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What to Do When the Motor in Your Woodworking Power Tool Fails

A typical woodworking shop has a considerable number of electric motor driven power tools. In a lot of cases the motor is the most expensive part in the tool. Some of these motors are replaceable and some are not. Some are repairable and some are not. It is always bad news when a motor fails. Here are some tips on what to do if a motor goes on one of your machines. There are some cases when it is cost effective to repair a bad motor, but unfortunately there are also cases when not only the motor but the whole tool needs to be replaced.

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There are basically three types of motors found in the power tools of a typical wood shop. What to do if one of these fails depends primarily on which type of motor it is.

Universal Motors

Most small hand held tools have what are called universal or AC/DC motors. The most common failure in these is that the brushes wear out. Symptoms of brush failure are generally failure to start, intermittent starting, and more sparking when running than is normal. New brushes are cheap and are usually accessible enough to be replaced by the user. A good rule of thumb is that if the manual for the tool contains info on brush replacement then you can probably do it yourself. Get the brushes from the tool manufacturer. If you can't figure out how to replace the brushes yourself, you may want to consider whether or not to send the tool in for repair or just pitch it and replace it. Brush replacement and minor maintenance for a hand held tool will generally take one shop hour. In this part of the country at the time of this writing shop rates run about $60 per hour, so expect this work to cost a little more than that. It is often a better deal to simply pitch the tool and buy a new one.

Sometimes motors simply burn out. This is pretty easy to spot in a hand held tool as they usually smoke a lot and get hot when they do. It is usually most cost effective to simply pitch the whole tool if the motor burns out.

Fractional Horsepower AC Motors

Small stationary woodworking machines often have AC motors of less than one horsepower. Some of these are standard NEMA motor frame sizes but there are a lot of machines, particularly low end imported ones, that use odd-ball motor sizes. It almost never makes sense to attempt to repair motors of this size as the cost of replacement is invariably lower than the cost of repair. That is not to say the cost of repair or motor replacement will be cheap. The manufacturer of the tool should be able to supply you with a replacement motor, but don't be too shocked if the cost of the motor is pretty steep. If the tool is an imported cheap thing, don't be surprised if the manufacturer can't (or won't) provide the replacement part. In a lot of cases it is cheaper to simply replace the whole tool.

Single Phase AC Motors

This table saw motor has two capacitors under the hump-like covers on top.

Most stationary machines found in the small shop will have a single phase AC motor of from one to three horsepower. AC motors have few parts and few failure modes. They can be repaired by any motor shop, but whether a motor is worth repairing depends on what is wrong with it and its size (actually, its replacement cost).

First things first. Before considering what may be wrong with the motor you should check out other reasons why the motor may not be working. Did it trip a breaker? Does the switch work? After you've determined there is power to the motor, check to see if its thermal overload switch has popped. In the picture of the table saw motor you can see a red button. That is the thermal overload switch. It may simply need to be reset by pushing the button. Of course, if you are continually tripping a breaker of if the thermal overload switch keeps popping then there is some load or supply voltage problem that must be fixed.

The three parts that commonly fail are the capacitor(s), the centrifugal starting switch, and the windings. Of these, capacitor failure is really the only one that most folks would want to attempt to fix themselves, although there are cases when you can effect a repair on the starter switch without much trouble. Capacitors generally fail pretty dramatically, emitting great plumes of acrid smoke when they do. A motor with a bad starter capacitor may fail to start (but will hum when power is applied) or will start slowly. If you suspect a bad capacitor, you can easily and cheaply replace it yourself. The capacitor(s) is located outside the motor housing, inside a hump-like metal cover. Make sure the power is turned off. Unscrew the cover and lift it up. If the cap is bad, you can expect the inside to be dripping with the oil that used to be inside the capacitor before it blew. Remove the capacitor (the wires just pull off) and write down the information on the label. You'll need that, as well as the physical dimensions of the capacitor (both length and diameter) to get a replacement. You can order replacements from industrial supply companies such as McMaster Carr. Install the new capacitor and replace the cover. The motor should start up. If not there is some other problem. If the new capacitor blows out in short order then it is likely that the centrifugal starting switch inside the motor is bad.

A motor start capacitor.

The centrifugal starting switch disconnects the starting windings once the motor gets up to speed. If it is stuck closed the starting windings (which are wired through the starting capacitor) will never disconnect and the capacitor and/or the starting windings will eventually cook. Some motors are totally sealed up, which keeps debris out of the inside of the motor. But motors that are not sealed may offer some access to clean gunk out of the starting switch. If your motor has obvious vent holes in the end, you can try applying a shop vac to them to see if you can clean out the switch. Tapping the other end of the motor shaft with a ruber mallet while vacuuming may help to dislodge built up gunk. Thanks to David Maulik for sharing these tips on starter switch cleaning. His family owns a business that does motor repair and they often can clean the starter switch without opening the motor case.

If the capacitor is not the problem and if the motor is sealed, whatever is wrong with the motor will require the thing to be opened up, something best left to a motor repair shop. Whether it is worth trying to fix the motor at all or not depends on what it would cost to replace it. In my part of the country at the time of this writing the minimum cost to open up a motor is about $100. The motor shop will always replace the bearings once they are in there. Starter switches are relatively cheap, but the windings are not, and most shops will not want to put the motor back together without new windings if the old ones are burnt even a little – even if they still work. So what it comes down to is that standard motors of less than about three HP are probably best replaced rather than attempting a repair that requires opening them up.

These motors are almost always in standard NEMA frame sizes and so replacements can be had from any place that sells AC motors. There are a number of online motor shops that can help you out. Get all of the info from the motor's ID plate before shopping for a replacement.

More Info on Motors

McMaster Carr has a nice, easy to understand page describing AC motors. Type “AC Motor” in the search box.

Leeson Electric has a good page on basic motor troubleshooting. Look under the Service & Support / Troubleshooting menu.