What is a synchronous motor? What are asynchronous motors and how they work

Synchronous motors and asynchronous motors are mainly distinguished by their working principle.

Synchronous motors and DC motors work on a similar principle, relying on the establishment of a magnetic field to operate.

Asynchronous motors mainly rely on the principle of induction motors.

The working principle of synchronous motor is to rely on the three-phase alternating current of the stator to generate a rotating magnetic field, which will cut the rotor conductor and generate an induced electromotive force to realize the conversion of electrical energy to mechanical energy.

The rotor speed of a synchronous motor is the same as the speed of the stator rotating magnetic field, so it is called a synchronous motor.

In contrast, asynchronous motors work on a slightly different principle.

When the stator windings are connected to a three-phase power supply, a rotating magnetic field is generated.

This magnetic field cuts the rotor conductor and generates an induced electromotive force, which generates an electric current.

This current, in turn, creates a magnetic field that interacts with the stator's rotating magnetic field to rotate the rotor.

Since the rotational speed of the rotor is lower than the rotational speed of the stator rotating magnetic field, it is called an asynchronous motor.

A synchronous motor is an AC motor with the same rotor speed as the stator rotating magnetic field, and its speed is constant, unadjustable, and does not change with load. The asynchronous motor can be said to be relative to the synchronous motor, and its rotor and the rotating magnetic field are never synchronized. The classification of asynchronous motors includes single-phase, two-phase, three-phase, etc., and the structure of different phases is also different.

After the motor is energized, the coil winding produces a rotating magnetic field, the rotor induces an electromotive force and an induced current, and the rotor generates a starting torque in the magnetic field, and rotates randomly, when the rotating magnetic field is as fast as the speed of the rotor, it is called synchronous, and when the rotor speed lags behind the rotating magnetic field, that is, it is not synchronized, it is called asynchronous.

To put it simply, the speed of the asynchronous motor is not fixed, and it is basically kept at about the rated speed, and the speed of the synchronous motor is fixed, and the rated speed on the nameplate is how much.

Both synchronous and asynchronous motors are commonly used AC motors.

The characteristic of synchronous motors is that when running in a steady state, there is a constant relationship between the speed of the rotor and the frequency of the grid.

Rotational speed = 60 * power frequency number of poles of the motor. If the frequency of the grid is constant, the speed of the synchronous motor is constant at steady state, regardless of the size of the load.

An asynchronous machine is a motor in which the rotor rotates at a different speed than the rotation speed of the rotating magnetic field generated by the stator, and there is a difference (out of sync). We call it deterioration. The ratio of this slip to the rotational speed of the rotating magnetic field generated by the stator is called the slip rate.

The difference between a synchronous machine and an asynchronous machine is:

1. From the aspect of power supply, the asynchronous machine only adds voltage on the stator side (there is also a voltage on the rotor), while the synchronous machine needs to add voltage to both the stator and the rotor (the rotor of the permanent magnet synchronous motor relies on the permanent magnet to excite, and does not need to be energized to excite, so the rotor does not need to add voltage), that is to say, the asynchronous machine is unilateral excitation, and the synchronous machine is bilateral excitation.

Second, in terms of speed, the speed of the asynchronous machine is only related to the load size (of course, there is a certain range), while the speed of the synchronous machine is only related to the frequency of the power grid.

3. Structurally speaking, the structure of the rotor of the synchronous motor and the asynchronous machine is also different. The rotor of the asynchronous machine is composed of a steel sheet and an aluminum strip (or a steel sheet and a coil), while the synchronous machine is generally composed of several magnets or coils.

Of course, there are many differences, such as process requirements, design issues, and so on.

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A synchronous motor is also a type of AC motor. It is mainly used for generators, but also for motors, and is generally used for production machinery with large power and speed that does not require adjustment, such as large water pumps, air compressors and mine ventilators. In recent years, due to the development of permanent magnet materials and electronic technology, micro synchronous motors have been more and more widely used.

One of the characteristics of synchronous motors is that there is a strictly constant relationship between the speed n during stable operation and the frequency f1 of the stator current, ie.

n=60f1/p=n0

The rotational speed n of the synchronous motor is the same as the rotational speed n0 of the rotating magnetic field. Hence the name "synchronicity".

