Rewinding and Reconnecting
At times, electric motors may be rewound to change the operating volt age to permit them to operate on a different line voltage. It may also become necessary to change the phases on a particular motor- usually in an emergency situation.
The voltage of any individual motor winding varies directly with the number of turns it has connected in series. There are, of course, certain practical limits beyond which this change of voltage should not be car ried. For example, a motor originally operating at 240 V might possibly be changed-by increasing the number of turns in series-to a point where the winding would stand 2400 V, but it is doubtful whether the in sulation would stand so high a voltage.
It is almost always permissible to reconnect a winding to operate on a lower voltage than it has been designed for, but when reconnecting a motor to increase its operating voltage, the insulation should always be considered. The usual ground test for the insulation of such equipment is to apply an alternating-current voltage of twice the machine’s rated volt age plus 1000 V. This voltage should be applied from the winding to the frame for at least 1 min, and a test should be made after the winding is reconnected or on any new winding before it is placed in operation. When a winding is reconnected for a different voltage, it should be arranged so that the voltage on each coil group will remain unchanged.
The diagram in Fig. 23-1 shows four coil groups connected in series to accommodate 240 V, which places 60 Von each coil group, causing an
assumed 5 A of current to flow. To be used on a 120-V circuit, the coils could be arranged as shown in Fig. 23-2. In this arrangement, two coil groups are in series in each of the two parallel circuits. When 120 V are applied to these two parallel groups, there will still be 60 V/coil, and the motor will operate normally on 120 V; even the same amount of current will flow. The rotating magnetic field will not be affected any differently as long as the amount of current per coil is not changed and the polarity of the coils is kept the same.
In connecting three-phase windings, all phases must be connected for the same number of circuits, and when connecting the groups for a winding having several circuits, extreme care should be taken to obtain the correct polarity on each group.
It is common practice for motor manufacturers to design machines that can readily be operated on either of two common voltages, that is, 120/240, 240/480, etc. This is accomplished by a series or parallel arrangement which can be changed by varying the lead connections to the motor-usually in the terminal junction box. For example, one three phase motor may have one connection where the pole groups in each phase are connected in series to operate on 480 V. Another connection ar rangement for this same motor could have the pole groups for each phase connected half in series and half in parallel for operation on half the volt age of the first arrangement, or 240 V. In this arrangement, however, to maintain the same horsepower at half the voltage, the amperes for the 240-V connection would double at full load. This presents no problem since in the 240-V connection with two circuits in parallel, twice the cross sectional area of copper exists than in the series circuit for the 480-V ar rangement.
In most dual voltage motors, each winding is divided into two parts with suitable leads from each section brought outside the motor. These leads can then be conveniently changed for either one or two voltages.
CHANGE OF PHASES AND FREQUENCY
In certain emergency cases, it is desirable to know how to change a motor from, say, three phase to two phase or vice versa. This can be accom plished by changing the number of coils per groups. As a brief example, a certain two-phase motor having three coils per group could be made to operate on three-phase power by arranging the coils to two per group.
Sometimes it is desirable to change a motor which has been operat ing on one frequency so that it will operate on a circuit of another fre quency. The most common frequency for alternating current in the United States is 60 Hz; in England, 50 Hz; etc. But occasionally (al though very infrequently) some other odd frequency may be encountered.
When an induction motor is operating, a rotating magnetic field is set up in the stator, and it is this field which induces the secondary cur rent in the rotor and produces the motor torque. This same rotating field cuts across the coils in the stator itself and generates in them a counter voltage which opposes the applied line voltage and limits the current through the winding. The speed of field rotation governs the strength of the countervoltage (CEMF) and therefore regulates the amount of cur rent which can flow through the winding at any given line voltage.
