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| CHOOSING AN ELECTRIC MOTOR |
Although you should expect to see higher price tags on better quality motors and the machines equipped with them, it's not always as simple as "You get what you pay for." A TEN POINT CHECKLIST 1. If you're comparing horsepower between motors or machines, make sure both ratings are for continuous-duty use, not maximum developed horsepower. 2. Ignore guideline #1, sort of. For a clearer picture of the motor's output, check the amperage and full load efficiency ratings on the nameplate. The higher the amperage number, the more powerful the motor is. 3. The motor should have it's own thermal overload protection to safeguard against damage from overheating, even if the machine's on/off switch is also protected. The reset function should be manual only, never automatic. 4. Determine whether the motor is rated for continuous use or only for intermittent use. Intermittent duty motors are designed for run times under sixty minutes, and will likely have shorter service lives than industrial quality motors. They're fine for home shop use if you let them rest between tough workouts. Don't be confused by the nomenclature -- an intermittent-duty motor can still have a continuous-horsepower rating, even though the insulation, windings, and air circulation won't be on a par with those of a continuous-duty motor. 5. A TEFC (totally enclosed, fan-cooled) motor housing will cost more but will fair better in a wood shop. You'll pay less for an open, drip-proof enclosure, but wood dust can accumulate inside and interfere with the motor's internal switches, electrical contacts, and even bearings. Explosion-proof motors have totally sealed housings so the electrical arcs can't ignite vapors. Except for use in dedicate finishing rooms or spray booths, their extra cost usually isn't warranted. 6. The best motors feature sealed, pre-lubricated ball bearings on both the load end (where the pulley mounts) and the idler end of the rotor shaft. Less expensive motors will have ball bearings on the load end only and a metal sleeve bearing downstream. 7. Look for a capacitor-start, induction-run motor, the most common for wood shop applications. Usually housed in a small piggyback enclosure on the motor, the capacitor shifts the phase of the incoming current and feeds a separate starter winding. As the motor comes up to speed, an internal centrifugal switch open, shutting off current to the starter winding. Avoid split-phase motors. Their low starting torque makes the too anemic for use in woodworking machines. 8. Make sure the motor's frame size and baseplate are compatible with the mounting plate on your machine. In the United States, the National Electrical Manufacturers Association (NEMA) sets standards, but imported motors will often have equivalent sizes. 9. This may seem obvious, but make sure the motor's rpm rating (either 1725 or 3450) is appropriate for the machine. Drill presses, lathes and bandsaws typically get the slower motors, while table saws, jointers, and planers run with the higher-speed powerplants. Check the manual or ask the tool manufacturer. 10. Check the nameplate for the insulation class rating, stated as the letter A, B, H, or F, in ascending order of resistance to heat. Class B insulation is adequate for most shop motors; a Class A rating may indicate that the motor is designed for intermittent use only. Note: For lots more on electric motors, see Workbench Volume 53, Number 3 (June 1997). Return to Workbench |