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The electrical motor discovery first took place in 1821 by the British scientist Michael Faraday when he discovered the torque caused by a magnetic field. It wasn't until after 1870 that we started to see electrical motors used in the field. At the time, they were all brushed DC motors. Since then, different types of DC and AC motors have been developed for different applications. This article will, very briefly, discuss some of these motor types and highlight the basic differences. To start, we would like to give our reader a quick look at different common motors types currently used.
There are four basic DC motor types: shunt motors, series motors, permanent magnet motors, and separately excited motors. Shunt motors have a stator and rotor windings connected in parallel while series motors have the stator and windings connected in series. The series motors tend to achieve higher torque. The most common motor is the permanent magnet motor which has a stator as a permanent magnet. Separately excited motors have a stator and rotor that feed from different power sources allowing more controls of the stator and rotor independently. In general, DC permanent magnet motors are constructed using an electrical coil surrounded by a permanent magnet. When DC current flows through the coil, it generates an electromagnetic field causing it to react with the permanent magnet field, resulting in rotation. These motors use brushes to conduct the current to the coil through the armature. Early on, most DC motors used brushes. Brushed DC motors rely on a mechanical contact between the brushes and the armature to conduct electricity while the armature is rotating. This mechanical contact usually fails due to wear and damage. However, due to electronics advancements, brushless motors are becoming more and more attractive due to their many benefits.
Brushless DC motor
There are several types of brushless DC motors. In short, a brushless DC motor has several coils controlled by an electronic module to accurately turn the coils ON/OFF for a precisely controlled time frame to cause the magnetic field to turn the armature. DC brushless motors use an electromagnet either at the centre or on the outside of the motor. These motors tend to be quiet and more efficient. Brushless motors are also known as ECM motors (electronically communicated motors). They have been combined with advanced electronics to create servo motors, stepper motors, and other type of motors with different advantages. Whether brushless or not, DC motors are very common and will be around a very long time to come as one their main advantages is that they can run on a battery.
As you may have guessed by now, there are also several types of AC motors. Induction motors are motors that have no permanent magnet. In these motors, the electromagnetic field is generated in a winding in the rotor and they are typically slow! Synchronous motors are made from a DC motor rotor and an induction motor stator; they are also called squirrel cage induction motors. There are different variations of each one of these motors and a few other types that will not be discussed in this blog. Some AC motors run on single phase voltage while others run on multiple phases. The most common motors are the single phase motors which are found in home furnaces and other appliances. These motors are very reliable and inexpensive. These motors require an initial boost the get them started. They typically have an internal switch to change windings once the motor starts turning. Capacitors are also used to provide an initial boost, but are also the main source of failure for these motors. Multiphase motors are motors that run on more than one phase (typically 3 phase). These motors are primarily used in industrial settings. Along with a motor drive, one can change the speed and direction of these motors and most of all maintain high torque at low RPM when needed. These advantages are not possible in a single phase AC motor.
Some companies have developed what is called a universal motor. These motors can work on both DC and AC power, but are not commonly used.
Both DC and AC motors convert electrical energy to work (mechanical energy). The efficiency of a motor will dictate how much heat is generated while converting the electrical energy to mechanical work. It is important to have the proper ventilation to keep the motor cool. It is also important to keep in mind that stall currents are very high and a stalled motor will generate more heat than it can dissipate. Using the proper protection (fuses, circuit breakers, etc.) to avoid damage to the motor is very important. When choosing a motor, one has to look at the load and properly size the motor to the load. Under-sizing will cause damage to the motor and over-sizing will increase cost. A motor should work at 65% to 80% of its load capacity but this should be evaluated depending on the application and the duty cycle of the motor.