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What are AC Motors?

John Sunshine
John Sunshine

There are many different types and sizes of electric motors. Electric motors can be divided into two types: Alternating Current (AC) motors and Direct Current (DC) motors. An AC electric motor requires an alternating current, while a DC motor requires direct current.

AC motors are further subdivided into single phase and three phase motors. Single phase AC electrical supply is what is typically supplied in a home. Three phase electrical power is commonly only available in a factory setting. The most common single phase AC motor is known as a universal motor because it can also run with DC current.

Among other small applications, AC motors find use in mixers.
Among other small applications, AC motors find use in mixers.

This type of motor is very inefficient but can be very inexpensively made. It is also used almost exclusively for small factional horse power AC motors. The other advantage that it has is that the rotational speed of the motor can be easy changed. This type of motor is commonly found in mixers, hand drills, and any other application requiring variable speed and low cost and small size.

For larger single phase AC motors, a electrical component known as a capacitor is used to create a second phase from the single phase AC current. This type of motor is known as an induction motor, and there are two basic types: a capacitor start motor and a capacitor run motor. It is the interaction between the two phases of this type of motor that causes it to turn.

This introduction of a second phase eliminates the need for the brushes used in a universal AC motor. This greatly increases the both the efficiency of the motor and increases its life expectancy, as brushes are a major source of wear and failure. This type of motor is a fixed speed motor, and it is commonly used as the drive for refrigerator compressors, shop air compressors, and for other general purposes.

AC motors are usually sized in horsepower. The most common sizes are what are called fractional horsepower motors, such as 1/2 horse power or 1/4 horsepower. Larger motors are typically only found in factories, where they can range in size to thousands of horsepower.

There are also various speed ratings for AC motors, with speed usually specified as rotations per minute (RPM) at no load condition. As the motor is loaded down, the speed will slow down. When it is running at its rated power draw, the speed of the shaft measured in RPM is the full load speed. If the electric motor is loaded too heavily, the motor shaft will stop, which is known as the stall speed and should be avoided. All of these speeds are typically listed on the specification sheet for the motor.

Before a consumer orders an AC motor, he or she should determine the mounting type required, the start up torque, the type of enclosure required, and the type of shaft output needed. There are many choices in each of these categories. If the shopper just needs to replace an existing motor that has failed, the salesperson can help him or her find a direct replacement. Otherwise, specifying the correct AC motor can be a difficult task.

Discussion Comments


DC motors used to be common in industry because it was easy to control their speed. AC machines were regarded as fixed speed machines. Now, however, with frequency inverters, the speed of AC machines can also be easily controlled and since AC is the readily available mains voltage, it makes sense to use AC motors. Plus, AC induction motors do not have parts which require replacing, like the brushes in DC machines. -- DW


Please differentiate between ac and dc motors and list the advantages of ac motor over dc motor and vice versa.


@win199 - Yeah that is true, three phase motor systems carry three alternating currents which reach their limit value at different times. Using the first conductor as the reference, the other two currents have a delay in time of one cycle of the electrical current. This delay between phases make the unit seem as though it is using constant power to transfer over each cycle of the current load. As the rotation reaches its limit, it generally creates a magnetic field in an electric motor.


@ellaesans - Although single phase motors use more conductor material to pull a load; a three-phase system is generally more economical to transmit electric power at the same voltage.


Three-phase motors also use alternating current. This is commonly known as a poly-phase system, and they are mostly used in split unit air conditioning.

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    • Among other small applications, AC motors find use in mixers.
      By: VL@D
      Among other small applications, AC motors find use in mixers.