Single-phase Electric Motors: Characteristics & Applications

by Kevin Heinecke, LEESON Electric Corporation

Single-phase AC motors are as ubiquitous as they are useful -- serving as the prime power sources for a seemingly limitless array of small-horsepower applications in industry and in the home. Knowing how to apply the various types is the key to successful design.

Where three-phase power is unavailable or impractical, it's single-phase motors to the rescue. Though they lack the higher efficiencies of their three-phase siblings, single-phase motors -- correctly sized and rated -- can last a lifetime with little maintenance.

Occasionally a manufacturing defect can result in early motor failure. However, most failures result from inappropriate application. Pay careful attention to the application requirements before choosing a motor for replacement of a failed one or for a new design application. Not choosing the correct motor type and horsepower can cause repeated motor failure and equipment downtime. Obviously, you don't want to specify a motor too small for the application, thus resulting in electrical stresses that cause premature motor failure. But neither should you specify a motor too powerful -- either because of its power or its inherent design characteristics. It can also have serious effects. For example, a motor with high locked-rotor and breakdown torques can damage the equipment it drives. Also, running a motor at less than full rated load is inefficient, costing you money for power wasted.

The key: First, size the motor to the application but, just as importantly, understand the characteristics of the major types of single-phase motors -- characteristics that go right to the heart of matching a motor to an application.

In general, an ac polyphase squirrel-cage motor connected to a polyphase line will develop starting torque. A squirrel-cage motor connected to a single-phase line develops no starting torque, but having been started by some external means, it runs approximately like a polyphase motor. The many types of single-phase motors are distinguished by the means by which they are started.

Split-phase

The split phase motor, also called an induction-start/induction-run motor, is probably the simplest single-phase motor made for industrial use, though somewhat limited. It has two windings : a start and a main winding, Figure 1. The start winding is made with smaller gage wire and fewer turns relative to the main winding to create more resistance, thus putting the start winding's field at a different angle than that of the main winding, and causing the motor to rotate. The main winding, of heavier wire, keeps the motor running the rest of the time.

A split-phase motor uses a switching mechanism that disconnects the start winding from the main winding when the motor comes up to about 75% of rated speed. In most cases, it is a centrifugal switch on the motor shaft.

The split-phase motor's simple design makes it typically less expensive than other single-phase motor types made for industrial use. However, it also limits performance. Starting torques are low, typically 100% to 175% of rated load. Also, the motor develops high starting currents, approximately 700 to 1,000% of rated. Consequently, prolonged starting times cause the start winding to overheat and fail; so don't use this motor if you need high starting torques.

Other split-phase motor characteristics: Maximum running torques range from 250 to 350% of normal. Plus, thermal protection is difficult because the high locked-rotor current relative to running current makes it tricky to find a protector with trip time fast enough to prevent start-winding burnout. And, 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 power needs from 1/20 to 1/3 HP. Avoid any applications requiring high cycle rates or high torques.

Capacitor start/induction run

Here is a true wide-application, industrial-duty motor. Think of it as a split-phase motor, but with a beefed-up start winding that includes a capacitor in the circuit to provide a start "boost", Figure 2. 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 about 75% 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, typically 200 to 400% of rated load. And the starting current, usually 450 to 575% of rated 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.

The cap-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. Use them on a wide range of belt-drive applications like small conveyors, large blowers and pumps, as well as many direct-drive or geared applications. These are the "workhorses" of general-purpose single-phase industrial motors.

Permanent split capacitor

A permanent split capacitor (PSC) motor, Figure 3, has neither a starting switch, nor a capacitor strictly for starting. Instead, it has a run-type capacitor permanently connected in series with the start winding. This makes the start winding an auxiliary winding once the motor reaches running speed. Because the run capacitor must be designed for continuous use, it cannot provide the starting boost of a starting capacitor. Typical starting torques of PSC motors are low, from 30 to 150% of rated load, so these motors are not for hard-to-start applications. However, unlike split-phase motors, PSC motors have low starting currents, usually less than 200% of rated load current, making them excellent for applications with high cycle rates. Breakdown torque varies depending on the design type and application, though it is typically somewhat lower than with a cap start motors.

