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Motors Are Made To Move

Motors which the layman generally deals with are luckily of simple design and accessible construction—here's what you have to know
 
It is fortunate that motors of rather simple design and accessible construction are used in the labor-saving appliances that get the most service in a home . . . and therefore are most likely to need attention. Vacuum cleaners, portable drills, fans, mixers, juicers, sewing machines, hair driers, etc., generally are powered by "universal" motors that work equally well on AC or DC. They are easy to identify by the presence of two "brush holders," diametrically mounted on the motor frame close to one end. These holders have slotted caps, which, when screwed open with a coin or a screwdriver, spring out to reveal small bars of carbon. The latter bear against a series of copper strips on the rotating shaft, called the "commutator." The carbon brushes being relatively soft and the commutator relatively hard, the former are bound to wear out eventually. Sometimes they shatter when the appliance is accidentally dropped, a not unlikely occurrence with portable machines.

The first step in inspecting a vacuum cleaner motor is to get to it; this is accomplished with the average tank type (such as this GE) by removing six bottom screws.

With the bottom pan off the vacuum cleaner, the switch becomes accessible; to facilitate checking with the Handitester, disconnect two motor wires from the main body of the cleaner. Some connectors at this point are of the screw type; others must be twisted off.

The switch which controls the motor of the vacuum cleaner is held by one hex nut; it is simple to dismount for testing purposes. Next step, below.
 
Blower motor is concealed inside a perforated cover in the main body of the cleaner. Remove holding screws, lilt cover carefully. See next step below.

Switch can now be checked quickly for opens or shorts; switch usually suffers physical rather than electrical damage. Replacement should be strong.
 
The motor is mounted on a sponge rubber ring; with the cover off and the wires disconnected from switch, the whole unit lifts out in one solid piece.

Carbon brushes have been a staple in hardware and electrical supply stores for more than half a century. There must now be hundreds of different sizes, in round, square and rectangular cross section. It's helpful to have a segment of an old brush when buying a replacement, but if you haven't, don't worry. Get slightly oversize brushes, and shape them to size by rubbing them gently on a piece of fine sandpaper or emery cloth.

Because AC has replaced DC power in virtually all American homes, the ability of a "universal" motor to work on DC is now purely incidental. The main importance of the "AC-DC" notation on an appliance's name plate is that it tells you positively that the motor is of the basic brush-commutator type and not of one of the more complicated straight AC types.

Universal motors represent an economical way of obtaining medium mechanical power in small packages. They start quickly, and develop very high speeds. Turn on a vacuum cleaner or a portable drill, and it's screaming away at several thousand revolutions per minute before you can take your hand away from the switch. The speed, however, is usually pretty sensitive to the load. It's fairly steady with a fixed load, such as the blades of a fan, the blower of a vacuum cleaner, etc.; but it drops when the machine or tool has to do work at an increased rate. For example, you've probably noticed that a portable drill bites into wood with only a slight reduction in speed, but groans noticeably when cutting metal or hard plastics.

The brush holders on top of the motor frame are insulated by fiber strips; unhook one end, and screw holding brush in place becomes visible; remove with small screwdriver, hold hand over end to catch brush.
 
Deliberate speed control is easy with AC-DC motors, and requires only the use of a variable or adjustable wire-wound resistor connected in simple series with the power line. This resistor is usually called a "rheostat". The commonest example is the foot treadle for speed control of sewing machines. This is nothing more than a resistor of about 150 ohms, adjustable in five or six steps from this maximum value (lowest motor speed) to zero (highest speed).

The Rheostat

The addition of an external control rheostat to a small fan (10-inch size or under) greatly increases the usefulness of the machine for both summer and winter purposes. Throttled down to about one-third speed, the fan does a fine job of breaking up the stale air found in many rooms equipped with window type air conditioners. Aim the blades toward the ceiling, where the naturally lighter hot air accumulates. During the heating period, aim the fan directly at the biggest radiator, and note how much more comfortable the room becomes. Remember, a fan doesn't "cool"; it can only move air, and moving cool air and moving warm air are equally important in season.

Rheostats in suitable sizes are sold by radio and surplus firms. The maximum resistance should be between 150 and 200 ohms, and the power rating not less than 25 watts and preferably 50 or 75 watts. The control develops an appreciable but not dangerous amount of heat as part of its normal operation.

When an appliance equipped with an AC-DC motor stops working, you have to open the case or frame to gain access to the end of the power cord, the switch, and the motor itself. In many such appliances, particularly fans, sewing machines, driers, small vacuum cleaners, etc., the brush holders are on the outside and the brushes can therefore be checked in a few seconds. In other instances, the motor may be enclosed in the shell of the appliance. Examine the latter carefully. If screw heads are in sight, you can be pretty sure that you have to loosen them to take the machine apart. Just remember where they came from, and keep them safely in a cigar box.

