Contents:
Where a fixed speed is acceptable or required, the single phase induction
motor is often an ideal choice. It is of simple construction and very
robust and reliable. In fact, there is usually only one moving part which
is a solid mass of metal.
Most of the following description applies to all the common types of
induction motors found in the house including the larger fractional
horsepower variety used in washing machines, dryers, and bench power
tools.
Construction consists of a stationary pair of coils and magnetic core called
the 'stator' and a rotating structure called the 'rotor'. The rotor is
actually a solid hunk of steel laminations with copper or aluminum bars running
lengthwise embedded in it and shorted together at the ends by thick plates.
If the steel were to be removed, the appearance would be that of a 'squirrel
cage' - the type of wheel used to exercise pet hamsters. A common name for
these (and others with similar construction) are squirrel case induction
motors.
These are normally called single phase because they run off of a single
phase AC line. However, at least for starting and often for running as
well, a capacitor or simply the design of the winding resistance and
inductance, creates the second (split) phase needed to provide the rotating
magnetic field.
For starting, the two sets of coils in the stator (starting and running
windings) are provided with AC current that is out of phase so that the
magnetic field in one peaks at a later time than the other. The net
effect is to produce a rotating magnetic field which drags the rotor
along with it. Once up to speed, only a single winding is needed though
higher peak torque will result if both windings are active at all times.
Small induction motors will generally keep both winding active but larger
motors will use a centrifugally operated switch to cut off the starting
winding at about 75% of rated speed (for fixed speed motors). This is
because the starting winding is often not rated for continuous duty
operation.
For example, a capacitor run type induction motor would be wired as shown
below. Interchanging the connections to either winding will reverse the
direction of rotation. The capacitor value is typical of that used with
a modest size fan motor.
1
Hot o------+------------+
| )||
| )|| Main winding
| 2 )||
Neutral o---+---------------+
| |
| | C1 3 C1: 10 uF, 150 VAC
| +----||------+
| )||
| )|| Phase winding
| 4 )||
+---------------+
Speed control of single phase induction motors is more complex than
for universal motors. Dual speed motors are possible by selecting
the wiring of the stator windings but continuous speed control is
usually not provided. This situation is changing, however, as the
sophisticated variable speed electronic drives suitable for induction
motors come down in price.
Direction is determined by the relative phase of the voltage applied
to the starting and running windings (at startup only if the starting
winding is switched out at full speed). If the startup winding is
disconnected (or bad), the motor will start in whichever direction
the shaft is turned by hand.
This type of motor is found in larger fans and blowers and other
fixed speed appliances like some pumps, floor polishers, stationary
power tools, and washing machines and dryers.
These are a special case of single phase induction motors where only a single stator winding is present and the required rotating magnetic field is accomplished by the use of 'shading' rings which are installed on the stator. These are made of copper and effectively delay the magnetic field buildup in their vicinity just enough to provide some starting torque. Direction is fixed by the position of the shading rings and electronic reversal is not possible. It may be possible to disassemble the motor and flip the stator to reverse direction should the need ever arise. Speed with no load is essentially fixed but there is considerable reduction as load is increased. In many cases, a variable AC source can be used to effect speed control without damaging heating at any speed. This type of motor is found in small fans and all kinds of other low power applications like electric pencil sharpeners where constant speed is not important. Compared to other types of induction motors, efficiency is quite poor.
