Woodworker's Journal 1993-17-2, страница 21

inexpensive and durable, bui are relatively inefficient for their size, so they're used mainly for small electric appliances and devices such as can openers, record players, clocks, fans, and the like. Three phase motors are used to drive heavy industrial equipment, including some large woodworking machines, but they have special current and voltage requirements so they won't run on standard 115V or 230V residential service. Electric companies usually supply three phase power to industrial and commercial buildings only.

Troubleshooting

Troubleshooting electric motor problems is much like tracking down problems in a car engine. You start with a visual inspection and check the obvious things first. If your car won't start, you check to see if there's gas in the gas tank, or if the battery is dead. Likewise, if you flip the switch on a power tool

Volt! ohm meter is used to make several tests on electric motors.

or machine and nothing happens, you check to see if there's power at the electrical outlet. After ruling out the obvious, you use your eyes and ears to track down the problem. The following text covers some common problems.

Motor Won't Start

If you plug in a tool or machine and nothing happens, the first thing to do is check to see if power is being supplied to the motor. First, check the electrical outlet in the wall. Test the outlet with a test lamp or an electrical device you know is working. If there's no power, replace the fuse or reset the circuit breaker. If you continue to trip the breaker or blow fuses, the circuit is either overloaded or there's a short circuit somewhere in the motor.

Next, check the power cord for damage. On heavy cords especially, a broken or shorted wire inside the cord's insulated jacket is nearly impossible to detcct by visual inspection alone, so you'll need to perform one of the following tests. First, unplug the cord and disconnect it from the motor leads. If you

March/April 1993

have a volt/ohm meter, test the cord for continuity as shown in Fig. 3. Set the meter to the RX1 scale, zero adjust the ohms scale <see meter manual for instructions) and attach the metal leads to the cord prongs. Connect the black and white cord wires with a ""jumper" as shown. A zero reading on the meter means the cord is okay; a high reading indicates the cord is defective. Volt-ohm meters are sold at many electronics store (such as Radio Shack) and auto supply shops.

If you determine that the cord is defective, replace it with an identical cord from the tool manufacturer (don't guess at wire gauges or cord lengths).

Next, check the ON/OFF switch. After unplugging the tool. set your volt-ohm meter to the RXI scale, then attach the probes to the switch terminals, as shown in Fig. 4. Turn the switch on and off: if the switch is working, the meter will read zero ohms in the ON position, and high ohms in the OFF position. If the ohm meter reads zero in both the ON and OFF positions, then the switch is stuck in the ON mode. If the ohm meter reads high in both positions, then a contact is not being made. If you can acccss the switch contacts, clean them. Note that if you must disassemble the motor to access the switch terminals, you should leave this test to a repair shop.

If the above tests fail to identify the problem, you'll need to lake the motor into a shop and have the interna! circuitry checked for loose connections or open circuits inside the motor.

Motor Hums or Won't Reach Full Speed

If the motor makes a humming noise and won't turn, or if it starts slowly or won't come up to full speed, the problem may be either electrical or mechanical.

First, check to see if the rotor shaft is turning freely by unplugging the tool and turning the projecting end of the shaft by hand. If the shaft doesn't turn freely, the bearings may be at fault, or the motor may simply be clogged with sawdust. Sawdust is the major cause of motor problems: it can literally suck the grease out of bearings, foul up the centrifugal switch in single phase motors, and block air vents, causing the motor to overheat. So. before conducting any tests, vacuum or blow out any accumulated sawdust in the motor.

Other mechanical problems thai keep a motor from reaching speed may be a bent rotor shaft, misaligned pulleys or arbor, an overtight belt, or mechanical problems with the machine to which the motor is attached. A scraping noise inside the motor usually indicates a loose fan or rotor.

Next, check the power supply for low line voltage or for a drop in line voltage during motor startup. The voltage supplied to the motor leads should be within about 8 percent of that stated on the motor nameplate. or about 106 volts minimum for a 115V motor. You can have a licensed electrician check this for you.

If the tool is connected to an extension cord, make sure the length and wire gauge of the cord match that specified in the tool owner's manual. If the cord is too long or the w ire gauge too small, electrical resistance in the cord will result in a voltage drop, which will cause the motor to start slowly, fail to come up to full speed, and cause overheating in the cord and

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