Inspecting an AC Motor

I got another single phase AC motor for free.  This one is an older GE single phase dual voltage 1hp motor.  The cradle mount is missing and I’ll have to deal with that later.  Currently, I’d like to check it out to see if it will run.  Here’s a picture of the motor.  It was missing the cover over where the wires connect and I made a new one from some scrap.

Sure, I could just hook the motor up and flip the switch but that’s not the best approach.  You could damage the motor or shock yourself if something is wrong.  Both are not desirable. There are a few checks that can be done first before trying to run the motor.

The first is a simple mechanical test.  Does the motor spin freely?  On a single phase motor, you will hear a slight rubbing from the centrifugal switch but other than that the armature should spin easily.  Are there any nuts or bolts missing from the front or back.  Usually the ends of the motor are held on with long bolts that go from one end of the motor to the other.  There should be a nut on the end of each bolt.  Sometimes the end bells are directly bolted to the middle of the motor.  Either way, make sure the end bells are attached to the rest of the motor.

After the brief mechanical inspection is done, it’s time to move on to an electrical inspection with a multimeter.  Most of this can be done without disassembling the motor but I’m going to for clarity.  Disassembling this motor is pretty simple.  First, the nuts are removed from the long bolts to allow the bolts to be removed.  Next, the end bells are removed by tapping them off with a hammer.  This motor has little pockets on the end bells where a screwdriver can be placed to tap the end bells off.  You’ll want to mark the orientation of the end bells to the body of the motor before removal.

Sometimes with an open motor you’ll want to take apart the motor just to clean it out.  Dirt and other gunk can get sucked into the motor which can cause overheating.  Keep an eye out for any shims that may be in the motor.  You’ll want to put the mall back in in the same orientation as they came out.

In theory an induction motor is a pretty simple device.  There are loops of wire called windings that AC voltage passes through that are fixed to the body of the motor known as the stator.  In the middle are more loops of wire on the rotor which turns.  The AC voltage in the stator creates a rotating magnetic field that interacts with the rotor causing it to spin.  A single phase motor won’t start on its own.  One way to start one is to add some more windings (start windings) on the stator in series with a capacitor.  The capacitor shifts the phase of the current in these start windings which helps start the rotor turning.  The start windings and start capacitor will burn up if left running though.  To remove them from the circuit, a centrifugal switch is used which switches when the motor comes up to speed.  Sometimes single phase motors have a second capacitor called a run capacitor.  It’s not the same as a start capacitor and does keep working when the motor is running.

On my motor there is also an over overload device that will shut the motor down if it overheads or draws too much current.  It is a normally closed switch which opens if a small piece of metal becomes too hot.

This motor is currently set up for 115V though it can be setup to run on 230V.   The voltage is selected by moving some small metal links which change the way the windings are connected.  For 115V the windings are wired in parallel and for 230V the windings are wired in series.   Here’s a little drawing of the wiring in the motor.  The ovals with two dots represent the metal links for this motor.  With the diagram, you can see multiple things can be tested such as continuity through both windings and the start winding, operation of the centrifugal switch, continuity through the overload devices, and the capacitor.  Note that any  resistance values are specific to my motor and probably don’t apply to others.Here’s a view of the end of the stator with all the interesting bits.  The black circular object is the overload device.  The wishbone looking thing is the electrical part of the centrifugal switch.  Finally, there’s all the wires, metal links, and posts for the motor line connections.

The first electrical test, is to check for continuity through the windings.  This is done by checking the resistance with the meter hooked up to the two line connection posts.  With the windings wired in parallel there’s a possibility that one of the windings is burned out.  To check, the metal links would need to be switched to the 230V setting and continuity checked again.  Alternatively, if you have a diagram of the motor, each winding can be checked separately.  I look for a low, but not zero, resistance value.  An open would indicate a failed winding or bad connection.

Next, is to check for continuity between the windings and the motor casing.  This is done by checking continuity between one line connection post and the casing.  You’ll need to find bare metal on the casing such as a screw hole or screw threads to connect to the motor casing.  For this test, I expect an open result otherwise the winding is contacting the motor casing which should trip a breaker or shock you.

The electrical part of the centrifugal switch is a simple switch.  When the motor is stopped the switch should be closed and opens when the motor is up to speed.  On my motor, without the mechanical part of the centrifugal switch in place the electrical switch is open.  I can close it by gently pushing on the wishbone.  If it doesn’t respond as expected check for loose wires or dirty contacts. The mechanical part of the centrifugal switch consists of some spring loaded weights that fly out when up to speed pulling the plastic piece down on my motor.  It should be checked to make sure it opens freely though not effortlessly by pushing down on the plastic piece.

On a capacitor start motor, the capacitor is usually on the outside of the motor under a metal cover.  Before you start messing with the capacitor be sure it is drained by putting a good sized resistor across it or connecting the two posts with a piece of metal that you’re insulated from such as a screwdriver.    Ideally, you’d check the capacitor with a capacitance checker but not everyone has one of those.  If you don’t there are a few makeshift tests you can do.  After the capacitor has been drained, disconnect and remove it.  A multimeter can be used to measure the resistance of the capacitor.  You shouldn’t get a low or open reading.  On mine I got around 14 megaohms.  Another way to test is to drain the capacitor and check the voltage.  It should read zero.  Then hook a 9V battery up to the capacitor for around 10 seconds.  Remove the battery and check the voltage across the capacitor.  You should see that the voltage is no longer zero and somewhere near 9V.  Mine was around 7V and slowly decreasing.  If you short the capacitor you should see a weak spark.  The results indicate the capacitor should probably work.  These kinds of tests are about all you can do without a capacitance tester and none check the capacitor at the kind of voltage it will operate at.

The overload device on my motor can also be checked for continuity.  It has three posts and should show continuity between all three sets of two wires.  If it does, it will let the motor run.  I suppose it could fail open or closed but there is no way I can think of to check the operation of it without specialized equipment.

I did some of the tests before opening up my motor and thought it should run.  I plugged it in and it ran.  It operated as expected but the bearings were noisy.  I ordered some replacement bearings and pulled the old ones.

The new bearings were pressed on with my arbor press. After that I did some more of the tests covered above before putting the motor back together. 

If you have a single phase capacitor start motor that won’t start but had previously been running well, there’s another simple test you can do.  Remove the load from it and turn it on.  If it still doesn’t start, quickly turn it off.  Find a piece of string and wrap it around the motor shaft.  Quickly pull the string off to get the motor shaft spinning and turn the motor on again.  If it starts up it’s probably the capacitor as the centrifugal switch isn’t likely to work fine one day and fail the next.  Obviously, don’t wrap the string around yourself or try to start the motor before the string is free of the shaft.

There are other tests that can be done on motors but hopefully these simple ones will help you get your motor running.



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One Response to Inspecting an AC Motor

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