DC MOTORS

WHAT ARE DC motors

DC motors are used in those drives where a wide range of speed control is required, greater accuracy in maintaining the speed of rotation of the drive, speed control up from the nominal.

DC Motor Diagram

How DC motors are Operate:

The operation of an electric DC motor is based on the phenomenon of electromagnetic induction. From the basics of electrical engineering, it is known that a force determined by the rule of the left hand acts on a conductor with a current placed in a magnetic field:

F = BIL

where I is the current flowing through the conductor, B is the magnetic field induction and L is the length of the conductor.

When the conductor crosses the magnetic lines of force of the machine, an electromotive force is induced in it, which is directed against it with respect to the current in the conductor, therefore it is called reverse or counteracting (counter-emf). Electric power in the engine is converted into mechanical power and is partially spent on heating the conductor.

Structurally, all DC electric motors consist of an inductor and an armature separated by an air gap. The inductor of the DC motor is used to create a stationary magnetic field of the machine and consists of a bed, main and additional poles. The bed serves for fixing the main and additional poles and is an element of the magnetic circuit of the machine. At the main poles are the excitation windings designed to create a magnetic field of the machine, at the additional poles - a special winding that serves to improve the conditions of switching. The anchor of a DC motor consists of a magnetic system assembled from separate sheets, a working winding laid in grooves, and a collector serving to supply a working DC winding.

The collector is a cylinder mounted on an engine shaft and selected from isolated from each other copper plates. On the collector there are tabs-cockerels to which the ends of the sections of the armature winding are soldered. The current is removed from the collector using brushes that provide sliding contact with the collector. Brushes are fixed in brush holders that hold them in a certain position and provide the necessary brush pressure on the surface of the collector. Brushes and brush holders are mounted on a traverse connected to the motor housing.

Switching in DC motors During operation of the DC motor of the brush, sliding on the surface of the rotating collector, sequentially move from one collector plate to another. In this case, the parallel sections of the armature winding are switched and the current in them changes. A change in current occurs at a time when the winding coil is short-circuited by a brush. This switching process and the phenomena associated with it are called switching.
At the time of switching in the short-circuited section of the winding under the influence of its own magnetic field induced EMF self-induction. Resulting EMF causes an additional current in the short-circuited section, which creates an uneven distribution of current density on the contact surface of the brushes. This fact is considered the main cause of sparking of the collector under the brush. The quality of switching is estimated by the degree of sparking under the running edge of the brush and is determined by the scale of degrees of sparking.
Methods for exciting DC motors under the excitation of electric machines

Understand the creation in them of a magnetic field necessary for the operation of the electric motor. The excitation circuits of DC motors are shown in the figure.

Excitation schemes for DC motors:

 a – independent

b – Parallel

c – Serial

d - Mixed

According to the method of excitation, DC electric motors are divided into four groups:

1. with independent excitation, in which the excitation winding is powered by an external DC source.

2. with parallel excitation (shunt), in which the excitation winding of the Motor is connected in parallel with the power source of the armature winding.

3. With sequential excitation (serial), in which the excitation winding of the Motor is connected in series with the armature winding.
4. Motors with mixed excitations (compound), which have serial and parallel excitation windings.

Types of DC Motors

DC motors primarily differ in the nature of the excitation. Motors can be independent, sequential and mixed excitation. Parallel excitation cannot be considered. Even if the field winding is connected to the same network from which the armature circuit is powered, then in this case the field current does not depend on the armature current, since the supply network can be considered as a network of infinite power, and its voltage is constant.
The field winding is always connected directly to the network, and therefore the introduction of additional resistance into the armature circuit does not affect the excitation mode. The specifics that exist during parallel excitation in generators cannot be here. In DC motors of low power, magneto electric excitation from permanent magnets is often used. At the same time, the engine switching circuit is greatly simplified, copper consumption is reduced. However, it should be borne in mind that, although the excitation winding is excluded, the dimensions and mass of the magnetic system are not lower than with electromagnetic excitation of the machine. The properties of engines are largely determined by their excitation system.

The larger the dimensions of the engine, the naturally larger the moment it develops and, accordingly, the power. Therefore, with a higher rotation speed and the same dimensions, you can get more engine power. In this regard, as a rule, DC motors, especially of low power, are designed for a high speed of 1000-6000 rpm.

However, it should be borne in mind that the rotation speed of the working bodies of production machines is significantly lower. Therefore, between the engine and the working machine you have to install a gearbox. The higher the engine speed, the more complex and expensive the gearbox is. In high power installations, where the gearbox is an expensive unit, the engines are designed at significantly lower speeds.
It should also be borne in mind that a mechanical gearbox always introduces a significant error. Therefore, in precision installations, it is desirable to use low-speed engines, which could be coupled with the working bodies either directly or through simple transmission. In this regard, the so-called high-torque engines at low speeds appeared. These engines are widely used in metal-cutting machines, where they are articulated with moving organs without any intermediate links by means of ball-screw gears.
Electric motors also differ in design with signs related to their working conditions. For normal conditions, the so-called open and protected engines are used, cooled by the air of the room in which they are installed. Air is blown through the channels of the machine through a fan located on the motor shaft. In aggressive environments, closed engines are used, the cooling of which is due to the external ribbed surface or external blowing. Finally, special engines for explosive atmospheres are available.
Specific requirements for structural forms of the engine are presented if it is necessary to ensure high speed - the rapid occurrence of acceleration and braking processes.