Reasons for Trouble

The cause of the trouble might be any of the following:

1) A mashed attachment plug or broken or shorted flexible cord. Easily tested with the Lightester or Handitester as explained in the section entitled "Don't Let a Cord Tie You into Knots."

2) A broken switch. Quite common with vacuum cleaners because it is invariably kicked on and off by the user's foot. Many of  the  small  toggle  switches  found   on cleaners just are not heavy enough to with stand this punishment very long.

3) Broken,    shattered    or   worn    out brushes. Easy to find and easy to replace.

4) Burned out "field" winding, the coil of wire fixed inside the frame of the motor.

Next the jack-in-box carbon and spring slide out of the holder for inspection. Put back in same position, so that worn end of brush matches diameter of copper commutator bars it presses against.

The commutator is actually a small drum of copper segments; clean with a strip of fine sandpaper; this will reduce sparking. Caution: Do NOT use emery cloth for this purpose. Emery dust that gets into the motor bearings can ruin them after a few hours of running time. Avoid this trouble.

The small tubular device mounted on the side of the motor is a capacitor which functions to kill radio interference; if this is shorted, cleaner will not run, and will blow fuses. Continuity check with meter shows up a short quickly. One wire of capacitor must be disconnected from motor line for this test.

Cleaner cord will suffer less damage if kept rolled up when not in use. Storage reel can be made from plywood discs.

Two discs of plywood or hardboard about 6" in diameter and a center piece about 3" are used to make the reel; the three sections are held by one bolt through them and the side of the cleaner case.
 
Easily checked for continuity with the Lightester or Handitester. Happens once in a blue moon. An "open" more usually is due to a break in the connection between the flexible line cord and the field winding. The line cord is wiggled around a great deal and such a break is common.

5) Burned out "armature" winding. This is the wire on the rotating member of the motor. The winding actually consists of a number of separate coils, each represented by a pair of diametrically opposite copper bars on the "commutator," the member against which the carbon brushes press.
 
Happens once in a green moon, but easily checked with either tester mentioned above.

6) Short-circuited interference eliminator. One of those concealed jokers that turns up only after you have wasted two hours testing everything else. This eliminator is merely an ordinary paper capacitor of the kind found by the dozen in radio and television receivers. It is sized as ".05 mfd." ("mfd." stands for microfarad, or one-millionth of a farad the unit of measurement of capacitors), and rated for operation up to 250 volts. You can buy one in radio supply stores for as little as a dime. It is connected directly across the power cord, and its purpose is to prevent the radiation of interfering signals created by the sparking of the brushes against the commutator. This interference is heard in radio receivers as a high pitched whine and is seen on TV screens as a series of short white lines or streaks.

In small hand-type vacuum cleaners, motor brushes  are out in  the  open and  always  accessible for inspection. The screwdriver points to the slotted end cap of the brush holder; check similarity of yours.
 
If this capacitor merely goes "open", the vacuum cleaner continues to run perfectly well, although neighbors may wonder what's causing the racket in their sets. More often, however, when it goes bad it usually goes thoroughly bad by developing an internal short circuit. Since it is connected across the power line, it blows the fuses and prevents the cleaner from running. To check a suspected capacitor, you must disconnect either of its leads from the line, and then apply the Handitester. A complete or partial short circuit shows up instantly as a very low resistance reading on the LO OHMS scale. If the capacitor is in normally good condition, the meter needle flickers slightly on the HI OHMS scale and then returns to the extreme left-hand position. An open capacitor will cause no reading at all. It is not always possible to distinguish between a good and an open capacitor with a simple continuity test, but with either capacitor the cleaner runs, and that's the main consideration. Depending on the cleanliness of the motor commutator and the smooth fit of the brushes, the capacitor may not really eliminate all interference, but it usually does reduce it considerably. You can readily determine its effectiveness by running the machine with it and then without it, near a radio or television receiver.
 
The accompanying series of photos shows disassembly, testing and checking operations on two vacuum cleaners of entirely different types and on a typical sewing machine. The larger clearer, a popular swivel-top model, started to blow fuses . . . not always, but about three times out of five when turned on. Wiggling the line cord seemed to bring on the trouble, so it was thought that a short circuit had developed inside the cord where it enters the case, a common occurrence. However, everything proved to be intact until the interference capacitor was isolated and tested, and that was it!

If a vacuum cleaner motor runs, but at what seems a lower than usual speed, the likelihood is that the blower unit is fouled up. Dust and dirt pass through quickly into the bag, but string and pieces of sewing materials often wrap themselves firmly around the blades and stay there until you pick them out. An occasional but thorough cleaning of a vacuum cleaner is a good idea.

