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Getting a handle on single-phase electric motors

A clear understanding of characteristics will help avoid application problems

Kevin Heinecke, EE, Electrical Design Engineer, LEESON Electric Corp. Grafton, Wisconsin
Considered as broad categories, only two types of AC motors exist in the plant: three-phase and single-phase. Three-phase motors are the choice where connecting to that type of electrical power is practical, which usually means in "permanently wired" applications. This includes, among other things, most larger machinery, air-moving and liquid-moving systems.
Three-phase motors have the most straightforward construction. Because three-phase power naturally creates a rotating magnetic field, no additional windings or switches are needed within the motor for starting. And three-phase motors have the highest efficiencies.
Nevertheless, in any plant there are dozens of applications where it's simply impractical to run three-phase lines. Often this involves portable or smaller horsepower machines, fans, pumps, compressors and material handling systems. Here, the versatility of single-phase motors becomes an advantage. Though somewhat more complicated in construction and lacking the ultimate efficiencies of three-phase motors, single-phase motors can provide years of reliable power with little or no maintenance.
The majority of single-phase motor failures result from inappropriate application--from not choosing the right type of single-phase motor. It is more involved than just matching the obvious horsepower and mounting requirements. You must also consider specific environmental and load conditions when deciding on the best single-phase motor solution.

Variety of enclosures
Single-phase motors made for industrial use are available with the same types of enclosures as their three-phase cousins. Open drip-proof (ODP) enclosures are an economical choice as long as the atmosphere is clean. Totally enclosed fan-cooled (TEFC) motors provide better protection for many in-plant uses in which dirt and dust levels are higher. Totally enclosed non-ventilated (TENV) enclosures provide sufficient cooling without an external fan for small, fractional horsepower motors. Or, in some air-moving applications, TENV motors are designated as totally enclosed air-over (TEAO), meaning that a portion of their cooling must come from being mounted in the driven fan's air stream.
In addition, single-phase motors are readily available with specialized enclosures meeting UL and CSA explosion-proof standards, and in designs that extend motor life in harsh environments such as washdown or chemical areas.

Major single-phase motor types
A clear understanding of what makes one type of single-phase motor different from another is a critical application factor. Because single-phase power does not naturally create a rotating magnetic field, motors operating on single-phase power must include additional means to start the rotor moving. Once moving, the rotor induces a rotating magnetic field that sustains operation. Therefore, in some types of single-phase motors, the starting circuit is "opened" after the rotor reaches a predetermined speed. In other types, the start circuit remains engaged. How single-phase motors are started--and the effect this has on torque--is often the factor that determines which motor type best fits the application.

The split-phase motor, also called an induction-start/induction-run motor, is perhaps the simplest kind of single-phase motor you'll find in the plant. It has two windings: a start and a main winding. The start winding is made with smaller gauge wire than the main winding and has much higher resistance. This results in different currents and magnetic fields in the two windings. These two magnetic fields, displaced from each other, form a rotating field that causes the rotor to turn.
Split-phase motors use a switching mechanism that disconnects the start winding when the motor comes up to approximately 75 percent of rated speed. Thereafter, the main winding operates independently. A centrifugal switch on, the motor shaft is most common.
The split-phase motor's simple design is typically less expensive than other single-phase motor types made for industrial use. However, the simplicity limits performance. Starting torque is low; 100 to 175 percent of rated load. In addition, the split-phase motor develops high starting current relative to motor horsepower, also called locked rotor current. In addition, because of the small wire used in the start winding, prolonged starting times can cause the motor to overheat and the start winding to fail. Therefore, don't use this motor if high starting torque is required.
Other split-phase motor limitations include relatively low breakdown torque (200 to 300 percent of rated load), and unreliable thermal protection due to the high locked rotor current relative to running current. Also, these motors usually are designed for single voltage, limiting application flexibility.
Good applications for split-phase motors include small grinders, small fans and blowers, and other low starting torque applications with horsepower requirements ranging from 1/20 to 1/3 hp. Avoid any applications requiring high cycle rates or high torque.

