What are the ways of excitation of synchronous motors? Working principle of synchronous motor excitation device

Four excitation methods for synchronous motors, including DC generator excitation, AC generator excitation, thyristor excitation and third harmonic excitation, are described along with the characteristics of different excitation methods for synchronous motors.

Synchronous motor excitation method

1. DC generator excitation

A smaller DC shunt generator and the synchronous motor are installed on the same shaft, and the electricity generated by the DC generator is used to excite the synchronous motor.

2. AC generator excitation

A smaller AC generator is coaxial with the synchronous motor and has a rotating armature structure. The AC power it generates is rectified and then supplied to the synchronous motor for excitation.

Because the rectifier is coaxial with the armature, the excitation current can be directly sent to the excitation winding of the synchronous motor, thereby eliminating the collector ring conductive device and achieving brushless excitation.

3. Thyristor excitation

The alternating current is rectified by thyristors and used to excite the synchronous motor. Its output voltage is easy to adjust.

4. Third harmonic excitation

A set of third harmonic windings is specially installed in the stator slots to cut the third harmonic of the air gap field and generate an electromotive force of three times the fundamental frequency, which is rectified to provide excitation for the synchronous motor.

By utilizing the third harmonic magnetic flux, energy is saved, efficiency is improved, and it has an automatic voltage stabilization effect.

What is the motor excitation method?

How the magnetic field is generated in a rotating electrical machine.

Most modern motors are based on electromagnetic induction, which requires a magnetic field in the motor. This magnetic field can be generated by permanent magnets or by passing current through a coil using an electromagnet.

The coil group in the motor specially designed to generate a magnetic field is called the excitation winding.

Due to the limitations of permanent magnetic material properties, the magnetic field established using permanent magnets is relatively weak, and it is mainly used in small-capacity motors.

With the emergence of new permanent magnet materials, especially rare earth materials with high magnetic energy product such as rare earth cobalt and neodymium iron boron, permanent magnet motors with a capacity of hundreds of kilowatts have begun to be developed.

Excitation method of synchronous motor

The DC excitation current of the synchronous motor needs to be provided from the outside. The device that supplies the excitation current is called the excitation system. The method of obtaining the excitation current is called the excitation method.

According to the rectifier device used, the excitation system can be divided into the following categories.

1. DC generator excitation system

This is the traditional excitation system, which is powered by a small DC generator mounted on the shaft of the synchronous motor. This DC generator dedicated to excitation is called an exciter.

2. Static rectifier excitation system

This excitation method is to convert the AC power generated by the coaxial AC exciter (small-capacity synchronous generator) or the main generator into DC power through a static rectifier, and then introduce it into the main generator excitation winding through a slip ring to supply the required DC excitation.

3. Rotating rectifier excitation system

This excitation method makes the coaxial AC exciter into a rotating armature type, and fixes the rectifier device on this armature to rotate together, forming a rotating rectifier excitation system. After the AC power output by the AC excitation generator is rectified, it is directly supplied to the excitation winding. In this way, the sliding contact devices such as collector rings and brushes can be completely omitted, becoming a brushless excitation system. This excitation system is widely used in large-capacity generators.

Working principle of synchronous motor excitation device

1. Demagnetization circuit

When the synchronous motor is started, the excitation winding cannot be open-circuited or short-circuited.

An open circuit will induce overvoltage in the field winding, thus destroying its insulation; a short circuit will cause a large current to flow through the field winding.

In order to prevent the excitation winding from being damaged by higher voltage or larger current during startup, a demagnetizing resistor of appropriate resistance should be connected in series with the excitation winding to form a closed loop. This closed loop can prevent the friction voltage of the excitation winding from being too high and the current from being too large.

After the synchronous motor is put into excitation, the demagnetization resistor automatically exits. In order to achieve this circuit effect, a demagnetization link is added to the excitation circuit. In the figure, v is the excitation voltmeter, KP1 and KP2 are demagnetization thyristors (the text symbols are the same as the excitation device). During the period from the start of the synchronous motor to the start of excitation, the excitation device does not send a trigger signal to the thyristor on the three-phase fully controlled bridge. The thyristor of the three-phase fully controlled bridge is in a blocked state, and there is no DC output.

