Thyristor exciters of synchronous motors: device and modes of operation

Electronic excitation control devices are widely used in industry. They are necessary to supply voltage to the excitation and control winding. Designed for automatic regulation of field currents during direct or reactor start-up from a frequency converter or network. Realizes stable operation in the mode of synchronous and emergency operation of powerful synchronous motors. The advantages of such systems are ease of management, compactness, integration into systems electronic regulation in automatic control systems, where remote change is applied parameters. Next, we will talk in detail about what thyristor exciters are, what types they are and how they work.

Description and installation diagram

Thyristor exciters are economical, not difficult to operate and set up. Made in the form of a freestanding cabinet.

Below is a diagram and description of an electronic installation with a thyristor control, of which it is clear what the device consists of:

The design of the device is:

• A controlled rectifier that supplies power to the excitation winding of a synchronous motor. Represents a block of thyristors with a pulse-phase control system.
• The reactor representing the input transformer.
• Field blanking module.
• Testing system.
• A measuring unit that controls the current level at the output of the exciter voltage and stator current.
• Protection module and signaling unit. Provides protection for indication of malfunction of automatic control and diagnostic systems.

Supplied together with a relay-contact control unit for starting the engine. Has a digital or analog control system.

The thyristor exciter allows:

1. Apply voltage to the field windings in the non-working state of the electric motor, for the test mode.
2. In direct-on-line mode, energizes the field windings to maintain the stator current and slip current function.
3. With reactor start-up, excitation is supplied after the shunt switch is turned on.
4. Smooth (asynchronous) start-up with a high-voltage soft starter.
5. Provides synchronous starting using a high-voltage frequency converter.

An electronic exciter monitors and maintains normal operation. At the same time, it ensures the safety of equipment, for which a protection unit is needed:

• Protects output circuits when the excitation current exceeds the initially set value.
• It protects the input circuits when the mains currents are exceeded.
• Damage to the insulating circuit.
• Emergency shutdown.
• Phase sequence error.
• Lack of power voltage.
• Errors in synchronizing the motor with the mains parameters.
• In case of an emergency, the electronic voltage unit.
• Long run, different from the specified one. The duration of the start is programmed. The overrun time is considered an error.
• Asynchronous progress alert.
• From external emergencies.
• Protection against control errors is performed.

If the exciter is equipped with protection against a decrease in the insulation resistance of the external circuit, it is completed additionally:

• A unit for constant monitoring of insulation resistance parameters with display on the display.
• The presence of a dry contact in the event of a decrease in the insulation resistance, less than two, constant values ​​that are set by the installer.

The presence of the control unit makes it possible to keep within the tolerance the voltage in the stator, as well as the coefficient of performance or excitation in automatic mode. The characteristics are set during commissioning or remotely.

The exterior and internal structure is shown in the photo:

Modes of operation

The device provides three operating modes, automatic, manual and emergency. It is possible to change modes while the engine is running. The transition from one to the other is not accompanied by current surges. Below we will see how the device works.

Auto mode

The preset parameters are maintained using the excitation coordination unit - ARV. Parameters are set using buttons on the remote control or remotely.

ARV supports the set parameters:

• Mains voltage.
• Motor power factor (cosⱷ).
• Stable engine operation when the load exceeds the maximum.
• Regulates the stator voltage when the load is less than the nominal.

Manual control mode

The device allows you to change the parameters in manual mode, set by the operator from the engineering panel.

In this case, the block provides:

• Direct starting with automatic excitation of the synchronous motor coils as a function of stator current and slip.
• Reactor start-up. In automatic mode, the stator current is regulated.
• Stabilization of the excitation current during sudden load changes.
• Maintaining the stabilization current within 5% when the supply voltage changes by 70-110% of the nominal. With changes in the temperature regime of the windings.
• The ability to smoothly adjust the current. If necessary, which can be quickly adjusted.
• Protection of the rotor against prolonged overloads.
• Rapid damping of the rotor field during prolonged voltage drop. In this case, an extinguishing signal must be given.
• Increase in voltage by 1.75 from the nominal. At normal mains voltage supplying the exciter.
• Voltage limitation by minimum values.
• Current limitation by maximum values.

Emergency mode

Designed for emergency operation of the engine. The analog exciter adjusts the currents from zero to boost value. There is an adjustment within the specified limits.

It contains a module that protects circuits when:

• Short circuit of the electronic converter circuits.
• Turn off the excitation of a running electric motor.
• Continuous asynchronous stroke.
• Insulation breakdown to ground.
• Overloads exceeding the specified values.
• Multiple engine starts.
• Failure of a group of contacts in the switch module.
• Undervoltage stator.
• Changing the direction of power.
• Increased voltage in the field windings.
• When the starting resistor overheats.

Electronic exciters are oriented to supply voltage to the field winding circuit and to regulate field currents in automatic mode. They are used for high power synchronous electric motors.

What are they and where are they used?

The industry has been producing thyristor exciters for many years. Upgraded computer-controlled devices are now being produced.

The devices are designed to power the field windings. With automatic current control for direct, reactor, frequency and smooth starts.

The table shows the types of pathogens with characteristics:

The scope of application is wide enough, they are used at hydroelectric power plants, electrical, metallurgical, petrochemical, chemical and food industries.