Hawkins Electrical Guide v. 03 (of 10) / Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications :: CHAPTER XXXIV HEATING

CHAPTER XXXIV HEATING

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The excessive heating of the parts of dynamos and motors is probably the most frequent and annoying fault which arises in operation. When the machine heats, it is a common mistake to suppose that any part found to be hot is the seat of the trouble. Hot bearings may cause the armature or commutator to heat, or vice versa.

All parts of the machine should be tested to ascertain which is the hottest, since heat generated in one part is rapidly diffused. This is best done by starting with the machine cold; any serious trouble from heating is usually perceptible after a run of a few minutes at full speed with the field magnets excited.

Heating may be due to various electrical or mechanical causes, and it may occur in the different parts of the machine, as in:

The connections;
The brushes and commutator;
The armature;
The field magnet;
The bearing.

Ques. How is heating detected?

Ans. By applying the hand to the different parts of the machine if low tension, or a thermometer if high tension, and also by a smell of overheated insulation, paint, or varnish. Ques. What should be done if the odor of overheated insulation, paint or varnish be noticeable?

Ans. It is advisable to stop the machine at once, otherwise the insulation is liable to be destroyed.

Ques. What is the allowable rise of temperature in a well designed machine?

Ans. It should not exceed 80° Fahr., above the surrounding air, and in the case of the bearings, this temperature ought not to be reached under normal conditions of working.

If this limit be exceeded after a run of six hours or less, it indicates a machine either badly designed and probably with the material cut down to the lowest possible limit with a view to cheapness, or some fault or other which should be searched for and remedied as early as possible, otherwise the machine will probably be destroyed.

Ques. How should the rise of temperature be measured?

Ans. It is not sufficient to feel the machine with the hand, but special thermometers must be placed on the armature winding, immediately on stopping the machine, covering them with cotton or wool to prevent cooling. Readings must be taken at short intervals, and continued till no further rise of temperature is indicated.

Heating of Connections.--A rise of temperature of the connections may be due to either excessive current, or bad contacts, or both. The terminals and connections will be excessively heated if a larger current pass through them than they are designed to carry. This nearly always proceeds from an overload of the dynamo, and if this be rectified, the heating will disappear.

If the contacts of the different connections of the dynamo be not kept thoroughly clean and free from all grit, oil, etc., and the connections themselves be not tightly screwed up, heating will result, and the connections may even become unsoldered.

Heating of Brushes, Commutator and Armature.--When heating occurs in these parts, it may be due to any of the following causes: 1, excessive current; 2, hot bearings; 3, short circuits in armature or commutator; 4, moisture in armature coils; 5, breaks in armature coils; 6, eddy currents in armature core or conductor.

Fig 727Ventilated commutator sectional view showing air ducts Air is frequently circulated

Fig. 727.--Ventilated commutator; sectional view showing air ducts. Air is frequently circulated through a commutator in order to maintain it at a sufficiently low temperature, suitable openings being provided for this purpose as shown.

Ques. What may be said with respect to excessive current?

Ans. When a dynamo is overloaded, the temperature of the armature will rise to a dangerous extent, depending upon the degree to which the safe capacity of the machine is exceeded, and heavy sparking of the brushes will also result. If the overload be not removed, the insulation of the armature may be destroyed.

Ques. State some causes of hot bearings.

Ans. Lack of oil; presence of grit or other foreign matter in the bearings; belt too tight; armature not centred with respect to pole pieces; bearings too tight; bearings not in line; shaft rough or cut.

Fig 728Self-oiling and self-aligning bearing The self-oiling feature consists of rings which

Fig. 728.--Self-oiling and self-aligning bearing. The self-oiling feature consists of rings which revolve with the shaft, and feed the latter with oil continually, which they bring up from the reservoir below. The dirt settles to the bottom, and the upper portion of the oil remains sufficiently clean for a long time, after which it is drawn off, and a fresh supply poured in through holes provided in the top. These latter are often located directly over the slots in which the rings are placed, so that the bearings can be lubricated immediately by means of an oil cup if the rings fail to act or the reservoir become exhausted. The bearing is made self-aligning by providing the bearing proper with an enlarged central portion of spherical shape, held in a spherical seat formed in the pedestal by turning, milling, or by casting Babbitt or other fusible metal around it, thus allowing the bearing to adjust itself to the exact direction of the shaft. The upper half of the box can be taken off to facilitate renewal, etc., and to permit the armature to be removed.

