Synchronous motor: principle of operation, device, purpose

Synchronous electric motors (SM) are not as common as squirrel cage induction motors. But they are used where a large torque is needed and frequent overload. Also, this type of engine is used where high power is needed to drive mechanisms, due to the high power factor and the ability to improve the power factor of the network, which will significantly reduce energy costs and load on lines. What is a synchronous motor, where is it used and what are its pros and cons we will consider in this article.

Definition and principle of operation

In simple terms, an electric motor is called a synchronous motor, in which the speed of rotation of the rotor (shaft) coincides with the speed of rotation of the stator magnetic field.

Let us briefly consider the principle of operation of such an electric motor - it is based on the interaction of a rotating stator magnetic field, which is usually created by three-phase alternating current and constant magnetic field rotor.

The constant magnetic field of the rotor is created by the excitation winding or permanent magnets. The current in the stator windings creates a rotating magnetic field, while the rotor is in operation is a permanent magnet, its poles rush to the opposite poles of the magnetic stator fields. As a result, the rotor rotates synchronously with the stator field, which is its main feature.

Recall that for asynchronous motor the speed of rotation of the stator MF and the speed of rotation of the rotor differ by the amount of slip, and its mechanical characteristic "humped" with a peak at critical slip (below its rated speed rotation).

The speed at which the stator magnetic field rotates can be calculated using the following equation:

N = 60f / p

f is the frequency of the current in the winding, Hz, p is the number of pole pairs.

Accordingly, the same formula determines the speed of rotation of the shaft of a synchronous motor.

Most of the AC motors used in production are designed without permanent magnets, and with field winding, while low-power AC synchronous motors are made with permanent magnets on rotor.

The current to the field winding is supplied by the rings and the brush assembly. Unlike a collector motor, where a rotating coil is used to transmit current collector (a set of longitudinally arranged plates), on the synchronous rings are installed across one of the ends stator.

The current source of constant excitation current is thyristor exciters, often called "VTE" (after the name of one of the series of such devices of domestic production). Previously, the "generator-motor" excitation system was used, when a generator (aka exciter) was installed on the same shaft with the engine, which through resistors supplied current to the excitation winding.

The rotor of almost all synchronous DC motors is performed without an excitation winding, and with permanent magnets, although they are similar in the principle of operation on AC LEDs, but in the way of connecting and controlling them, they are very different from the classic three-phase machines.

One of the main characteristics of an electric motor is its mechanical characteristic. For synchronous electric motors, it is close to a straight horizontal line. This means that the load on the shaft does not affect its speed (until it reaches some critical value).

This is achieved precisely due to the DC excitation, therefore the synchronous motor is excellent maintains constant speed under changing loads, overloads and voltage dips (up to a certain limit).

Below you see a legend on the diagram of a synchronous machine.

Rotor design

Like any other, a synchronous electric motor has two main parts:

• Stator. There are windings in it. It is also called an anchor.
• Rotor. Permanent magnets or an excitation winding are installed on it. It is also called an inductor because of its purpose - to create a magnetic field).

To supply current to the excitation winding, 2 rings are installed on the rotor (since excitation with direct current, one of them is supplied with "+", and the other with "-"). The brushes are attached to the brush holder.

Rotors for AC synchronous motors are of two types, depending on the purpose:

1. Explicit pole. The poles (coils) are clearly visible. Used at low speeds and a large number of poles.
2. Implicit - looks like a round blank, in the slot on which the winding wires are laid. They are used at high speeds of rotation (3000, 1500 rpm) and a small number of poles.

Synchronous motor start

The peculiarity of this type of electric car is that it cannot be simply plugged into the network and wait for it to start. In addition, for the operation of the LED, not only an excitation current source is needed, it also has a rather complex starting circuit.

Start-up occurs as in an induction motor, and to create a starting torque, in addition to the excitation winding, an additional short-circuited winding "squirrel cage" is also placed on the rotor. It is also called the "damping" winding because it increases stability during sudden overloads.

