The most common faults in DC machines

Brush sparking of DC machines.

Brush arcing can be caused by a variety of reasons that require service personnel to closely monitor the sliding contact system and brush apparatus. The main of these causes are mechanical (mechanical arc) and electromagnetic (electromagnetic arc).

The mechanical causes of sparking are independent of the load. Brush arcing can be reduced by increasing or decreasing brush pressure and, if possible, decreasing peripheral speed.

With a mechanical spark, green sparks spread across the entire width of the brush, burning collector not naturally, disorderly. Mechanical sparking of the brushes is caused by: local or general beating, scratching of the sliding surface of the collector, scratches, protruding mica, bad groove of the collector (cutting the mica between the collector plates), tight or loose fitting of the brushes in the brush holders, the flexibility of clamps causing brush vibrations, machine vibrations, etc.

Electromagnetic causes of brush sparking are more difficult to identify.Sparking caused by electromagnetic phenomena varies in proportion to the load and depends little on speed.

The electromagnetic spark is usually blue-white. The sparks are spherical or in the form of drops. The burning of the collector plates is natural, by which it is possible to determine the cause of the sparking.

If a short circuit occurs in the winding and equalizers, the soldering is broken or a direct break occurs, the spark will be uneven under the brushes, and the burnt plates will be located along the collector at a distance of one pole.

If the brushes under the clamps of one pole spark more than under the clamps of the other poles, this means that there was a rotation or short circuit in the windings of individual main or additional poles; the brushes are not positioned correctly or their width is wider.

In addition, additional violations can be observed in DC machines:

• displacement of the crosshead of the brush from neutral causes sparking and heating of the brushes and the collector;

• the deformation of the sliding surface of the collector causes vibrations and sparks of the brushes;

• the asymmetry of the magnetic field causes a decrease in the reactive EMF threshold, impairs the switching ability of the machine, which in turn causes sparking of the brushes. The magnetic field of the machine is symmetrical if the correct circular pitch between the lugs of the main and auxiliary poles is strictly observed and the calculated clearances under the poles are maintained.

For large machines, the adjustment of electromagnetic circuits is carried out by the spark-free zone method.

Increased heating of the DC machine.

In a DC machine, there are several heat sources that heat up all its elements.

The concept of increased heating of the insulation includes passing through the permissible limit of the heat resistance classes of the insulation accepted in the electrotechnical industry.

In the practice of electrical engineering plants in our country, a rule has been introduced to create a certain margin for the heat resistance of the insulation by taking working temperatures with a class lower than the insulation used. Most machines are now manufactured with class F thermal insulation; this means that the permissible temperature rises for the windings must be the same as for class B, i.e. approximately 80 ° C. This rule was introduced due to the accidental destruction of the insulation of the windings of roller machines due to high temperatures.

Overheating of DC machines can be caused by various reasons.

When machines are overloaded, general overheating occurs due to the heat generated by the armature winding, additional poles, compensating winding and field winding. The load on large machines is monitored by an ammeter, and the heating of the windings is controlled by devices connected to sensors mounted in various isolated elements of the machine — armature winding, additional poles, compensating winding, excitation winding. For particularly critical large cylinder engines operating under severe conditions, signals are displayed in the operator's control room and in the engine room, warning that the temperature of the machine has risen to the limit value.

Overheating can be caused by the high temperature of the room where the machines are installed.This may be due to improper ventilation in the engine room. All air ducts must be serviceable, clean and transportable. The filters must be cleaned systematically by pulling the sieves through mineral oil.

Air coolers are sometimes clogged with microorganisms that impede the flow of water. Periodically, the air coolers are backwashed.

Dirt (dust) entering the machine contributes to heating. So, the conducted studies of electric motors showed that coal dust with a layer of 0.9 mm falling on the windings contributes to a temperature increase of 10 ° C.

Clogging of the windings, ventilation ducts of active steel, the outer shell of the machine is unacceptable, as this creates thermal insulation and stimulates an increase in temperature.

Overheating of the armature winding of the DC machine.

The largest amount of heat can be released in the armature. The reasons can be different.

Overloading the entire machine, including the armature, will heat up. If the machine works at low speeds, but is made as self-ventilated, the ventilation conditions deteriorate, the armature will overheat.

The collector, as an integral part of the fixture, will help warm up the machine. The collector temperature can rise significantly under the following circumstances:

• constant operation of the machine at maximum power;

• incorrectly selected brushes (hard, high coefficient of friction);

• in the engine room, where the electrical machines are installed, the air humidity is low. In this case, the friction coefficient of the brushes increases, the brushes accelerate and heat the collector.

