Repair of a direct current motor (DMC) is a special discipline in electrical engineering, in which the brush-commutator unit plays a key role. Efficiency, service life, and the very performance of the machine depend on its condition. In this article, Elektromotors engineers from Tashkent analyze in detail the DMT device, typical faults, the sequence of brush replacement, commutator grooving technology, armature balancing and final tests. The material is aimed at chief power engineers, technical directors and shop foremen operating cranes, rolling mills, excavators and traction drives.
DPT device: armature, stator, commutator and brushes
The DC motor consists of a stationary part (stator with main and additional poles) and a rotating part (armature with winding, commutator and shaft). The current is supplied to the armature winding through the brush-collector assembly - it is this that distinguishes the DPT from an asynchronous machine and at the same time is the most vulnerable point.
- Stator - a cast or welded frame with main poles on which the field windings are located (series, parallel or mixed), and additional poles to improve commutation.
- Anchor - a package made of sheet electrical steel mounted on a shaft, with grooves for winding sections and a collector.
- Collector is a cylinder of copper plates (lamellas) separated by micanite spacers. Through it, the voltage is removed from the rotating winding.
- Brush assembly - brush holders, springs, graphite or electrographite brushes, traverse with adjustable position.
The main and additional poles must be strictly centered relative to the armature: displacement of the poles causes sparking and burning of the lamellas. In Tashkent, on crane DPTs, the position of the traverse often “floats” due to aging, and the geometry of the neutral is violated.
Typical DPT malfunctions
Over the years of practice, we have systematized the most common breakdowns. Below is a summary table with signs and causes.
| Fault | External signs | Cause |
|---|---|---|
| Sparking under the brushes | All-round fire, carbon deposits on the commutator | Brush wear, commutator runout, neutral displacement |
| Anchor runout | Vibration, hum, bearing heating | Imbalance, bearing wear, shaft bending |
| Breakdown of the armature winding | Protection triggered, burning smell | Overheating, insulation aging, moisture |
| Scorched commutator | Dark rings on the lamellas, micanite protrusion | Wrong brand of brushes, overload |
| Short circuit of the lamellas | Drop in speed, overheating | Copper dust in the inter-lamella grooves |
| Break in the field winding | Acceleration to dangerous speed | Overheating, vibration, poor soldering |
Most problems are diagnosed visually and with a megohmmeter. More details about the methods can be found in the material about diagnostics of electric motors.
Replacing brushes: brands EG-74 and EG-841
Brushes are consumables, and their choice determines the service life of the commutator. There is no universal brand: each mode has its own composition. Electrographite brushes are most often used.
- EG-74 is an electrographite brush of medium hardness. Used on crane, rolling and metallurgical DFCs with voltages up to 440 V. Works well under shock loads and reverse. Permissible linear speed of the collector is up to 40 m/s.
- EG-841 - harder, for heavy duty conditions with frequent reverse and overloads. It is used on traction, shaft and load-lifting DPTs. Gives less sparking, but requires a stiffer brush holder spring.
- EG-4 - for low and medium power engines with uniform load.
- EG-14 - for traction motors with high voltage between the lamellas.
It is strictly forbidden to mix brands of brushes in one bracket - one brush works, the other sparks, and as a result both and the commutator burn. When replacing, new brushes are ground with glass sandpaper (not emery cloth) to the shape of the commutator, after which the engine is idled for 2-4 hours to break in. The spring pressure on the brush is set using a dynamometer within 15-25 kPa, depending on the brand.
Grooving the commutator and milling the inter-lamella insulation
If grooves are visible on the collector, the ellipse exceeds 0.03 mm, or the lamellas are worn out unevenly, a groove is required. The anchor is installed in the centers of a lathe or a special flow device, and the cutter removes the minimum required layer of copper - usually 0.3-0.8 mm. Feed 0.05-0.1 mm/rev, speed 80-150 m/min, carbide cutter with a small tip angle.
After grooving, the collector is ground with fine glass sandpaper, and then the inter-lamella grooves must be milled: the micanite should be deepened 0.8-1.5 mm below the surface of the copper. If this is not done, the hard micanite will protrude as the copper wears and lift the brushes, causing sparking. After milling, remove the burrs, blow with compressed air and check the runout with an indicator - no more than 0.02-0.03 mm is acceptable.
At the same time, the condition of the armature winding is checked. If there is a breakdown or an interturn short circuit, rewinding of the rotor is performed, followed by soldering of the collector cockerels with silver-containing solder PSR-15.
Balancing the armature after repair
DFCs operate at speeds from 500 to 3000 rpm and above, and any armature imbalance leads to runout, commutator spalling and premature bearing wear. After rewinding and grooving, balancing is required.
The anchor is installed on a balancing machine and checked in two correction planes. The permissible residual imbalance for medium-power engines is no more than 4-6 g mm/kg. Corrective masses are attached to special grooves in the balancing rings or to the ends of the armature package. The technology is described in more detail in the material about electric motor rotor balancing.
At the same time, the condition of the bearings is checked. DBTs mainly use angular contact roller and ball bearings of accuracy classes 5-6. Worn ones or with a play of more than 0.05 mm are replaced with new ones - this is included in the standard scope of replacement of bearings in an electric motor.
Tests after repair
The repaired DPT undergoes a mandatory test cycle in accordance with GOST 11828. In the Elektromotors workshop in Tashkent, this cycle includes:
- Measurement of insulation resistance with a 500 or 1000 V megohmmeter - at least 1 MOhm for machines up to 1000 V.
- Testing the electrical strength of insulationwith increased voltage 2Unom+1000 V for 1 minute.
- Measurement of winding resistance to direct current to check the integrity and absence of interturn short circuits.
- Idling - checking vibration, noise, heating of bearings, absence of sparking under the brushes.
- Load test 70-100% of the rated value with current, speed and temperature measurements.
- Checking commutation based on the degree of sparking of the brushes - no more than 1.5 points on the GOST 183 scale are acceptable.
After successful testing, a protocol is drawn up and the engine is shipped to the customer with a guarantee. For urgent orders of crane DPTs in metallurgy and construction, an urgent repair service with an engineer visiting the site is available.
Prevention: how to extend the life of the brush-collector assembly
To avoid having to deal with major repairs less often, we recommend:
- check the length of the brushes once a month and replace them when worn more than 60% of the original height;
- once a quarter, blow out the collector with dry compressed air and remove coal dust;
- monitor the pressure of the brush springs;
- do not allow overloads of more than 25% of the nominal value;
- If a persistent spark appears, immediately stop the machine and check the neutral.
Do you need a DPT repair or diagnostics of a brush-collector unit? Elektromotors engineers in Tashkent perform a full cycle of work: from replacing brushes to rewinding the armature and balancing. Contact us for a consultation or order a specialist visit through the services section.