Types of DC and AC motors, their differences

How motors work

The principle of operation of all types of electric motors is the interaction of the magnetic fields of the rotor and stator. In this case, the magnetic field can be created by a permanent magnetic or winding (coil-electromagnet).

Depending on the power and type of the motor, the windings can be located only on the stator or both on the stator and on the rotor. Let's try to explain the device and the principle of operation for dummies in electrics.

Let's start by looking at the design of collector motors. For example, in small collector DC motors, like for radio models, permanent magnets are located on the stator, and coils of copper wire are wound in the rotor. The current to the rotor coils of such an electric motor is supplied through a brush assembly consisting of brushes and a collector. The collector has lamellas to which the winding leads are connected.

After turning on the power, the rotor (armature) starts to rotate, the collector is fixed on it, and the stationary brushes alternately touch different pairs of collector lamellas. Through the brushes and lamellas, current is supplied to the rotor windings first to one winding, then to the other, thus creating a changing magnetic field that interacts with the field of the magnet. As a result, the poles of the rotating and stationary electromagnets are attracted, which is why the rotation occurs.

If we omit some of the nuances, then the greater the rotor current, the greater this field and the faster the rotor rotates. However, this is mainly applicable for DC and AC collector machines (they are universal).

If we talk about an asynchronous motor (AM) with a squirrel-cage rotor, this is an AC motor without brushes. In it, the windings are located on the stator (a), and the rotor consists of rods (b), short-circuited by rings - the so-called squirrel cage.

In this case, the rotating magnetic field of the stator generates a current in the rotor bars, which also generates another magnetic field. What happens when there are two magnets next to each other?

They are repelled or attracted to each other. Since the rotor is fixed at the ends in bearings, the rotor begins to rotate. AD is intended only for alternating current, and its shaft rotation speed depends on the frequency of the current and the number of poles in the stator windings, we will consider this issue in more detail in the article on asynchronous electric motors.

But to start the rotation of the shaft of such an engine, it is important either to push it (to give an initial speed), or to create a rotating magnetic field. It is created using windings arranged in a certain way, connected to a three-phase power network. (for example, 380V), or using starting and operating capacitors (including capacitor asynchronous engines).

In addition to the interaction of magnetic fields in the rotation of the electric motor shaft, and ampere force.

Therefore, you need to understand that the moment on the shaft of an abstract motor and the number of revolutions depend on the design and type of the electric machine, as well as on the strength of the current and its frequency. I repeat that in this article we will not go deep into the details of the device of each of the types and types of electric motors, but we will make separate articles for this.

It should be noted that asynchronous and universal collector motors are most common in everyday life and in production, in drives of construction machines. They are used everywhere, both for the movement of industrial mechanisms, and for cars, electric vehicles and used in household appliances, up to the electric toothbrush.

Main classification

So, electric motors are mainly divided into machines that operate on direct current as well as on alternating current. What is the difference between alternating current and direct current, we told in the article: https://samelectrik.ru/chem-otlichaetsya-peremennyj-tok-ot-postoyannogo.html. We will consider the types of electric motors from machines running on recess.

AC motors

Most of the electric machines used in production and in everyday life to drive elevators, in other types of electric drive are powered by alternating current.

AC motors can be classified as follows:

• asynchronous;
• synchronous.

In this case, asynchronous motors are distinguished either by the design of the rotor:

• with a squirrel-cage rotor (most common with any number of phases);
• with phase rotor (only three-phase).

And by the number of phases:

• single-phase (with a starting capacitor) are used in household electric fans and other low-power devices;
• capacitor or two-phase (these are single-phase with a capacitor that does not turn off during operation, due to which "Second" phase) are used in small pumps, ventilation, on "baby" type washing machines and old production models THE USSR;
• three-phase are the most common and are used everywhere in production.

There are different designs of single-phase IM, the list contains two main options!

A feature of all asynchronous electric motors is that the rotor speed is slightly less than the rotational speed of the stator magnetic field and is equal to:

where n is the number of revolutions per minute, f is the frequency of the supply network, p is the number of pole pairs, s is the slip, and "60" is the seconds per minute.

Thus, the rotor speed is determined by the frequency of the supply network, the design of the windings, or rather the number of pole pairs (coils) in it and the amount of slip.

Slip is a quantity that characterizes how much less the rotor speed is relative to the frequency of the rotating magnetic field. Under normal operating conditions, it is in the range of 0.01-0.06. In simple terms, the field in a stator with one pair of poles rotates at a speed:

60 * 50/1 = 3000 rpm

With two pairs - 1500 rpm, and with three pairs - 1000 rpm.

When sliding, for example, at 0.05, the rotor speed will be equal to:

3000 * (1-0.05) = 2850 rpm

To adjust the speed of such electric motors, use frequency converters, since we cannot influence the rest of the variables of the above formula.

The most common in Russia are asynchronous motors with a supply voltage of 220V for connecting windings in a delta pattern and 380V in a star pattern.

If in a three-phase electric machine the rotating field of the stator is created by the arrangement of the windings and a phase shift in the network by 120˚, then in single-phase such an effect is not observed. The shaft will rotate if you give it an initial rotation by turning the shaft by hand or installing a phase-shifting capacitor, which will create a phase shift on the starting winding.

Two-phase capacitor motors are arranged in a similar way, but the second winding does not turn off after starting, but continues to work through capacitor. Therefore, the name "two-phase" rather refers to the design and connection scheme, and not to the supply circuits. Both two-phase and single-phase are designed to operate from a 220V network.

