Motors come in all shapes and sizes but the ones we need are "single phase induction motors"

Determining the actual type of motor is the first step toward being able to test to see if it is being powered properly or if there is a fault in the motor itself.

Open frame motors in line operated appliances with a single coil off to one side are almost always shaded pole induction motors. To confirm, look for the copper 'shading rings' embedded in the core. There will usually be either 1 or 2 pairs of these. Their direction is determine by the orientation of the stator frame (position of the shading rings).

For enclosed motors, first check to see if there are carbon brushes on either side of a commutator made of multiple copper bars. If so, this is almost certainly a series wound 'universal' motor that will run on AC or DC though some may be designed for DC operation only.

If there are no brushes, then it is likely a split phase induction or synchronous motor. If there is a capacitor connected to the motor, this is probably used for starting and to increase torque when running.

Where there is a capacitor, it is likely that how this is wired to the motor determines the direction of rotation - make sure you label the connections!

Very small motors with enclosed gear reducers are usually of the synchronous type running off the AC line. Their direction of rotation is often set by a mechanical one-way clutch mechanism inside the casing.

Motors used in battery operated tools and appliances will usually be of the permanent magnet DC type similar to those found in toys and electronic equipment like VCRs and CD players. Most of these are quite small but there are exceptions - some electric lawnmowers use large versions of this type of motor, for example. These will be almost totally sealed with a pair of connections at one end. Direction is determined by the polarity of the DC applied to the motor.

For universal and DC permanent magnet motors, speed control may be accomplished with an internal mechanical governor or electronic circuitry internal or external to the motor. On devices like blenders where a range of (useless) speeds is required, there will be external switches selecting connections to a tapped winding as well as possibly additional electronic circuitry. The 'solid state' design so touted by the marketing blurb may be just a single diode! A similar approach may also be used to control the speed of certain types of induction motors (e.g., ceiling fans) but most are essentially fixed speed devices.

Once identified, refer to the appropriate section for your motor.

The Universal motor is the most common type of high speed motor found in appliances and portable line operated power tools. Typical uses include vacuum cleaners, floor polishers, electric drills, routers, and sewing machines. They are likely to be found anywhere medium power, high speed, and/or variable speed control are required capabilities. Note that quiet operation is NOT a feature of these motors. Therefore, they will not often be found in electronic equipment.

Construction consists of a stationary set of coils and magnetic core called the 'stator' and a rotating set of coils and magnetic core called the 'armature'. Incorporated on the armature is a rotating switch called a 'commutator'. Connection to the armature is via carbon (or metal) contacts called 'brushes' which are mounted on the frame of the motor and press against the commutator. Technically, these are actually series wound DC motors but through the use of steel laminated magnetic core material, will run on AC or DC - thus the name universal.

Speed control of universal motors is easily achieved with thyristor based controllers similar to light dimmers. However, simply using a light dimmer as a motor speed controller may not work due to the inductive characteristics of universal motors.

Changing direction requires interchanging the two connections between the stator and the armature.

This type of motor is found in blenders, food mixers, vacuum cleaners, sewing machines, and many portable power tools.

These motors can fail in a number of ways:

Test the field coils for continuity with an ohmmeter. An open winding is bad and will require replacement of the entire stator assembly unless the break can be located. Compare the resistance of the two windings - they should be nearly equal. If they are not, a short in one of the windings is likely. Again, replacement will be necessary.

An open or shorted armature 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 it 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. Any erratic readings may indicate the need for brush replacement or cleaning. 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.

A motor can be tested for basic functionality by disconnecting it from the appliance circuit and running it directly from the AC line (assuming it is intended for 115 VAC operation - check to be sure). CAUTION: series wound motors can overspeed if run without a load of any kind and spectacular failure may result due to centrifugal disassembly of the armature due to excess G forces. In other words, the rotor explodes. This is unlikely with these small motors but running only with the normal load attached is a generally prudent idea.

A commutator is essentially a rotating switch which routes power to the appropriate windings on the armature so that the interaction of the fixed (stator) and rotating (armature) magnetic fields always results in a rotational torque. Power is transferred to the commutator using carbon brushes in most motors of this type. The carbon is actually in the form of graphite which is very slippery as well. Despite that fact that graphite is a relatively soft material, a thin layer of graphite is worn off almost immediately as the motor is started for the first time and coats the commutator. After this, there is virtually no wear and a typical set of carbon brushes can last thousands of hours - usually for the life of the appliance or power tool.

A spring presses the brush against the rotating commutator to assure good electrical contact at all times. A flexible copper braid is often embedded in the graphite block to provide a low resistance path for the electric current. However, small motors may just depend on the mounting or pressure spring to provide a low enough resistance.

The typical universal motor will have between 3 and 12 armature windings which usually means a similar number of commutator segments. The segments are copper strips secured in a non-conductive mounting. There are supposed to be insulating gaps between the strips which should undercut the copper. With long use, the copper may wear or crud may build up to the point that the gaps between the copper segments are no longer undercut. If this happens, their insulating properties will largely be lost resulting in an unhappy motor. There may be excessive sparking, overheating, a burning smell, loss of power, or other symptoms.

Whenever checking a motor with a commutator, inspect to determine if the commutator is in good condition - smooth, clean, and adequately undercut. Use a narrow strip of wood or cardboard to clean out the gaps assuming they are still present. For larger motors, a hacksaw blade can be used to provide additional undercutting if needed though this will be tough with very small ones. Don't go too far as the strength of the commutator's mounting will be reduced. About 1/32 to 1/16 inch should do it. If the copper is pitted or worn unevenly, use some extra fine sandpaper (600 grit, not emery cloth or steel wool which may leave conductive particles behind) against the commutator to smooth it while rotating the armature by hand.

Since the carbon brushes transmit power to the rotating armature, they must be long enough and have enough spring force behind them to provide adequate and consistent contact. If they are too short, they may be unstable in their holders as well - even to the point of being ripped from the holder by the commutator causing additional damage.

Inspect the carbon brushes for wear and free movement within their holders. Take care not to interchange the two brushes or even rotate them from their original orientation as the motor may then require a break-in period and additional brush wear and significant sparking may occur during this time. Clean the brushes and holders and/or replace the brushes if they are broken or excessively worn.

Too bad that the Sears lifetime warranty only applies to hand (non-power) tools, huh?

Which part of the motor is bad? The armature or stator? How do you know? (A smelly charred mess would probably be a reasonable answer).

Rewinding a motor is probably going to way too expensive for a small appliance or power tool. Finding a replacement may be possible since those sizes and mounting configurations were and are very common.

However, I have, for example, replaced cheap sleeve bearings with ball bearings on a couple of Craftsman power drills. They run a whole lot smoother and quieter. The next model up used ball bearings and shared the same mounting as the cheaper sleeve bearings so substitution was straightforward.

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.

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.