DC brushed motor: design and principle of operation

Collector motors are quite common in everyday life and at work. They are used to drive various mechanisms, power tools, in cars. Part of the popularity is due to the simple adjustment of the rotor speed, but there are some limitations of their use and, of course, disadvantages. Let's figure out what a DC collector motor (DCM) is, what types of this type of electric motors are and where they are used.

Definition and device

Reference books and encyclopedias give the following definition:

“A collector motor is an electric motor in which the shaft position sensor and the winding switch are the same device - the collector. Such motors can operate either only on direct current, or on both direct current and alternating current. "

The collector electric motor, like any other, consists of rotor and stator. In this case, the rotor is the anchor. Recall that the armature is the part of the electric machine that consumes the main current, and in which the electromotive force is induced.

What is the collector for and how is it arranged? The collector is located on the shaft (rotor), and is a set of longitudinally spaced plates, isolated from the shaft and from each other. They are called slats. The taps of the armature winding sections are connected to the lamellas (you can see the armature winding device of the KDPT on group of figures below), or rather each of them is connected to the end of the previous and the beginning of the next section windings.

The current is supplied to the windings through the brushes. The brushes form a sliding contact and, during the rotation of the shaft, come into contact with one or the other lamella. Thus, the armature windings are switched, for this a collector is needed.

The brush assembly consists of a bracket with brush holders, and graphite or metal-graphite brushes are installed directly in them. To ensure good contact, the brushes are pressed against the collector by springs.

Permanent magnets or electromagnets (field winding) are installed on the stator, which create a magnetic field of the stator. In the literature on electrical machines, instead of the word "stator", the terms "magnetic system" or "inductor" are often used. The figure below shows the design of the DCT in different projections. Now let's figure out how a brushed DC motor works!

Operating principle

When current flows through the armature winding, a magnetic field arises, the direction of which can be determined using gimlet rules. The constant magnetic field of the stator interacts with the field of the armature, and it begins to rotate due to the fact that the poles of the same name are repelled, being attracted to the opposite ones. Which is perfectly illustrated in the figure below.

When the brushes move to other lamellas, the current begins to flow in the opposite direction (if we consider the above example), the magnetic poles change places and the process is repeated.

In modern collector machines, a two-pole design is not used due to uneven rotation; at the moment of switching the direction of the current, the forces acting on the armature will be minimal. And if you turn on the engine, the shaft of which has stopped in this "transitional" position, it may not start to rotate at all. Therefore, there are significantly more poles and sections on the collector of a modern DC motor. windings laid in the grooves of the laminated core, thus achieving optimal smoothness of movement and torque on the shaft.

The principle of operation of the collector motor in simple language for dummies is disclosed in the following video, we strongly recommend that you familiarize yourself.

KDPT types and winding connection diagrams

According to the excitation method, collector DC motors are distinguished of two types:

1. With permanent magnets (low-power motors with a capacity of tens and hundreds of watts).
2. With electromagnets (powerful machines, for example, on hoisting mechanisms and machine tools).

There are such types of KDPT according to the method of connecting the windings:

• Sequential excitation (in the old Russian literature and from old electricians you can hear the name "Serial", from the English. Serial). Here, the field winding is connected in series with the armature winding. A high starting torque is the advantage of such a scheme, and its disadvantage is a drop in speed with an increase in the load on the shaft (soft mechanical characteristic), and the fact that the engine running out of control (uncontrolled increase in speed with subsequent damage to the support bearings and armature) if they are idling or with a load on the shaft in less than 20-30% of nominal.
• Parallel (also called "shunt"). Accordingly, the field winding is connected in parallel with the armature winding. At low revs on the shaft, the torque is high and stable over a relatively wide range of revs, and with increasing revs it decreases. The advantage is stable revolutions in a wide range of load on the shaft (limited by its power), and the disadvantage is that if the excitation circuit is broken, it can go haywire.
• Addicted. The field windings and armatures are powered from different sources. This solution allows you to more accurately control the shaft speed. Features of work are similar to DCT with parallel excitation.
• Mixed. Part of the field winding is connected in parallel, and part in series with the armature. Combine the advantages of serial and parallel types.

You can see the conventional graphic designation in the diagram below.

In foreign and modern domestic literature, as well as on the diagrams, you can find another representation of the UGO for KDPT, as shown in the previous figure in the form of a circle with two squares, where the circle denotes an anchor, and two squares - brushes.

Connection diagram and reverse

The connection diagram of the stator and rotor windings is determined during manufacture, and, depending on where a particular motor is used, an appropriate solution must be chosen. In certain operating modes (braking mode, for example), the winding switching circuits can be changed or additional elements introduced.

Includes low-power DC brushed motors using: semiconductor switches (transistors), toggle switches or buttons, specialized driver microcircuits or using low-power relay. Large powerful machines are connected to the DC network through two-pole contactors.

Below you see a reversible circuit for connecting a DC motor to a 220V network. In practice, in production, the circuit will be similar, but there will be no diode bridge in it, since everything lines for connecting such motors are laid from traction substations, where alternating current straightens up.

The reverse is carried out by changing the polarity on the field winding or on the armature. It is impossible to change the polarity both there and there, since the direction of rotation of the shaft will not change, as is the case with universal collector motors when operating on alternating current.

To smoothly start the engine, an adjusting device is introduced into the power circuit of the armature winding or the armature winding and the excitation winding (depending on the circuit of their connection), for example, a rheostat, the shaft speed is also regulated in the same way, but instead of a rheostat, a set of constant resistors is more often used, connected using a set contactors.

In modern applications, the speed is changed using pulse width modulation (PWM) and semiconductor key, this is exactly how it is done in a cordless power tool (screwdriver, for example). The efficiency of this method is much higher.

Scope of application

DC collector motors are used everywhere both in everyday life and in industrial devices and mechanisms, let's briefly consider their field of application:

• In cars, 12V and 24V collector DCTs are used to drive the wiper blades (wipers), in the window lifters, to start motor (starter is a brushed DC motor of series or mixed excitation) and drives of other destination.
• In lifting mechanisms (cranes, elevators, etc.), KDPT are used, which operate on a direct current network with a voltage of 220V or any other available voltage.
• In children's toys and low-power radio-controlled models, KDPTs with a three-pole rotor and permanent magnets on the stator are used.
• In a hand-held cordless power tool - a variety of drills, grinders, electric screwdrivers, etc.

Note that not collector, but brushless electric motors are installed in modern expensive power tools.

Advantages and disadvantages

Let's analyze the pros and cons of a DC brushed motor. Advantages:

1. The ratio of dimensions to power (weight and dimensions).
2. Easy to adjust the speed and implement a soft start.
3. Starting torque.

The disadvantages of the KDPT are as follows:

1. Wear of brushes. Highly loaded motors that are regularly operated require regular inspection, replacement of brushes and maintenance of the manifold assembly.
2. The collector wears out due to the friction of the brushes.
3. Arcing of brushes is possible, which limits the use in hazardous places (then explosion-proof KDPT is used).
4. Due to the constant switching of the windings, this type of DC motor introduces noise and distortion into the supply circuits. or the power grid, which leads to malfunctions and problems in the operation of other elements of the circuit (especially important for electronic schemes).
5. In DC motors with permanent magnets, the magnetic forces weaken over time (demagnetized) and the efficiency of the motor decreases.

So we examined what a collector DC motor is, how it works and what its principle of operation is. If you have any questions, ask them in the comments below the article!

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