How DC motors are repaired: the complete technological process

A typical DC motor repair includes:

1. 01Intake of the motor, checking the nameplate and documentation.
2. 02Initial fault detection: visual inspection, insulation resistance measurement on the armature and field windings, inspection of the commutator and brushes, checks of shaft run-out and bearing play.
3. 03Teardown: pulling the armature out of the frame bore on a dedicated rig, removing the commutator when needed, disassembling the bearing units.
4. 04Detailed electrical diagnostics: megger testing, a surge test on the armature coils, a core loop test, checking the main and commutating pole coils and the field winding.
5. 05Removing the damaged armature winding and, where needed, the pole coils.
6. 06Manufacturing new parts: armature coils to the original winding data, a commutator, pole coils.
7. 07Laying the new armature winding and correctly connecting the coil leads to the commutator bars.
8. 08Impregnating and heat-treating the armature winding.
9. 09Mechanical restoration of the shaft, bearings, seating surfaces and housing.
10. 10Finishing the commutator: turning, undercutting, polishing.
11. 11Assembly: refitting the armature, setting the brush-holder rocker relative to the neutral, adjusting brush spring pressure.
12. 12Dynamic balancing of the armature.
13. 13Final testing: no-load run, checking commutation, load and high-voltage testing, checking vibration, and issuing test protocols.

The exact scope depends on the motor’s power, construction, the extent of the damage and its duty — for example, a traction, crane, or rolling-mill motor.

In induction or synchronous machines, almost all of the "winding" repair work concerns the stationary stator. A DC machine is the opposite — the winding that most often needs rewinding rotates with the shaft and is continuously switched by the brushes through the commutator.

That is why almost every "winding" stage of a DC motor repair is specific to it: checking, removing, manufacturing and laying the armature winding, working on the commutator, and setting the brush rocker. Repairing the field winding on the poles follows the same principles as repairing stator coils, so this article gives it less attention.

On intake the nameplate data and accompanying information are recorded:

• the motor type and manufacturer;
• the serial number;
• rated power, armature and field voltage;
• rated armature and field current;
• rated speed and its control range;
• the excitation scheme — separately excited, shunt, series, or compound;
• the insulation class and enclosure rating;
• the mounting type and whether a half-coupling or pulley is fitted;
• whether a tachogenerator or speed sensors are fitted;
• the reason the motor was sent for repair.

If the nameplate is damaged or missing, winding data are recovered from the datasheet, the drawing, earlier repair reports, or the markings on the armature or pole windings themselves.

Information about duty and the failure

Because the armature and the commutator are especially sensitive to overload, load swings and incorrect supply, the customer is asked:

• whether the motor is fed from a thyristor or transistor converter;
• whether brush sparking was visible before the shutdown, and what it looked like;
• whether the brushes, commutator or winding were replaced before;
• how many reversals and starts happen per hour;
• whether there was a burning smell, smoke, or a flashover;
• whether the load is of a shock nature — cranes, rolling mills, excavators;
• when the commutator was last turned.

Before teardown, a set of checks is carried out that gives an initial picture of the armature, commutator and field winding condition.

Visual inspection

• the commutator’s condition — the color of the running surface, burning, grooves, high bars;
• the brushes’ condition — length, cracks, even wear, the color of any charring;
• the position of the brush rocker and brush holders;
• the condition of the armature winding’s end turns — discoloration, cracked bands;
• the condition of the main and commutating poles, visible damage to the field winding;
• traces of overheating, moisture or oil on the winding;
• the condition of the fan and cooling passages.

Insulation resistance measurement

A megohmmeter is used to measure the insulation resistance of the armature winding against the frame (via the brushes or directly at the commutator) and of the field winding against the frame and against the armature. Measurements are taken both before and after cleaning, since a dirty commutator often understates the reading.

Inspecting the brush-commutator assembly

Brush spring force, the free movement of brushes in their holders and the condition of the flexible leads are checked separately, along with commutator run-out and ovality using a dial indicator while the shaft is turned slowly.

Mechanical checks

• radial and axial run-out at the shaft end;
• axial rotor play;
• unusual noise while turning the shaft;
• bearing condition by sound and by feel, and any play.

The results of the initial fault detection determine whether the repair can be limited to the commutator and the bearings, or whether a full teardown with detailed electrical diagnostics of the armature winding is required.

Before teardown, the relative positions of the end shields, the rocker, the poles and the field-winding leads are marked — reinstalling the rocker in the correct position relative to the neutral is critical for commutation.

Pulling the armature out of the frame

The armature is pulled out of the frame bore on a dedicated rig, or with guides and lifting gear, so the armature core never touches the poles or the field winding.

• mandrels, roller supports or carts are used for large armatures;
• the commutator and the winding’s end turns are protected from impact during removal;
• for heavy armatures — traction, rolling-mill, or stepping motors — a crane and slings are used at the points provided for by the design;
• the armature must never be allowed to rest on the commutator or the winding during handling.

Removing the commutator

The commutator is taken off the armature only when necessary — for a full commutator replacement, an internal-joint repair, or access to the slots underneath it. Removal is accompanied by recording the connection scheme between the coils and the bars.