Shaded-Pole Motors

Brushless motors used for small fans and similar air-moving devices, and for clocks and automatic timers, are usually of the "shaded pole" type and work on AC only. They are favored for these applications because they are extremely simple in construction, cheap to manufacture, and rugged and reliable. The catch is that they have very low starting and running torque and are therefore suitable only for light jobs. When you turn on a fan using a shaded-pole motor, you can count almost to two before the blades pick up speed. If you stick a pencil into them they'll probably stop dead, without breaking the pencil. A compensating feature to this low efficiency is that a shaded-pole motor rarely if ever bums out itself or the line fuses if it stalls, as practically all other types do. It might get a bit warmer than usual, but that's all.

High speed of small cleaner motors makes brushes wear rapidly; it's a good idea to keep a spare pair on hand. Spring maintains pressure of brush against the motor commutator.

Remove the end casing of the cleaner; you may be surprised at the amount and type of material tangled in the blades; occasional cleaning of the blades  maintains  cleaner efficiency.
 
About the only thing you can do to a shaded-pole motor is lubricate it once in a while. Over-oiling can't hurt it, because there are no internal switches, commutators or contacts. The rotating member is called a "squirrel cage" because it resembles one.

Both the universal and the shaded-pole type motors are usually found in only small sizes in home appliances, and are easy to take apart for inspection and cleaning. With an AC-DC motor, first remove the brushes, and then the bolts holding the framework together; with a shaded-pole unit, it is only necessary to loosen the body bolts. Watch out for spacing washers on the ends of the shafts.

Motors for Power

For applications calling for better starting torque and constant speed under varying load, straight AC motors of quite different construction are used. In the sizes used in the home or small shop, from about 1/6 to 1 horsepower, by far the most common type is called the "split phase." One form of split-phase motor using a capacitor (or "condenser") is particularly popular because it develops high starting torque with relatively low line current. Capacitor motors are almost universally used for refrigerators, freezers, air conditioners and home workshop power tools.

Above, the brushes of a sewing machine motor are in the open at the end of the frame. Since the motor is run only intermittently, brush wear is slight;   tighten   holder   and   caps   occasionally.

Above right, the rubber drive wheel on the end of the shaft has a tendency to loosen; clean shaft of possible oil slick bom bearings, and tighten set screw firmly; it may be helpful to flatten end of the shaft slightly with a file in order to give   a   better   bite   to   the   screw   holding   it.

Motor is usually spring-loaded to make drive wheel press firmly against the large hand wheel; check  the  tension  occasionally  for  drive  action.
 
There are numerous variations of the split-phase motor, designed to meet particular requirements. In its basic form it consists of two fixed windings of wire on the inside of the frame, one called the "starting" winding and the other the "running" winding. The rotating member is again a "squirrel cage," and has no external connections. However, it is fitted with a spring-loaded, centrifugally-oper-ated switch. When the motor is at rest, the contacts of this switch are closed, and connect the starting winding to the line; the running winding is "on" independently of the switch. When the line switch is closed to start the motor, the shaft starts to turn. When the motor reaches a predetermined speed, usually in a second or so, the centrifugal switch opens, removing the starting winding from the power line. Only the running winding is then in the circuit. When the motor is turned off, the switch closes just before the shaft comes to rest, and the machine is ready for another cycle. A clicking or slapping sound made by the switch as it opens and closes is characteristic of split-phase motors.

Repulsion-Induction Motors

Another common type of motor is the "repulsion-induction," which is noted for its very powerful starting torque, needed for pumps and other "stiff" machines. This type has one fixed running winding on the frame, and also a wire-wound armature complete •with commutator and brushes, just like an AC-DC motor but on a larger scale. The armature winding and its accessories are needed only for starting.  When  the  motor  reaches  its  rated speed, a weighted governor lifts the brushes away from the commutator and at the same time moves in a circular ring or "necklace" which short circuits all the commutator segments to each other.

Foot treadle speed control of sewing machine motor is tapped  150-ohm resistor; keep connections tight.

Many small electric fans are driven by "shaded-pole" motors, which work on AC only; this type can be recognized by an absence of brush holders. Construction is simple; pencil points to one of two screws holding motor case together; box on right contains speed control rheostat. This is connected in series with 115-volt line, exactly as when a universal AC-DC motor is used; see Diagram 1 on page 107 for details.

Shaded-pole motor comes apart readily. Above left, end bell; center, main body holds windings through which current passes; right, rotating armature,   which   has   no   external   connections.

The speed control rheostat is mounted conveniently in a small radio shield box; it must be enclosed to protect user against possible contact with the bare wire on it. It's a handy unit for a fan.

