Practical direct current (DC) motors were invented in the early 19th century following experiments by Michael Faraday and others. DC motors continued to be improved upon along with DC generators to the time of Thomas Edison's first power plant and DC power distribution system in 1882.
This was not a practical solution for the electrification of the United States however because of the large conductors required to transmit the low voltage power from the power plant to the customer.
George Westinghouse backed Nicolas Tesla's promotion of alternating current (AC) that could be transformed from low voltage to high voltage for efficient long distance transmission, then back down again to supply homes and industry. AC electrification of the US was the only practical solution.
Despite the dominance of alternating current today however, both AC and DC motors have practical applications. AC or DC motors have two interacting magnetic fields at least one of which is wound and electrified. The other magnetic field can be created by magnets making it a permanent magnet motor type.
AC motors generally fall into one of two categories, synchronous or asynchronous types. Synchronous motors synchronize with the frequency of the alternating current whereas asynchronous motors run at a slightly slower speed relative to synchronous speed. The difference in output speed of asynchronous motors compared to synchronous speed is referred to as slip.
The speed of a synchronous motor is determined by frequency of the AC current and the number of poles in the motor by the following: Speed in revolutions per minute (rpm) = 120 x frequency (Hz)/ number of motor poles.
Asynchronous motors are brushless as are many synchronous motor types but some synchronous motors include brushes and a slip ring. Brushless motors by definition will have lower maintenance because the only parts to wear from friction are the shaft bearings.
Also referred to as an induction motor, the asynchronous AC motor was invented by Nikola Tesla in 1882 and is considered to be one of the 10 greatest discoveries of all times. Induction motors are among the most common in the world powering machinery and many applications in the home today including common appliances like refrigerator and air conditioner compressors, washing machines, dryers and fans.
Common examples of synchronous motors are AC electric clocks and timer motors in appliances. A special kind of synchronous motor is a stepper motor. Stepper motors are used in a variety of open loop motion control applications as a low cost alternative to a brushless servomotor. A brushless servomotor is another type of synchronous motor having permanent magnets and an integrated feedback device that continuously monitors the shaft motion, and in combination with a servo drive regulates the commanded motion very precisely.
DC motors have many applications where DC power is readily available or when duty cycles are low like for vehicle starter motors. Unlike AC motors all DC motors have brushes and a commutator that work together to automatically switch current to different windings in order to cause rotation. DC torque motors are designed to provide smooth controllable torque to a load typically below 1000 rpm, and often within a limited angle of motion. Torque motors may also be of the AC type. Examples include feed or take up rolls on industrial equipment or as an actuator to vary the opening of a valve or move a flight surface.
With so many motor types to choose from and the tens of thousands of standard models available it can be a challenge to choose the right one. Fortunately there are resources like on-line tutorials, motor sizing software, and application engineers who are happy to offer assistance.
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