Since their construction is so simple and quite robust, there is little to go bad. Many of these - particularly the shaded pole variety - are even protected from burnout if the motor should stall - something gets caught in a fan or the bearings seize up, for example. Check for free rotation, measure voltage across the motor to make sure it is powered, remove any load to assure that an excessive load is not the problem. If an induction motor (non-shaded pole) won't start, give it a little help by hand. If it now starts and continues to run, there is a problem with one of the windings or the capacitor (if used). For all types we have: * Dirty, dry, gummed up, or worn bearings - if operation is sluggish even with the load removed, disassemble, clean, and lubricate with electric motor oil. The plain bearings commonly used often have a wad of felt for holding oil. A add just enough so that this is saturated but not dripping. If there is none, put a couple of drops of oil in the bearing hole. * Open coil winding - test across the motor terminals with your ohmmeter. A reading of infinity means that there is a break somewhere - sometimes it is at one end of the coil and accessible for repair. For those with starting and running windings, check both of these. * Shorted coil winding - this will result in loss of power, speed, and overheating. In extreme cases, the motor may burn out (with associated smelly byproducts) or blow a fuse. The only way to easily test for a winding that is shorted to itself is to compare it with one from an indentical good motor and even in this case, a short which is only a few turns will not show up (but will still result in an overheating motor). * Coil shorted to the frame - this will result in excessive current, loss of power, overheating, smoke, fire, tripped breaker or overload protector, etc. If any of these faults are present, the motor will need to be replaced (or rewound if economical - usually not for typical appliance motors). The only exception would be if the location of the open or short is visible and can be repaired. They usually are not. For capacitor run type: * A bad capacitor may be the cause of a motor which will not start, has limited power, excessive hum, or overheats. A simple test with your ohmmeter on the high resistance scale can give some indication of whether the capacitor is good. remove at least one lead of the capacitor and measure across it. A good capacitor will show an initially low reading which will quickly climb to infinity. If there is no low reading at all or it remains low, then the capacitor is bad (open or shorted respectively). This does not really prove the capacitor is good but if the test shows open shorted, it is definitely bad. Substitution is best. For larger induction motors with centrifugal starting switches: * A centrifugal switch which does not activate the starting winding will result in a motor that will not start on its own but will run if it is rotated initially by hand. A centrifugal switch that does not cut off when the motor is up to speed will result in excessive power use, overheating, and may blow a line fuse or trip a circuit breaker. These are usually pretty simple and a visual inspection (may require disassembly) should reveal broken, worn, or otherwise defective parts. Check for proper switch contact closing and opening with a continuity tester or ohmmeter. Inspect the rotating weights, springs, and the sliding lever for damage. * Bad rotor - this is somewhat rare but repeated heating and cooling cycles or abuse during starting can eventually loosen up the (supposedly) welded connections of the copper bars to the end rings. The result is a motor that may not start or loses power since the required shorted squirrel cage has been compromised. One indication of this would be a rotor that is asymmetric - it vibrates or has torque at only certain large angular positions indicating that some of the bars are not connected properly. Normally, an induction motor rotor is perfectly symmetric.
The description below assumes that the construction is of an enclosure with an integral stator and brush holder. For those with an internal structural frame, remove the outer casing first. For the case of induction motors, ignore any comments about brushes as there are none. With shaded pole motors, the entire assembly is often not totally enclosed with just stamped sheet metal brackets holding the bearings. Follow these steps to minimize your use of 4 letter expletives: 1. Remove the load - fan blades, gears, pulleys, etc. If possible, label and disconnect the power wiring as well as the motor can them be totally removed to the convenience of your workbench. 2. Remove the brushes if possible. Note the location of each brush and its orientation as well to minimize break-in wear when reinstalled. Where the brushes are not easily removable from the outside, they will pop free as the armature is withdrawn. Try to anticipate this in step (6). (Universal motors only). 3. Confirm that there are no burrs on the shaft(s) due to the set screw(s) that may have been there. For motors with plain bearings in particular, these will need to be removed to allow the shaft(s) to be pulled out without damage to the bushing. For ball bearing motors, the bearings will normally stay attached to the shaft as it is removed. 4. Use a scribe or indelible pen to put alignment marks on the covers so that they can be reassembled in exactly the same orientation. 5. Unscrew the nuts or bolts that hold the end plates or end bells together and set these aside. 6. Use a soft mallet if necessary to gently tap apart the two halves or end bells of the motor until they can be separated by hand. 7. Remove the end plate or end bell on the non-power shaft end (or the end of your choice if they both have extended shafts). 8. Remove the end plate or end bell on the power (long shaft) end. For plain bearings, gently ease it off. If there is any resistance, double check for burrs on the shaft and remove as needed so as not to damage the soft bushing. 9. Identify any flat washers or spacers that may be present on the shaft(s) or stuck to the bushings or bearings. Mark down their **exact** location and orientation so that they may be replaced during reassembly. Clean these and set aside. Inspect all components for physical damage or evidence of overheating or burning. Bad bearings may result in very obvious wear of the shaft or bushings or show evidence of the rotor scraping on the stator core. Extended overloads, a worn commutator, or shorted windings may result in visible or olfactory detected deterioration of wire insulation. While it is apart, brush or blow out any built up dust and dirt and thoroughly clean the shaft, bushings, commutator, and starting switch (present in large induction motors, only). Relubrication using electric motor oil for plain bearings and light grease for non-sealed ball/roller bearings. CAUTION: cleanliness is absolutely critical when repacking bearings or else you will be doing this again very soon. Badly worn ball bearings will need replacement. However, this may be better left to a motor rebuilding shop as they are generally press fit and difficult to remove and install. Reassemble in reverse order. If installation of the brushes needs to be done before inserting the armature, you will need to feed them in spring end first and hold them in place to prevent damage to the fragile carbon. Tighten the nuts or bolts evenly and securely but do not overtighten.