Needless to say, the stator of the synchronous motor also needs to be energized to keep it synchronized with the stator. (The rotor of the micro synchronous motor can be made of permanent magnet magnets).

Asynchronous motors are different, the stator windings are energized to produce a rotating magnetic field, which induces a current in the rotor to rotate the motor. The structure is simpler than that of synchronous motors.

What is an asynchronous motor?

The asynchronous motor is an AC motor based on the interaction between the air-gap rotating magnetic field and the induced current in the rotor winding to generate electromagnetic torque, so as to realize energy conversion. Since the rotor winding current is induced, there is a certain difference between its rotational speed and synchronous rotational speed.

Difference Between Asynchronous Motor and Synchronous Motor:

The number of revolutions is fixed during normal operation of the synchronous motor and does not change due to the change of load.

The number of revolutions is not fixed during normal operation of the asynchronous motor, but will change slightly due to the change of load. Synchronous motors have complex circuits and reversible energy (motors and generators are theoretically interchangeable) and are mostly used as generators.

The speed of the rotor of the asynchronous motor is lower than the speed of the rotating magnetic field, and the rotor winding induces electromotive force and current due to the relative motion between the magnetic field and the magnetic field, and interacts with the magnetic field to generate electromagnetic torque to achieve energy conversion.

The rotational speed of the rotor of the synchronous motor motor and the rotational speed of the current of the stator are synchronized.

Synchronous motors are used to generate electricity.

Asynchronous motors are used on electric motors.

1. The difference between the design of synchronous motor and asynchronous motor.

The biggest difference between synchronous motor and asynchronous motor is whether their rotor speed is consistent with the stator rotating magnetic field, the rotor speed of the motor is the same as the stator rotating magnetic field, which is called synchronous motor, and vice versa, it is called asynchronous motor.

In addition, the stator windings of synchronous motors and asynchronous motors are the same, the difference is in the rotor structure of the motor. The rotor of an asynchronous motor is a short-circuited winding that generates an electric current by electromagnetic induction. The rotor structure of the synchronous motor is relatively complex, with DC excitation winding, so it needs to add an external excitation power supply to introduce current through the slip ring; Therefore, the structure of the synchronous motor is relatively complex, and the cost and maintenance cost are relatively high.

2. The difference between synchronous motor and asynchronous motor in terms of reactive power.

Compared with asynchronous motors, which can only absorb reactive power, synchronous motors can emit reactive power and can also absorb reactive power!

3. The difference between synchronous motor and asynchronous motor in terms of function and use.

The speed of the synchronous motor is synchronized with the electromagnetic speed, and the speed of the asynchronous motor is lower than the electromagnetic speed, regardless of the load size of the synchronous motor, as long as it does not lose step, the speed will not change, and the speed of the asynchronous motor will always change with the change of the load size.

The synchronous motor has high precision, but the construction is complex, the cost is high, and the maintenance is relatively difficult, while the asynchronous motor is slow to respond, but it is easy to install and use, and at the same time it is cheap. Therefore, synchronous motors are not as widely used as asynchronous motors.

Synchronous motors are mostly used in large generators, while asynchronous motors are almost used in electric airports.

There are three main modes of operation of synchronous motors, namely, as generators, motors, and compensators.

Running as a generator is the most important mode of operation of synchronous motors, and running as a motor is another important mode of operation of synchronous motors. The power factor of the synchronous motor can be adjusted, and the application of a large synchronous motor can improve the operating efficiency in the case where speed regulation is not required.

The synchronous motor can also be connected to the grid as a synchronous compensation machine. At this time, the motor does not carry any mechanical load, and sends the required inductive or capacitive reactive power to the power grid by adjusting the excitation current in the rotor, so as to achieve the purpose of improving the power factor of the grid or regulating the grid voltage.

How it works. When the synchronous motor is working, three symmetrical currents are introduced into the three-phase windings of the stator, and DC currents are introduced into the excitation windings of the rotor. When a three-phase alternating current is introduced into the three-phase symmetrical winding of the stator, a rotating magnetic field is generated in the air gap.

