What is the difference between a synchronous motor and an asynchronous motor for dummies, briefly, in simple words, a comparison in design and principle of operation – ElektrikExpert.ru

An electric motor is a device that converts electrical energy into mechanical energy. Structurally, the unit consists of a stator (fixed) and a rotor (rotates). The first creates a magnetic flux, and the second spins under the influence of an electromotive force (EMF).

Difference – in short simple words

In short and simple words, synchronous and asynchronous motors differ in the design of the rotors. Outwardly, it is almost impossible to understand which electric motor is in front of you, with the exception of the presence of additional cooling fins for asynchronous electric motors.

In a device operating on a synchronous principle, a winding with independent voltage supply is provided on the rotor.

In an asynchronous motor, current is not supplied to the rotor, but is formed using a magnetic stator field. At the same time, the stators of both units are identical in design and have a similar function – the creation of a magnetic field.

Additionally, in a synchronous motor, the magnetic fields of the stator and rotor interact with each other and have equal speed.

Asynchronous units in the rotor slots have short-circuited metal plates or slip rings that provide the difference in the magnetic field of the rotor and stator mechanism by the amount of slip.

Despite the apparent simplicity, it is unlikely that it will be possible to deal with this issue immediately, so we will consider the issue in more detail. Let’s talk about the features and differences between asynchronous and synchronous machines.

Synchronous motor (SM)

A synchronous motor is a unit with an individual rotor design and a permanent magnet inductor. It features improved characteristics of power, torque and inertia. It has a number of design features and principle of operation.

Устройство

Structurally, it consists of two elements: a rotor (rotates) and a stator (fixed mechanism). The rotor assembly is located inside the stator, but there are designs when the rotor is located on top of the stator.

The composition of the rotor includes permanent magnets, characterized by increased coercive force.

Structurally, LEDs are divided into two types according to the poles:

1. Implicitly expressed. They differ in the same inductance along the transverse and longitudinal axes.
2. Explicitly expressed. The transverse and longitudinal inductances have different parameters.

Structurally, rotors come in different devices and designs.

In particular, magnets are:

1. outdoor installation.
2. Built-in.

The stator conditionally consists of two components:

1. Casing
2. Core with wires.

The winding of the stator mechanism is of two types:

1. distributed. Its difference lies in the number of slots per pole and phase. It is two or more.
2. Focused. In it, the number of grooves per pole and phase is only one, and the grooves themselves are distributed evenly over the surface of the stator part. The pair of coils forming the winding can be connected in parallel or in series. The disadvantage of such windings is the impossibility of influencing the EMF line.

The form of the electromotive force of an electric synchronous motor is in the form:

1. Trapeze. Typical for devices with a pronounced pole.
2. Sinusoids. It is formed due to the bevel of the tips on the poles.

Generally speaking, a synchronous motor consists of the following elements:

• assembly with bearings;
• core;
• sleeve;
• magnets;
• anchor with winding;
• sleeve;
• “plate” made of steel.

Principle of operation

First, a direct current is applied to the excitation windings. It creates a magnetic field in the rotor part. The stator of the device contains a winding to create a magnetic field.

As soon as a variable current is applied to the stator winding, a torque is created according to Ampère’s law, and the rotor begins to rotate at a frequency equal to the frequency of the current in the stator assembly. In this case, both parameters are identical, therefore the motor is called synchronous.

Rotary EMF is formed thanks to an independent power source, which allows you to change the speed and not be tied to the power of the connected consumers.

Due to the peculiarities of operation, a synchronous electric motor cannot start on its own when connected to a three-phase current source.

Scope

The synchronous type electric motor has a wide scope, due to the constancy of the speed.

This feature expands the scope of its application:

• energy: sources of reactive power to maintain voltage, maintaining the stability of the network during emergency drawdowns;
• mechanical engineering, for example, in the manufacture of guillotine shears with high impact loads;
• other areas – the rotation of powerful compressors or fans, generators at power plants, ensuring the stable operation of pumping equipment, etc.

Advantages and disadvantages

After considering the design features, the principle of operation and the scope of the SD, let’s summarize the positive / negative features.

Pros:

1. Opportunity to work with cosine Phi equal to one (the ratio of useful power to total). This feature improves the cosine Phi of the network. When operating with leading current, synchronous machines generate reactive power, which is supplied to asynchronous motors and reduces the consumption of “reagent” from the generators of power plants.
2. High efficiency, reaching 97-98%.
3. Increased reliability due to large air gap.
4. Availability of regulation of overload characteristics, due to a change in the current supplied to the rotor.
5. Low sensitivity to voltage changes in the network.

Cons:

1. More complex design and, accordingly, high manufacturing cost.
2. Difficulties with starting, because for this you need special devices: exciter, rectifier.
3. The need for a direct current source.
4. Application only for mechanisms that do not need to change the speed.

Example SD2-85/37-6U3, 500kW, 1000rpm, 6000V.

Asynchronous motor (HELL)

An asynchronous (induction) electric motor having a different frequency of rotation of the magnetic field in the stator and rotor speed. Depending on the type and setting, it can operate in motor or generator mode, XX mode or electromagnetic brake.

Design features

Structurally, asynchronous mechanisms are difficult to distinguish from synchronous ones. They also consist of two main components: the stator and the rotor. In this case, the rotor assembly can be phase or short-circuited. However, there are some minor design differences.

Consider what an asynchronous motor consists of:

• core;
• case fan;
• bearing;
• box with terminals;
• triple winding;
• slip rings.

In view of the foregoing, one of the main differences is the absence of armature windings (with the exception of phase HELL). Instead of a winding, the rotor contains rods shorted to each other.

