Chapter 9: AC motors
Chapter Overview
Chapter 9 will cover AC motors. Three-phase AC motors are the most common type of motor used in commercial and industrial applications because they are simple in construction, require little maintenance, and cost less to operate than a DC motor. It is important to understand how AC motors operate in order to properly design, service and install them correctly in the field. Useful information Link.
Key points to remember:
- AC motors contain two main components: Rotor– or rotating part, and Stator– or stationary part.
- Split-phase motors are fractional horsepower motors usually between 1/20 to 1/3 horsepower and are used to operate washing machines, oil burners, and small pumps and blowers.
- Heat is the most common problem contributing to failure of AC motors.
Overview of AC Motors.
AC motors are very similar to DC motors in that they both utilize electromagnetic induction to operate but utilize. Due to the widespread availability of AC electrical systems in existing electrical infrastructures, AC motors are widely utilized throughout the industry. AC motors are relatively less complicated than their DC counter parts, especially three-phase motors that do not require auxiliary components to help them start. This simplified design contributes to minimal maintenance and increased working life, although AC motors typically do not produce as much torque as DC motors. As an electrician, you will encounter a range of AC motors used in residential, commercial and industrial settings that control everything from common appliances to complex industrial equipment. It is important to have a solid understanding of their unique operating and maintenance characteristics being that AC motors are so commonly relied upon in every aspect of the industry.
Single-phase Motors.
Single-phase AC motors operate on one single phase and are not self-starting, so they require some sort of “starting aid” to begin the initial rotation of the motor shaft. Single-phase motors are easily identified due to the presence of the starting aids on the outside of the motors housing, something that would not be present on a three-phase motor. Single-phase motors are commonly used to power smaller loads and can be described as split-phase, capacitor start, and shaded pole motor types.
A “split” phase motor refers to the separate starting coil that produces the auxiliary magnetic field necessary to start single-phase motors. Split-phase motors contain a separate winding for starting the motor. The starting winding is made of smaller wire used only to help the motor start and so it has a higher resistance than the running winding which is sized relative to the motor load. When the motor gets up to speed, a centrifugal switch is used to disconnect the starting winding after it is no longer needed to overcome the motors initial resistance to starting i.e. inertia.
Capacitor start motors use a capacitor to help the motor start and overcome the initial resistance to starting known as inertial resistance. This type of starting aid is used to provide increased starting torque and allows the motor to be cycled on and off rapidly. The capacitor should be checked first when trouble shooting these types of motors as the capacitor is prone to failure.
Shaded pole motors are a smaller type of single-phase motor, usually less than 1/20th horsepower, typically used for exhaust fans. They contain a “shaded” starter pole positioned 90-degrees in relation to the main stator-pole. The shaded pole sets up an offset magnetic field that determines the direction of rotation, which helps start the motor but also prevents the motor from being reversed.
Editing note: insert flash cards for split-phase, capacitor start, and shaded pole motors.
Three-phase AC Motors
Three-phase AC motors are self-starting and do not require a starting aid to begin the initial rotation of the motor shaft due to the presence of three separate phases or coils spaced 120-degree apart. This simplifies their construction and ultimately leads to less maintenance and longer effective operating life. Three-phase motors are typically designed to operate at higher voltages allowing for smaller wire and conduit requirements along with increased efficiency over their working life span.
Editing note: insert “click here for more information on motor connection diagrams”: AC Motor Types | Working Principle | Single & Three Phase AC Motors
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Looking Deeper
Troubleshooting an AC motor should start at the motor starter, as this is a central point where an electrician can determine if the input voltage, coil voltage or operating signal, and motor load current levels are appropriate regardless of the distance between the motor and panelboard supplying the circuit. Remember that heat is the largest contributor to a shortened working life for any motor. Over cycling or repeatedly turning a motor on and off along with overloading the motor are major contributors to excessive heat that can prematurely destroy motor insulation and shorten operating lifespan.
Keep in mind:
Dual voltage three-phase nine-lead motors are commonly used in commercial and industrial settings because they can be wired to operate at either 208/230 volts or a higher, more cost-efficient voltage of 480 volts, which is typically available in larger commercial and industrial systems.
A device that converts electrical energy into rotational movement through the principle that current carrying conductor moving through a magnetic field will tend to be forced out of the field at right angles.
An assembly consisting of many turns of wire wound together with the purpose of creating an electric magnet strong enough to be used to open and close sets of contacts. Referred to as the “heart” of a contactor or motor starter.
A type of switch used in single-phase motors to disconnect the starting winding when the motor gets up to its rated speed.