Industrial Motors / Motor Monitoring

Get schooled: AC motor basics

In this installment of Automation Zone, understand how AC motors work to figure out which motor and drive will work for you.

By Jeff Payne, AutomationDirect

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. AC motors have replaced DC motors in many instances and are now the preferred solution for many manufacturing applications, but how exactly is an AC motor able to function as it does? Let’s review that question and consider some common industrial applications of AC motors.

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.

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. 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.

VFDs are used to control the speed, torque, and direction of AC motors. 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 to drive the centrifugal load, and one-half the speed would require one-eighth of the power.

Specifying AC motors


If you are specifying a motor for a new application, start by determining the required voltage, speed, and horsepower. If you are replacing a properly sized motor in an existing application, you can find all of the information you need on the nameplate of the existing motor. The need to control motor speed and position will define whether the motor load is constant or variable horsepower and torque.

AC motors can work well in constant-speed, variable-speed and some position-control applications, but each type of application places different demands on horsepower and torque. Constant-speed applications are a common use of AC motors. With conveyors, cranes and gear pumps, for example, horsepower needs may vary at constant speeds, but the torque load remains the same.

If the torque is variable, such as in a web unwind or rewind machine where the load increases with the diameter of the roll and vice versa, other motor options with a wider torque range, such as DC or servo motors, may work better. However, adding a VFD and closing the speed loop with encoder feedback works well for many of these applications.

An AC motor with inverter duty ratings is a good choice in variable-speed applications such as with fans, pumps, and mixers/agitators. As the motor speed increases, so does the motor load. An AC motor and a VFD handle the variable horsepower and variable torque needed in these applications.

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 also result in significant energy savings, along with improvements to the product. 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. 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.

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.