There are two factors that govern the speed of rotation in this mag netic field. These are the number of poles in the winding and the frequen cy of the applied alternating current. Any change that is made in the fre quency of the current supplied to a motor should be offset by a change of voltage in the same direction and in the same proportion. For example, a motor changed from 30 to 60 Hz should have the magnetic field rotate twice as fast and the CEMF doubled. To maintain the same current value in the stator coils, the line voltage should also be doubled. If the winding is to be operated on the same voltage at this higher frequency, the number of turns in each group across the line should be reduced to one half the original number to allow the same current to flow.
This procedure should, of course, be reversed when changing a motor to operate on a lower frequency.
When the frequency of a given motor is varied and the stator flux kept constant, the horsepower rating will vary directly with the change in speed. In other words, the horsepower of any motor is proportional to the product of its speed and torque.
CHANGING NUMBER OF POLES AND SPEED
The speed of an induction motor is inversely proportional to the number of poles; that is, if the number of poles is increased to double, the speed will decrease to one half, or if the poles are decreased to one-half their original number, the speed will increase to double.
When changing the number of poles of an induction motor, if the voltage is varied in the same direction and same proportion as the change produced in the speed, the torque will remain practically the same, and the horsepower will vary with the speed. Therefore, the horsepower in creases with the higher speeds and decreases at lower speeds in exact pro portion to the change of speed.
With induction-type motors, a change in speed invariably involves a change in the number of poles set up by the winding, and since this im plies a variation in the coil span, rewinding is usually required.
COMPLETING THE WINDING JOB
Once the windings are positioned in the motor being overhauled, as dis cussed in previous chapters, the connecting of the leads to the commu tator is the next process. When the bottom leads are laid in the slots as the coils are wound, proper sequence is ensured. When the coils are all wound, then the top leads are put in their proper segments- which are adjacent to the bottom leads on lap-wound armatures and with the proper commutator span on wave-wound armatures.
Some armatures are wound completely before the bottom leads are placed into their segments, followed by the top leads. Another method is
when the bottom leads are installed through the same slot as the top leads. This type of winding is frequently used on lap-wound armatures where the top and bottom leads swing out of line with the coil itself.
Therefore, bringing the bottom leads out of the bottom coil side would necessitate extra long bottom leads. By combining the two leads, the shorter bottom leads will not have a tendency to raise up and cause a short on the revolving armature.
The top and bottom leads can be laid one after the other on some armatures if care is taken to insulate between leads with cotton tape or oiled linen; these leads should also be sleeved. This insulation is interwoven between the leads as shown in Fig. 23-3 and then brought up next to the commutator where the leads need the most insulation because of their closeness.
To correctly connect the leads, it is necessary to use a test lamp or continuity meter to perform the following tests. At this point, the com mutator bars should have the bottom leads connected to them. Place one test lead onto one commutator bar (with a bottom lead connected to it) and then touch the unconnected top leads with the test lamp or meter until you come to the lead that lights up the lamp or deflects the meter. Connect this lead as shown in the data collected from the motor prior to beginning the overhaul.
For instance, the top lead, if on a lap winding, will be connected to the bar adjacent to that of the bottom lead. If dealing with a wave-wound armature, count the armature span over from the bottom lead to where the top lead connects and put it in the bar. Take the next bar and follow the same procedure and then continue on with each of the succeeding bars until all leads have been connected. Always continue in the direction taken, taking each bar in succession and connecting the top lead for its respective bottom lead.
When checking for top leads of lap-wound armatures with cross connectors in back of the commutator, care must be taken when finding a top lead for a corresponding bottom lead connected in a bar. With the cross connectors there will be two top leads where the lamp lights- since the current travels over the cross connector or jumper- to another bar where there is a bottom lead connected. The current will follow this path and then travel through this coil, having the bottom lead in the same slot as the jumper out to the second top lead. Knowing from the data that this
armature is cross-connected, test the leads near the coil from which the bottom lead comes.
When making the final connections to any motor, refer to the data card frequently. Always determine the bottom lead connection for one coil and then lay in the remaining bottom leads in succession as the coils progress.
Where the bottom leads are not placed into the bars as the armature is being wound, find the bottom lead for one coil and place it in its proper slot; then continue as discussed previously. These bottom leads should be marked in any way that is convenient so long as the marking does not in jure the insulation in any way.