Many motors have a wiring diagram on their nameplate. However, where this is
not the case, some educated guessing and experimentation will be necessary.
Here is an example for a common multispeed furnace blower motor. In this
case there is no capacitor and thus there are few unknowns.
" Here's the problem - I have a squirrel cage fan that I would like to wire
up. Unfortunately, there's only these four wires hanging there and I would
hate to burn it up trying combinations. Here's what I know:
* The motor came out of a furnace.
* It's marked with three amp ratings (4.5, 6.1, 7.5) - three speeds, right?
* The wires look like they were white, black, red and blue.
* With a ohm meter set on 200, I tried the following combinations:
White Black Blue Red
------------------------------------
White 0 1.5 2.2 2.9
Black 1.5 0 .7 1.3
Blue 2.2 .7 0 .7
Red 2.9 1.3 .7 0
So, how do I connect the motor?"
From the resistance readings, it would appear that the Black, Blue, and Red
are all taps on a single winding. My guess (and there are no warranties :-)
would be: White is common, black is HIGH, blue is MEDIUM, red is LOW.
I would test as follows:
Put a load in series with the line. Try a 250 W light bulb. This should
prevent damage to the motor if your connections are not quite correct.
Connect each combination of White and one other color. Start with black.
It should start turning - not nearly at full speed, however. If it does
turn, then you are probably safe in removing the light bulb.
Alternatively, if you have a Variac (variable autotransformer) of sufficient
ratings, just bring up the voltage slowly.
If it does not make any effort to start turning - just hums, go to plan B.
It may require a starting/running capacitor and/or not be a 3 speed motor.
These are constructed like small versions of universal motors except that the stator field is provided by powerful ceramic permanent magnets instead of a set of coils. Because of this, they will only operate on DC as direction is determined by the polarity of the input voltage. Small PM DC motors are used in battery or AC adapter operated shavers, electric knives, and cordless power tools. Similar motors are also used in cassette decks and boomboxes, answering machines, motorized toys, CD players and CDROM drives, and VCRs. Where speed is critical, these may include an internal mechanical governor or electronic regulator. In some cases there will be an auxiliary tachometer winding for speed control feedback. This precision is rarely needed for appliances. As noted, direction is determined by the polarity of the input power and they will generally work equally well in either direction. Speed is determined by input voltage and load. Therefore, variable speed and torque is easily provided by either just controlling the voltage or more efficiently by controlling the duty cycle through pulse width modulation (PWM). These motors are usually quite reliable but can develop shorted or open windings, a dirty commutator, gummed up lubrication, or dry or worn bearings. Replacement is best but mechanical repair (lubrication, cleaning) is sometimes possible.
These motors can fail in a number of ways: * Open or shorted windings - this may result in a bad spot, excess current drain and overheating, or a totally dead motor. * Partial short caused by dirt/muck, metal particle, or carbon buildup on commutator - this is a common problem in CD player spindle and cassette deck motors but not as common a problem with typical appliances. * Dry/worn bearings - this may result in a tight or frozen motor or a motor shaft with excessive runout. The result may be a spine tingling squeal during operation and/or reduced speed and power.
