Get Schooled: AC Motor Basics

This AC motor and drive information will aid in selection and application.

Jeff Payne, product manager for the drives & motors group at AutomationDirect, wrote an article for the January 2019 edition of Plant Service magazine titled Get schooled: AC motor basics

Here’s a summary, click on the link above for the full text. Payne points out that alternating current (AC) motors are widely used in industry, primarily because of their high efficiency and ability to produce constant torque up to rated speed. Because of these and other attributes, their use is becoming more widespread.

How AC Motors Work

The two basic parts of a three-phase inductive motor are the stator and the rotor. The stator is the stationary outer drum; the rotor is the rotating inner portion of the motor attached to and driving the motor shaft.

AC motors have replaced DC motors in many instances and are now the preferred solution for many manufacturing applications, including industrial machinery, conveyors, pumps, and blowers. The two most widely used types of AC motors are induction (asynchronous) and synchronous motors.

Rotating magnetic fields are present in both the stator and the rotor. The sinusoidal nature of the alternating current flowing through the stator produces its magnetic field. The rotor’s magnetic field is created in several ways, either by permanent magnets positioned in a circle around the rotor, reluctance saliency, or additional electrical windings and an alternating electric current.

In a manufacturing plant, these motors are often automated by simple on/off control using a motor starter, says Payne, with other options added as needed.

Soft starters and variable frequency drives (VFDs) are used for more advanced applications and to help improve efficiency. Each of these types of components requires separate overcurrent protection devices and proper wiring, including motor branch-circuit protection from short circuits and motor overload protection. However, this protection may be electronically built in to some of the components.

The on/off control of a contactor or manual motor starter toggles AC power to a motor. Soft starters add an acceleration and deceleration function for smooth operation by temporarily reducing the load and torque during motor start-up. With less inrush of energy and torque, soft starters prevent excess temporary electrical demand, and reduce wear on motors and driven mechanical equipment.

Payne says VFDs are used to control the speed, torque and direction of AC motors. This provides a number of benefits in certain applications.

This is particularly beneficial when AC motors are used to drive conveyors, fans, pumps and other centrifugal equipment with varying loads. This is because of the affinity laws, whereby the power requirement varies by the cube of the change in speed. Therefore, two times the speed would require eight times the power, and one-half the speed would require one-eighth of the power to drive the centrifugal load.

Back-To-Basics Application

A common goal in applications where an AC motor is paired with a VFD is energy cost savings resulting from improved efficiency. Running fans and certain pumps at a speed matching the load, or changing conveyor speed to match demand, can provide substantial savings. It is also important to consider how these motor/drive installations can work with the automation system to improve control functionality, and ultimately the final product.

Changing speed in drilling, honing, grinding, sanding and buffing applications can result in significant energy savings, along with improvements to the product. Many older machines have only one speed, or maybe a few speeds depending on gearbox selection. Upgrading to an AC motor-and-drive package to provides basic control of tool or belt RPM and feed speeds is worth considering in these cases, Payne notes.

For example, the operation of a honing machine that provides a final surface finish to the inside diameter of large cylinders can be improved with programmable speed control from an AutomationDirect controller. In this application, the honing stone rotates while reciprocating axially through the cylinder. Controlling the rotation speed of the honing stone using an AC motor and variable-frequency drive will improve the surface by creating an optimal finish, even with open-loop control. Adding encoder feedback to the drive moving the stone through the cylinder can add short-stroke capability and programmable end-of-travel limits to optimize the process. With the proper speeds and feeds determined and controlled by the motor and drive, not only will the process improve, but the cycle time may as well.

AC Motor Basics

Adding this AutomationDirect DURApulse GS4VFD to a honing machine provided speed and position control for an AC motor to improve the final product.

In this and other applications, optimized AC motor and drive use can improve the final product, save energy, and reduce wear and tear on motors and driven loads.

To read more articles about AC Motors, click here.

To learn more on choosing an AC Motor check out our video!