In this case, the engine must have a special geometry - a small diameter of the armature with a large length.
To reduce the inductance of the winding, it is laid not in the grooves, but on the surface of the smooth armature. The winding is fastened with adhesive compounds such as epoxy resin. With a small inductance of the winding, the conditions for switching on the collector are significantly improved, there is no need for additional poles, a smaller collector can be used. The latter further reduces the moment of inertia of the engine armature. The use of a hollow anchor, which is a cylinder of insulating material, provides even greater opportunities for reducing mechanical inertia. On the surface of this cylinder there is a winding made by printing, stamping or from wire according to the template on a special machine. The winding is fastened with adhesive materials.

Inside the rotating cylinder is a steel core, which is necessary to create paths for the passage of magnetic flux. In engines with smooth and hollow anchors, due to an increase in the gaps in the magnetic circuit due to the introduction of windings and insulating materials into them, the required magnetizing force for conducting the necessary magnetic flux increases significantly. Accordingly, the magnetic system is more developed.

Low-inertia engines also include engines with disk anchors. Disks on which windings are applied or glued are made of thin insulating material that is not subject to warping, such as glass. The bipolar magnetic system consists of two brackets, one of which contains the field windings. Due to the low inductance of the armature winding, the machine, as a rule, does not have a collector and current is removed by brushes directly from the winding. We should also mention the linear motor, which provides not rotational motion, but translational. It is an engine, the magnetic system of which is deployed as it were, and the poles are installed on the line of movement of the armature and the corresponding working body of the machine. Anchor is usually performed as low inertia. The dimensions and cost of the engine are large, since a significant number of poles are needed to ensure movement on a given stretch of track.

Starting DC Motors

At the initial moment of starting the engine, the anchor is stationary and counter EMF,

Back EMF=V-IaRa

the voltage in the anchor is zero, therefore

      0=V-IaRa                                         V=IaRa                                     Ia=V/Ra

The resistance of the armature circuit is small, so the inrush current is 10 to 20 times or more rated. This can cause significant electrodynamic efforts in the armature winding and its overheating, therefore, the engine is started using starting rheostats - active resistances included in the armature circuit.

Engines with power up to 1 kW allow direct start. The resistance value of the starting rheostat is selected according to the permissible starting current of the motor. The rheostat is performed step wise to improve the smoothness of starting the electric motor. At the beginning of the start, all the resistance of the rheostat is entered. As the speed of the anchor increases, a counter-emf occurs, which limits inrush currents. Gradually removing step by step the resistance of the rheostat from the armature circuit increases the voltage supplied to the armature.

DC motor speed control DC motor speed:

Equation of Dc Motor:

N=(V-IaRa)/kc Ф

where V is the voltage of the supply network,

 Ia - armature current, Ra – armature resistance, kc is the coefficient characterizing the magnetic system, Ф - magnetic flux of the electric motor. The formula shows that the rotational speed of the DC motor can be controlled in three ways: by changing the excitation flux of the electric motor,

by changing the voltage supplied to the electric motor,

and by changing the resistance in the armature circuit. The first two methods of regulation were most widely used, the third method is rarely used. It is uneconomical, and the engine speed significantly depends on load fluctuations.

The magnitude of the excitation current of the DC motor can be adjusted using a rheostat or any device whose active resistance can be changed in magnitude, for example, a transistor. With increasing resistance in the circuit, the excitation current decreases, the engine speed increases. When the magnetic flux is weakened, the mechanical characteristics are higher than the natural ones (i.e., higher than the characteristics in the absence of a rheostat). Increasing the engine speed causes increased sparking under the brushes. In addition, during operation of the electric motor with a weakened flow, the stability of its operation decreases, especially with variable loads on the shaft.

Therefore, the limits of speed regulation in this way do not exceed 1.25 - 1.3 of the nominal.

Voltage control requires a direct current source, such as a generator or converter. Such regulation is used in all industrial electric drive systems.

Braking DC Motors In electric drives with DC motors, three methods of braking are used: dynamic, regenerative and anti-inclusion braking. Dynamic braking of a DC motor is carried out by shorting the motor armature winding short-circuit or through a resistor. In this case, the DC motor starts to work as a generator, converting the stored mechanical energy into electrical energy. This energy is released as heat in the resistance to which the armature winding is closed. Dynamic braking ensures precise motor stop.
Regenerative braking of the DC motor is carried out in the case when the electric motor included in the network is rotated by the actuator at a speed exceeding the ideal idle speed. Then EMF induced in the motor winding, will exceed the value of the mains voltage, the current in the motor winding reverses direction. The electric motor goes to work in generator mode, giving energy to the network. At the same time, a braking torque occurs on its shaft. This mode can be obtained in the drives of lifting mechanisms when lowering the load, as well as when regulating the speed of the engine and during braking processes in DC electric drives.

Regenerative braking of a DC motor is the most economical way, since in this case there is a return to the electricity network.

Braking by the opposition of a DC motor is carried out by changing the polarity of the voltage and current in the armature winding. When the armature current interacts with the magnetic field of the field winding, a braking torque is created, which decreases as the motor speed decreases. When the motor speed decreases to zero, the motor must be disconnected from the network, otherwise it will begin to turn in the opposite direction.