A DC motor is an electrical machine that converts direct-current electrical energy into mechanical energy through the interaction of current in the armature winding with the magnetic field of the poles, with the commutator continuously reversing the direction of that current.
Although most industrial drives today are built around induction motors and variable-frequency drives, DC machines remain in service wherever a very high starting torque, deep and smooth speed control over a wide range, or precise torque control is required — in rolling mills, traction drives, cranes, excavators and the exciters of synchronous machines.
DC motors and the closely related DC generators are used, among other things, in:
The main reasons DC machines are still used alongside modern induction drives are:
At the same time, a DC motor is a substantially more complex machine than a squirrel-cage induction motor.
Its service life depends on the condition of several interdependent assemblies:
A repair that restores only one of these assemblies is not enough — the commutator, the armature winding and the bearings should be treated as parts of a single electromechanical system.
A DC motor is an electrical machine in which direct current in the armature winding interacts with the stationary magnetic field of the poles, while the commutator and brushes continuously reverse the current direction in the rotating armature conductors so that the electromagnetic torque always acts in the same direction.
In the most general terms, a DC motor works as follows:
The main types of DC motor by method of excitation are:
Each type has its own torque-speed characteristic and its own typical field of use, covered below.
A DC motor consists of a stationary magnetic system (the stator) and a rotating armature, linked through the air gap, together with a brush assembly that provides the electrical contact between the stationary and rotating parts of the machine.
The main assemblies are:
The frame is the motor’s outer housing and, at the same time, part of the magnetic circuit.
It performs several functions:
Frames are cast from steel or cast iron, or, in large machines, welded from structural steel plate; the material and wall thickness are chosen to give the cross-section the flux needs without excessive saturation.
The main poles are bolted to the frame and produce the machine’s main magnetic flux.
Each main pole consists of:
The field winding can be supplied from the same source as the armature (shunt- or series-wound motors), from a separate source (separate excitation), or combine a series and a shunt coil on the same pole (compound excitation).
Interpoles are mounted between the main poles, precisely on the machine’s geometric neutral.
Their winding is connected in series with the armature circuit, so the field they produce is proportional to the load current.
Their purpose is to compensate for the reactance voltage induced in the coil undergoing commutation, thereby keeping commutation sparkless across a wide load range.
Reversed polarity, an open circuit, or an interturn short in an interpole winding after a repair is one of the most common causes of heavy brush sparking even when the commutator itself is in good condition.
The brush assembly consists of the brushes, the brush holders and the rocker they are mounted on.
Spring-loaded brushes pressed against the commutator surface provide the sliding electrical contact between the stationary supply and the rotating armature.
The rocker lets the brushes be set precisely on the neutral axis and, when needed, allows the whole brush assembly to be rotated together during adjustment.
The shaft transmits the armature’s torque to the driven machine and carries the radial and axial loads through bearings fitted in the end shields.
Cooling is provided by a shaft-mounted fan, by an independently driven forced-ventilation fan, or, in high-power machines, by air-to-air or air-to-water heat exchangers — the choice depends on the motor’s power, duty cycle and the dust and moisture levels expected in the installation.
The armature core is built up from thin electrical-steel laminations insulated from one another with varnish or an oxide film to limit eddy-current losses, since, unlike the main poles, the armature core is continuously remagnetized as it rotates.
The armature winding is laid in slots cut into the core, and the laminated stack itself is pressed onto the shaft or onto a hub.
The armature winding is built as either a lap (parallel) winding or a wave (series) winding, with its ends connected to the corresponding commutator segments.
The choice of winding type depends on the machine’s rated current and voltage:
Coil sides are held in the slots with wedges or bands, and the end windings with binding wire or glass tape, since they experience considerable centrifugal force in operation.
The commutator is made up of copper segments, insulated from one another and from the shaft by mica or another heat-resistant dielectric.