An open or shorted winding may result in a 'bad spot' - a position at which the motor may get stuck. Rotate the motor by hand a quarter turn and try it again. If it runs now either for a fraction of a turn or behaves normally, then replacement will probably be needed since it will get stuck at the same point at some point in the future. Check across the motor terminals with an ohmmeter. There should be a periodic variation in resistance as the rotor is turned having several cycles per revolution determined by the number of commutator segments used. Any extremely low reading may indicate a shorted winding. An unusually high reading may indicate an open winding or dirty commutator. Cleaning may help a motor with an open or short or dead spot as noted below. Erratic readings may indicate the need for cleaning as well. Also check between each terminal and the case - the reading should be high, greater than 1M ohm. A low reading indicates a short. The motor may still work when removed from the equipment but depending on what the case is connected to, may result in overheating, loss of power, or damage to the driving circuits when mounted (and connected) to the chassis. A motor can be tested for basic functionality by disconnecting it from the appliance circuit and powering it from a DC voltage source like a couple of 1.5 V D Alkaline cells in series or a DC wall adapter or model train power pack. You should be able to determine the the required voltage based on the battery or AC adapter rating of the appliance. If you know that the appliance power supply is working, you can use this as well.
If the carcass of the device or appliance is still available, the expected voltage may be determined by examining the original power supply - batteries, voltage regulator, wall adapter, etc. The following applies to the common DC permanent magnet (PM) motors found in tape players and cassette decks used for the capstan. * This motor may have an internal speed regulator. In that case, you can determine the appropriate voltage by using a variable supply and increasing it slowly until the speed does not increase anymore. This will typically be between 2 and 12 V depending on model. The motor should then run happily up to perhaps 50% more input voltage than this value. Note that many motors are actually marked with voltage and current ratings. Internal regulators may be electronic or mechanical (governor). One way to tell if there is an internal electronic regulator is to measure the resistance of the motor. If it is more than 50 ohms and/or is different depending on which direction the meter leads are connected, then there is an electronic regulator. Motors without internal speed regulators are used for many functions in consumer electronics as well as toys and small appliances. * If it does not have an internal regulator, typical supply voltages are between 1.5 and 12 V with typical (stopped) winding resistances of 10 to 50 ohms. Current will depend on input voltage, speed, and load. It *cannot* be determined simply using Ohms law from the measured resistance as the back EMF while running will reduce the current below what such a calculation would indicate. The wire color code will probably be red (or warm color) for the positive (+) lead and black (or dark cool) color for the minus (-) lead.
Dirt or grime on the commutator can result in intermittent contact and erratic operation. Carbon or metal particle buildup can partially short the motor making it impossible for the controller to provide enough voltage to maintain desired speed. Sometimes, a quick squirt of degreaser through the ventilation holes at the connection end will blow out the shorting material. Too much will ruin the motor, but it would need replacement otherwise anyway. This has worked on Pioneer PDM series spindle motors. Another technique is to disconnect the motor completely from the circuit and power it for a few seconds in each direction from a 9 V or so DC source. This may blow out the crud. The long term reliability of both of these approaches is unknown. WARNING: Never attempt to power a motor with an external battery or power supply when the motor is attached to the appliance, particularly if it contains any electronic circuitry as this can blow electronic components and complicate your problems. It is sometimes possible to disassemble the motor and clean it more thoroughly but this is a painstaking task best avoided if possible. See the section: "Disassembling and reassembling a miniature PM motor".
