Skip to the content

What is an AC Motor and How Did It Come About?

Motor with caption "It all started in 1830..."

First, a Brief History.

The AC motor that’s in your plant right now has a long and storied history, one that goes all the way back to Michael Faraday’s 1830 discovery that a changing magnetic field can induce an electric current in a circuit.  Fifty-eight years later that discovery culminated in Nikola Tesla’s 1888 patent for the first AC commutator-free polyphase induction motor.  

What is an AC Motor?

At its most basic level, your AC motor is an electric motor driven by an alternating current (AC).  It commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft which produces a second rotating magnetic field.  The magnetic field generated by the alternating current is what turns the motor’s shaft, which “in turn” produces the motor’s mechanical energy that facilitates whatever functional purpose the motor has within your operating environment. 

What are the Main Types of AC Motors?

The two most common types of AC motors are induction motors and synchronous motors.

  • If you have an induction motor (or asynchronous motor), it always relies on a small difference in speed between the stator rotating magnetic field and the rotor shaft speed (called slip) to induce rotor current in the rotor AC winding.  As a result, your induction motor cannot produce torque near synchronous speed where induction (or slip) is irrelevant or ceases to exist.
  • In contrast, a synchronous motor at your plant or facility does not rely on slip-induction for operation and uses either permanent magnets, salient poles (having projecting magnetic poles), or an independently excited rotor winding. The synchronous motor produces its rated torque at exactly synchronous speed.

Other types of motors include eddy current motors, and AC and DC mechanically commutated machines in which speed is dependent on voltage and winding connection.

How is the Motor's Speed Determined?

The speed of your AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding.  This speed is derived from a common formula involving variables that include synchronous speed (in RPM), AC power frequency, and the number of poles per phase winding.

It turns out that the actual RPM for your induction motor will be less than its calculated synchronous speed by an amount known as slip (referenced above), that increases with the torque produced.  With no load, the speed of your motor will be very close to synchronous.  When loaded, a standard motor might have between 2–3% slip, and special motors may have up to 7% slip. 

Are You in Need of an AC Motor or Need one Repaired?

At RSAW we take a hands-on consultative approach, one where a personal project manager is there to inform you if we believe a motor repair or replacement is your best next step.  We repair motors up to 40 Tons, 144 inches in diameter, and 13,800 Volts.  We also stock new Baldor/Reliance and Toshiba motors up to 2000 horsepower that are ready for 24/7 delivery.  Give us a call or Contact Us today to learn more.

About the author



RSAW's team of thought-leaders on all things motor repair, sales, storage, and field services contributed this post.