KUSA circuit

from Wikipedia, the free encyclopedia

The KUSA circuit ( squirrel-cage rotor soft start circuit ) is an electrical circuit with which it is possible to reduce the starting torque of three-phase motors with squirrel- cage rotor , in order to enable the motor to start smoothly .

Circuit diagram

Layout and function

A damper resistor ( starting resistor ) or a choke coil is connected to one of the three motor supply lines during the start-up phase of the motor, which is bypassed again after the nominal speed has been reached. This reduces the voltage in the connected motor winding according to the size of the series resistor. However, the starting currents in the two supply lines are somewhat greater than with direct starting. This change in the current changes the magnitude of the magnetic flux , creating an asymmetry in the rotating field . Due to the asymmetry of the stator currents , the "rotating field" changes into an elliptical shape. The speed of the motor does not change with the KUSA circuit. The voltage imbalance at the motor connections decreases as the current intensity decreases. After the engine has started up, the starting resistor is bridged by means of a switching contact .

In the case of a motor protection relay that is sensitive to phase failure , the motor protection relay can trip during long start-up times , which is why motor protection relays are not suitable for single-phase short-circuit soft starts. Motor protection relays without phase failure detection are used here. Since the dimensioning and calculation is very imprecise in practice due to the different load conditions and controls of the motor, the "Kusa resistance" is generously dimensioned and optimally adapted to the load conditions on site. In many cases, a soft starter is used instead of the "Kusa resistor" . These devices work on the principle of phase control , which means that the current in the connected motor lead can be changed between zero and rated current .

Rotating field and torque consideration

The starting torque can be influenced in a wide range. With an infinitely large series resistor , the motor is only connected with two windings , the machine does not develop a rotating field, but only an alternating field . The swelling and decreasing alternating field can be compared with two rotating fields of the same size which have different directions of rotation . Since no starting torque is generated either, the motor does not start. The motor behaves as if two equally powerful motors were sitting on one shaft and rotating against each other. While one motor tries to turn the shaft, the other motor practically acts as a brake. Without a series resistor, a pure rotating field is created and the motor starts up with normal starting torque. With resistance values ​​that lie between the two extremes, a "rotating field" arises in the machine, which becomes weaker the greater the value of the starting resistance. Depending on the size of the series resistor, this field then changes the circular rotating field shape into an elliptical shape.

Advantages and disadvantages

advantages
  • smooth, jerk-free start
  • simple circuit construction
  • Bumps and impacts in gears or transmission parts are avoided

Source:

disadvantage
  • asymmetrical load on the outer conductor
  • only suitable for motors up to a maximum of 5 kW
  • in public networks mostly only approved for motors up to 2.2 kW
  • The phase failure protection can be triggered

Source:

Areas of application

The KUSA circuit is used for motors up to a maximum of 5 kW if the starting torque must not exceed a certain limit value. The main area of ​​application is in textile machines, as these machines require a particularly smooth and smooth start-up. This circuit is not used for larger motors due to the losses in the series resistor.

Individual evidence

  1. ↑ a b FANAL circuit practice. 7th edition, Metzenauer & Jung GmbH, Wuppertal.
  2. ↑ a b c Helmut Greiner: Starting, braking, positioning with three-phase motors. Danfoss Bauer GmbH, Esslingen 2001, online (PDF; 9.5 MB) (accessed May 7, 2015).
  3. ↑ a b c d e Günter Boy, Horst Flachmann, Otto Mai: The master's examination in electrical machines and control technology. 4th edition, Vogel Buchverlag, Würzburg 1983, ISBN 3-8023-0725-9 , p. 129.
  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. ↑ a b Detlev Roseburg: Electrical machines and drives. Fachbuchverlag Leipzig in Carl Hanser Verlag, 1999, ISBN 3-446-21004-0 .
  6. ↑ a b c Klaus Fuest, Peter Döring: Electrical machines and drives. 6th edition, Friedrich Vieweg Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden 2004, ISBN 3-528-54076-1 .
  7. ^ A b Franz Moeller, Paul Vaske (ed.): Electrical machines and converters. Part 1 structure, mode of operation and operating behavior, 11th revised edition, BG Teubner, Stuttgart 1970, p. 140.
  8. ^ Gregor D. Häberle, Heinz O. Häberle: Transformers and electrical machines in systems of energy technology. 2nd edition, Verlag Europa-Lehrmittel, Haan-Gruiten 1990, ISBN 3-8085-5002-3 .