Disassembling the bearing units

Bearings are removed with the force applied to the ring that has the interference fit, using pullers or controlled induction heating. At the same time, the seals, labyrinths and grease chambers are checked.

After teardown, an in-depth check of the armature, commutator and field winding uncovers defects that the initial inspection could not.

Megger testing

The insulation resistance of the armature winding and of every field winding — separately excited, shunt, series, commutating-pole, compensating — is measured against the frame separately.

Interturn short test (surge test and growler test)

The armature coils are checked for an interturn short with an impulse surge tester or on an induction-type growler. Both methods find shorted or damaged turns that a plain insulation-resistance reading cannot.

• a surge test compares the waveform between every pair of commutator bars — a distorted waveform points to a short in that coil;
• a growler test finds shorted coils with a steel strip that vibrates or is pulled down over the slot holding the damaged coil;
• both methods are used alongside bar-to-bar resistance measurements.

Checking the armature core (loop test)

The armature core is checked for interlaminar shorts with a ring-flux loop test or thermal imaging — local overheating of the stack points to shorted laminations, which have to be fixed before the new winding is laid in.

Checking the main and commutating pole coils

The main pole coils (the field winding) and the commutating-pole coils are checked for resistance, an interturn short, and continuity. Pole polarity sequence is checked separately — it must match the factory scheme, or reassembly can mix up the N-S-N-S sequence, which causes heavy sparking even in a sound armature.

Checking the field winding

On motors with series, shunt, or compound excitation, the continuity and resistance of the series winding (high-current, few turns of heavy wire) and the shunt winding (many turns of fine wire) are checked separately, along with their insulation against the frame and against each other.

The results of every check are logged in the incoming electrical diagnostics report, which becomes the baseline for comparison with the final test results.

If diagnostics confirm an interturn short, an open circuit, a ground fault, or mechanical damage in the armature winding, the old winding is removed.

Recording the winding data before removal

Before the winding is removed, the following are recorded:

• the number of armature slots and commutator bars;
• the winding type — lap or wave, simplex or multiplex;
• the slot pitch and the commutator pitch;
• the number of coils, turns per coil, and parallel paths;
• the diameter and number of strands per conductor;
• the connection scheme between the coil leads and specific commutator bars;
• the type and thickness of the slot insulation and the banding material.

Removing the armature winding

The old winding is removed by controlled burn-out in an oven, mechanically, or by a combination of both, without damaging the teeth or the interlaminar insulation of the core. Once removed, the slots are cleaned and the core is checked again.

Removing the pole coils

The main or commutating pole coils are taken off the frame only once damage — an open circuit, an interturn short, or an insulation breakdown — is confirmed. Before removal, the number of turns, wire gauge and polarity of each coil are recorded.

Manufacturing the armature coils

New armature coils are wound to the recorded winding data — the same pitch, turn count, wire gauge and grade. Coils are wound on formers that reproduce the slot shape and end-turn geometry of the original.

• keeping to the winding scheme (lap or wave) and pitch;
• making every coil identical in shape and turn count;
• leaving enough lead length to reach the commutator;
• checking the wire insulation before it is laid in.

Manufacturing or repairing the commutator

Depending on its condition, the commutator is:

• turned to remove run-out, ovality, and traces of sparking;
• undercut to bring the inter-bar insulation to the correct depth;
• repaired by replacing individual damaged bars, where technically possible;
• fully remanufactured — assembling new bars, V-rings and insulating cones — when it cannot be restored.

Manufacturing pole coils

New main pole, commutating pole, or compensating winding coils are made to the original turn count, wire gauge and connection scheme when needed, keeping strictly to the polarity sequence.

Laying the new armature winding is the most demanding stage of the repair, because it decides whether the machine will commutate correctly.

Slot insulation and laying the coils

• new slot insulation of the correct thermal class goes into the slots;
• coils are laid in strictly the same sequence and pitch as the original winding;
• crossed conductors, damaged enamel and pinched insulation are not tolerated in the slot;
• slots are closed with wedges once the coils are in.

Connecting coil leads to the commutator bars

Every coil lead is connected to a specific, predetermined commutator bar, following the scheme that matches the winding’s commutator pitch. The joint is made by soldering or welding, giving a low contact resistance and mechanical strength.

Banding

The winding’s end turns are secured with fiberglass tape or wire banding at the tension called for in the drawing — too little tension lets the winding shift under centrifugal force, too much damages the wire insulation.

After laying and banding, an interim electrical check follows: insulation resistance, winding resistance, and the symmetry of resistance between neighboring commutator bars.

The new armature winding is impregnated with varnish or resin to raise its dielectric strength, its mechanical resistance to centrifugal force, and its protection against moisture and contamination.

Impregnation methods

• dip-and-drain, letting excess varnish run off;
• vacuum pressure impregnation (VPI) — the most complete way to fill the winding’s pores and voids;
• multiple impregnation cycles for critical or high-speed armatures.

Curing and drying

After impregnation the armature is baked in an oven on a cycle matched to the varnish or resin, with temperature, dwell time, and insulation resistance monitored throughout the bake.

