Asynchronous thyristor cascade converters for wound-rotor induction motors

Asynchronous thyristor cascade (ATC) is designed to replace traditional AC drives in electric drive systems of shaft hoisting mechanisms, main ventilations, etc. equipped with wound-rotor induction motor.

ATC5 cabinets

Application of recuperative asynchronous thyristor cascade ATС5 ensures:

  • Smooth acceleration within a given period of time and with no power losses in induction motor rotor circuit;
  • Highly accurate formation of multicycle velocity diagram required for hoister, regardless of load changes;
  • Speed regulation during steady motion stages;
  • Clearance take-up in power transmission and smooth cable tension in intervals between drive startups;
  • Balancing electric motors load (in twin-engine drives) in all intervals of motion;
  • Regenerative braking and deceleration of masses with regeneration of current;
  • Highly accurate shutdown without having to use mechanical brakes;
  • Drive operation at low speeds in inspection mode without inefficient energy losses in the rotary control station
  • Drive motoring and braking modes with regeneration of current to the supply network.

As a result of above-mentioned advantages, ATС5 enables a decrease in drive’s power consumption by more than 30% as compared to a traditional electric drive based on a starting resistor-contactor station.

Additionally to energy saving, ATC allows to improve the electric drive performance by reducing the intervals of low speed motion while speed diagrams are formed with high accuracy.
By ensuring high accuracy of control, ATC eliminates vessel shocks, prolongs the service life of mechanisms and reduces operating costs.
Application of ATС5 in main ventilation drives makes it possible to reduce capacity of power matching transformer in rotor circuit by more than twice as compared with the drives based on asynchronous valve cascade due to available control of the rotor rectified voltage.

    ATC consists of:

  • rotor controlled rectifier;
  • smoothing inductors;
  • network controlled rectifier;
  • matching network transformer;
  • control systems.

ATC ensures operation of a single-motor as well as a twin-motor drive.

Operating principle

ATС5 contains two thyristor rectifiers – mains and rotor rectifiers connected in series.
On the AC side, the mains rectifier is connected to the low-voltage windings of the matching transformer, and the rotor rectifier is connected through contact rings to the motor windings. The DC terminals are connected through a smoothing reactor, which limits rectified current ripple. As a result of this connection, a regenerative thyristor converter is formed that provides for a two-way exchange of active power between the motor rotor and the network with minimal losses of slip power. By adjusting the amplitude of active rotor current, the torque and speed of motor shaft are adjusted.
Change of direction of the motor’s active rotor power ensures a sine change of torque developed by the electric motor and its transfer to the generator mode and back to the motor mode.
Current control in both modes is carried out by a network rectifier.
This principle of drive control allows to obtain the torque reverse, as well as the reverse of the motor shaft rotation direction (for inspection modes -0.1nnom <n <0.1nnm) without changing the stator phase sequence.

Cooling system

A combined cooling system is implemented in ATC. During low loads power thyristors are cooled naturally by air. When the temperature rises above the controlled limit, the system turns on forced cooling. This system allows to minimize the equipment’s maintenance during operation under severe contamination condition.

Control system (CS)

The CS provides the user with a number of service opportunities:

  • quick access to all adjustable parameters via the console terminal. The parameters are displayed in numerical decimal format on the console terminal screen,
  • displaying a message re. the cause of malfunction, if any, on the console terminal screen via embedded self-diagnostic program,
  • specified indication of triggered protections. When a protection is activated, a message is displayed in a text form on the console terminal with respect to the first activated protection followed by all subsequent types;
  • automatic recording of an emergency “track” via a specific built-in software for recording of given parameters to a file, with possible further handling of this data on a PC.

Protection system

ATC ensures the following types of protection and blocking:

  • external and internal faults;
  • protection against unacceptable by value and duration overload currents of power circuit elements and powered electric motors (time-current protection);
  • cooling system malfunction protection;
  • power system overvoltage and undervoltage protection;
  • protection against phase failure of mains supply:
  • motor excess EMF protection;
  • motor overspeed protection;
  • “closed standstill” mode;
  • protection against unacceptable decrease of station auxiliaries voltage;
  • protection against ATC start with velocity demand;
  • protection against ATC start with rotating motor;
  • power sources failures protection;
  • interlock against turn-on with alarming and warning signals

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