There is no excitation current in the rotor winding at start-up, and when it accelerates to subsynchronous speed (3-5% less synchronous), the excitation current is supplied, after which it and the stator current oscillate, the motor enters synchronism and goes to Work mode.

To limit the starting currents of powerful machines, the voltage at the terminals of the stator windings is sometimes reduced by connecting an autotransformer or resistors in series.

While the synchronous machine starts up in asynchronous mode, resistors are connected to the excitation winding, the resistance of which exceeds the resistance of the winding itself by 5 to 10 times. This is necessary so that the pulsating magnetic flux arising under the action of the currents induced in the winding during start-up does not slow down the acceleration, and also so as not to damage the windings due to the EMF induced in it.

Views

There are a lot of types of such machines, the design of an alternating current synchronous electric motor with field windings, as the most common in production, was described above. There are other types as well, such as:

• Permanent magnet synchronous motors. These are various electric motors, such as PMSM - permanent magnet synchronous motor, BLDC - Brushless Direct Current and others. The differences between them are in the control method and the shape of the current (sinusoidal or trapezoidal). They are also called brushless or brushless motors. Used in machine tools, radio-controlled models, power tools, etc. They do not work directly from direct current, but through a special converter.
• Stepper motors are synchronous brushless motors, in which the rotor precisely holds a given position, they are used for positioning working tool in CNC machines and for controlling various elements of automatic systems (for example, the position of the throttle valve in car). They consist of a stator, in this case, field windings are located on it, and a rotor, which is made of soft magnetic or hard magnetic material. Structurally, they are very similar to the previous types.
• Reactive.
• Hysteresis.
• Reactive hysteresis.

The last three types of LEDs also do not have brushes, they work due to the special design of the rotor. Reactive SMs have three designs: a cross-stratified rotor, a rotor with pronounced poles and an axial-stratified rotor. The explanation of the principle of their work is rather complicated and will take a lot, so we will omit it. In practice, you are likely to see such electric motors infrequently. These are mainly low-power machines used in automation.

Scope of application

Synchronous motors are more expensive than asynchronous motors, and they also require an additional source direct current excitation - this partly reduces the width of the field of application of this type of electrical machines. However, synchronous electric motors are used to drive mechanisms where overloads are possible and precise maintenance of stable speed is required.

Moreover, they are most often used in the field of high power - hundreds of kilowatts and units of megawatt, and, at the same time, starting and stopping occur quite rarely, that is, the machines work around the clock for a long time. This application is due to the fact that synchronous machines operate with cosphi close to 1, and can produce reactive power into the network, as a result of which the power factor of the network is improved and its consumption is reduced, which is important for enterprises.

Advantages and disadvantages

In simple terms, any electric car has its pros and cons. The positive aspects of a synchronous motor are:

1. Operation with cosPhi = 1, due to direct current excitation, respectively, they do not consume reactive power from the network.
2. During operation, with overexcitation, reactive power is transferred to the network, improving the power factor of the network, voltage drop and losses in it, and the KM of generators in power plants increases.
3. The maximum torque developed on the shaft of the SD is proportional to U, and for the IM - U² (quadratic dependence on voltage). This means that the LED has a good load capacity and stability of operation, which are preserved in the event of a voltage drop in the network.
4. As a consequence of all this, the rotation speed is stable during overloads and dips, within the limits of the overload capacity, especially when the excitation current is increased.

However, a significant disadvantage of a synchronous motor is that its design is more complicated than that of an induction motor with a short-circuit rotor; an exciter is needed, without which it cannot work. All this leads to higher cost compared to asynchronous machines and complexity in maintenance and operation.

Perhaps this is where the advantages and disadvantages of synchronous electric motors end. In this article, we have tried to summarize the general information about synchronous motors. If you have something to add to the material - write in the comments.

Related materials:

• What is rotor and stator
• How electricity is transmitted over distances without wires
• What is a frequency converter