A feature of synchronous electric motors is that the magnetic flux and the rotor have the same rotation speed. For this reason, the rotor of an electric motor does not change its speed when the load increases. There is a winding on the rotor that creates a magnetic field.
Sometimes powerful permanent magnets are used. Usually in synchronous machines there are as many windings on the rotor as there are on the stator. So it turns out to equalize the speed of rotation of the magnetic flux and the rotor. The load that is connected to the motor does not affect the speed at all.
Electric motor design
The device of a synchronous motor consists of the following elements:
- The fixed part is the stator, on which the windings are located.
- Mobile rotor, sometimes called inductor or armature.
- Front and rear covers.
- Rotor mounted bearings.
There is free space between armature and stator. Windings are laid in the grooves, they are connected instar. As soon as voltage is applied to the motor, current begins to flow through the armature winding. A magnetic field is formed around the inductor. But the stator is also energized. And this is where the magnetic flux comes in. These fields are offset from each other.
How a synchronous motor works
In synchronous machines, the electromagnets on the stator are poles, since they operate on direct current. In total, there are two schemes by which the stator windings are connected:
- Salifole.
- Implicit pole.
In order to reduce the magnetic resistance and optimize the conditions for the passage of the field, cores made of ferromagnets are used. They are available in both the stator and the rotor.
They are made from special grades of electrical steel, which contains a huge amount of such an element as silicon. With this, it is possible to significantly reduce the eddy current, as well as increase the electrical resistance of the metal.
The operation of synchronous electric motors is based on the interaction of the stator and rotor poles. When starting, it accelerates to the speed of the flow. It is under such conditions that the electric motor operates in synchronous mode.
Starting method with auxiliary electric motor
Previously, special starting motors were used, which were connected to the motor using mechanical devices (belt drive, chain, etc.). During start-up, the rotor began to rotate and, gradually accelerating,reached the synchronous speed. After that, the motor itself began to work. This is exactly the principle of operation of a synchronous motor, regardless of design and manufacturer.
A prerequisite is that the starting motor must have a power of about 15% of that of the accelerated motor. This power is quite enough to start any synchronous motor, even if a small load is connected to it. This method is quite complicated, and the cost of the entire equipment is greatly increased.
Modern launch method
Modern designs of synchronous motors are not equipped with such overclocking circuits. A different trigger system is being used. Approximately in this way the synchronous machine is turned on:
- With the help of a rheostat, the rotor windings are closed. As a result, the armature becomes short-circuited, as on simple induction motors.
- The rotor also has a squirrel-cage winding that is soothing and prevents the armature from swinging during synchronization.
- As soon as the armature reaches its minimum rotational speed, direct current is connected to its windings.
- If permanent magnets are used, external starting motors must be used.
There are cryogenic synchronous electric motors that use a reverse type design. The excitation windings are made fromsuperconducting materials.
Advantages of synchronous machines
Asynchronous and synchronous motors have very similar designs, but there are still differences. In the latter there is a clear advantage in that the excitation occurs from a direct current source. In this case, the motor can operate at a very high power factor. There are also other benefits of synchronous motors:
- They work at an inflated rate. This allows you to reduce power consumption, and also significantly reduces current losses. The efficiency of a synchronous machine will be much higher than that of an asynchronous motor with the same power.
- Torque directly depends on the voltage in the mains. Even if the voltage in the network decreases, the power will remain.
But still, asynchronous machines are used much more often than synchronous. The fact is that they have great reliability, simple design, do not require additional maintenance.
Disadvantages of synchronous motors
It turns out that synchronous machines have much more disadvantages. Here are just the main ones:
- The circuit of a synchronous motor is quite complex, it consists of a large number of elements. It is for this reason that the cost of the device is very high.
- Be sure to use a constant source to power the inductorcurrent. This greatly complicates the whole construction.
- The procedure for starting an electric motor is rather complicated than for asynchronous machines.
- It is possible to adjust the rotor speed only by using frequency converters.
In general, the advantages significantly outweigh the disadvantages of synchronous motors. For this reason, they are very often used where it is necessary to conduct a continuous continuous production process, where it is not necessary to stop and start the equipment frequently. Synchronous machines can be found in mills, crushers, pumps, compressors. They rarely turn off, they work almost constantly. Through the use of such motors, significant energy savings can be achieved.
