Introduction: Make an Electric Motor Run Again

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first to…

You have an electric motor designed to run on alternating current, but it does not run. What can you do yourself? This Instructable concerns a triage procedure for getting your capacitor start motor running again in a series of easy and logical steps.

I am assuming you have a single phase 1/4 or 1/2 horsepower motor, even possibly up to 1 horsepower. It may have one capacitor, but there may also be two. The motor's design may be capacitor start, or capacitor run. These types of motors are what you will find in most home and workshop applications. Special purpose industrial motors (DC, three phase) will have some differences and I still know nothing about them.

The most common cause of HVAC failure during the hot summer is a motor capacitor. (Your air conditioner does not run, but just hums with a buzzing sound.) Knowing that, you can use what you find in this Instructable to check your system and replace the capacitor even before it fails.

Note: This Instructable is written to share some helpful things I learned, as well as some tools and resources you will find helpful. The purpose is to give the reader enough help to make some careful tests and solve some basic problems. Please do not ask me to diagnose your motor from a distance and tell you how to fix it. I am not equipped to do that. Get a book like the one I bought (see step 6), seek out some articles on the Internet, and watch some videos. Then make some careful tests in a logical order. Reason things out. Be prepared to renounce your assumptions. Be patient. You may get your motor running again, too.


  • Bearing grease
  • 16-3 power cord and plug (if needed to replace an old cord)


  • Sharply pointed round prying tool (for lifting bearing seals)
  • Hammer, punch, and flat surface (for smoothing bearing seals before reinstalling)
  • Soldering gun and solder
  • Screwdrivers
  • Nut drivers
  • Volt-Ohmmeter
  • Capacitance meter (optional, but nice to have--See step 5.)

Step 1: Spin by Hand

Turn the motor shaft by hand. Does it turn freely enough to run? There will always be a little drag, but the shaft should spin relatively freely.

Still, an older motor may have bearings with dried grease. If the bearings are not rusted inside, you can probably work new grease into them. and improve them greatly. I sometimes gently pry a bearing seal off of one side, work some bearing grease into them, undo any damage to the bearing seal, and tap it back into place. It is not a recommended procedure, but I figure it is suitable for a motor I use only occasionally in my workshop.

The bearings may also be bronze sleeve bearings. At the worst, you may need to go to a bearing shop and buy a match for a badly stuck ball bearing, but those are not cheap. Hopefully, your motor will run like new again with no or very little money spent.

Step 2: Add Power for a Few Seconds

What happens when you apply power to the motor? Hopefully, there are no sparks or shocks from the motor frame, and the circuit breaker does not immediately trip. If you have any of those things, look for frayed wires in the power cord touching another frayed wire or shorting to the iron frame of the motor.

Apply power for only a couple of seconds. Coil windings inside the motor quickly heat up on a motor that does not run and the coils can be destroyed. Then the motor needs to be rewound to work again. Chances are the motor made a growling noise when you added power. It is also probable that the starting windings are not working properly, but there could be more than one reason for that.

The graphic shows the circuit diagram for this type of motor. The start winding portion is energized for only a couple of seconds. Then the centrifugal switch locks out the components of the starting winding circuit. In the next steps tests will be done to eliminate probable causes.

The second photo shows the motor running while connected to an AC Ammeter. The name plate on the motor says it runs with a current draw of 7.3 Amps. The meter is set to the 15 Amp. range. The actual reading is just a little below 7 Amps. Taking such a reading while the motor is actually running confirms that all is well. This photo was made after the motor was fully functioning as it should. An earlier reading when the starting winding was not working as it should pegged the needle, which also indicated a problem.

Step 3: Check for Shorts and Opens

Unplug the motor from its power source. Access the connections between the power cord and the motor windings. Remove the junction box cover plate, if there is one. (Make some photos showing the connections so you can make them correctly again, if you need to take them apart.) If there is no junction box on the motor, remove the end of the motor where the power cord connects to the windings.

Also, bridge the two terminals of the capacitor with a screwdriver to remove any charge that could shock you. Remove the wire from one side of the capacitor to prevent spurious readings caused by current passing through a back channel.

Use an Ohmmeter. Many suggest a setting around 2000 Ohms. Connect one lead to the iron frame of the motor. Connect the other lead to each of the wires coming out of the motor, including those that connect to the capacitor. All readings should indicate open circuits (no pathway for electricity). If any of the readings indicate a pathway for electricity, double check to be certain you were getting an accurate reading. If you still find evidence of a pathway for electricity, one of the windings is shorted to the motor frame. This could indicate the motor needs to be rewound. But, first do a visual inspection to check for a simpler explanation. Once I snagged a thin enamelled wire from one of the windings when I was inserting the rotor. The wire got a scrape that left it bare in one spot. Then it was pushed close to the iron frame of the motor where it shorted and showed a leak pathway. I was able to coat the wire with an insulating liquid and push it out of the way.