Capacitor start/induction run
The capacitor-start/induction-run design results in a wide-application motor for industrial use. In general, it is similar to a split-phase motor, but has a much heavier start winding with a capacitor in the circuit to provide a starting "boost." Like the split-phase motor, the capacitor-start motor also has a starting mechanism, either a mechanical or solid-state electronic switch. This disconnects not only the start winding, but also the capacitor when the motor reaches approximately 75 percent of rated speed.
Capacitor-start/induction-run motors have several advantages over split-phase motors. Since the capacitor is in series with the start circuit, it creates more starting torque. Typical starting torque ranges from 200 to 400 percent of rated load. Breakdown torque is usually boosted a bit in capacitor start designs, too--up to 350 percent of rated load. Plus, the starting current is much lower than the split-phase due to the larger wire in the start circuit. This allows higher cycle rates and reliable thermal protection.
A capacitor-start/induction-run motor is more expensive than a comparable split-phase design because of the additional cost of the start capacitor. But the application range is much wider because of higher starting torque and lower starting current relative to motor horsepower. Capacitor-start motors are ideal for a wide range of belt-driven and geared applications like small conveyors, medium-sized blowers and pumps, as well as many direct-drive applications. These are the "workhorses" of general-purpose industrial motors.

Permanent split capacitor
Permanent split capacitor (PSC) motors do not have a starting switch or a capacitor strictly for starting. Instead, permanent split capacitor motors have a run-type capacitor that is permanently connected in series with the start winding. This makes the start winding an auxiliary winding after the motor reaches running speed.
Because the run capacitor must be designed for continuous use, it cannot provide the short-term "boost" of a starting capacitor. Therefore, starting torque of a PSC motors is low, ranging from 30 to 150 percent of rated load, which makes the motors unsuitable for hard-to-start loads. However, unlike split-phase motors, PSC motors have low starting currents, usually less than 200 percent of rated full-load current, making them excellent for applications with high cycle rates.
Permanent split capacitor motors have several advantages: since they do not require a starting mechanism, they can be designed for easy reversing. They can also be designed for optimum efficiency and high power factors at rated load. They are considered to be the most reliable single-phase motors, primarily because a starting switch is not required.
Permanent split capacitor motors have a wide variety of applications depending on the design. Examples include direct drive fans, blowers with low starting torque requirements and intermittent cycling applications such as adjusting mechanisms, valve actuators, gate operators and garage door openers, many of which also require reversing.

Capacitor start/capacitor run
This motor design combines the best of the capacitor-start/induction-run motor with the best of the permanent split capacitor motor. Like a capacitor-start motor, it has a start-type capacitor in series with the auxiliary winding, which produces high starting torque. Like a PSC motor, it also has a run-type capacitor in series with the auxiliary winding after the start-capacitor is switched out. Plus, because each capacitor is specific-purpose, performance is further optimized with high breakdown torque, lower full-load current and higher efficiency.
Capacitor-start/capictor-run motors command a higher price, which is mostly the result of one or more additional capacitors (plus a starting switch). If the ultimate single-phase performance is required in demanding applications, this motor is the choice. Such applications include woodworking machinery, air compressors, high-pressure water pumps, vacuum pumps and other high-torque applications requiring up to 10 horsepower.

Though shaded-pole motors are more suited to household appliance use, they are appropriate for some air-moving applications in the plant. Unlike the previous single-phase motors, shaded-pole motors have only one main winding and no start winding. Starting is accomplished through a design that uses a copper ring around a small portion of each motor pole. This "shades" that portion of the pole, causing the magnetic field in the ringed area to lag the field in the non-ringed portion. The reaction of the two fields initiates rotation.
Because it lacks a start winding, starting switch or capacitor, the shaded pole motor is electrically very simple and inexpensive. Plus, speed can be controlled merely by varying voltage (or through a multi-tap winding). These motors offer poor starting torque, typically 25 to 75 percent of rated load, and very low efficiency. But their low initial cost makes them good for small-horsepower or light-duty applications. The most common application is probably household ventilation fans.

The preceding information establishes guidelines for determining the proper single-phase motor type to use for your application. However, there are always special cases and applications. Make it a point to check with your motor manufacturer or supplier for technical support when required.