Operating principle

In an asynchronous motor, a magnetic field is created due to the current in the stator winding located on special grooves. On the rotor, as noted above, there are no windings, but instead of them, shortly combined rods. This feature is typical for a squirrel-cage rotor mechanism.

In the second type of rotor (phase), windings are provided on the rotor, the current and resistance of which can be regulated by a rheostat unit.

In simple terms, the principle of action can be decomposed into several components:

1. When voltage is applied, a magnetic field is created in the stator.
2. A current appears in the rotor, interacting with the stator EMF.
3. The rotor mechanism rotates in the same direction, but with a lag (slip) of 1 to 8 percent.

Scope

Asynchronous electric motors are in great demand in everyday life, due to the simplicity of design and reliability in operation.

They are often used in household appliances:

• washing machines;
• fan
• hood;
• concrete mixers;
• lawn mowers, etc.

They are also used in production, where they are connected to a 3-phase network.

This category includes the following mechanisms:

• compressors;
• ventilation;
• Pumps
• gate valves of automatic type;
• cranes and winches;
• woodworking machines, etc.

Asynchronous machines are used in electric transport and other areas. They have found application in tower cranes, elevators, etc.

Example Three-phase AIR 315S2 660V 160kW 3000rpm.

Advantages and disadvantages

An asynchronous type electric motor has weak and strong points that must be remembered.

Advantages:

1. Simple design, which is due to the three-phase connection scheme and the simple principle of operation.
2. Lower cost compared to synchronous counterpart.
3. Possibility of direct start.
4. Low energy consumption, which makes the engine more economical.
5. High degree of reliability due to simplified design.
6. Versatility and the ability to use in areas where there is no need to support the speed, or there is a feedback control scheme.
7. Possibility of application when connected to one phase.
8. Successful self-starting of the IM group in case of loss and subsequent supply of voltage to them.
9. Minimum operating costs. All that is required is to periodically clean the mechanism from dust and stretch the contact connections. Bearings can be changed every 15-20 years if the manufacturers’ requirements are met.

Disadvantages:

1. The presence of a sliding effect, which ensures that the rotor rotation lags behind the field rotation frequency inside the mechanism.
2. Heat loss. Asynchronous motors tend to overheat, especially under heavy load. For this reason, the body of the product is made ribbed to increase the cooling area (for SD, this is not used on all models). Additionally, a fan can be installed to blow the surface.
3. Voltage only for 220V and above. Due to the design features, such electric motors are not produced for an operating voltage of less than 220 V. Hydraulic or pneumatic drives are often used as a replacement.
4. Low efficiency at start-up and high reactivity. For this reason, the motor may overheat already at start-up. This limits the number of starts in a certain time period.
5. The synchronous speed cannot be more than 3000 rpm, because otherwise the use of a turbocharged drive or a step-up gearbox is required.
6. Difficulties in regulating devices that are set in motion by “synchronous”.
7. Increased starting current is one of the main problems of asynchronous motors with a power of over 10 kW. At the moment of starting, the current load can exceed the nominal one by six to eight times and last up to 5-10 seconds. For this reason, a direct connection is not recommended for “asynchronous”.
8. When a short circuit appears near the tires with the engine running, a current feed appears.
9. Sensitivity to voltage changes. If this parameter deviates by more than 5%, the performance of the electric motor deviates from the nominal values. In the case of a decrease in voltage, the moment of blood pressure decreases.

Comparison of synchronous and asynchronous motors

In conclusion, we can summarize what are the main differences between asynchronous (AD) and synchronous (SD) motors.

Let’s highlight the basic points:

1. The rotor of asynchronous motors does not require current supply, and the induction at the poles depends on the stator magnetic field.
2. The speed of the IM under load lags behind by 1-8% of the rotation speed of the stator field. In SD, the number of revolutions is the same.
3. The “synchronizer” provides an excitation winding.
4. Structurally, the SD rotor is a magnet: permanent, electric. In AD, the magnetic field in the rotor mechanism is induced by induction.
5. A synchronous machine has no starting torque, so an asynchronous start is needed to achieve synchronization.
6. Synchronizers are used in cases where it is necessary to ensure the continuity of the production process and there is no need for frequent restarts. HELL are needed where a large starting torque is required and there are frequent stops.
7. The LED needs an additional current source.
8. “Asynchronous” wear out more slowly, because in their design there are no slip rings with brushes.
9. For blood pressure, as a rule, not a round number of revolutions is characteristic, but for SD – rounded.

About reactive power

Synchronous motors generate and simultaneously consume reactive power. Features and parameters of the “reagent” depends on the current in the exciting winding. At full load, the cosine Phi is equal to 1. In this mode, the SM does not consume the “reagent” from the network, and the current in the stator winding is minimal.

It is important to understand here that reactive power worsens the parameters of the power system. A large parameter of inactive currents leads to an increase in fuel consumption, an increase in losses and a decrease in voltage.

In addition, the “reagent” loads power transmission lines, which leads to the need to increase the cross-section of cables and wires, and, accordingly, increase capital costs.

Today, one of the main tasks of power engineers is reactive power compensation. Its main consumers include AD, consuming 40% of the “reagent”, electric furnaces, converters, power lines and power transformers.

Which is better

When comparing asynchronous and synchronous motors, it is difficult to say which is better. In terms of design and reliability, the IM wins, which, under moderate load, has a longer service life. In SD, the brushes wear out quickly, which requires their replacement.

Otherwise, these are two similar in design, but differing in the principle of operation of the mechanism, which have individual areas of application.