You have checked for shorts. Now check for opens. Follow the wires as best you can. The starting windings use a thinner wire than the running windings. Use the Ohmmeter to check for breaks in both windings. The resistance on each winding will be relatively low, but the starting winding will show a little higher resistance. If one of the windings shows an open circuit, look closely at where the wires attach to a terminal or to a heavier wire. Such junctions are the most common places for an open junction to happen. (I once repaired a "dead" Dremel tool. There was no indication of an open circuit in any of the armature coils. But, the field coil showed an open circuit between the two terminal pins. I looked closely and saw a break where the coil wire attached to one of the terminal pins. I scraped enamel from the wire as best I could and made a solder bridge to the terminal pin. It was not a perfect solder joint, but the Dremel worked again.) If you are not able to find the break in the coil circuit and repair it, the motor will need to be rewound.

Check with an Ohmmeter to be certain the high temperature protection reset provides and electrical path through it and that it is not broken so as to open the circuit and keep the motor from running.

Step 4: The Centrifugal Switch

Capacitor start electric motors use a starting coil and a capacitor to create an advancing magnetic field in the stator (outer frame of the motor with its coils). This advancing magnetic field gives the rotor something to chase, causing the rotor to spin. (For example, put a bar magnet on top of a glass table. Bring another bar magnet up to it from under the table. Move the magnet under the table. The magnet above the table chases the magnet under the table in a crude form of an electric motor. The advancing magnetic field in the stator replaces the movement of your hand under the table.) That starting coil will be destroyed if it remains powered up in the circuit more than a very few seconds. Most motors use a centrifugal switch to disconnect the starting winding from the circuit as soon as the motor shaft builds most of its speed.

See the first graphic. It is really not possible to make a photo of the centrifugal switch in all of its parts as it is in place inside the motor, so I made a side view drawing that shows the relationship of the parts. See the text boxes.The centrifugal switch normally presents a closed circuit, if it is working properly. In an older motor fully assembled the switch's contacts may have become pitted and burned, or its parts may be worn so that there is not enough pressure to close the contacts when the motor is not running or is first starting up. Look closely at your motor and find the wires that lead to the stationary switch contacts. (One may go to the capacitor and one may be on a post in the junction box.) Connect an Ohmmeter to the contact points wires. Is there an indication of an electrical path through the switch contacts? If not, open the motor and try dragging some very fine sandpaper through the contact points to clean and polish both.

If the switch is worn, the plastic spool on the motor shaft may not be exerting enough pressure on the plate that holds the contacts for them to close fully. See the second photo. I tested my switch contacts with the motor fully assembled. I inserted a screwdriver through an opening on the front of the motor and gently applied a little pressure toward the end of the shaft (away from the motor) to close the contacts while connected to an Ohmmeter.

Be sure the rotor is properly centered end-to-end in the motor. Thrust washers may be used to move it a little, if there is space for the washers.) I made a different washer from 16 gauge steel sheet to fit on the end of the spool. The contacts on my motor now close and the motor runs. But, I also want to weld tabs to it that will fold around the end of the spool so I am certain the washer retracts when the centrifugal weights pull the spool away from the stationary contacts. I do not want that washer to float into the contacts and inadvertently press them closed while the motor is running. That could overheat and destroy the starting windings. (I looked again at my book [See step 6.]. Another option is to put a spacer under the stationary part of the centrifugal switch to bring it nearer to the spool.)

It may be possible to buy a new stationary portion of a centrifugal switch, although they are not all the same. I have not seen any listed anywhere. It is also possible to buy an electronic switch to replace the centrifugal switch. An electronic switch does not respond to the speed of the motor shaft like a centrifugal switch does, but has a timed disconnect. It allows the starting windings to be engaged for 7.4 seconds and then breaks the circuit. Electronic switches cost a bit more than $40.

Step 5: The Capacitor

It is not uncommon for an electrolytic capacitor to dry out and fail in audio equipment after 20 years or less. But, replacing a start capacitor without first checking for shorted or open windings, an open reset, and a faulty centrifugal switch will not make your motor run, if the capacitor is not really your problem.

Many motors have a domed cover on the outside of the motor, and the capacitor is under it. Motor capacitors are usually a cylinder with terminals on top. But, some capacitors in older motors may also be flat, like a short stack of 4 x 6 index cards. These may be located in the base of the motor so that outward appearances make it seem the motor has no capacitor.

A capacitor may bulge or leak when it is failing. It may even split open. But, it may also look perfectly normal. There are various test procedures for capacitors, but those tests are not foolproof. A capacitor can pass several tests and still fail under a load.

If you have not done so already, use a screwdriver to short any residual charge in your motor's capacitor. Do this a couple of times, just to be safe.

If your capacitor definitely needs to be replaced, copy the voltage and capacitance numbers hopefully still legible on it. You can always use a replacement capacitor rated for a higher voltage than your motor's original capacitor, but the capacitance figures should match as closely as possible. So, at 230 volt AC capacitor can replace a 125 volt AC capacitor. The capacitance will give a range, like 220 microfarad to 260 microfarad. A capacitor rated 210 microfarad to 250 microfarad should be close enough to work well. (If you see ratings in millifarads, 1 millifarad equals 1000 microfarad.)

