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AC Motor vs DC Motor

Motor Basics: AC Motors vs DC Motors

The electric motor in its simplest terms is a converter of electrical energy to useful mechanical energy. While many types of motors are produced today, alternating current induction motors (AC motors) and direct current motors (DC motors) are among the most commonly used in commercial and industrial applications.


AC Motors

Alternating current (AC) induction motors are divided into two electrical categories based on their power source – single phase and polyphase (three phase).


Types of single-phase motors are distinguished mostly by the way they are started and the torque they develop.


Shaded Pole motors have low starting torque, low cost, low efficiency, and no capacitors. There is no start switch. These motors are used on small direct drive fans and blowers found in homes. Shaded pole motors should not be used to replace other types of single-phase motors.


PSC (Permanent Split Capacitor) motors have applications similar to shaded pole, except much higher efficiency, lower current (50% - 60% less), and higher horsepower capability. PSC motors have a run capacitor in the circuit at all times. They can be used to replace shaded pole motors for more efficient operation and can be used for direct drive fan applications, but are characteristically not applied to belted fan applications, due to the low starting torque .


Split Phase motors have moderate to low starting torque (100% - 125% of full load), high starting current, no capacitor, and a starting switch to drop out the start winding when the motor reaches approximately 75-80% of its operating speed. With moderate starting torque, typical applications would encompass easy-to-start belt drive fan and blower, light-start centrifugal pump, grinder and small drill press equipment. 


Capacitor Start motors are designed in both moderate and high starting torque types with both having moderate starting current, high breakdown torques. Moderate-torque motors are used on applications in which starting requires torques of 175% or less or on light loads such as fans, blowers, and light-start pumps. High-torque motors have starting torques in excess of 300% of full load and are used on compressors, wood working, industrial, commercial and farm equipment. Capacitor start motors use a start capacitor and a start switch, which takes the capacitor and start winding out of the circuit when motor reaches approximately 75-80% of its operating speed.


Capacitor Start/Capacitor Run motors have applications and performance similar to capacitor start except for the addition of a run capacitor (which stays in circuit at all times) for higher efficiency and reduced running amperage. Generally, start/ capacitor run motors are used in 1HP and larger single-phase applications such as conveyors, pressure washers and pumps.


Polyphase, or three-phase, induction motors have a high starting torque, power factor, high efficiency, and low current. They do not use a switch, capacitor, relays, etc., and are suitable for larger commercial and industrial applications. Polyphase induction motors are specified by their electrical design type: A, B, C, D or E, as defined by the National Electrical Manufacturers Association (NEMA). These designs are suited to particular classes of applications based upon the load requirements typical of each class. Because of their widespread use throughout industry and because their characteristics lend themselves to high efficiencies, many types of general purpose three-phase motors are required to meet mandated efficiency levels under the U.S. Energy Policy Act.

Overall, three-phase motors offer improved efficiency and power factor than a comparable single phase motor and are also more reliable, lighter and require less maintenance, as it does not require a capacitor or internal mechanical switch. Three-phase induction motors have extensive uses in many applications, because they are self-starting, powerful, and efficient. Three-phase motors also offer the potential to be controlled with a variable frequency drive (VFD), offering additional benefits in applications requiring speed control.  For example, various automation processes in the industry require variable speed control utilizing a VFD.  Adding a VFD to a three-phase induction motor permits the speed of the motor to be adjusted based on a motor’s load or application requirement, which saves energy. Voltage and frequency are provided via the VFD in a precise method as the motor starts eliminating energy waste and providing a soft start to the application, offering an increased lifespan for the motor and an economical method for speed control. 


DC Motors

Another commonly used motor in industrial applications is the direct current motor. The DC motor is the oldest member of the electric motor family. Technological breakthroughs in magnetic materials, as well as solid state electronic controls and high-power-density rechargeable batteries, have all revitalized the versatile DC motor. DC motors are designed for applications requiring motor speed adjustment, constant torque and reversing capabilities. These applications range from packaging, printing, turntable equipment, sports equipment and medical equipment.

DC motors are always single phase and have extremely high torque capabilities. They can be used in conjunction with relatively simple solid state control devices to give programmed acceleration and deceleration over a wide range of selected speeds. Because the speed of a DC motor is not dependent on the number of poles, there is great versatility for any constant or variable speed requirement.


Permanent Magnet DC motors use high strength permanent magnets to create the magnet field, which have replaced traditional field coil windings. The magnets require no current from the power supply. This improves motor efficiency and reduces internal heating. In addition, the reduced current draw enhances the life of batteries used as power supplies in mobile or remote applications. Permanent magnet DC designs are generally used for motors that produce less than 5 HP.

DC motors can be operated from rectified alternating current or from low-voltage battery or generator source. This is a low-voltage design, which includes external connection lugs for the input power.


Selecting and maintaining the proper motors is key to ensuring operational efficiency and reduced downtime.


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Commercial Systems|Industrial Systems