Synchronous electric machines have a number of advantages over other types of units. But at the same time, you cannot connect them directly to the network under load. Therefore, in this article we will consider the methods of starting and wiring a synchronous motor.
Start methods
Due to the significant inertia of the rotor, it is not able to move under the load by the stator field. If the operating voltage is applied, it will not be possible to obtain a stable magnetic connection and the rotation will not start. To solve this problem, methods are used to start the rotor up to a certain speed of rotation. Typically, this is the number of revolutions that approaches the value in synchronous operation.
Among the most common ways to set a synchronous motor in motion are:
- Asynchronous start - this method is provided by introducing steel elements in the form of a squirrel cage into the rotor structure. When voltage is applied, an EMF is induced in the cell and magnetic interaction occurs. The main disadvantage of this method is the large starting currents, several times higher than the nominal mode of the synchronous motor. Therefore, the startup scheme uses reactors or autotransformers to reduce the negative impact.
- Frequency start - provided by means of frequency converters. Which reduce the frequency of the supply voltage on the working windings. This slows down the rotation speed of the magnetic field of the synchronous motor. Due to this, the rotor begins to rotate.
- Motor start - to start the movement, the shaft of the synchronous unit is connected to the accelerating engine. At the start phase, rotation is provided by an electric drive machine. As soon as the main motor reaches the subsynchronous speed, the booster unit is taken out of operation.
For each of the methods, appropriate circuits and equipment are used to optimize the operating mode. Therefore, below we will consider several typical examples for each launch method.
Asynchronous start
In this method, synchronous motors of a special type are used, but the rate of current rise and its magnitude in the working windings is forcibly reduced. For this, reactors or autotransformers are installed.
As you can see in the diagram, a reactor is installed in the power circuit of each phase winding of the synchronous motor. When the contactor K2 is turned on, the voltage is applied to the windings, the current in the reactor cannot grow abruptly. Therefore, the start of the electric motor is smoother than in the case of direct connection. When the electric machine accelerates to the subsynchronous speed, the bypass switch K1 removes the inductive element from the circuit and the unit operates in normal mode.
In this scheme, the voltage on the working windings of the synchronous motor is automatically reduced due to the autotransformer. The P3 regulator smoothly increases the potential difference to the established value, while the current increases proportionally. After reaching the rated torque, the K1 switch will bypass the autotransformer. This method makes it possible to reduce starting currents with a significantly greater force than in the case of using reactors.
Frequency start
The basis of modern frequency starting are circuits on semiconductor elements, as a rule, thyristor converters. Such devices reduce the frequency of change of the voltage curve, but practically do not violate the effective value.
This starting method shortens the acceleration time of the synchronous motor and reduces the value of the current load at the moment of starting. However, the modern frequency start circuit has a much more complex implementation:
Motor start
The method of motor start provides for the simultaneous installation of a synchronous and an accelerating engine on one shaft. The start of rotation is provided by an asynchronous accelerating motor, which easily picks up speed under load. The synchronous unit is put into operation when the subsynchronous rotation speed is reached.
However, a significant disadvantage of this method is a long period of time from start to the moment the electric machine enters synchronism.
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