Note: for motors with carbon brushes, refer to the section: "Disassembling and reassembling a universal or induction motor". This procedure below is for those tiny PM motors with metal brushes. Unless you really like to work on really tiny things, you might want to just punt and buy a replacement. This may be the strategy with the best long term reliability in any case. However, if you like a challenge, read on. CAUTION: disassembly without of this type should never be attempted with high quality servo motors as removing the armature from the motor may partially demagnetize the permanent magnets resulting in decreased torque and the need to replace the motor. However, it is safe for the typical small PM motor found in appliances and power tools. Select a clean work area - the permanent magnets in the motor will attract all kinds of ferrous particles which are then very difficult to remove. Follow these steps to minimize your use of 4 letter expletives: 1. Remove the load - fan blades, gears, pulleys, etc. Label and disconnect the power wiring as well as the motor will be a whole lot easier to work on if not attached to the appliance or power tool. Note: polarity is critical - take note of the wire colors or orientation of the motor if it is directly soldered to a circuit board! 2. Confirm that there are no burrs on the shaft(s) due to the set screw(s) that may have been there. For motors with plain bearings in particular, these will need to be removed to allow the shaft(s) to be pulled out without damage to the bushing. 3. Use a scribe or indelible pen to put alignment marks on the cover so that it can be replaced in the same orientation. 4. Make yourself a brush spreader. Most of these motors have a pair of elongated holes in the cover where the power wires are connected to the commutator. These allow the very delicate and fragile metal brushes to be spread apart as the armature is removed or installed. Otherwise, the brushes will get hung up and bent. I have found that a paper clip can be bent so that its two ends fit into these holes and when rotated will safely lift the brushes out of harm's way. 5. Use a sharp tool like an awl or dental pick to bend out the 2 or 3 tabs holding the cover in place. 6. Insert the brush spreader, spread the brushes, and pull the cover off of the motor. If done carefully, no damage will be done to the metal brushes. 7. The armature can now be pulled free of the case and magnets. 8. Identify any flat washers or spacers that may be present on the shaft(s). Mark down their **exact** location and orientation so that they may be replaced during reassembly. Clean these and set aside. Inspect all components for physical damage or evidence of overheating or burning. Bad bearings may result in very obvious wear of the shaft or bushings or show evidence of the rotor scraping on the stator core. Extended overloads, a worn commutator, or shorted windings may result in visible or olfactory detected deterioration of wire insulation. Check that the gaps in the commutator segments are free of metal particles or carbonized crud. Use a sharp instrument like an Xacto knife blade to carefully clear between the segments. Clean the brushes (gentle!), shafts, and bushings. When reassembling, make sure to use your brush spreader when installing the cover.
These are a variation on the small DC motors described above and uses a rotating permanent magnet and stationary coils which are controlled by some electronic circuitry to switch the current to the field magnets at exactly the right time. Since there are no sliding brushes, these are very reliable. DC brushless motors may be of ordinary shape or low profile - so called pancake' style. While not that common in appliances yet, they may be found in small fans and are used in many types of A/V and computer equipment (HD, FD, and CD drives, for example). Fortunately, they are extremely reliable. However, any non-mechanical failures are difficult to diagnose. In some cases, electronic component malfunction can be identified and remedied. Not that common in appliances but this is changing as the technology matures. Direction may be reversible electronically (capstan motors in VCR require this, for example). However, the common DC operated fan is not reversible. Speed may be varied over a fairly wide range by adjusting the input voltage on some or by direct digital control of the internal motor drive waveforms. The most common use for these in appliances are as small cooling fans though more sophisticated versions are used as servo motors in VCRs and cassette decks, turntables, and other precision equipment.
This is the type you are likely to encounter - modify this procedure for other types. 1. Remove the fan from the equipment, label and disconnect the power wires if possible. 2. Remove the manufacturer's label and/or pop the protective plastic button in the center of the blade assembly. Set these aside. 3. You will see an E-clip or C-clip holding the shaft in place. This must be removed - the proper tool is best but with care, a pair of fine needlenose pliers, narrow screwdriver, dental pick, or some other similar pointy object should work. Take great care to prevent it from going zing across the room. 4. Remove the washers and spacers you find on the shaft. Mark down their positions so that they can be restored exactly the way you found them. 5. Withdraw the rotor and blades from the stator. 6. Remove the washers and spacers you find on the shaft or stuck to the bushings. Mark down their positions so that they can be restored exactly the way you found them. For fans with plain bearings, inspect and clean the shaft and the hole in the bushing using a Q-tip and alcohol or WD40 (see there is a use for WD40!). Check for any damage. Lubricate with a couple drops of electric motor oil in the bushing and any felt pads or washers. For fans with ball bearings, check the bearings for free rotation and runout (that they do not wobble or wiggle excessively). If bad, replacement will be needed, though this may not be worth the trouble. These are generally sealed bearings so lubrication is difficult in any case. On the other hand, they don't go bad very often. Reassemble in reverse order.