Ques. What is the effect of hot bearings?

Ans. Besides giving trouble themselves, the heat may be conducted along the armature shaft and core, thus giving rise to excessive heating of the armature. POINTS RELATING TO HOT BEARINGS

Use good oil;
See that oil cups or reservoirs are full and all oil passages clear;
In self-oiling and splash systems where the oil is used over again, it should be kept in clean condition by frequent straining;
Keep bearings clean and properly adjusted;
Maintain bearings in good alignment;
Avoid tight belts;
Examine the air gap or clearance between armature and pole faces and see that they are uniform.

Ques. What troubles are encountered with short circuits in the armature or commutator?

Ans. This results in sparking at the brushes, and in the heating of one or more of the armature coils, and even in the burning up of the latter if a bad case.

When the armature is overheated, and the defect does not proceed from an overload or the causes mentioned below, the dynamo should be immediately stopped and tested for this fault.

Ques. What will happen with an overheated commutator?

Ans. It will decompose carbon brushes and cover the commutator with a black film, which offers resistance and increases the heat.

Ques. What should be done if carbon brushes become hotter than the other parts?

Ans. Use higher conductivity carbon. Reduce length of brush by adjusting holder to grip brush nearer the commutator. Reinforce brushes with copper gauze, sheet copper or wires, or use some form of combined metal and carbon brush. Increase size or number of brush if necessary, so the current does not exceed 30 amperes per square inch of contact.

Brushes heat sometimes due to too much friction. They should not press against the commutator more than is necessary for good contact.

Ques. Give some causes for heating of armature.

Ans. Eddy currents; moisture; short circuits; unequal strength of magnetic poles; operation above rated voltage, and below normal speed.

Ques. What trouble is encountered with eddy currents?

Ans. Considerable heating of the whole of the armature results, which may even extend to the bearings.

Fig 729Eck Manchester type motor It is a very small size unit and is designed for special

Fig. 729.--Eck Manchester type motor. It is a very small size unit and is designed for special purposes where very little room is available. The motor occupies a space of 2-1/2" × 4-3/8" between bearings and develops 1/16 horse power at 2,000 R. P. M. The frame of this motor is made of high permeability steel so as to reduce the weight to a minimum. The armature is of the hand wound bipolar type built up of thin punchings. The armature, after being wound, is baked at high temperature for a prolonged period and then dipped while hot in insulating varnish. Pulley is one inch in diameter and takes a 1/4 inch round belt. Weight of motor 5-1/2 pounds.

Ques. How can this be overcome?

Ans. There is no remedy for eddy currents other than the purchase of a new armature, or reconstruction.

The fault may be detected by exciting the field magnets and running the machine on open circuit, with the brushes raised off the commutator for some time, when the armature will be found to be excessively heated.

Ques. How does moisture in the armature coils affect the armature?

Ans. The effect of this fault being to practically short circuit the armature, a heating of the latter results. In bad cases, steam or vapor is given off.

Ques. What is the effect of short circuits in the armature?

Ans. It produces overheating.

Ques. What trouble is likely to occur when the armature is not centered in the armature chamber?

Ans. A heating of the bearings is liable to be occasioned through the attractive forces developed by the center of the armature core not being parallel with the centre of the armature chamber or bore, or through the core being nearer one pole piece than the other.

This may result from unequal wearing of the bearings, and therefore the bearings should either be relined or the bolt holes of the bearings readjusted, or the bearings packed up until the armature is correctly centered.

Ques. What happens in case of breaks in the armature coils?

Ans. This fault results in local heating of the armature, for the reason that resistance is interposed in the path of the current at the fracture. It always results in sparking at the brushes, and the heating being confined to the neighborhood of the break.

