Split-phase motor (upper right) appropriately serves small fan that needs no high cycle rate or high torque capability. Capacitorstart/induction-run motor (directly above) works on sausage-linking machine. This application also requires washdown-duty design, which here has an encapsulated electronic starting switch. Capacitorstart/ capacitor-run motor (lower right) works well on grain-handling equipment, typically subject to hard-to-start, hard-to-move loads.
Select figure to enlarge.

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 come from inappropriate application. Pay careful attention to 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 squirrelcage motor connected to a polyphase line will develop starting torque. A squirrelcage 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 mostly 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 electrical 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 for industrial use. However, it also limits performance. Starting torque is low, typically 100 to 175% of rated load. Also, the motor develops high starting current, 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 torque.

Other split-phase motor characteristics: Maximum running torque ranges 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 startingtorque applications with power needs from 1/20 to 1/3 hp. Avoid applications requiring high cycle rates or high torque.

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