Miniature synchronous motors are used in mechanical clock drives as found in older clock radios or electric clocks powered from the AC line, appliance controllers, and refrigerator defrost timers. These assemblies include a gear train either sealed inside the motor or external to it. If the motor does not start up, it is probably due to dried gummed up lubrication. Getting inside can be a joy but it is usually possible to pop the cover and get at the rotor shaft (which is usually where the lubrication is needed). However, the tiny pinion gear may need to be removed to get at both ends of the rotor shaft and bearings. These consist of a stator coil and a magnetic core with many poles and a permanent magnet for the rotor. (In many ways, these are very similar to stepper motors). The number of poles determines the speed precisely and it is not easily changed. Direction is sometimes determined mechanically by only permitting the motor to start in the desired direction - they will usually be happy to start either way but a mechanical clutch prevents this (make note of exactly how is was positioned when disassembling). Direction can be reversed in this manner but I know of no actual applications where it would be desirable. Others use shading rings like those in a shaded pole induction motor to determine the direction of starting. Speed, as noted, is fixed by construction and for 60 Hz power it is precisely equal to: 7200/(# poles) RPM. Thus, a motor with 8 poles will run at 900 RPM.
The best approach is usually replacement. In some designs, just the rotor and gear unit can be replaced while retaining the stator and coils. However, if your motor does not start on its own, is sluggish, or squeals, cleaning and lubrication may be all that is needed. However, to get to the rotor bearing requires removal of the cover and in most cases the rotor as well. This may mean popping off a press-fit pinion gear. 1. Remove the motor from the appliance and disconnect its power wires if possible. This will make it a lot easier to work on. 2. Remove the cover. This may require bending some tabs and breaking an Epoxy seal in some cases. 3. Inspect the gears and shafts for gummed up lubrication. Since these motors have such low torque, the critical bearing is probably one for the main rotor. If there is any detectable stiffness, cleaning and lubrication is called for. 4. You can try lubricating in-place but this will usually not work as there is no access to the far bearing (at the other end of the shaft from the pinion gear). I have used a small nail or awl to pop the pinion gear from the shaft by gently tapping in the middle with a small hammer. 5. Withdraw the rotor from the motor. 6. Identify any flat washers or spacers that may be present on the shaft. Mark down their **exact** location and orientation so that they may be replaced during reassembly. Clean these and set aside. Inspect and clean the shaft and bushings. Lubricate with electric motor oil. Reinstall the rotor and washers or spacers. Then press the pinion gear back onto the shaft just far enough to allow a still detectable end-play of about .25 to .5 mm. Check for free rotation of the rotor and all gears. Replace the cover and seal with household cement once proper operation has been confirmed.
A dry or worn bearing can make the motor too difficult to turn properly or introduce unacceptable wobble (runout) into the shaft as it rotates. Feel and listen for a dry bearing: The shaft may be difficult to turn or it may turn with uneven torque. A motor with a worn or dry bearing may make a spine tingling high pitched sound when it is turning under power. A drop of light machine oil (e.g. electric motor oil) may cure a dry noisy bearing - at least temporarily. Runout - wobble from side to side - of a motor shaft is rarely critical in a small appliance but excessive side-to-side play may result in noise, rapid bearing wear, and ultimate failure.
If the noise is related to the rotating motor shaft, try lubricating the motor (or other suspect) bearings - a single drop of electric motor oil, sewing machine oil, or other light oil (NOT WD40 - it is not a suitable lubricant), to the bearings (at each end for the motor). This may help at least as a temporary fix. In some cases, using a slightly heavier oil will help with a worn bearing. See the section: "Lubrication of appliances and electronic equipment". For AC motors in particular, steel laminations or the motor's mounting may be loose resulting in a buzz or hum. Tightening a screw or two may quiet it down. Painting the laminations with varnish suitable for electrical equipment may be needed in extreme cases. Sometimes, the noise may actually be a result of a nearby metal shield or other chassis hardware that is being vibrated by the motor's magnetic field. A strategically placed shim or piece of masking tape may work wonders.Go to [Next] segment
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