The Complete Guide to AC Motors

An AC motor is a common type of electric motor, driven by an alternating current. Like the majority of effective practical motors used in everyday industrial applications (as well as in hobbyist projects, domestic goods, and all manner of other professional equipment and consumer products), AC motors offer a relatively efficient method of producing mechanical energy from a simple electrical input signal.

AC motors are distinguished from many other types of electrical motor - and from the somewhat more familiar DC (direct current) motors in particular - by several important criteria. The most fundamental of these is the fact that an AC motor relies specifically on alternating the flow of current around its circuit to produce efficient mechanical energy. We’ll discuss this unique process in slightly more depth over the following sections of this guide.

AC motors are also distinct from DC motors in that most AC motors do not include brushes. This means there’s often a greatly reduced need for maintenance and parts replacement on an AC motor, and most users generally expect them to have a longer average life expectancy as a result. Also unlike DC motors, the output speed for many types of AC motor is typically dictated by a frequency drive control - again, we’ll briefly outline a range of potential variations to the basic AC motor model a little further on in this guide.

AC motors are frequently used in a diverse catalogue of very familiar consumer products and industrial equipment, thanks mainly to their durability, low manufacturing costs, general affordability and ease of operation. Some typical uses of AC motors at home and in the workplace might include:

As noted above, the key characteristic that really marks AC motors out as distinct from many other motor types - most notably DC motors - is the fact that they specifically run on alternating current. There are other differences too, but this one is key in gaining a basic understanding of exactly how AC motors work.

An alternating current or charge - generally abbreviated to AC, hence AC motor - is one whose flow direction around a circuit is reversed at regular intervals. (This trait of switching current direction also means that the voltage on an AC circuit changes periodically.) By contrast, a DC or direct current only flows one way around a circuit, and thus voltage on a DC circuit remains relatively constant.

AC currents, and by extension AC motors, rely on a device called an alternator to produce this alternating charge direction. An alternator is a specialised type of electrical generator, in which an electromagnetic field (EMF) is typically created when electricity is passed through a spinning shaft (the rotor), which itself turns around or within a set of static wire coils (the stator). The resulting EMF switches direction, or polarity, as the rotor turns in relation to the stator.

Because an EMF created by a charged rotor turning on fixed axis will switch polarity at set points relative to the stator, the periodic reversal of current direction in an AC motor happens at regular and predictable intervals. In practice, the alternator and current on the AC circuit behave a bit like a piston or paddle moving water around a ducting system - as the piston moves in and out at a constant speed, it in turn pushes then pulls the water back and forth through the conduit.

As outlined above, an induction motor (or a rotating transformer, as they’re less commonly called) is a specific type of AC motor assembly that relies on a spinning, electrically charged rotor to create an EMF around a stator, and thus produce the vital alternating current that the AC motor can then convert into mechanical energy.

Induction AC motors of this kind are also known as asynchronous motors, because the output rotor generally turns at a slower rate than the frequency being supplied to it at any given time. In other words, the motor spins ‘out of sync’ with the power being supplied.

This is necessary because of another key characteristic of a true induction motor; namely that a charge created through electromagnetic induction is the only source of electrical ‘excitement’ the armature receives. If the rotor turned at the exact same speed as the rotating magnetic field on the stator, no current would be induced, and the armature would require another source of excitement in order to generate power. This is what happens in a DC motor, where the current is directly conducted to the armature, or in a synchronous AC motor (see below).

A synchronous AC motor is a special type in which the output rotor speed is directly aligned - ’in sync’, as it were - with the rate of the alternating current being supplied to it. Whereas this would tend to induce no current under a standard AC induction or asynchronous motor construction, the synchronous AC motor typically has additional components fitted, known as slip rings, which allow for current transmission between the motor’s rotating and fixed parts.

Slip rings are electromechanical devices that allow electrical signals to be transmitted through electromechanical systems constructed from components that need to rotate while generating power. In the case of a synchronous AC motor, the slip rings are what generate the necessary magnetic field around the rotor, and thus allow it to turn at the same rate as the alternating current being supplied without breaking the flow of current.

Squirrel cage motors are a subtype of asynchronous AC induction motor that use cage rotors - a simple, rugged design featuring a cylinder made from solid metal bars used for conducting current through the rotor - instead of a series of wound coils.

Because the conductive rotor bars involve no moving contacts within the rotor mechanism itself (they’re permanently short-circuited by being embedded into end rings), squirrel cage motors are seen as the more durable, less maintenance-heavy option, and are considerably cheaper to produce and buy than wound or slip ring rotors.

The downside is that the permanent short-circuit construction of a squirrel cage means that no external resistors can be connected in series to the circuit - in other words, you can’t control the current induced in the rotor winding beyond its fixed state. In practical terms, this generally means that they require a high starting current, and initially generate quite low torque until up to full speed.

Shaded-pole AC motors are a subtype of single-phase squirrel cage motor, defined by their use of an auxiliary rotor winding composed of either a copper ring or bar - this is known as a shading coil.

Unlike standard squirrel cage motors, they’re well-suited to operating at multiple speeds, albeit only producing a relatively small starting torque as compared to their torque at full speed. Shaded-pole motors are typically economical to make and cheap to buy, whilst being impressively reliable due to their simple, rugged construction.

For all of these reasons, shaded-pole motors are often one of the leading choices for AC motor assemblies to power fan arrays, as well as a range of other fractional horsepower devices with loads that are easily started such as:

• Record players
• Toys
• Electric clocks
• Hair dryers
• Other small instruments