Electric motors convert electrical power into mechanical power and come in all shapes and sizes. Motors can be characterized as begin alternating current (AC) or direct current (DC) types. There are literally millions of standard motors to choose from. These include motors large and small for standard industrial or commercial applications, or operation in extreme conditions like deep sea submersible motors for propulsion or robotic arms, or motors designed to operate in the vacuum of space like on the Space Shuttle.
In a volatile environment like a paint booth or grain elevator an explosion proof motor may be required to prevent the possibility of a spark igniting the flammable environment. Even with all the available motors there is often the need for a custom motor to meet a specific torque and speed requirement, environmental condition or operate from a specific voltage and current. These might be either AC or DC types based on the specific application.
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.
Asynchronous motors are brushless as are many synchronous types but some synchronous motors include brushes and a slip ring. Brushless motors by definition will have lower maintenance since 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 electric clocks and appliance timers. Since the frequency of the AC supplied to these motors is precisely controlled they keep very accurate time. Some synchronous motors especially those with permanent magnet rotors are used in combination with variable frequency drives to control of speed over a wide range, from zero speed to maximum speed. Commonly referred to as AC brushless servomotors, these motors use feedback in combination with a servo drive to precisely control the commanded speed, or torque. Induction motors can also be driven with a variable frequency drive but are not as controllable as permanent magnet synchronous types however are less expensive.
DC motors have many applications where DC power is commonly available or when duty cycles are low like automobile 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 generate motion. A common example is a golf cart motor run from batteries. Another application for DC motors when battery power is available is for traction motors. These motors provide traction to the ground and are common for vehicles, human transporters and automatically guided vehicles (AGVs) to move product in large industrial plants. More recently however permanent magnet brushless AC motors are being used for all kinds of traction applications to lower maintenance and improve efficiency to minimize battery requirements.
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