Ques. What are the effects of operation above the rated voltage and below normal speed?

Ans. Voltage above normal is a possible cause of heating, and operation below normal speed calls for an increase of field strength and reduces the effective ventilation, thus tending to cause heating.

Fig 730Forced system of lubrication as applied to engine of the generating set shown in

Fig. 730.--Forced system of lubrication as applied to engine of the generating set shown in fig. 443. In engines employing the forced system of lubrication the crank pit, which is formed by the columns, is accessible through doors in the front and back of the engine. The base of the engine forms an oil tank to which is attached a small plunger pump driven by an eccentric on the shaft. The lubricant is carried under pressure to the various parts of the engine by the mechanism shown in the accompanying diagram. The oil is forced by a pump to a groove in the main bearing, and a drilled hole in the shaft connects this groove with the crank pin. From the crank pin box the oil is further forced to the wrist pin through the pipe running along the side of the connecting rod. The passage in the crosshead allows the oil to be forced from the wrist pin to the guides. As the oil is forced from one bearing to another, it is quite important that the bearing caps be set tight, otherwise the oil will escape before reaching the last bearing. After passing through the bearings, the oil is collected in the base, strained and used again. The oil should be free from foreign substances, and to guard against the introduction of any foreign matter, a strainer, which may be taken out for examination or cleaning, is attached to the suction valve of the pump. An oil pressure of from 10 to 20 lbs. should be maintained, and may be regulated by adjusting the set screw on the relief valve of the oiling system. The pressure gauge need not remain in the circuit continuously. Only mineral oils should be used for lubrication. A heavy oil gives better results and prevents knocking more effectively than thin oil. An oil which has been found to give good results, consists of two-thirds red engine oil and one-third heavy cylinder oil. As the oil passes through the bearings repeatedly, it gradually loses its lubricating properties, becoming thick and gritty, and should be occasionally run through a filter and mixed with new oil. The frequency of this change depends on the oil, as well as the number of hours the engine is in operation, and can easily be determined by observation. The oil in the reservoir should stand about 2 inches over the suction and discharge valves, and no water should be allowed to mix with it. Should any water accumulate in the base, it should be drawn off by the cock provided for the purpose before starting the engine.

Ques. How may the field magnets become heated?

Ans. By excessive field current; eddy current in pole pieces; moisture; short circuits.

Ques. What may be said with respect to excessive field current?

Ans. When heating results from this cause, all the exciting coils will be heated equally. It may be due to excessive voltage, in the case of shunt dynamos; or to an overload in the case of compound and series dynamos. In either case it may be remedied by reducing the voltage or overload. If due to the coils being incorrectly coupled up, that is, coupled up in parallel instead of in series, it will be necessary to rectify the connections or insert a resistance in series.

Ques. State the causes of eddy currents in the pole pieces?

Ans. This fault may be due to defective design or construction of the armature. Slotted armatures are particularly liable to cause this fault, if the teeth and air gap be not properly proportioned. The defect may also be occasioned by variation in the strength of the exciting current.

If due to this latter cause, it will be accompanied by sparking at the brushes. If a shunt dynamo, insert an ammeter into the shunt circuit, and note if the deflection be steady. If this be not the case, the variation in the current most probably proceeds from imperfect contacts thrown into vibration.

Ques. How is the insulation affected by moisture?

Ans. Moisture tends to decrease the insulation resistance, thus in effect producing a short circuit with its attendant heating.

Ques. How is moisture in the field coils detected?

Ans. It is easily detected by applying the hand to the coils, when they will be found to be damp, and in addition steam or vapor will be given off where the machine is working.

The fault may be remedied by drying and varnishing the coils.

Ques. What is the indication of short circuits in the field coils?

Ans. This fault is characterized by an unequal heating of the field coils. If the coils be connected in series, the faulty coil will be heated to a less extent than the perfect coils; if connected in parallel, the faulty coil will be heated to a greater extent than the perfect coils. The former can thus be easily located.

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