Here are some ways to test your capacitor. Choose those that fit what you have available.

Procedure A--With at least one wire removed from the capacitor and no power to the motor circuit, attach an Ohmmeter to both terminals of the capacitor. An analog meter is preferred, but not mandatory. The reading should rise to a high number and drop suddenly to zero or an open circuit. If there is a steady reading of some value, the capacitor is shorted. If the reading does not rise initially, something inside the capacitor is broken and there is an open circuit.

Procedure B--Remove both wires from the capacitor. Connect it to lamp cord and in series with an incandescent light bulb about 60 Watts in size. Connect it to a wall outlet. The bulb should light, although may be dimmer than usual.

Procedure C--You can get a meter that reads the capacitance value of a capacitor here for less than $20 plus shipping. The tests above give you an idea about whether a capacitor is working, but give you no hint about the actual capacitance of the capacitor. (A dried out electrolytic capacitor may appear good, but its capacitance is too low to start the motor.) A meter changes that. Search Instructables for capacitance meter circuits. At least one uses an Arduino module. About 25 years ago I had an electronics magazine with a home built circuit for a capacitance meter based on a 555 IC. (Here is a similar device you can make.) I now have a digital multimeter with capacitance reading. Some capacitance meters use a high frequency signal generator that is part of the meter. These can be used "in circuit" and give an accurate reading without feedback through other parts of the circuit.

Capacitors can give good readings on a meter and still be weak or failed. An ESR meter measures internal resistance that effects actual performance.

Procedure C'--The book mentioned in the next step provides another test. It involves measuring the current flow (amperage) used by the motor when it is powered up. A mathematical formula tells how many microfarads your capacitor yields, given the parameters of the test. It is helpful because it is a test under load.

Procedure D--It is not always feasible to buy several pieces of test equipment you may not use more than once or twice. If all else (shorts and opens, centrifugal switch, reset, etc.) checks out in your motor and the capacitor gives indications it should be good, but the motor still does not run, a new capacitor is $10 to $20 shipped to your door. At the worst, you will be out a relatively small amount of money, and it may be your capacitor has a flaw that does not show itself in the tests you are able to conduct. At the best, the motor may run.

When finished, restore the connections to the capacitor, either the old one or a new one.

Step 6: Personal

I looked at price and content descriptions before deciding to buy this book. (I do not benefit in any way if you buy the book, nor do any of my friends or relatives.) It has been very helpful. Where something lacked enough detail for me I went on-line and looked for additional material or for videos at YouTube. The book is now drenched in ink from my underlinings and marginal notes. I bought the book because some people I know somehow think I know enough to help them with their electric motor problems.

I have the 61 year old 1/2 HP Craftsman single phase capacitor start motor shown in many of the photos. (There is a 10 56 [October 1956] stamp on the motor plate.) It worked when I bought it at a used tool sale about two years ago, but the ball bearings sounded rough when the shaft was turned by hand. I opened the motor and packed the bearings with new grease. I smoothed out the seals as best I could and put them back. The motor had lots of fine sawdust packed inside. I cleaned that out. The motor shaft spun freely and smoothly when turned by hand with no power.

When I assembled the motor and put power to it, the shaft turned about 1/8 of a turn, froze, and growled. I checked for shorts and opens, but the coils are fine. I am always suspicious of older capacitors, and this one is 61 years old. I did several tests of the capacitor. It passed every one of them. I was still absolutely certain the capacitor is faulty.

I slept on the problem. I realized I made an assumption the centrifugal switch is working. I had oiled the weights so they move freely. I cleaned so the spool slides freely. When I connected an Ohmmeter to the wires that go to both sides of the stationary part of the centrifugal switch, the contact points did not provide an electrical pathway, even though I had cleaned and polished them with fine sandpaper. I inserted a screwdriver at an angle from the front of the fully assembled motor. I was able to nudge the contact plate a little with the screwdriver. Suddenly my Ohmmeter indicated a good path for electricity. I had seen a couple of wear spots on the contact plate where the spool rubs on it. I made a washer to act as a spacer between the contact plate and the spool. My motor purrs like a contented kitten. But, the washer could move to put pressure on the centrifugal switch contacts and engage the starting winding while the motor is running. That could cause the starting winding to burn out. I checked my book and another option is to add a spacer between the motor frame and the stationary part of the centrifugal switch. I finally did that, using a common washer for each mounting screw, and I removed the washer I had made.

After 61 years the rubber covered cord has several cracks. I bought new cord. The old cord was two wire. I bought three wire and will connect the ground wire to the motor frame to bring it up to present day standards..

I have had quite a bit of experience with household wiring over most of my life, but electric motors were still a mystery beyond a little very basic understanding. The book I bought has filled in the gaps. I now realize some logical basic testing and thoughtful deduction will help me to solve the basic motor problems I can expect to see.

Make It Move Contest 2017

Runner Up in the
Make It Move Contest 2017