Synchronous machines and motors

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They are the machines whose speed for a given number of poles is unique and is determined by the network frequency. The frequency being the number of cycles per unit of time. Each loop goes through a north pole and a south pole.


In Europe and in most of the world the frequency of industrial networks is 50Hz and in the USA and some other countries it is 60Hz)

When it works as a generator, the speed of the machine must be perfectly constant.

From the formula it follows that for a synchronous machine, working as a motor, to rotate at different speeds, it must be fed with a variable frequency, which is specific for each speed. But since the electrical current supplied by industrial networks has a fixed frequency, a frequency inverter is needed.


  • It works with a very high power factor that does not need to be corrected, saving energy and money.
  • It maintains constant speed, it is self-synchronized, even for load variations.
  • It has high performance and is very stable.
  • The motor torque is proportional to the voltage and in the asynchronous motor it is proportional to the square of the voltage. Thus, the repercussions of voltage drops in the network are less
  • The air gap is relatively large, which increases mechanical safety.

Due to these advantages, in drives of Megawatts and constant speed, the synchronous motor fed from the network is of great application.


  • A synchronous motor cannot start by itself. For it to work we have to bring them up to sync speed. Therefore we need additional installations for booting.
  • If there are sudden variations in load, the speed of synchronism between frequency and speed can be lost and the machine stops.
  • Boot difficulties and stability problems.

A synchronous machine powered directly from the industrial network is, in principle, not very suitable for use as a motor.

When we see the machine in generated mode, although from a theoretical point of view it is indifferent to have the inductor poles in the stator and the loop in the rotor with the loop placed in the stator slots and the inductor poles fed with direct current through two slip rings and brushes or vice versa. At a technological and constructive level it is not the same and the configuration is used.

For proper operation, the AC voltages need to be as similar as possible to a sine wave. For this, on the one hand, the spatial configuration of the induction wave is modified and, on the other, the loop is replaced by a more complex winding.

With the configuration of three diametric coils with three independent outputs and 120º out of phase to obtain a three-phase system.

In recent years, due to the discovery and improvement of materials with very good magnetic properties with samarium, cobalt and rare earths, synchronous motors with permanent magnets are being used, without winding in the excitation.

Advantages of permanent magnet motors

Absence of slip rings and brushes. Maintenance problems associated with moving parts, which were the most critical parts of the engine, disappear

As there is no excitation winding, Joule losses in the rotor are eliminated, improving performance and making it easier to cool.

Disadvantages permanent magnets

Tendency to demagnetization of the magnets due to the large currents in the armature and due to the high temperatures reached during motor operation

The excitation is fixed and this value cannot be changed. Which reduces engine operating settings.

It may interest you control and protection of industrial electric motors.

at industry level

The field of application par excellence of synchronous machines is the generation of electrical energy.

Almost all the electrical energy that is produced is generated through synchronous machines in their generator version. An average synchronous generator can have between 3 and 100 MVA and in nuclear power plants up to 300 – 1000 MVA. With 1500KV outputs and currents of the order of kA.

As a motor, they are widely used in the range of 3 to 30 Megawatts, competing with the asynchronous ones.

Today, with convertomachines, an electronic converter assembly plus a synchronous machine, they are competitive even for powers below 10kW, even with variable speeds. It is a more expensive mount due to the converter. but it is already industrially profitable for those powers. competing with DC motors and asynchronous motors.


  • Fundamentals of rotating electrical machines. Luis Serrano Iribarnegaray

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