When the top and bottom leads are installed together, find the bar where the bottom lead of a coil connects and then connect the single lead to a top and bottom lead to that bar. Take the lead from the next coil and connect it to the next bar and so on, being sure to connect the leads to the bars in the same direction as the coil progresses. When two single wires are encountered which are in separate sleeves and are the beginning and ending leads of the winding, connect them to the same bar.
Once the armature has been wound and is ready for connecting, some projects will require some building up between the end of the armature winding and the commutator so the bottom leads will have some means of support. This is accomplished by installing cotton tape around the shaft between the commutator and the winding to fill up the gap. This tape should be wound until it builds up even with the bottom of the slot in the segment. Be sure to keep it as smooth as possible to avoid hav ing to bend the leads out of shape to travel over it. Next, oiled linen or cotton tape should be placed over the winding, covering an area from where the winding leaves the slots to its end so that leads do not rest directly upon it. If leads were allowed to run over the winding with no insulation between, a short circuit might develop due to friction while the motor is running.
The bottom leads are laid in place next, being sure to interweave cot ton tape between each lead so they will not cause a short circuit. More cotton tape is used to cover them to prevent the top leads from resting directly upon them. Finally the top leads are placed in their proper position.
Once all leads are in place, all insulation should be scraped off where they make contact with the bars. The leads are then placed in the bars, and any excess lead is cut off. When the bottom leads are placed in the bars first, they are tapped down in the slots with a lead sinker to flatten the lead so it becomes tight in the slot as shown in Fig. 23-4. The top leads are treated in the same manner, being careful in both cases not to crush the leads so they will break off in back of the commutator. Once both leads are in the slot, it is sometimes recommended that the slots be peened over to prevent the leads from rising out. While this peening will secure the leads, it makes them very difficult to remove the next time that the motor needs overhauling.
SOLDERING LEADS
Once the leads have been cleaned of insulation and placed in their proper slots, they are ready to be soldered. A temporary protective band should be installed over the leads- especially against the back of the commu tator- so when the leads are soldered, none of the solder will flow down in the back of the commutator and short the bars together. A soldering iron or gun is used for the job, but the tips must be well tinned for proper soldering and should not be allowed to become too hot.
Resin-type soldering paste is used to coat the tops of the bars at points where the leads will connect. It is best not to use a paste with an acid base. When the soldering iron is at the right temperature, touch it to the commutator bar and heat until the paste runs down between the leads and the slot in the segment. The bar should be hot enough to allow the solder to take, but be careful not to overheat. Some solder should also be touched against the iron and be allowed to flow into the slot between the leads. Then lift the soldering iron off the bar with a quick jerk as this will leave a smooth finish and will also keep the solder from bridging between commutator bars. Never use so much solder on the iron or the bar to cause the solder to stack up on the top of the bar; not only does this make a bad looking job, but it increases the chances of the solder running over onto the next bar, shorting the two bars together.
Experienced motor repairmen like to allow the soldering gun to be heating part of the next bar as it is heating the bar being worked on at the moment. One way to accomplish this is to use an extralarge soldering iron, one that will produce enough heat to heat the next bar while one is being soldered.
Should the solder run across between the two commutator bars so it covers the mica separating them, it should not be melted off because the bar may become hot enough to cause the solder to run down in back of the connector. It is best to use a hacksaw or a small file to cut the solder over the mica so the mica will again separate the bars so as not to be shorted.
Should any solder run down the front of the commutator, this usually presents no problems so long as it does not short two bars together. The commutator may then be turned in a lathe to even the surface.
BAKING AND VARNISHING
All windings, whether de or ac, should be thoroughly impregnated with a good grade of insulating varnish before they are put into service.
This varnish serves several very important purposes. When proper ly applied, it penetrates to the inner layers of the coils and acts as extra insulation of the conductors, thereby increasing the dielectric strength of
the insulation between them. This compound within the coils and in their outer taping greatly reduces the liability of short circuits between con ductors and of grounds to the slots or frame.
