Synchronous motors excitement control

The control system is designed for replacement and reconstruction of the existing conventional control system.

The present situation - the synchronous motors are provided with exciters mostly Russian of origin, built entirely on technically and materially old base of elements which creates obstacles at repairing-recovering works. Serious problems occur when the power supply of a company needs to be reconstructed, because these systems do not provide possibilities for control of the consumed (generated) reactive power, as well as auto-control of facilities, set in motion by the motors.
The new system of control is built on a contemporary base of elements, available for a sufficient term. The programmable structure of the built-in controllers allows for adaptation to the production characteristic features, the experience and particular practice of the specialists in the company. It can be done at commissioning, as well as within operation. Thus the building of an entire system can be done on stages, within a long period, at comparatively low costs for modernization of the equipment which allows a homogenous system to be built, irrespective of the building term.

Structure
We offer two solutions: the first is a brand-new exciter, replacing the existing one; the second is a system for control and recycling the existing exciter.
The excitement control system is mounted in the exciter cabinet or a separate metal cabinet (at reconstructions) with IP65 protection degree and overall dimensions 400х500х200mm. A display is mounted on the cabinet door for viewing the current values of the exciting current, cos(ф), a lamp of switched-on power supply, key for sampling and reset button.
In the cabinet there is a second letter-numeric display with two lines of 16 characters and eight buttons for monitoring all the values reported, for setting all the parameters, connected to system operation. After entering the password, the parameters related to the current and voltage scales, phase deviation and cos (ф) measurement can be readjusted.

Technical data:
Voltage for powering the protective circuits - 220V;
Rated direct current – depending on the motor;
Rated voltage of the rectifier - depending on the motor;
Continuous work regime

The exciter has three working regimes - automatic, manual and emergency.

• At manual mode the exciter provides:
  • Start of the synchronous motor with automatic excitement as a function of stator’s current or slipping;
  • Smooth excitement current adjustment from 0.3 to 1.75 of the rated one, with an option for limit values adjustment;
  • Minimum excitement voltage limitation within 0 - 0.5 of the rated value;
  • Maximum excitement voltage limitation within 0.8 - 1.75 of the rated value;
  • Rotor protection from continuous overcurrent.
• At automatic mode the exciter provides motor control through one of the laws below or a combination between them:
  • By excitement current;
  • By cos j (decrease loses of power transfer);
  • By stator’s voltage (resistant to temporary stator’s voltage falls);
  • By entire stator’s current (steady operation at load changes);
  • By stator’s reactive current (lowers loses in the motor);
  • By stator’s active current (dispenses the active load);
  • By the loading angle of the motor (compensates the rotor’s fluctuations);
• At emergency mode the exciter provides only smooth excitement current adjustment from zero to its rated value. The control system works off the following protections:
  • From internal short-circuits;
  • From external direct current short-circuits;
  • From overheating the start resistor;
  • From continuous asynchronous run of the motor.

 

Design and operation.
The exciter is a thyristor rectifier. Powered by the AC 380V~50Hz mains through an automatic switch and coherence power transformer.

• The electronic system for control consists of various separate units which in their unity provide:
  • Impulse signals formation for starting the thyristors at given point of time;
  • Impulse signals entering for starting the thyristors at given point of time;
  • Automatic excitement delivery within the starting process of a synchronous motor;
  • Contactless commutation of the starting resistor and protecting it from overheating;
  • Inverter mode at normal and emergency switching-off the synchronous motor;
  • Rotor protection from continuous overload.
• The digital system for control is realized on a contemporary base of elements. It allows easy adjustment of the exciter’s basic parameters and provides high reliability of work and protection of the synchronous motor. The system is adapted for connection to industrial networks, bus type, with physical environment RS485. As an option, connecting to a PROFIBUS network is provided.