Start and synchronize synchronous motors - Electric drive

Starting and synchronizing synchronous motors

The starting and synchronization of synchronous motors differs depending on the features of the technological process in which the electric drive participates. There are light and heavy start of the synchronous motor. Easy start of the synchronous motor occurs at small moments of inertia of the electric motor and small moments of resistance on the shaft of the electric motor. Heavy starting is carried out at relatively large moments of inertia of the electric motor and the moments of resistance . Heavy starting is carried out for a considerable time, and the entry of the engine into synchronism is complicated.

For powerful engines, the circuits of power circuits are practically reduced with insignificant variations to one, the basic circuit of which is shown in Fig. 5.52.

The synchronous motor is started in asynchronous mode. In most cases, a synchronous motor with a power of up to several hundred kilowatts is allowed to be directly connected to the network. Multiplicity of the starting current for direct start

When starting synchronous motors with a power of several megawatts, it becomes necessary to limit the starting currents. Methods for limiting starting currents flow from the equation of short-circuit current of an induction motor

(5.75)

where - the phase voltage of the stator winding of the synchronous motor in the asynchronous start mode; - the resistance and inductive resistance of the stator winding; - the active resistance and inductive resistance of the scattering of the rotor winding, reduced to the stator winding.

Synchronous Motor Circuit Diagram

Fig. 5.52. Synchronous Motor Circuit Diagram

From the analysis of the expression for the short-circuit current (5.75), there are three possible ways of limiting the current when the synchronous motor is started asynchronously:

• introduction of the additional active resistance in the stator winding circuit for the time of starting;

• introduction of the additional reactance in the stator winding circuit for the time of starting;

• short-term reduction for the start-up time of the phase voltage of the stator windings.

The most common limitation for starting synchronous motors is the use of reactors L , included in the chain of stator windings. In some cases active resistors are used instead of reactors L . A short-term reduction in the voltage of the stator windings is achieved by including transformers or autotransformers in the circuit. A variant of the stator current limitation circuit for starting a synchronous motor using an autotransformer is shown in Fig. 5.53.

Schematic diagram of synchronous motor circuits with autotransformer inrush current limitation

Fig. 5.53. Schematic diagram of synchronous motor circuits with autotransformer inrush current limitation

The static electromechanical characteristics that explain the start-up of a synchronous motor with current limitation are shown in Fig. 5.54.

The motor starts with characteristic 1, with an additional inductance L in the stator winding circuit or under voltage of the stator windings. After a certain time, when the inrush current decreases to the switching current, additional inductances (see Figure 5.52) from the stator winding circuit are output, and the starting process continues along characteristic 2.

Static Electromechanical Characteristics Explaining the Synchronous Motor Startup Process

Fig. 5.54. Static electromechanical characteristics explaining the starting process of a synchronous motor

When starting in asynchronous mode, the control pulses for thyristors VS2 ... VS8 are not applied and the voltage of the controlled rectifier is zero. In the excitation winding of the synchronous motor, the variable emf of the slip is induced, under the action of which, through the zener diodes VD1 , VD2 , and VD3 , VD4 auxiliary thyristors VS 1 and VS2 are opened. During the asynchronous start-up, the synchronous motor excitation winding is short-circuited to the discharge resistance R. When the motor reaches a speed close to the subsynchronous , The emf of the slip decreases, the voltage on the control electrodes of the thyristors VS1 , VS2 decreases and they cease to turn on. The discharge resistance is disconnected from the field winding. After that, a direct current is supplied to the excitation winding from the controlled rectifier KS3 ... PS 8.

The starting squirrel cage of the synchronous motor is designed for short-term operation, as a rule, s., long work in asynchronous mode is unacceptable. In addition to providing a start-up mode, the squirrel cage plays the role of a damping winding, stabilizing the transients during engine operation in synchronous mode.

For synchronous motors up to several hundred kilowatts, direct starting can be initiated into the grid without intermediate triggering characteristics. An approximate view of the transient processes of the moment M and the speed when the synchronous motor is directly started taking into account the electromagnetic transients is shown in Fig. 5.55. The synchronous motor is accelerated in asynchronous mode to the subsynchronous speed , after which the excitation voltage and the motor retracts into synchronism. In principle, the process of entry into synchronism is affected by the moment of connection of the voltage to the field winding. The most favorable moment of switching on the excitation voltage is the one at which the instantaneous value of the induced EMF in the excitation winding will be zero. However, as shown by special studies [11], the relative position of the rotor relative to the magnetic field created by the stator windings is of little practical significance either from the point of view of the quality of the transient process or the time of its termination. Therefore, in most practical cases, the control scheme is not complicated by introducing devices that enable excitation to be activated at the most favorable time.

Curves of the transient processes of the moment M and the speed ω at the start of the synchronous motor

Fig. 5.55. Curves of the transient processes of the moment M and the speed ω at the start of the synchronous motor

You can check the synchronization condition by using the expression

(5.76)

where - the maximum moment of the synchronous machine, - the total moment of inertia of the drive driven to the motor shaft.

The process of retraction depends mainly on two parameters: the values ​​of the subsynchronous speed and the total moment of inertia of the drive driven to the motor shaft .

thematic pictures

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