such. B. chains, too high loads. In the case of short braking processes, switching off the countercurrent contactor is usually not precise enough. As a result, a higher circuit complexity is required. If counter-current braking is used frequently, the stator current must be reduced. This is done by lowering the stator voltage or by means of armature resistors . However, due to the high temperature rise in the motor, countercurrent braking is hardly suitable for large motors.

Use with DC motors

In the case of DC motors , countercurrent braking occurs either by switching the armature voltage or by changing the polarity of the field excitation. In DC series motors , as the traction motors are used for electric trains, the regenerative braking is not suitable. Since the source voltage supports the supply voltage in DC motors, the motor current can become very high. In particular, when switching over while driving, the current is higher than with the corresponding drive level at a standstill. The current increases rapidly according to the driving speed. Due to the high current, the motors are subject to high thermal loads. In DC series motors, counter-current braking causes strong brush fire . In order to limit the current when switching the armature voltage, resistors are connected in series. As soon as the motor comes to a standstill, the supply voltage must also be switched off here.

Use in single-phase AC motors

With single-phase AC motors, there is no excessive current increase when using reverse current braking. The reason for this is the predominantly inductive voltage drop in the motor. However, when using alternating current motors as traction motors, self-excitation with direct current can occur even at lower driving speeds . Due to the low ohmic resistance of the transformer , this leads to high currents, which makes further operation impossible.

Individual evidence

  1. ↑ a b c Helmut Greiner: Starting, braking, positioning with three-phase asynchronous motors. Danfoss Bauer GmbH, Esslingen 2001 Online (accessed March 5, 2012; PDF; 9.5 MB).
  2. ^ A b c Germar Müller, Bernd Ponick: Fundamentals of electrical machines. 9th edition, Wiley-VCH Verlag GmbH & Co KGaA., Weinheim 2006, ISBN 3-527-40524-0 .
  3. ^ Georg Flegel, Karl Birnstiel, Wolfgang Nerreter: Electrical engineering for mechanical engineering and mechatronics. Carl Hanser Verlag, Munich 2009, ISBN 978-3-446-41906-3 .
  4. ↑ a b Ernst Hörnemann, Heinrich Hübscher: Electrical engineering specialist training in industrial electronics. 1st edition. Westermann Schulbuchverlag GmbH, Braunschweig, 1998, ISBN 3-14-221730-4 .
  5. ^ Detlev Roseburg: Electrical machines and drives. Fachbuchverlag Leipzig in Carl Hanser Verlag, 1999, ISBN 3-446-21004-0 .
  6. ↑ a b c Jens Weidauer: Electrical drive technology. Publicis Corporate Publishing, Erlangen 2008, ISBN 978-3-89578-308-1 .
  7. ↑ a b c d Manfred Rudolph, Ulrich Wagner: Energy application technology. Springer Verlag, Berlin Heidelberg 2008, ISBN 978-3-540-79021-1 .
  8. ↑ a b Heinz M. Hiersig (Ed.): VDI-Lexikon Maschinenbau. VDI-Verlag GmbH, Düsseldorf 1995, ISBN 9783540621331
  9. ^ A b Zarko Filipovic: Electric railways . 4th revised edition, Springer Verlag, Berlin Heidelberg New York 2005, ISBN 3-540-21310-4

See also