When a DC current is introduced into the rotor excitation winding, a stationary magnetic field with constant polarity is generated.

If the number of pole pairs of the rotor magnetic field is equal to the number of pole pairs of the stator magnetic field, the rotor magnetic field rotates synchronously with the stator rotating magnetic field due to the magnetic pull of the stator magnetic field, that is, the rotor rotates at a speed and direction equal to the rotating magnetic field, which is the basic working principle of the synchronous motor.

Synchronous motor, synchronous generator is to use the prime mover (water turbine, steam turbine) to drive the rotor to rotate, so that the excitation stator winding generates a magnetic field, and converts mechanical energy into electrical energy.

When the rotor speed is synchronized with the stator rotating magnetic field, the rotor and the stator have no relative motion, and the Kairan rotor winding does not have the feeling of cutting the magnetic flux, so it is called a synchronous generator.

The working principle is:When the DC motor is energized, the rotor winding produces a magnetic field, and the magnetic field direction of the magnetic field is opposite to the magnetic field direction of the main magnetic field, and an air gap magnetic field is generated, which cuts the rotor conductor, and the rotor conductor induces the electromotive force e and the generated current i to produce an electromagnetic force (braking force) in the conductor that is opposite to the direction of rotation of the rotor, and the physical expression is f=bli.

At the same time, there is an electromagnetic force (driving force) in the air gap that is in the same direction as the rotation of the rotor, and the expression is F-drive = BLS.

If the motor is designed so that the driving force is equal to or slightly greater than the braking force, the enameled wire is tightly pressed against the upper surface of the groove, and the maximum torque of the rotor is limited to a certain value.

The synchronous motor is mainly used as a generator, and the main reason is that the synchronous motor cannot start by itself under the grid voltage, and the stationary rotor pole has an average torque of zero under the action of the rotating magnetic field. However, in recent years, with the development of power electronics technology, the problem of synchronous motor starting and speed regulation has been solved, so that the synchronous motor appears as the prime mover in electric drag.

The asynchronous motor is connected to the three-phase AC power supply from the stator side, and the three-phase symmetrical current flowing through the stator winding generates a three-phase magnetodynamic potential (stator rotating magnetodynamic potential) and generates a rotating magnetic field. The function is mainly used for motors, and generally not for generators.

The difference between synchronous motor and asynchronous motor is:

1. In terms of power supply, the asynchronous motor only adds voltage on the stator side (there is also a voltage on the rotor), while the synchronous motor adds voltage to both the stator and the rotor.

2. Structurally speaking, the structure of the rotor of synchronous motor and asynchronous motor is also different. The rotor of an asynchronous motor is composed of a piece of steel and aluminum (or a sheet of steel and a coil), while a synchronous motor is generally composed of several magnets and coils (there are also hidden pole types that are different).

1. The direction of the magnetic field generated by the two is different

The rotor of the synchronous motor itself generates a magnetic field in a fixed direction (generated by a permanent magnet or a DC current), and the stator rotates the magnetic field"Dragging"The magnetic field of the rotor (rotor) rotates, so the speed of the rotor must be equal to the synchronous speed, so it is called a synchronous motor.

The asynchronous motor (induction motor) is an induced current in the rotor through the rotating magnetic field of the stator to generate electromagnetic torque, and the magnetic field is not directly generated in the rotor. Therefore, the speed of the rotor must be less than the synchronous speed (without this difference, that is, the slip rate, there is no rotor induced current), so it is called an asynchronous motor.

2. The speed of the two is not the same

The rotor of the synchronous motor rotates with the rotating magnetic field under the action of the rotating magnetic field. If the rotational speed of the rotor is exactly the same as the rotational speed of the rotating magnetic field.

Under the action of the rotating magnetic field, the speed of the rotor of the asynchronous motor is less than the speed of the magnetic field, that is to say, the two are not synchronized.

3. The structure of the rotor of the two is different

The rotor of the synchronous motor has a DC excitation winding, so an external excitation power supply is required to introduce current through the slip ring.

The rotor of an asynchronous motor is a short-circuited winding that generates an electric current by electromagnetic induction.