When the synchronous motor starts, the rotor excitation winding induces an alternating voltage. When the induced voltage is in the positive half cycle at the excitation winding B end, the diode D3 is turned on, and the induced voltage forms a loop through RF2, D3, and RF1.

Since the resistance of the discharge resistors RF1 and RF2 is small, the induced voltage is very small after being discharged through the loop. Also, due to the existence of the discharge resistors RF1 and RF2, the current in the excitation winding is limited to a safer range of values. When the induced voltage is positive at the A end of the excitation winding (see the figure below) in the half cycle, the diode D3 is cut off. At the beginning of this half cycle, the induced voltage amplitude is small and cannot reach the conduction voltage of the thyristors KPI and KP2. The induced current forms a loop through the resistors RF1, R1, R2, potentiometer RP1 and resistors R3, R4, potentiometer RP2, resistor RF2 and other components.

Since the resistance of this loop is relatively large, which is several thousand times the DC resistance of the rotor excitation winding, it is equivalent to starting in an open circuit state, and the induced voltage rises sharply. When the induced voltage reaches a certain value, the voltage regulator tubes DW1 and DW2 break down and turn on (DW1 is broken down by the voltage drop on the potentiometer RP1, and then the trigger current is provided to the thyristor KP2 through the diode D1, and the thyristor KP2 is turned on: the mechanism of the voltage regulator tube DW2 breakdown and the thyristor KP1 conduction is similar to this). Thyristor KP2 and KPI are turned on, and the induced voltage of the excitation winding passes through thyristors KP2 and KP1. With the discharge resistors RF1 and RF2, a discharge circuit with a small resistance is formed for discharge. Until the end of this half cycle, thyristors KP1 and KP2 are automatically turned off due to the voltage crossing zero.

Adjust the resistance of potentiometers RP1 and RP2. In fact, what is adjusted is the value at which the induced voltage of the excitation winding reaches the value that makes the thyristors KP2 and KPI conduct.

Note that during adjustment, the two thyristors should be turned on as synchronously as possible.

The button SB in the figure below can be used to detect whether the demagnetization circuit is normal. During the detection, the excitation device is in the debugging state. The excitation voltage and excitation current should be the set values. At this time, operate the button SB to close its contacts, and the resistor R5 is connected in parallel with R1 and R2. R6 is connected with R3.

R4 is connected in parallel. Since the resistance of R5 and R6 is small, the voltage drop on the potentiometers RP1 and RP2 is relatively increased, and the demagnetization thyristor is easier to conduct. Therefore, the excitation voltmeter indicates zero at this time: after releasing the button to reset it, the voltmeter returns to normal value.

During the starting process of the synchronous motor, the voltage waveform amplitude of the rotor excitation winding after demagnetization has been greatly reduced and is limited to a safe value range.

The Zener diodes DWI and DW2 act as switch controls for the thyristors KP2 and KP1. After excitation, the voltage drop of the DC excitation voltage on the potentiometers RP1 and RP2 is lower than the breakdown voltage of the Zener diodes DWI and DW2. The Zener diodes cannot be turned on, and the thyristors KP2 and KPI are in the off state.

In the figure below, the common end of KPI and KP2 is connected to the C phase of the three-phase fully controlled rectifier bridge. This connection line is called the demagnetization line. When the excitation is turned on, KPI and KP2 must be turned off. Otherwise, the excitation circuit will provide current for the demagnetization resistor. After the excitation is turned on, the two rectifier thyristors connected to the C phase will be turned on successively, which will inevitably cause the thyristors KPI and KP2 equivalently connected in parallel to be short-circuited and turned off within one power cycle, and the demagnetization resistor will automatically exit the circuit.

The dual measures described above can ensure that the demagnetization circuit of the excitation device exits the working state in time after the synchronous motor is excited.