A feature of synchronous motors is that the magnetic flux and rotor rotational speeds are the same. For this reason, the rotor of an electric motor does not change its speed with increasing load. On the rotor is a winding that creates a magnetic field.
Powerful permanent magnets are sometimes used. Usually in synchronous machines on the rotor there are as many windings as on the stator. So it turns out to align the speed of rotation of the magnetic flux and rotor. The load that is connected to the motor does not affect speed at all.
Motor design
The synchronous motor device consists of the following elements:
- The fixed part is the stator on which the windings are located.
- A movable rotor, it is sometimes called an inductor or an anchor.
- Front and rear covers.
- Bearings mounted on the rotor.
There is free space between the anchor and the stator. In the grooves, the windings are laid, they are connected into a star. As soon as voltage is applied to the motor, current flows through the armature winding. A magnetic field is formed around the inductor. But the stator is also energized. And here comes the magnetic flux. These fields are offset relative to each other.
How the 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:
- Explicitly.
- Implicitly.
In order to reduce magnetic resistance and optimize the conditions of field passage, cores made of ferromagnets are used. They are available both in the stator and in 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 motors is based on the interaction of the poles of the stator and rotor. When starting, acceleration occurs to the speed of the flow. It is in such conditions that the electric motor operates in synchronous mode.
Start method using an additional electric motor
Previously, special engines were used to start, 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 value of synchronous speed. After that, the electric motor itself began to work. This is exactly the principle of operation of a synchronous electric motor, regardless of design and manufacturer.
A prerequisite is that the starting motor should have a power of about 15% of the same characteristics of the accelerated motor. Such power is quite enough to start any synchronous electric motor, even if a small load is connected to it. This method is quite complicated, and the cost of all equipment is significantly increased.
Modern startup method
Modern designs of synchronous electric motors are not equipped with similar schemes for acceleration. Another startup system is used. In approximately this way the synchronous machine starts up:
- Using a rheostat, the rotor windings are closed. As a result, the anchor becomes short-circuited, as on simple asynchronous electric motors.
- On the rotor there is also a short-circuited winding, which is soothing, with its help the armature swing during synchronization is prevented.
- As soon as the armature reaches the minimum speed of rotation, a direct current is connected to its windings.
- If permanent magnets are used, then external starting motors will be required.
There are cryogenic synchronous electric motors that use a reversed type design. Field windings are made of superconducting 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 that excitation from a direct current source occurs. In this case, the motor can operate at a very high power factor. There are also other advantages of synchronous motors:
- They work with an overestimated coefficient. This allows you to reduce power consumption, and also significantly reduces current loss. The efficiency of a synchronous machine will be much higher than that of an asynchronous motor with the same power.
- Torque directly depends on what 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 machines. The fact is that they have great reliability, simple design, do not require additional care.
Disadvantages of Synchronous Motors
It turns out that synchronous machines have many more shortcomings. Here are just the main ones:
- The synchronous motor circuit is quite complex, it consists of a large number of elements. For this reason, the cost of the device is very high.
- Be sure to use a DC source to power the inductor. This greatly complicates the entire design.
- The procedure for starting an electric motor is quite complicated than for asynchronous machines.
- It is possible to adjust the rotor speed only by using frequency converters.
In general, the advantages significantly cover 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 frequently stop and start equipment. 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.