To ensure the operation of powerful electric drives are used synchronous motors. They have found application in compressor units, pumps, systems, rolling mills, and fans. They are used in metallurgical, cement, oil and gas and other industries where it is necessary to use high-power equipment. In this article, we decided to tell the readers of the site Electrician himselfhow synchronous motors can be started.
Content:
- Advantages and disadvantages
- Start methods
- Starting with a booster motor
- Asynchronous start
- Frequency start
- Excitation systems
Advantages and disadvantages
Structurally, synchronous motors are more complicated than asynchronous motors, but they have a number of advantages:
- The operation of synchronous motors is less dependent on fluctuations in the supply voltage.
- Compared to asynchronous ones, they have higher efficiency and better mechanical characteristics with smaller dimensions.
- The rotation speed is independent of the load. That is, load fluctuations in the operating range do not affect the speed.
- They can work with significant overloads on the shaft. If short-term peak overloads occur, these overloads are compensated by increasing the current in the field winding.
- With an optimally selected excitation current mode, the electric motors do not consume or deliver reactive energy to the network, i.e. cosϕ is equal to one. Motors working with overexcitation are capable of generating reactive energy. This allows them to be used not only as motors, but also as compensators. If the generation of reactive energy is required, an overvoltage is applied to the field winding.
With all the positive qualities of synchronous electric motors, they have a significant drawback - the complexity of starting up. They have no starting torque. Special equipment is required to launch. This limited the use of such engines for a long time.
Start methods
Synchronous electric motors can be started in three ways - using an additional motor, asynchronous and frequency starting. When choosing a method, the design of the rotor is taken into account.
It is performed with permanent magnets, electromagnetic excitation or combined. Along with the excitation winding, a short-circuited winding is mounted on the rotor - a squirrel cage. It is also called a damping winding.
Starting with a booster motor
This starting method is rarely used in practice because it is technically difficult to implement. An additional electric motor is required, which is mechanically connected to the rotor of the synchronous motor.
With the help of the accelerating motor, the rotor spins up to values close to the rotation speed of the stator field (to the synchronous speed). After that, a constant voltage is applied to the rotor field winding.
The control is carried out by light bulbs that are connected in parallel with the switch, which supplies voltage to the stator windings. The switch must be disconnected.
At the initial moment, the lamps blink, but when the rated speed is reached, they stop burning. At this moment, voltage is applied to the stator windings. Then the synchronous electric motor can operate independently.
Then the additional motor is disconnected from the mains, and in some cases it is disconnected mechanically. These are the features of starting with an accelerating motor.
Asynchronous start
The asynchronous start method is the most common today. This launch became possible after a change in the design of the rotor. Its advantage is that there is no need for an additional accelerating motor, since in addition to the excitation winding short-circuited squirrel cage rods were mounted in the rotor, which made it possible to run it in an asynchronous mode. Under this condition, this method of launching is widespread.
We immediately recommend watching a video on the topic:
When voltage is applied to the stator winding, the motor accelerates in asynchronous mode. After reaching the speed close to the nominal, the excitation winding turns on.
The electrical machine enters synchronism mode. But not everything is so simple. During start-up, a voltage arises in the excitation winding, which increases with increasing speed. It creates a magnetic flux that affects the stator currents.
In this case, a braking torque arises, which can halt the acceleration of the rotor. To reduce the harmful effect of the field windings are connected to a discharge or compensation resistor. In practice, these resistors are large heavy boxes, where steel spirals are used as a resistive element. If this is not done, insulation breakdown may occur due to the rising voltage. What will lead to equipment failure.
After reaching the subsynchronous speed, the resistors are disconnected from the excitation winding, and a constant voltage is supplied to it from generator (in the generator-motor system) or from a thyristor exciter (such devices are called VTE, TVU, and so on, depending on series). As a result, the motor goes into synchronous mode.
The disadvantages of this method are large inrush currents, which causes a significant voltage drop in the supply network. This can lead to the shutdown of other synchronous machines operating on this line, as a result of the operation of the low voltage protections. To reduce this effect, the stator winding circuits are connected to compensation devices that limit inrush currents.
It can be:
- Additional resistors or reactors that limit inrush currents. After acceleration, they are shunted, and mains voltage is applied to the stator windings.
- The use of autotransformers. With their help, the input voltage is reduced. When the rotation speed reaches 95-97% of the working speed, switching occurs. Autotransformers are disconnected and AC mains voltage is applied to the windings. As a result, the motor enters synchronization mode. This method is technically more complex and expensive. And autotransformers often fail. Therefore, in practice, this method is rarely used.
Frequency start
Frequency starting of synchronous motors is used to start high-power devices (from 1 to 10 MW) with an operating voltage 6, 10 kW, both in the mode of easy start (with fan-type load), and with heavy start (drives of ball mills). For these purposes, frequency soft starters are produced.
The principle of operation is similar to high-voltage and low-voltage devices operating according to the frequency converter circuit. They provide starting torque up to 100% of the nominal, and also provide starting of several motors from one device. You see an example of a circuit with a soft starter below, it is turned on for the time the engine starts, and then it is removed from the circuit, after which the engine is connected directly to the network.
Excitation systems
Until recently, an independent excitation generator was used for excitation. It was located on the same shaft with a synchronous electric motor. This scheme is still used in some enterprises, but it is outdated and is no longer applied. Nowadays, thyristor exciters BTE are used to regulate the excitation.
They provide:
- optimal starting mode for a synchronous motor;
- maintaining the specified excitation current within the specified limits;
- automatic regulation of the excitation voltage depending on the load;
- limiting the maximum and minimum excitation current;
- an instant increase in the excitation current with a decrease in the supply voltage;
- suppression of the rotor field when disconnected from the supply network;
- monitoring of the state of insulation, with notification of a malfunction;
- provide a check of the state of the field winding when the electric motor is not running;
- work with a high-voltage frequency converter, providing asynchronous and synchronous starting.
These devices are highly reliable. The main disadvantage is the high price.
In conclusion, we note that the most common way to start synchronous motors is asynchronous starting. Starting with the help of an additional electric motor practically did not find application. At the same time, frequency starting, which automatically solves starting problems, is quite expensive.
Related materials:
- How to choose a frequency converter
- Smooth inclusion of incandescent lamps
- How an induction motor works