Unlike the stator’s stationary winding, a rotating armature is under constant centrifugal load, which is why impregnation quality here is especially critical for the long-term integrity of the banding and end turns.

Shaft repair or replacement

The shaft is checked for run-out, bending, cracks and worn seating surfaces. Depending on the defect, it is straightened, built up by welding or metallized at the seats, fitted with a repair sleeve, or replaced outright.

Bearing replacement

Bearings removed to gain access for armature fault detection are, as a rule, replaced with new ones — reusing a bearing that has been pulled off an interference fit, or that shows even minor wear on its raceways, raises the risk of an early failure.

Restoring seating surfaces

Worn bearing seats on the shaft and in the end shields are restored by metallizing, thermal spraying, weld buildup, or a repair sleeve, followed by machining to the required diameter, roundness and finish.

Frame repair

The frame is checked for cracks, deformed feet, and the condition of the pole and end-shield seats; welding, machining of the mounting surfaces, and re-cutting threads are carried out as needed.

Once the armature winding is laid, impregnated and the commutator fitted, its final machining takes place.

Turning

The commutator is turned to its final diameter on a lathe or a dedicated commutator lathe, removing the minimum stock needed to clear run-out, ovality, and traces of earlier wear or sparking.

Mica undercutting

The inter-bar insulation (mica) is recessed below the copper to the correct depth — too shallow and the mica stays proud and breaks brush contact, too deep and carbon dust builds up faster in the grooves.

Polishing

The running surface is polished to the required finish, bar edges are chamfered, and copper burrs left over from undercutting are removed.

The armature is fitted into the frame with the same care used to remove it — no contact between the core and the pole shoes, and no impact loads on the commutator.

Setting the brush rocker relative to the neutral

The position of the rocker, and with it the brushes, relative to the geometric and magnetic neutral is one of the most important assembly parameters. The rocker is set to the factory marking, or located by the induced-voltage (millivoltmeter) method, or by finding the point of minimum sparking during a trial run.

Adjusting brush spring pressure

Spring force is equalized across every brush of a given polarity and checked with a gauge, not by hand. Too little force lets a brush bounce and spark; too much accelerates wear on both the brush and the commutator.

Bedding in the brushes

New brushes are bedded to the commutator profile with glass paper of the right grit — never sandpaper with a metallic or silicon abrasive — until the contact face shows an even imprint along its whole length.

Balancing the armature is mandatory after rewinding, replacing the commutator, repairing or replacing the fan, straightening the shaft, or any other work that changes the mass distribution of the rotating part.

• balancing is done fully assembled — with the commutator, the fan, and the key fitted;
• the balance quality grade specified for the motor’s type and speed is observed;
• imbalance is corrected in the planes provided by the design, by removing metal or by adding balance weights;
• the results are recorded in a report stating the initial and residual imbalance.

The assembled motor goes through a set of acceptance tests that confirm the quality of the armature, commutator and related work.

No-load run

The motor is started unloaded, checking smooth acceleration, direction of rotation, noise and vibration levels, bearing temperature, and field current symmetry.

Checking commutation

Brush sparking is assessed first unloaded, then under load — light, even sparking with no blue or white flashes counts as normal; any increase in sparking as load or speed changes points to an uncorrected commutation defect.

Load testing

The motor is loaded to its rated value and, where the test program calls for it, up to overload, while armature and field current, speed, winding heating and commutator temperature are monitored.

High-voltage (HiPot) testing

The armature and field winding insulation is tested with an elevated power-frequency voltage against the frame — this confirms the dielectric strength of the insulation restored during the repair.

Vibration check

Vibration is measured at reference points on the end shields, both unloaded and under load, and compared against the limits for the motor’s balance quality grade.

Test protocols are produced from every result and attached to the motor’s accompanying documentation.

How long a DC motor repair takes depends chiefly on the amount of work needed on the armature and commutator, not just on the motor’s power.

These timeframes are indicative and are refined after fault detection — building a non-standard commutator or sourcing an unusual wire grade can extend a repair considerably.

• How electric motors are repaired: the complete process
• Winding insulation resistance testing
• Rotor balancing
• Why do an electric motor’s brushes spark?
• What does a capital repair of a motor include?
• High-voltage testing
• Post-repair motor testing
• The motor commutator: design, repair, undercutting
• The DC motor: construction and operating principle

EPR (Elektropromremont) carries out the full cycle of DC motor repair in-house — from armature and commutator fault detection to manufacturing new coils, commutators and pole windings, balancing, and acceptance testing.

The scope of work includes:

• fault detection on the armature, commutator, and field winding;
• testing the armature winding for an interturn short — surge and growler checks;
• rewinding the armature to the original winding data;
• manufacturing and repairing commutators, turning and undercutting;
• manufacturing main pole, commutating pole and compensating-winding coils;
• vacuum pressure impregnation and heat treatment of windings;
• shaft repair, restoring bearing seats, and bearing replacement;
• dynamic balancing of armatures;
• no-load, load, and high-voltage testing.

• DC electric machine repair
• All services