An AC motor is an electric motor that converts electrical energy from an alternating current supply into mechanical rotation, using electromagnetism rather than mechanical contact or commutation to turn a shaft. AC motors are the largest single category of rotating equipment in most industrial facilities, and by extension, one of the biggest drivers of the electric bill. Machine-driven equipment such as pumps, compressors, conveyors, fans, and mixers accounts for roughly 54% of the electricity used in U.S. manufacturing, according to the U.S. Department of Energy, and the vast majority of that load is driven by AC motors. Despite how common they are, what actually happens inside an AC motor and what separates the two main types are often glossed over in spec sheets.

In this article, we'll cover how AC motors work, how induction motors differ from synchronous motors, and where each fits in an industrial operation.

Every AC motor relies on the same basic exchange of alternating current in and rotating force out. What differs between motor types is exactly how each one produces and sustains that rotational speed, and that distinction is what separates an induction motor from a synchronous motor.

How an AC Motor Works

Every AC motor is built around two main components: a stator and a rotor. The stator is the stationary ring of windings that surrounds the rotor; the rotor is mounted to the output shaft and is the part that actually turns. When alternating current is applied to the stator windings, which are spaced around the stator at fixed intervals, the current in each winding peaks at a slightly different point in the electrical cycle. The combined effect of that sequencing is a magnetic field that appears to sweep continuously around the inside of the motor housing at a constant rate, known as the rotating magnetic field.

The rotating field is what does the work. As it sweeps past the rotor, it induces a current in the rotor through electromagnetic induction, the same underlying principle that makes a transformer work, and that induced current generates its own magnetic field around the rotor. The interaction between the stator's rotating field and the rotor's induced field produces torque and pulls the rotor around after it. That mechanism is common to every AC motor. What changes between motor types is how the rotor's magnetic field gets created in the first place, and that's the line that separates induction motors from synchronous motors.

Induction Motors and Slip

Induction motors, sometimes called asynchronous motors, generate the rotor's magnetic field entirely through induction. There's no separate power connection to the rotor and no permanent magnet involved, just a set of conductive bars, typically arranged in what's known as a squirrel-cage rotor pattern, that carry whatever current the stator's rotating field induces in them.

That design is also where slip comes from. For a current to be induced in the rotor at all, there has to be relative motion between the rotor and the rotating magnetic field, which means the rotor can never fully catch up to the field's speed. If it is, there would be no relative motion, no induced current, and no torque to keep it turning. So the rotor always settles at a speed slightly below the field's synchronous speed, and that gap, expressed as a percentage of synchronous speed, is called slip. In practice, slip is small: on the order of 2-3% for large industrial motors running at rated load, and somewhat higher for smaller motors.

Slip isn't a design flaw but rather a mechanism that enables an induction motor to run in the first place. Because the rotor needs no external excitation, induction motors are simple to build, rugged, and comparatively inexpensive, which is a large part of why they're the default motor across most industrial equipment.

Synchronous Motors and Why They Run at Fixed Speed

Synchronous motors solve the rotor's magnetic field in a different way. Rather than relying on induction, the rotor's field is created directly, either by permanent magnets mounted on the rotor or by a separately excited winding. Because that field already exists independent of the stator, the rotor locks onto the stator's rotating field and turns at exactly the same speed as that field, with no slip involved.

The defining trait of a synchronous motor is its speed, fixed by the supply frequency and the number of poles, which it maintains regardless of load, within its torque limits. An induction motor's rotor speed will always sag slightly as load increases; a synchronous motor won't. Eliminating slip also eliminates the rotor losses that come with it, which is why synchronous motors, particularly permanent-magnet designs, tend to run more efficiently than a comparable induction motor. That combination of speed precision and efficiency is why synchronous motors show up in processes where holding an exact speed, or squeezing out efficiency at partial load, matters more than upfront cost.

Choosing Between Induction and Synchronous Motors

For most industrial buyers, the decision comes down to a handful of factors:

• Precise speed control: If a process requires the motor to maintain an exact, constant speed regardless of load, a synchronous motor provides that natively. An induction motor's speed will always drift slightly with load.
• Efficiency: Because synchronous motors don't lose energy to slip, they generally run more efficiently, which compounds over the life of a motor in continuous-duty, high-hour applications.
• Cost: Induction motors are simpler to manufacture and typically cost less upfront, which keeps them the default choice wherever slight speed variation isn't a problem.
• Load characteristics: Simple, continuous loads that don't require tight speed regulation, such as typical fans and pumps, are well served by an induction motor's ruggedness. Loads that need to track a process variable precisely, or that justify chasing every point of efficiency, are where a synchronous motor's premium pays off.

Neither type is a universal answer, and most industrial facilities run a mix of both: induction motors covering the bulk of general-purpose equipment, and synchronous motors reserved for specific applications where their strengths deliver a real return.

Where AC Motors Are Used in Industrial Settings

AC motors are the default power source behind the equipment that keeps an industrial process running. Pumps moving fluid through a system, compressors pressurizing air or refrigerant, conveyors and material-handling equipment, fans and blowers, and mixers and agitators. In nearly all of these, a motor is turning a shaft continuously or near-continuously, and the reliability and simplicity of an AC motor, most often an induction motor, is exactly what that kind of duty calls for.

Controlling AC Motor Speed

An AC motor's speed is set by the supply frequency and the number of poles it has, so on its own it runs at essentially one speed straight off the line. When a process needs the motor to start more gently or run at a speed other than the fixed speed, that's handled by adding a separate control device rather than changing anything about the motor itself.

For applications that only need a smoother, lower-stress startup, use a soft starter, which ramps the voltage up gradually to limit inrush current without changing the motor's running speed. For applications that need the motor to operate across a range of speeds, an AC drive is required, which varies output frequency to control motor speed on the fly. The right choice between the two depends on whether the application needs to control only the starting current or the ongoing speed as well.

Frequently Asked Questions

How long do AC motors last?

Most industrial AC motors are built for a service life of 15 to 20 years, and well-maintained units in favorable operating conditions often reach 20 to 30 years. Because induction motors have no brushes or commutators to wear out, the components that ultimately limit their life are the bearings and the winding insulation. Motors running in hot, dirty, or high-vibration environments, or ones that go without regular lubrication, typically fall well short of that range. 

What maintenance do AC motors need?

AC motors need comparatively little maintenance relative to other motor types, but a few items matter over the long run. Lubricating bearings on a schedule based on run hours and motor speed rather than the calendar, keeping the motor clean and dry so dust and moisture don’t compromise the winding insulation, and periodically testing insulation resistance to catch winding degradation before it causes a failure. Keeping an eye out for unusual heat, noise, or vibration is also a simple, low-cost way to catch a developing problem before it takes the motor down.

Ready to Source Your Next AC Motor?

Understanding how an AC motor works and what distinguishes an induction motor from a synchronous one makes it easier to have an informed conversation about which is best suited to a given application. From there, matching horsepower, frame size, and enclosure to the job is the next step, and it's one that Wistex's team can help you through directly. Explore Wistex's AC motor inventory to compare induction and synchronous options across the specifications and configurations most industrial applications require.