PSC motors have several advantages. They need no starting mechanism and so can be reversed easily. Designs can be easily altered for use with speed controllers. They can also be designed for optimum efficiency and high power factor at rated load. And they're considered to be the most reliable of the single phase motors, mostly because no starting switch is needed.

Permanent split capacitor motors have a wide variety of applications depending on the design. These include fans, blowers with low starting torque needs, and intermittent cycling uses such as adjusting mechanisms, gate operators and garage door openers, many of which also need instant reversing.

Capacitor start/capacitor run

This type, Figure 4, combines the best of the capacitor-start/induction-run motor and the permanent split capacitor motor. It has a start-type capacitor in series with the auxiliary winding like the capacitor-start motor for high starting torque. And, like a PSC motor, it also has a run-type capacitor that is in series with the auxiliary winding after the start capacitor is switched out of the circuit. This allows high breakdown or overload torque.

Another advantage of the capacitor-start/capacitor-run type motor: It can be designed for lower full-load currents and higher efficiency. Among other things, this means it operates at lower temperature than other single-phase motor types of comparable horsepower.

The only disadvantage to a cap-start/cap-run motor is its higher price -- mostly the result of more capacitors, plus a starting switch. But it's a real powerhouse, able to handle applications too demanding for any other kind of single-phase motor. These include woodworking machinery, air compressors, high-pressure water pumps, vacuum pumps and other high torque applications requiring 1 to 10 hp.

Shaded-pole

Unlike all the previous types of single-phase motors discussed, shaded-pole motors have only one main winding and no start winding, Figure 5. Starting is by means of a design that rings a continuous copper loop 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 unringed portion. The reaction of the two fields gets the shaft rotating.

Because the shaded pole motor lacks a start winding, starting switch or capacitor, it is electrically simple and inexpensive. Plus, speed can be controlled merely by varying voltage, or through a multi-tap winding. Mechanically, shade-pole motor construction allows high-volume production. In fact, these are usually considered "disposable" motors, meaning they are much cheaper to replace than to repair.

The shaded pole motor has many positive features, but it also has several disadvantages. Its low starting torque is typically 25 to 75% of full load torque. It is a high slip motor with running speed 7 to 10% below synchronous speed. Also, it is very inefficient, usually below 20%.

Low initial cost suits shaded pole motors to low-horsepower or light-duty applications. Perhaps their largest use is in multi-speed fans for household use. But low torque, low efficiency, and less sturdy mechanical features make shaded-pole motors impractical for most industrial or commercial uses where higher cycle rates or continuous duty are the norm.

The preceding information establishes guidelines for determining the proper motor type for your application. However, there are always special cases and applications in which it is acceptable to vary from these guidelines. Make it a point to check with your motor manufacturer for technical support in these areas.

Capacitor Details

Start Capacitor: The electrolytic start capacitor helps the motor achieve the most beneficial phase angles between start and main windings for the most locked-rotor torque per locked-rotor ampere. It is disconnected from the start circuit when the motor reaches about 75% of full-load speed.

The start capacitor is designed for short-time duty. Extended application of voltage to the capacitor will cause permature failure, if not immediate destruction. Typical ratings for motor start capacitors range from 100 to 1,000 microfarad (uF) capacitance and 115 to 125 volts AC.

However, special applications require 165 to 250-Vac capacitors, which are physically larger than capacitors of lower voltage rating for the same capacitance. Capacitance is a measure of how much charge a capacitor can store relative to the voltage applied.

Run Capacitor: These are constructed similarly to start capacitors, except for the electrolyte. They are designed to serve continuously in the run circuit of a capacitor-start /capacitor-run motor. They withstand higher voltages, in the range of 250 to 370 Vac. They also have lower capacitance, usually less than 65 uF.

Kevin Heinecke, AC Motor Group at LEESON Electric Corporation.