9. BATTERY CHARGING EQUIPMENT 

9.1 MOTOR GENERATOR SETS

Diverter-pole motor-generator sets used for battery charging,, if properly installed and maintained, will give long and satisfactory service. Shunt generators with voltage regulating equipment are used as battery chargers in some older installations. 

Operation of the diverter-pole generator is similar to that of a shunt-wound generator, except the field rheostat is set to maintain floating voltage. The diverter-pole generator will maintain voltage within close limits and will automatically adapt to fixed or fluctuating loads within its capacity and will charge the battery at the proper rate. 

A. Communication

Common problems with motor-generator sets involve the commutation system, and frequent checking of commutators is desirable. After several months of operation, a commutator should develop a smooth, burnished surface, free from grooves, burns, or pit marks, and should be kept in this condition. Never use oil or any other lubricant on the commutator. 

Once each month, remove brushes from the holders and wipe off the commutator, brushes, and brush holders with a clean, dry cloth. If oil or grease are found on these parts, clean thoroughly with a cloth dripped in an approved solvent. Follow with a cloth saturated with hydrogen peroxide and thoroughly wipe dry with a clean cloth. The lower brushes have a natural tendency to collect dirt and must be cleaned each month. If grooves develop in the commutator, brushes should be lifted from holders, and the faces of the brushes should be carefully inspected for embedded specks of copper. These particles of copper or other hard spots should be carefully removed with a knife point. 

The grooves between commutator bars should be cleaned regularly. Remove copper or carbon dust or other foreign matter. A thin, flat piece of wood or fiber is recommended for this purpose. 

Mica should always be located about 1/32 inch below the surface of the commutator bars. When undercutting the mica becomes necessary, a knife or broken hacksaw blade on which the "set" of the teeth has been ground off is effective in scraping the mica to the required depth. Do not undercut the mica more than 1/32 inch. Be sure that slots are left clean and that no small flakes of mica project above the surface of the commutator, particularly on the sides of the bars. 

All sharp corners on the edges of the commutator bars should be rounded off to prevent cutting or scraping the brushes. 

Commutation is best when the commutator is perfectly smooth and "seasoned" to a chocolate brown color. The commutators should not be stoned or ground except when necessary to remove grooves or scoring, and they should not be turned down in a lathe except when allowed to become badly out of round or after commutators have been dismantled and rebuilt. Excessive cutting or grinding should be avoided. Cut barely enough to accomplish the required result. 

When stoning, use the finest grit that will serve the purpose and finish by polishing with a canvas pad and rouge. Mica between bars must be undercut after turning and after repeated stoning to keep within the 1/32- inch depth mentioned above. 

New brushes must be carefully fitted to the commutator to ensure good contact. Place a strip of 00 sandpaper between the commutator and the brushes with the sanded side against the brushes and draw the sandpaper back and forth until the brushes are hollowed to fit the commutator. Increase the spring tension to its highest point to speed this preliminary fitting, but the tension must be restored to its original position for the final fitting. The correct operating pressure is about 2-1/2 pounds per square inch of brush surface area. 

For the final fitting of the brushes, pull the sandpaper in one direction only. When the brush holders are of the reaction (Baylis) type, the sandpaper must be pulled against the direction of rotation. When the brush holders are of the radial type, the sandpaper must be pulled in the direction of rotation. 

Before starting the motor-generator after sanding the new brushes, be sure to wipe all sand and carbon dust from the brushes, brush holders, and commutator. See that each brush makes firm contact with the commutator and is free in its holder. Clean and tighten all connections in the field circuit. Clean the rheostat contact points and contact arm, and see that they make firm contact. 

B. Positioning  Brushes of  Diverter-Pole Generators

For best commutation, brushes should be set so that the armature coils short circuited by the brushes are under the influence of the diverter-pole flux. 

Moving brushes in the direction of rotation may improve commutation but may reduce compounding action of the diverter poles. Increased strength of the diverter poles is then necessary to produce the same compounding action. Increased strength of the diverter poles increases the flux change in the poles and results in greater curvature of the voltage curve. Best results are obtained when this curvature is at a minimum. 

Brushes should be set as far back (against direction of rotation) as perfect 

commutation at full load will permit. Then the strength of the diverter-pole winding should be adjusted to produce the flattest possible curve without showing a rising voltage characteristic. Each point on the voltage curve should be slightly lower as load is added. If the voltage rises at any point, operation with a battery will be unstable. 

The strength of the diverter-pole coils is adjustable by means of an adjustable shunt connected in parallel with the diverter-pole circuit. This shunt consists of resistance wire wound on an insulating tube and is usually mounted inside one of the arms of the commutator end bearing bracket. Clamp-type sliders provide means for adjusting the resistance of this circuit. 

