Universal Electric Motors
A look at how to choose one and how to keep it running smoothlySynopsis: Universal electric motors are used to power portable power tools as well as shop vacuums and small stationary and benchtop tools. They tend to be lighter and less expensive than the induction electric motors found on large stationary tools. In this article, Barry Wixey explains how universal motors work, tells you what to look for in a motor, and offers motor maintenance tips.
Universal electric motors are used to power portable power tools as well as shop vacuums and small stationary and benchtop tools. Compared with the induction electric motors found on large stationary tools (see FWW #167, pp. 96-98), universal motors tend to be lighter and less expensive. They also are noisier, run faster, and wear out quicker, but with some routine maintenance, you can keep them humming.
How universal motors work
Universal motors consist of a rotor surrounded by a stator, both made from copper wire and iron. The rotor contains a number of separate coils or windings of copper wire, and the ends of these windings are attached to the bars on the commutator. Electrical power is fed to the rotor via this commutator and two carbon brushes, and to the stator through conventional wires.
When current flows through the stator, it becomes an electromagnet. When power is applied to a bar on the commutator through the brushes, that particular winding causes the rotor to become an electromagnet, too, whose north and south poles are nearly aligned with the north and south poles of the stator. The north pole of the stator repels the north pole of the rotor, which causes the rotor to rotate. This in turn lines up a new set of bars and brushes, which move the magnetic fields back to their previous positions, and the poles are repelled over and over again, keeping the motor turning.
The speed of a universal motor is controlled in two different ways. Cheaper tools use a rheostat (like a light dimmer) to reduce the voltage supplied to the motor and hence to slow it down. This system will not maintain the motor’s speed when it is under load and is likely to be found on such items as inexpensive drills.
Some products use a constant motorfeedback system that either measures or calculates the motor speed and adjusts the voltage accordingly to try to maintain that speed. This type of system commonly is found in high-end routers.
From Fine Woodworking #175
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