DC Motor

Introduction:

A direct current (DC) motor is a type of electric machine that converts electrical energy into mechanical energy. DC motors take electrical power through direct current, and convert this energy into mechanical rotation.

DC motors use magnetic fields that occur from the electrical currents generated, which powers the movement of a rotor fixed within the output shaft. The output torque and speed depends upon both the electrical input and the design of the motor.

The Invention Of DC Motor: 

This amazing piece of electrical equipment has revolutionised our lives in many ways, but who invented the DC motor? As with all major innovations, there are many people who had a role to play through the development of similar mechanisms.

In the US, Thomas Davenport is widely celebrated as the inventor of the first electric motor, and undoubtedly he was the first to patent a useable electric motor in 1837. Davenport, however, was not the first person to build an electric motor, with various inventors in Europe having already developed more powerful versions by the time Davenport filed his patent.

In 1834, Moritz Jacobi had presented a motor that was three times as powerful as the one Davenport would later patent, while Sibrandus Stratingh and Christopher Becker were the first to demonstrate a practical application for an electric motor, by running a small model car in 1835.

The first practical DC motor was invented some years later in 1886 by Frank Julian Sprague, whose invention lead to the first motor powered trolley system in 1887, and the first electric elevator in 1892. Sprague’s DC motor was a hugely significant development, leading to a variety of applications which would reshape the face of industry and manufacturing.

Types of DC Motors:

So far, this guide has broadly explained how DC motors work, the history of these mechanisms, and what they look like. While the principles are the same across variants, there are actually several different types of DC motors, which offer specific advantages and disadvantages over each other.

This section of the guide will look at the four main types of DC motor - brushless, brushed, shunt, and series.

Brushless DC motors

Brushless DC motors are also known as electronically commutated motors, or synchronous DC motors, and differ to the brushed motor, thanks to the development of solid state electronics.

The key differences between brushless DC motors and other varieties is that they do not have a commutator, which is replaced by an electronic servomechanism that is able to detect and adjust the angle of the rotor.

The brushless DC motor has several advantages. Commutators use soft contacts called ‘brushes’ which wear down over time. A brushless DC motor is therefore more durable, and also safer than the more classical design.

All electric motors develop torque by alternating the polarity of rotating magnets attached to the rotor and stationary magnets on the surrounding stator. At least one of these set of magnets is an electromagnet, made from a coil of wire around an iron core.

In a DC motor, DC running through the wire winding creates the magnetic field. Each time the armature rotates by 180°, the position of the north and south poles are reversed. If the magnetic field of the poles remained the same, the rotor would not turn. To create torque in one direction in a DC motor, the direction of the electric current must be reversed with every 180° turn of the armature.

In a traditional brushed motor, this would be done by a commutator, but in a brushless DC motor, an electronic sensor instead detects the angle of the rotor, with controlled semiconductor switches either reversing the direction of the current or turning it off at the correct time in the rotation to create torque in one direction.

Brushed DC motors

The brushed DC motor is the original DC motor, dating back to Sprague’s initial design. As we have discussed already in this guide, the classic brushed motor features a commutator, to reverse the current every half cycle and create single direction torque.

While brushed DC motors remain popular for electrical propulsion, cranes, paper machines, and steel rolling mills, many have been phased out for the more efficient brushless model in recent years.

As has been explained in this guide already, brushed DC motors utilise soft contacts known as ‘brushes’ to ensure the motor rotates in one direction. Brushed motors’ speed can be varied by the operating voltage or the strength of the magnetic field.

There are a few different varieties of brushed DC motors, which differ depending on the connection to the armature, which will be discussed in the following sections.

Shunt DC motors

A DC shunt motor is a variety of brushed motor that has the field windings connected in parallel with the armature. Shunt wound DC motors have a lower current because of the parallel windings.

A shunt motor is used for applications that require a constant torque, where the load is not significantly altered by speed, such as conveyor belts, mixers and hoists.

The specific field windings provide unique shunt motor characteristics that make it such an effective choice for constant torque applications. Many shunt DC motors feature constant speed characteristics, with the small difference between no-load and full-load speed the main benefit of this type of motor.

Series DC motors

 

Series wound DC motors are the final variety of brushed motors in this guide. The key difference between this variety and the shunt motors discussed previously is that field windings are connected in a series. This means that the entire armature current passes to the field winding, creating much faster speeds.

As the supply voltage can’t be adjusted, series DC motors can’t regulate their speed particularly well.

While this is a problem for some applications, it makes them particularly useful for tasks that require high starting torque, such as power tools and sewing machines.

The specific characteristics of DC series motors offers a clear and distinct purpose. The Aulhaber Brushed DC Geared motor is a good example of the benefits of series wound motors, having high starting torque that is ideal for appliances.

Construction Of DC Motor:

The motor has a magnet. In the center of the magnet is a rectangular coil of wire on a sheet of cast iron. This cast iron plate is called armature. As a result of the flow of electricity, the armature rotates in symmetry between the two poles of the magnet. The two ends of the rectangular coil are connected by two slip rings. The slip ring rotates along the same axis of the two armatures. The two carbon brushes are placed in such a way that they touch the slip ring when the armature is rotating.

Procedure Of DC Motors: 

As soon as the motor is started, electricity flows from the outermost through the armature coil so that the coil starts to rotate. The action of two inverted magnetic balls causes rotation in the coil. Increasing the electric current in the coil also increases the speed of rotation. When the coil rotates, a small amount of electrical energy is absorbed. This time the coil rotates in that magnetic field and wants to position itself with the magnetic force line.

As the coil rotates, so does the commuter. When the surface of the coil comes in a vertical position, the coil travels a little further in the same direction due to motion inertia. In this state of the coil the position between the brush and the commutator changes so that the electric current acts on the coil and rotates the coil further in the same direction. In this way the electrical energy in the motor is converted into mechanical energy.

Applications of DC motors

Thanks to the different types of DC motors available, there are a wide variety of applications for DC motors. The previous section considered some of the various applications and circumstances where DC motors are used, and the benefits of the various types of motor.

While each type has its advantages, broadly speaking, there are various uses of a DC motor. At home, small DC motors are used in tools, toys and various household appliances. In retail, the applications of DC motors include conveyors and turntables, while in an industrial setting, large DC motor uses also include braking and reversing applications.