Each segment is connected to the corresponding armature coil, so the commutator is effectively a mechanical switch that, together with the brushes, acts as a rotating rectifier: it converts the alternating current in the armature coils into direct current in the external brush circuit.
After the commutator surface is turned, the inter-segment insulation must be recessed below the copper — an operation called undercutting; if the mica stays flush with or stands proud of the copper, the brush loses stable contact, which causes vibration and sparking.
A current-carrying conductor placed in a magnetic field experiences an electromagnetic force whose direction is given by the left-hand rule and whose magnitude equals the product of flux density, conductor length and current.
Because the armature conductors are distributed around the circumference under main poles of opposite polarity, the forces acting on them combine into a net rotating electromagnetic torque applied to the armature.
As the armature rotates, each coil passes in turn under poles of alternating polarity, so the current in it must periodically reverse — this is exactly what the commutator and brushes accomplish together.
The moment a brush bridges two adjacent segments, the corresponding coil is briefly short-circuited, and the current in it must fall to zero, reverse, and rise to its new value in the opposite direction — this process is called commutation.
The interpoles, whose winding carries the armature current, induce a compensating electromotive force in the coil being commutated that helps the current change in time; if this compensation is inexact, sparking appears under the brush.
As the armature turns in the magnetic field, an electromotive force is induced in its winding that opposes the applied voltage — the so-called back EMF, or armature EMF.
Its magnitude is approximately proportional to the product of the pole flux and the rotational speed:
E = k · Φ · n
where:
The voltage at the armature terminals equals the back EMF plus the voltage drop across the armature circuit resistance:
U = E + I · R
where I is the armature current and R is the combined resistance of the armature winding and brush contact.
The motor’s electromagnetic torque is approximately proportional to the product of the pole flux and the armature current:
M = kt · Φ · I
These relationships give the DC machine its basic mechanical characteristic: speed rises with applied voltage, falls as load increases (because the I · R drop grows), and rises as the pole flux is reduced — these three dependencies are exactly what the practical speed-control methods described below rely on.
How the field winding is connected to the armature circuit determines the motor’s torque-speed characteristic and, with it, the applications it suits.
The field winding is connected in series with the armature, so the magnetic flux depends on the same current that produces torque.
This gives the motor its characteristic properties:
A series-wound motor must never be run unloaded or with a disconnected drive link — losing the mechanical load with power still applied can drive the speed to a destructive runaway.
Because of its high starting torque, this type is traditionally used in traction drives — electric locomotives, trolleybuses, cranes and hoisting mechanisms — wherever a large torque at standstill matters most.
The field winding is connected in parallel with the armature (or, in a separately excited motor, fed from a separate source at the same rated voltage).
The magnetic flux is nearly independent of the armature load, so this motor’s speed stays almost constant over a wide load range — a characteristic close to a hard, or stiff, curve.
Shunt and separately excited motors are used wherever a stable speed and precise control are required — in metal-cutting machine tools, feed drives, and certain pumps and fans with special control requirements.
Structurally identical to a shunt motor, but the field winding is fed from an independent DC source rather than from the armature terminals.
This allows the field current and armature voltage to be controlled independently, which is exploited in deep, precise speed-control systems built around a Ward Leonard set or a controlled rectifier.
Has two windings on each main pole: a series winding and a shunt winding.
Depending on whether their magnetic fluxes add or subtract, two variants are distinguished:
Cumulatively compounded motors are used in drives that need both an elevated starting torque and a limited no-load top speed — for example, in certain crane and rolling-mill mechanisms.
The main poles are built from permanent magnets rather than a field winding, so the magnetic flux is essentially constant across the whole load range.
Advantages include a simpler construction, no field losses, and compact size; the drawback is that speed cannot be raised by field weakening, and the magnets risk partial demagnetization under heavy overload or overheating.
These motors are common in low- and medium-power drives — power tools, gate and valve actuators, small servo drives and certain electric vehicles.