If commutation is not perfect, as evidenced by burning of the commutator bars, shift brushes slightly in the direction of rotation. Moving the brushes too far will make the generator slow to recover voltage following an overload. Under this condition, the battery will regain voltage faster than the generator, which will result in a momentary discharge from the battery through the generator. This condition will cause any reverse-current device between the generator and the battery to trip open. When no reverse- current device is used, this same condition may cause surging of current back and forth between the battery and the generator 

C. Checking Voltage Regulation

When checking generator voltage regulation, always start the generator without load and with the field rheostat resistance all cut in. Raise the voltage slowly to the floating value, using care not to go beyond this value. If this value is exceeded, shut down the generator, allow it to come to a complete stop, and then start over. This procedure is important to eliminate the effect of hysteresis. 

Use a variable-resistance and increase the load in successive steps to a point where the generator voltage drops sharply, recording the generator volts and amperes on each step. Be sure that each successive step increases the load to eliminate the effect of hysteresis. The generator should always be run long enough to produce a perfect and full seating of the brushes on the commutator before taking final check test readings. 

The final voltage curve should show a drop as the first small increments of load are added in order to prevent a swing to discharge on light loads. The load should then level off to a very gradual drop over the major portion of the curve to ensure stability until cutoff point is reached, then the load should drop abruptly to protect the generator against overloads beyond its safe capacity. 

D. Motor Generator Bearing Maintenance

Once every 6 months, remove bearing covers; clean out old grease; flush the bearings and bearing chamber with kerosene; and repack the bearing loosely with clean, neutral mineral grease, filling about one-fourth full. Do not neglect the complete flushing of the bearing chamber and replacement of the grease every 6 months; otherwise, the grease will pack and harden, and the balls will drag and slide rather than roll on the races, resulting in overheating and scoring. Do not use grease containing vegetable or animal oils or graphite; vegetable and animal oils develop acids that are highly destructive to the finely polished surfaces of the balls and raceways. The rolling action of the balls will pack graphite into the surface of the raceways and close the necessary clearance between balls and races. The grease container should always be kept tightly sealed because dust and grit will quickly destroy any ball bearing. Also be sure that tools used to remove grease from the container are perfectly clean. Follow the above instructions carefully. More ball bearings are destroyed by the use of dirty or too much grease than from any other cause. 

The motor-generator should be shut down when adding oil because the oil level is always slightly lower when the machine is running, which can cause danger of overfilling and subsequent waste of oil and fouling of the windings. Proper oil level is about 1/16 inch below the top of the overflow fitting on the side of the bearing. The following schedule is recommended: 

Once each week Check the oil level and add oil if necessary. Inspect oil rings to see that they are rotating with the shaft and carrying oil to the journal. 

Once every month Drain the bearing chamber completely. Flush with a clean, light oil, and refill within 1/16 inch of the top of the overflow fitting using a high grade, medium machine oil. 

9.2 STATIC RECTIFIER CHARGERS

Static rectifier chargers are generally provided with automatic voltage- regulation features. Maintenance personnel must always refer to manufacturer's instructions and service manuals for correct operation, maintenance, and care procedures. This charger consists of a saturable reactor, a transformer, a silicon rectifier controlled by a sensing device, a magnetic amplifier, and a reference voltage. 

Battery chargers of this type exhibit a constant voltage output characteristic under changing load conditions and a-c supply fluctuations. This charger, when operating within its current rating, will maintain the float voltage or the equalizing voltage within 1 percent. The charger will do this with a-c supply voltage fluctuations of ±10 percent. 

The magnetic amplifier charger is capable of regulating the voltage down to zero load, but only when the battery is connected to the charger output. Chargers may be equipped with copper oxide rectifiers, selenium rectifiers, or silicon rectifiers. 

A. Trouble Shooting Static Rectifier Chargers

A common problem is low output voltage in chargers that have copper oxide rectifiers in the control circuits. Copper oxide rectifiers (and to a lesser degree selenium rectifiers) have a tendency to deteriorate after long use, causing a reduction in output voltage. When this condition occurs, rectifiers should be replaced with silicon rectifiers, which have a slow aging rate. 

Oscillations in output voltage can occur in some types of battery chargers during light or no-load conditions because of resonance between the battery and the charger. These oscillations can be damped out by connecting a small load resistor (about 100 watts) across the output of the charger. 

Always refer to the manufacturer's service manual for detailed instructions regarding operation, maintenance, and care.