The throw-out mechanism in split-phase and repulsion-induction motors is very reliable, but it cannot be called simple. The best thing you can do to a motor in running order is nothing. Even be stingy with lubrication, if any is required at all. Many high-grade motors have sealed bearings that are guaranteed to run practically forever. More damage than good is done by over-oiling. The lubricant is likely to get into the switch contacts or onto the commutator surfaces, and since oil and grease are good insulators they can cause real trouble.

When a motor with a starting winding is stalled to a dead stop by an overload, there is some danger that this winding will burn out if the power remains on. It won't if the branch fuse or circuit breaker is of the right size for the line; the protective device should kick out long before the copper wire in the motor has a chance to melt.

Many AC motors in the popular sizes between ¼ and 1 horsepower are comfortably underrated, and are known to develop as much as twice their rated values for short periods. Some induction-repulsion motors can produce three to five times their nominal ratings under heavy starting conditions. You have to mistreat a machine very badly and deliberately to make such rugged motors quit.

Occasional stalling is quite normal with some types of clothes washers when they go into the "spin" cycle with more wet wash in them than they're supposed to carry. An overload breaker is usually provided alongside the regular controls, and it only needs to be reset to start the machine again . .. after a few towels have been removed.

Because there are so many forms and variations of both split-phase and repulsion-induction motors, it is difficult to give anything but the most general suggestions in regard to servicing them. Before they give trouble, familiarize yourself with their wiring and construction. Copy off all the data on the name plates and write to the manufacturer for any instructions he has available. Some firms send back elaborate four-page folders, some put all the information on a tag, others just ignore such requests altogether. You have a lot to gain and nothing to lose by at least trying. In the absence of printed instructions, about all you can do is probe around carefully and hopefully.

Continuity Check

A continuity check may give some clue. If a meter like the Handitester shows no movement (that is, infinite resistance, or open circuit) when connected to the line plug, possibly there is only a break in one of the power leads, or in the on-off switch, if it's mounted directly on the motor. Some motors have built-in overheating controls, which open automatically when a predetermined temperature is reached. A defective   one   that   stays   open   prevents current from reaching the motor. The normal resistance of a motor in the 6- to 1-horsepower range is very low . . . only an ohm or two . . . but some motors will register a reading of about this value even if the starting winding is burned out and cut out of the circuit by a defective centrifugal throw out switch.

Motors of split-phase type are commonly used tor household machines, power tools, etc. li the motor requires lubrication, apply oil sparingly, since excess  might  get  into  internal  switch  contacts.
 
With four screws through motor frame removed, end bell can be pried off to reveal the switch mechanism in the end bell itself, and will also reveal the centrifugal weights on the motor shaft.

Nothing shows on the outside of motors of the aforementioned types except the shaft and the line cord. To check the internal parts, you must, of course, open the frame. This is a simple job, inasmuch as the end bells are held to the central body by only four long bolts, in 99 motors out of 100. Just proceed slowly and carefully and keep a sharp eye for spacing washers and lubricating rings on the shaft ends. What happens when you pry off the end containing the throw-out switch can't be predicted; every manufacturer has his own design of weights and contacts.

In split-phase (including capacitor) motors, the arrangement is not too forbidding, as the throw-out switch is after all only a single contact affair. In repulsion-induction motors the mechanical linkage can be somewhat on the Rube Goldberg side because it has to raise the brushes and also move in a circular ring. If nothing else, you'll have some fun investigating.

Suppose you get a motor apart, isolate the leads of the various windings, and determine pretty positively that one of the latter is burned out. Your nose will also help in the investigation, as burned insulation has a pungent and unmistakable odor. That's a polite expression; the motor will stink, no mistake about it! You can now do one of two things: 1) Have the motor rewound at a shop specializing in the work; 2) Buy a new motor and use the old one as a plaything. After getting an estimate on the repair job, you might readily decide that the second step is simpler and cheaper. New motors are surprisingly inexpensive and are available in a wide variety of sizes and types to suit all purposes. A recent Sears, Roebuck catalog lists no less than 38 of them. ·

Diagram 1 shows basic wiring diagram of universal AC-DC motor with speed control rheostat. Diagram 2 shows basic connections of split-phase AC motor; note that there are no connections to the rotor, which is pulled around by magnetic effect. Diagram 3 shows split-phase motor of capacitor type; capacitor is connected in starting-winding section only. Switch opens starting circuit when motor reaches operating speed.

For closer inspection of switch of split-phase motor, switch frame can be dismounted; it is held by screws. Pencil points to contacts of switch, which must be kept clean and free of dirt and oil.
 
This is squirrel-cage rotor of the split-phase motor; vanes are for cooling. Spring-loaded weights move out when motor attains speed, pressing against and thus opening switch connected to starting-winding.

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