Woodworker's Journal 1993-17-2, страница 22

In The Shop

Continued

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motor. Typically, the longer the cord, the heavier the wire gauge must be. Also, larger motors require heavier-gauge cords than smaller ones, to handle the higher amperage draw. If recommended extension cord sizes aren't listed in the tool ow ner's manual, contact the manufacturer for these figures.

Similarly, if you suspect a voltage drop, check the voltage at the wall outlet. In most areas of the country, the power company provides 115/230 volt service to the house (plus or minus a few volts) at the service panel, but line voltage may drop as much as 10 volts during times of peak use. Also, the actual voltage at any given outlet in the house may be less than that provided at the service panel, depending on the distance between the outlet and the panel. Outlets at the end of a branch circuit will have lower voltages than those near the panel. Faulty or insufficient wiring (especially in older homes) is another cause of voltage drop. First, contact the utility company and have them conduct a line voltage test at the service panel, then at the wall outlet servicing the tool in question ll the voltage drop between the service panel and outlet is more than about 4 volts, the wiring is insufficient.

Once you've ruled out mechanical and power-supply problems, you can suspect internal electrical problems. On single phase motors, the most common electrical failures ot c iii in the starting circuitry—the capacitor, centrifugal switch, or starting windings. If the motor hums but does not turn, the centrifugal switch may be stuck in the open position, or the capacitor has an open circuit. A motor that starts, but shuts down a few seconds later usually indicates a centrifugal switch stuck in the closed position. A shorted capacitor usually results in slow starting of the motor. In many cases, the sw itch is simply packed with sawdust, so you might be able to blow oui the motor with compressed air. If the switch or capacitor are bad. they'll need to be replaced. If the motor has an externally mounted capacitor, you can test it by carefully disconnecting the leads and attaching them to an identical capacitor (one with the same microfarad and voltage reading) that you know works. Bear in mind that some capacitors may carry a residual electrical charge, even after the machine is unplugged: to avoid electrical shock, disconnect the leads carefully and avoid direct contact with the capacitor terminals.

Motor Cuts Out During Use

Single phase induction motors often have a thermal overload protector that shuts down the motor when it overheats. This is normally indicated by the words "Thermally Protected" on the motor nameplate. If the motor stops suddenly during use. turn off the machine and allow the motor to cool down tor several minutes before restarting it. Most woodworking motors have a manual reset button on the motor housing, which you must depress before you can restart the motor. If the motor still w-on't start, check the fuse or breaker in the circuit supplying power to the tool.

While most universal motors don't have such overload protection, some have brushes that automatically disengage from the commutator when they wear dow n to a predetermined length. This feature is designed to prevent damage to the

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commutator by worn-out brushes. When the brushes disengage, the tool stops cold .i cue to replace the brushes.

Motor Stiills I ruler I j>atf

Hie same conditions that cause a motor to hum. start slow ly or fail to teach lull speed can also cause the motor to slow down or stall when a load is applied. An even more common cause is physically overloading the motor. A dull blade or cutter, or simply trying to force the tool beyond it's capacity can be tough on motors. Keep blades sharp: if the motors starts to labor w hile using ihe tool, slow down the feed rate.

If a stalled motor results in blown fuses or tripped circuit breakers in the branch circuit, this means that the breakers or fuses don't have sufficient capacity to handle the electrical devices attached to the circuit. If this is the case, you'll either need to reduce the number of appliances, machines, and lights on ihe circuit, or provide a separate, dedicated circuit for the machine, with the correct size fuse or breaker to handle it. An overloaded circuit can also result in a voltage drop.

Motor Overheats

Motors will overheat when subjected to the overload conditions described above. In a woodshop environment, a more common cause of overheating is insufficient air supply to the motor's cooling fan: that is. the motor vents are blocked with sawdust. The obvious fix is to unblock the vents. Then, check for a

22 The Woodworker's Journal