Practical Guide to
AC Motors and Control Options
AC motors are electric motors driven by alternating current (AC). AC motors are widely used in industry, primarily due to their high
efficiency, and their ability to produce constant torque up to the rated speed.
efficiency, and their ability to produce constant torque up to the rated speed.
AC Motor Types
The two most widely used types of AC motors are induction motors and synchronous motors.
How AC Motors Work
The two basic parts of an AC motor are the stator (the stationary outer drum) and the rotor; the rotating inner portion of the motor which is attached to (and drives) the motor shaft. Both the stator and the rotor produce rotating magnetic fields. In the windings of the stator, this rotating field is provided inherently by the sinusoidal nature of alternating current. In the rotor, the magnetic field is created by
permanent magnets, reluctance saliency, or by additional electrical windings.
Synchronous motors operate in lock step with the frequency of the supply current because their rotors have either permanent magnets
or electromagnets generating the rotating electromagnetic field.
or electromagnets generating the rotating electromagnetic field.
In an induction motor, the magnetic field in the windings of the rotor is “induced” by the magnetic field of the stator. In order for this
induction to produce torque, the speed of the rotor’s field must lag the field of the stator’s magnetic field. This speed differential is known as “slip”, and is the reason that induction motors will have a “Nameplate RPM” rating that is about 5% less than their synchronous speed.
induction to produce torque, the speed of the rotor’s field must lag the field of the stator’s magnetic field. This speed differential is known as “slip”, and is the reason that induction motors will have a “Nameplate RPM” rating that is about 5% less than their synchronous speed.
For example, an Ironhorse model MTRP-001-3DB18 (1hp, three phase, four pole, AC induction motor) has a synchronous speed rating of 1800 RPM (assuming 60hz power), but the “Nameplate RPM” rating is 1760. This motor shaft will turn at 1760 RPM when powered directly with the US standard of 60 Hz three-phase power. Differences Compared to DC Motors Industrial DC motors have historically been of the brush type. DC motors with brushes and commutators have a number of drawbacks when compared to AC motors: added maintenance (brush replacement), limited speed ranges and overall life expectancy is shorter. AC induction
motors have no brushes and have a much longer life expectancy. DC motor speed is controlled by varying the armature current, while AC motor speed control is achieved by varying the frequency of the alternating current, often with a variable frequency drive (VFD).
motors have no brushes and have a much longer life expectancy. DC motor speed is controlled by varying the armature current, while AC motor speed control is achieved by varying the frequency of the alternating current, often with a variable frequency drive (VFD).
Brushless DC motors have become available over the last several decades, primarily as a result of the advent of the semiconductor control circuitry required to operate them, and the availability of high-quality permanent magnets. Brushless DC motors require no brushes or physical commutator and thus have increased service life. They also overcome the speed limitations of the brushed versions.
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