Woodworker's Journal 1993-17-2, страница 20

In The Shop

Continued

brushes, damaged power cord or internal wire, poor electrical connections, or a short or open circuit in the rotor or field windings—in short, anything that interrupts or alters the proper flow of electrical current in the motor. Electrical problems can also be the result of insufficient line voltage or an incorrectly fused circuit at the power source (house wiring).

Mechanical problems relate to the moving parts of the motor: bearings, fan. rotor shaft, belts and pulleys. Sometimes, the two types of problems are interrelated. For example, a mechanical condition, such as sticky or frozen bearings, creates electrical resistance in the motor windings or other electrical components, causing them to overheat and eventually burn up. The same problem can happen if you physically overload the motor, such as cutting dense wood with a dull saw blade. The trick is to catch individual problems before they lead to further damage.

Types of Motors

To better understand what can go wrong with motors, it helps to have a basic know ledge of the various types of motors and their respective components.

The universal motor, used in portable power tools, is shown in Fig. I. while the induction motor, used in most stationary woodworking machines, is shown in Fig. 2.

As you can see from the illustrations, universal motors differ from induction motors in two ways: first, thev have windings in both the slator (field) and the rotor, whereas induction motors have windings in the stator only. Universal motors also have a set of carbon brushes that conduct current to the rotor by direct

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contact through a commutator: in induction motors, a rotational electromagnetic field is "induced" into the rotor without actual physical contact with the fteld windings.

The term universal means that this type of motor can be wired to run on AC or DC current; induction motors run on AC current only. DC motors are similar to universal motors, in that they have a wound rotor, a pair of carbon brushes, and a commutator. However. DC motors use permanent magnets instead of field windings in the stator. As the name implies, they run on DC current only, and are widely used in cordless (battery-powered) tools. On both types, the brushes require periodic replacement: instructions are usually provided in the tool owner's manual. A dirty, worn, or rough commutator is another potential source of electrical trouble.

Induction motors used for most woodworking equipment are referred to as single phase motors. These require separate field windings to provide the power necessary to start the motor. On most, additional starting power (torque) is developed through use of a starting capacitator. which is connected to an internal centrifugal switch on the motor shaft. When (he motor reaches speed, the centrifugal switch cuts off power to the secondary starting windings and capacitator to prevent them from burning up. Single phase motors range from fractional horsepower sizes up to about 10 horsepower. Although these motors have no brushes to replace, electrical problems common to them include a faulty centrifugal switch or capacitor.

Other types of induction motors include light-duty shaded pole motors at one end of the spectrum and industrial-type three phase motors at the other. Shaded pole motors arc

The Woodworker's Journal