The requirement to maintain adequate air humidity in machine rooms is dictated by the need to ensure the presence of a wet film between the brush and the sliding surface of the collector as a lubricating element.

An uneven air gap can be one of the causes of overheating of the armature winding. With an uneven air gap in the part of the armature winding, an emf is induced, as a result of which equalizing currents arise in the winding. With significant unevenness of the gaps, they cause heating of the coil and sparking of the brush apparatus.

Distortion of the magnetic field of a DC machine occurs, as noted, due to the unevenness of the air gaps under the poles, and also when the windings of the main and auxiliary poles are incorrectly turned on, a rotation of the circuit in the coils of the main poles, which causes equalizing currents, which cause heating of the coil and sparking of the brushes at one pole is stronger than the other.

In the case of a spin circuit in the armature winding, the machine cannot work for a long time, because due to overheating, the short-circuited section and the active steel may burn out in the center of the development of the spin circuit.

Contamination of the armature winding insulates it, impairs heat dissipation from the winding and, as a result, contributes to overheating.

Generator demagnetization and magnetization reversal. A parallel-excited DC generator can be demagnetized before its first start after installation. A running generator is demagnetized if the brushes are displaced from neutral in the direction of armature rotation.This reduces the magnetic flux generated by the parallel field coil.

Demagnetization, and then reversal of the magnetization of the parallel-excited generator, is possible when starting the machine, when the armature magnetic flux reverses the magnetization of the main poles and changes its polarity. excitation coil. This happens when the generator is connected to the mains at start-up.

The residual magnetism and polarity of the generator is restored by magnetizing the excitation coil from an external reduced voltage source.

When starting the engine, its speed increases excessively. The main faults in DC machines that cause the speed to increase excessively include the following:

• mixed excitation — parallel and series excitation windings are connected in opposite direction. In this case, when starting the electric motor, the resulting magnetic flux is small. In this case, the speed will increase sharply, the engine may switch to «different». The inclusion of parallel and series windings must be coordinated;

• mixed excitation — the brushes are shifted from neutral to rotation. This acts on the demagnetization of the motor, the magnetic flux weakens, the speed increases. Brushes should be set to neutral;

• series excitation — no-load starting of the motor is allowed. The engine will run out of speed;

• in parallel winding, turn circuit - the engine speed increases. The more turns of the field winding close to each other, the smaller the magnetic flux will be in the motor excitation system.Closed coils must be rewound and replaced.

Other malfunctions are also possible, for example.

The brushes are offset from neutral in the direction of engine rotation. The machine is magnetized, that is, the magnetic field increases, the engine speed decreases. The crosshead should be set to neutral.

Open or short-circuit the armature winding. The motor speed is reduced drastically or the armature does not turn at all. Brushes shine brightly. It must be remembered that if there is a break in the winding, the collector plates will burn out after two pole divisions. This is due to the fact that when there is a break in the winding in one place, the voltage and current under the brush doubles when the circuit is broken. If there is a break in two places next to it, the voltage and current under the brush are tripled, etc. Such a machine must be immediately stopped for repair, otherwise the collector will be damaged.

The motor "rocks" when the magnetic flux in the field coil is weakened. The motor works quietly up to a certain speed, then when the speed increases (within the passport data) due to the weakening of the field in the excitation coil, the motor begins to "pump" strongly, that is, there are strong fluctuations in current and speed. In this case, one of several malfunctions is possible:

• the brushes are offset from neutral to the direction of rotation. This, as stated above, increases the rotational speed of the armature.The weakened flux of the excitation coil is affected by the reaction of the armature, in this case there is an increase, then a weakening of the magnetic flux, and accordingly the frequency of rotation of the armature changes in the "swing" mode;

• with mixed excitation, the series winding is turned on in anti-parallel, as a result of which the magnetic flux of the machine will be weakened, the rotation speed will be high and the armature will enter the "swing" mode.

For the 5000 kW machine, the clearances of the main posts from the factory shape have been changed from 7 to 4.5 mm. The maximum speed used is 75% of the nominal. Then, after a few years, the rotation frequency increases to 90-95% compared to the nominal, as a result of which the armature begins to "swing" strongly in terms of current and rotation frequency.

It is possible to restore the normal position of a large machine only by restoring the air gap under the main pillars, according to the shape, from 4.5 mm to 7 mm. Any machine, especially a large one, should not be allowed to "sway".