When a winding is thoroughly saturated with insulating varnish and this varnish is properly hardened, it adds a great deal to the strength of the coils and holds the conductors rigidly in place. This prevents a great deal of vibration that would otherwise tend to wear and destroy the insulation, particularly in the case of alternating-current windings where the alternating flux tends to vibrate the conductors when in operation.
Insulating varnish also prevents moisture from getting in the coils and reducing the quality of the insulation, and it keeps out considerable dust, dirt, and oil that would otherwise accumulate between the coils. Keeping out moisture, dust, and oil greatly prolongs the life of the insula tion.
There are many grades of insulating varnish some of which require baking to set or harden them and others which have in them certain liq uids or solvents which make them dry and harden very quickly when ex posed to air. Varnishes of the first type are called baking varnishes, and the latter are called air-dry varnishes.
Good air-dry insulating varnish will set or harden in from 20 to 30 min, but it should be allowed to dry out thoroughly for about 24 h before the windings are put in service. Air-dry varnish is not considered quite as good as the better grades of baking varnish. Therefore, the latter should be used wherever a bake oven or some means of applying heat is avail able.
There are several methods used to apply insulating varnish to motor coils and windings. The most popular methods include dipping, brushing, and spraying.
Dipping is considered the best method and should be used for all small windings of stators and armatures and for armatures and stator coils and field coils. To dip these coils or windings, a pan or tank of the proper size and depth will be required. Before dipping the windings, they should be thoroughly dried out in a bake oven at about 212°F in order to drive out all moisture and to heat the coils so that when they are dipped the varnish will rapidly penetrate to their inner layers.
The coils should be allowed to remain in the varnish until all bub bling has ceased. When they seem to have absorbed all the varnish possi ble, they should be slowly withdrawn from the tank at about the same rate as the varnish flows from them of its own accord. This will give them a uniform coating with the least possible accumulation of varnish at the lower end. They should then be allowed to drain until the varnish stops dripping and becomes partially set. The time required for this will depend on the size of the winding or coils.
When dipping a large number of small coils, considerable time can be saved by arranging a drip board set at an angle, so the coils can be hung above it and the varnish which drips from them will run down the board and back into the tank. With this method other coils can be dipped while the first set is draining.
After all the surplus varnish is drained from the coils, they should be baked. When placing them in the oven, it is a good plan to reverse their positions, so that any excess varnish on the bottom ends will tend to flow back evenly over their surface when first heated.
When applying the varnish with a brush, the winding should, if pos sible, be preheated to drive out the moisture and permit the varnish to flow deeper into the coils. Varnish can be applied with an ordinary paint brush, and this method is used where the dipping tank is not large enough to accommodate the winding or where no dipping tank is avail able.
Spraying is used principally on large windings and gives a very good surface for a finishing coat.
The ends of coils should be given two or three coats of varnish as an added protection against mechanical damage and moisture and to help prevent flashovers to the frame of the machine.
Try not to get insulating varnish on the commutator, as it makes it hard to turn. Allow the armature to drain for a few minutes and then clean off all excess varnish from the commutator, shafts, and armature insulations with gasoline or AWA 1,1,1. It is necessary that all varnish be taken off the laminations so the armature will turn freely between the fields or stator. It must be taken off the shaft so oil may properly lubri cate. When this is done, put the armature back into the oven and bake it for a few hours, being careful not to get it too hot.
Manufacturers of insulating varnish usually furnish convenient tables with their product which give the proper temperatures and approx imate time for baking. Be aware that when baking complete armature windings, more time is required to thoroughly bake the larger sizes. As a rule, slower baking produces a more elastic and better-quality insulation than when baking quickly at higher temperatures.
Besides the insulating qualities, insulating varnish also provides a smoother surface on the windings and coils, making them much easier to clean either by means of a brush, compressed air, or washing them with some degreasing solutions such as AWA 1,1,1.