Synchronous electric motors (SM) are almost always performed with an excitation winding on the armature, and the current excitation is transmitted to it either through the brush assembly, or induced by an electromagnetic systems.

This is necessary in order for its shaft to rotate at a frequency that coincides with the rotation frequency of the stator field. That is, there is no such parameter as sliding in this case.

The excitation current is supplied from special excitation systems such as "generator-motor" or electronic converters based on thyristors or transistors. The most common at domestic enterprises are devices such as VTE, TVU, etc.

There is not always a field winding and brushes, for example, in a microwave oven, a synchronous permanent magnet motor is used to drive the rotation of the dish.

Synchronous machines are salient and implicit. The visual differences lie in the design of the rotor, in practice there is a difference in their characteristics, production methods and design. In practice, the average household electrician is unlikely to have to deal with them.

It remains to say the main thing about AC motors - they are difficult to adjust the speed of rotation due to the fact that their speed is tied to speed. Reduction of voltage (current) on the stator or excitation (for synchronous and asynchronous with a phase rotor) leads to a drop in the moment and an increase in the amount of slip (at AD), while the shaft can rotate slower. To control the speed of such motors, you need a frequency converter. We talked about how to choose a frequency converter in the article: https://samelectrik.ru/vybor-chastotnogo-preobrazovatelya.html.

DC motors (DC motors)

There are the following types and types of DC motors:

1. DC collector motors. They consist of magnets or an excitation coil and an armature, the current is transmitted to the armature winding using a brush assembly, the disadvantage of which is gradual wear.
2. Universal collector motors. They are similar to the previous ones, but they can operate on both direct and alternating current.
3. Brushless or brushless. It consists of stator windings, permanent magnets are installed on the rotor. It is connected to the DC circuit through a special controller that switches the stator windings.

Collector motors can be divided into groups according to the type of excitation:

• self-excited;
• with independent excitement.

According to the type of connection of the field windings, they are distinguished as follows:

1. Sequential excitation allows a high torque on the shaft, but the idle speed is also very high and can damage the engine (go into overshoot).
2. Parallel excitation - in this case, the speed is more stable and does not change under load, but the torque on the shaft is less.
3. Mixed excitement combines the merits of both types.

In low-power collector DCTs, excitation is most often organized using permanent magnets.

With independent excitation of the collector motor, the stator and rotor windings are not connected to each other, but in fact are powered from different sources. Thus, it is possible to organize the regulation of the torque or speed, as well as achieve greater energy efficiency.

Depending on the design, such an electric motor can operate either only on direct current, or operate on alternating and direct current. In the second case, they are called "universal collector motor". They are widespread in everyday life, used in kitchen appliances and power tools (grinders, drills, etc.).

Brushless motors are devoid of the drawbacks inherent in collector motors due to the absence of a brush assembly. The current is supplied to the three stator windings, and the windings are switched by the controller. In fact, brushless DC motors are powered by converted alternating current. You can find out how these motors work by watching the following video:

They are similar in design to synchronous motors, except that permanent magnets are used rather than electromagnets. To rotate such a motor and increase its efficiency, Hall sensors are used to determine the position of the shaft and the correct switching of the windings.

They are often called valve motors, and in English-speaking sources, such motors, depending on their design, are called PWSM or BLDC.

They are used in computer coolers, as a drive for radio-controlled models such as quadcopters, as well as in motorcycles for bicycles.

Additional classification

In addition to the engines discussed above, it should be said about other types, such as:

• stepper;
• servos;
• linear;
• ripple current motors (similar to a DC motor, the difference is that the power is supplied by a rectified ripple current).

Stepper motors and servos are used wherever it is necessary to position the assembly of some kind of mechanism. The simplest example is CNC, 3D printer, and so on. Also, with the help of "steppers" sometimes control the position of the throttle valve of the car - and this is only a small part of their application.

Description of the functions and features of these types of electric drives is a topic for a separate article. If you are interested, write comments and we will publish it!

A linear motor, unlike all of the above, the movement of its shaft is not rotational, but translational. That is, it does not spin, but moves "back and forth". They are different:

• alternating current on the principle of action similar to synchronous and asynchronous electric motors;
• direct current;
• piezoelectric;
• magnetostrictive.

In practice, they are rare, they are used as a drive for a monorail railway, for feeding a working body in various machines.

However, the classification given in the article was chosen from the point of view of practicality, while in the literature it is suggested to divide the electric drive according to the following criteria.

By the specifics of the generated torque:

• hysteresis;
• magnetoelectric.

The next classification option is based on the differences in design and features of their design.

By type and location of the shaft:

• with a horizontal shaft;
• with vertical shaft placement.

Protection from the actions of the external environment:

• protected from high humidity and dust;
• for use in explosive areas.

By the duration of the operating mode:

• intermittent (winches, cranes, gate valve motors);
• for continuous operation (pumps, ventilation, etc.).

In terms of power, it is also possible to distinguish between machines of small, medium, high power. However, it makes no sense to give the limits of these capacities, since somewhere 6 MW is the average power, and somewhere 1 kW is a colossal number.

It is impossible to consider in detail all types within one article, therefore we will consider each version separately. We hope that the classification provided in brief helped you understand what types of AC and DC electric motors are, as well as what are their differences and application features!

Related materials:

• How an alternating electric current is obtained
• Types of voltage regulators
• How to make the simplest electric motor with your own hands

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