What is an electric motor?

One ​​of the first, if not the first, rotary engine was built by Michael Faraday in 1821 and consisted of a freely suspended wire immersed in a vessel of mercury. A permanent magnet was placed in the middle of the container. When a current passed through the wire, the wire rotated around the magnet, showing that the current caused an increase in a circular magnetic field around the wire. This engine is often shown in school physics classes, but sometimes salt water is used instead of the toxic substance mercury. The classic DC motor has an armature of electromagnets. A rotary switch called a commutator reverses the direction of the electric current twice per cycle so that it flows through the armature and the electromagnets attract and repel the permanent magnet outside the motor. The speed of the DC motor depends on the combination of voltage and current passing through the motor windings and the motor load or braking torque. The speed of the DC motor depends on the voltage and its torque depends on the current. Usually, the speed is controlled by variable voltage or current flow and by using taps (a type of switch to change the state of the coil) in the motor winding or by having a variable voltage source. Because this type of engine can generate high torque at low speeds, it is usually used in traction applications such as locomotives. However, there are many limitations in the classical design, many of which are due to the need for brushes to connect to the commutator. The wear of the brushes and the commutator creates friction, and the higher the engine speed, the harder the brushes must be pressed to make a good connection. Not only does this friction lead to engine noise, but it also puts a higher limit on speed and means that the wipers will eventually wear out and need to be replaced. Imperfect electrical connection also produces electrical noise in the connected circuit. These problems are eliminated by moving the inside of the motor to the outside, placing the permanent magnets inside and the coils outside to achieve a brushless design.

The permanent magnets in the outer (stator) of a DC motor can be replaced with electromagnets. By changing the field current (the coil on the electromagnet) we can change the speed/torque ratio of the motor. If the field winding is placed in series with the armature winding, we will have a low-speed high-torque motor, and if it is placed in parallel, we will have a high-speed, low-torque motor. We can also reduce the field current to get even more speed but with the same less torque. This technique is ideal for electric traction and many similar applications, and the application of this technique can lead to the elimination of the equipment of a mechanical variable gearbox.

One of the types of DC motors is the wound field universal motor. The name of these motors is derived from the fact that these motors can be operated with both DC and AC current, although in practice these motors often operate on AC power. The principle of operation of these motors is based on the fact that when a field-wound DC motor is connected to an alternating current, the current in both the field winding and the armature winding (and the resulting magnetic fields) changes simultaneously, thus creating a mechanical force. It will always be unchanged. In practice, the motor must be specially designed to be compatible with AC current (impedance/reactance must be considered) and the final motor will generally be less efficient than an equivalent pure DC motor. The advantage of these motors is that AC power can be applied to motors that have the characteristics of a DC motor, especially since these motors have very high starting torque and a very compact design at high speeds. The downside of these motors is their maintenance and reliability problem caused by the commutator, and as a result these motors are rarely seen in industries, but the most common AC motors are in appliances such as blenders and power tools that are occasionally used.

The most common single-phase motor is the slotted-pole synchronous motor, which is often used in devices that require low torque, such as electric fans, microwave ovens, and other small household appliances. Another type of single-phase AC motor is the induction motor, which is often used in large appliances such as washing machines and clothes dryers. Generally, these motors can produce a larger starting torque by using a starting coil with a starting capacitor and a centrifugal switch. During start-up, the capacitor and start-up coil are connected to the power source through a set of spring-loaded contacts on the rotary centrifugal switch. The capacitor helps to increase the starting torque of the motor. When the motor reaches the rated speed, the centrifugal switch is activated, the contact group is activated, disconnecting the capacitor and the series starting coil from the power source, at this time the motor operates only with the main coil.

For applications requiring higher power, three-phase AC (or multi-phase) induction motors are used. These motors use the existing phase difference between the phases of the multi-phase electrical supply to create a rotating electromagnetic field inside them. Often, the rotor consists of a number of copper conductors embedded in steel. Through electromagnetic induction, the rotating magnetic field induces current in these conductors, which results in the creation of a balancing magnetic field and causes the motor to move in the direction of the field rotation. This type of motor is known as induction motor. In order for this motor to move, the motor must always rotate at a speed lower than the frequency of the power source applied to the motor, because otherwise, the balancing field in the rotor will not be created. The use of this type of motor in traction applications such as locomotives, where it is known as asynchronous traction motor, is increasing day by day. A separating field current is applied to the rotor windings to create a continuous magnetic field, which in a synchronous motor, the motor rotates synchronously with the rotating magnetic field caused by the three-phase AC power. We can also use synchronous motors as current generators. The speed of an AC motor depends primarily on the supply frequency, and the amount of slip, or the difference in rotational speed between the rotor and the stator field, determines the torque produced by the motor. Changing the speed in this type of motor can be made possible by having a bunch of coils or poles in the motor that change the speed of the rotating magnetic field by turning them on and off. However, with the advancement of power electronics, we can have more uniform control over the speed of the motors by changing the frequency of the power supply.

Another type of electric motor is the stepper motor, in which an internal rotor, consisting of permanent magnets, is controlled by a set of external magnets that are switched on and off electronically. A stepper motor is a combination of a DC electric motor and a solenoid. Simple stepper motors are kept in certain positions by part of a gear system, but relatively controlled stepper motors can rotate very smoothly. Computer-controlled stepper motors are one form of positioning systems, especially when they are part of a digital system with auxiliary steering control.

A linear motor is basically an electric motor that has been converted from rotary to produce a linear force by creating a traveling electromagnetic field along its length instead of producing a rotational torque. brought Linear motors are often induction or stepper motors. You can see a linear motor in a high-speed maglio train where the train flies over the ground.