What’s the Difference Between an AC Motor and a DC Motor?

Motor Basics: AC Motors Vs DC Motors

Electric motors convert electrical energy to useful mechanical energy. The most commonly-used motors are alternating current (AC) induction motors and direct current (DC) motors. AC and DC motors power devices in commercial and industrial applications.

AC Motors

The two categories of AC motors are single phase and polyphase (three phase). The power source of the motor determines the category.

Single-Phase Motors

Single-phase motors are distinguished how they start and the torque they develop.

Shaded Pole Motors

Shaded pole motors have low starting torque, low cost and low efficiency. They also have no capacitors and no start switch. You'll find shaded pole motors on small direct drive fans and blowers found in homes. Shaded pole motors should not replace other types of single-phase motors.

PSC (Permanent Split Capacitor) Motors

PSC motors have similar applications to shaded pole motors. But PSC motors feature much higher efficiency. They also have lower current (50% - 60% less), and higher horsepower capability. PSC motors have a run capacitor in the circuit at all times. For direct drive fan applications, you can replace a shaded pole motor with a PSC motor. Doing so will result in more efficient operation. Yet PSC motors are not found in belted fan applications, due to the low starting torque.

Split Phase Motors

Split phase motors have moderate to low starting torque (100% - 125% of a full load). They also have high starting current. Split phase motors do not have a capacitor but they do feature a starting switch. The switch drops out the start winding when the motor reaches about 75-80% of its operating speed. Because of the moderate starting torque, you'll find split phase motors in:

• Easy-to-start belt drive fans and blowers;
• Light-start centrifugal pump;
• Grinders; and
• Small drill press equipment.

Capacitor Start Motors

Capacitor start motors come in both moderate and high starting torque types. Both types have moderate starting current and high breakdown torques. Moderate torque motors power fans, blowers and light-start pumps. These applications need starting torques of 175% or less or on light loads. Compressors, wood working, industrial, commercial and farm equipment use high torque motors. With starting torques higher than 300%, high-torque motors are preferable in these applications. Capacitor start motors use a start capacitor and a start switch. When the motor reaches about 75 - 80% of its operating speed, the capacitor and start winding are taken out of the circuit.

Capacitor Start/Capacitor Run Motors

Capacitor start/capacitor run motors feature a run capacitor. The run capacitor stays in circuit at all times. These motors have similar applications and performance to capacitor start motors. However, capacitor start/capacitor run motors feature higher efficiency and reduced running amperage. These motors power 1 horsepower and higher single-phase applications. You'll find them in conveyors, pressure washers and pumps.

Polyphase (or Three-Phase) Induction Motors

Polyphase motors have a high starting torque, power factor, high efficiency, and low current. They do not use a switch, capacitor or relays. Polyphase motors are suitable for larger commercial and industrial applications. Polyphase induction motors are specified by their electrical design type (A, B, C, D, or E). The National Electrical Manufacturers Association (NEMA) defines these types. The designs are suited to particular classes of applications based on the typical load requirements of each class. The characteristics of polyphase motors lend themselves to high efficiencies. Many industries rely on polyphase motors. The U.S. Energy Policy Act mandates efficiency levels for many types of polyphase motors.

Polyphase motors offer improved efficiency and power factor over comparable single phase motors. The lack of capacitors and internal mechanical switches makes them more reliable and lighter. Polyphase motors also need less maintenance. Polyphase motors are self-starting, powerful and efficient. They have extensive uses in many applications.

You can control a polyphase motor with a variable frequency drive (VFD). This offers offering extra benefits in applications requiring speed control. For example, various industrial automation processes need variable speed control utilizing a VFD. Adding a VFD saves energy. With a VFD, the motor's load or application need will determine the speed. The VFD provides the voltage and frequency in a precise method as the motor starts. This eliminates energy waste and provides a soft start to the application. Benefits include an increased motor lifespan and a more cost-effective method for speed control.

DC Motors

DC motors are the oldest member of the electric motor family. Found in many industrial applications, DC motors are versatile. Innovations like magnetic materials, solid state electronic controls and high-power-density rechargeable batteries have kept the DC motor vital. They excel in 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 very high torque capabilities. You can use a DC motor together with relatively simple solid state control devices. These devices give programmed acceleration and deceleration over a wide range of selected speeds. The speed of a DC motor is not dependent on the number of poles. DC motors have great versatility for any constant or variable speed needs.

Permanent Magnet DC Motors

Permanent Magnet DC motors use high strength permanent magnets to create the magnetic field. These have replaced traditional field coil windings. The magnets do not need current from the power supply. This improves motor efficiency and reduces internal heating. Also, 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 horsepower.

You can operate DC motors from rectified AC or from a low-voltage battery or generator source. This low voltage design includes external connection lugs for the input power.

Want to ensure operational efficiency and reduce downtime? Select and maintain the proper motor for your application.