One of the main reasons DC machines are still used alongside variable-frequency induction drives is how simply and how deeply their speed can be controlled.
Three main methods are used in practice, and they are often combined.
Reducing the field current lowers the magnetic flux and, with the armature voltage held constant, raises the rotational speed.
This method extends speed above the rated value, but it reduces the available torque, since torque is proportional to both flux and armature current — hence this range is called the constant-power zone, as opposed to the constant-torque zone obtained with armature voltage control.
Varying the voltage applied to the armature while keeping the rated field flux constant gives smooth speed control from zero up to rated speed with practically no reduction in available torque — the constant-torque zone.
Historically the voltage was controlled with a Ward Leonard motor-generator set; modern installations use controlled thyristor or transistor DC converters instead.
A resistor added in series with the armature circuit lowers speed by increasing the I · R voltage drop, but it wastes significant energy in the resistor and is a less precise, less efficient method.
Today, resistance control is mostly used to limit starting current while the motor accelerates rather than as the primary means of running-speed control.
In practice, armature voltage control (for speeds from zero to rated) is most often combined with field weakening (for speeds above rated), giving a wide control range with minimal energy loss.
A DC motor’s nameplate typically lists:
| Parameter | What it shows |
|---|---|
| Rated armature voltage | The design supply voltage for the armature winding |
| Rated field voltage | The supply voltage for a separately excited field winding |
| Rated armature current | The maximum continuously permissible armature current |
| Rated power | The mechanical power at the shaft in the rated duty |
| Rated speed | The rotational speed at rated voltage, current and flux |
| Method of excitation | Series, shunt, separate, compound, or permanent magnet |
| Duty type | S1–S10, by load duration and pattern |
| Insulation class | The maximum permissible winding temperature rise |
| IP rating | Protection against contact, dust and moisture |
| Direction of rotation | The permitted direction, or whether reversal is allowed |
Beyond nameplate data, assessing a specific machine’s condition and repairability also calls for knowing:
Series- or compound-wound DC traction motors are traditionally used in electric and diesel-electric locomotives, trams and mine haulage equipment thanks to their inherently high starting torque and their ability to withstand significant short-term overloads when starting off and climbing grades.
In metallurgy, separately excited DC motors have long remained the drive of choice for the roughing and finishing stands of rolling mills, since they allow speed and torque to be controlled accurately and quickly in a cyclic, reversing duty with frequent overloads.
Crane and excavator drives call for a motor that tolerates frequent starts, reversals and stalls, along with the high torque overload capacity typical of series- and compound-wound machines.
DC machines are also used as generators — power sources for the excitation systems of synchronous generators and motors, welding sets, plating lines and laboratory equipment that need a controllable DC voltage.
Separately excited and shunt-wound motors are used in metal-cutting machine tools, in winders and unwinders in the textile and paper industries, and in other equipment where smooth speed control over a wide range at constant torque matters.
Advantages of DC machines:
Below is a brief overview of the most common faults in DC machines; each is covered in more depth in a dedicated article.
Sparking results from disturbed commutation — a misaligned rocker, a faulty interpole, a dirty or eccentric commutator, overload, or the wrong brush grade; left undiagnosed, heavy sparking can escalate into a flashover, a powerful arc across the commutator surface that destroys segments and brush holders.
Accelerated brush wear is usually tied to incorrect spring pressure, a brush sticking in its holder, the wrong material grade, or a contaminated commutator surface; length, free movement in the holder, and even wear across all brushes of one polarity should all be checked.
Caused by aged or mechanically damaged conductor insulation, overheating, moisture or vibration; it shows up as localized heating in one coil, characteristic sparking at the same point every revolution, and a deviation on a surge (impulse) test of the winding.
An open circuit, an interturn short, or a breakdown to frame in a field or interpole winding alters the flux and disturbs commutation; an interpole fault often masquerades as "ordinary" brush sparking and only shows up under a targeted check.
Produces increased noise and vibration, armature run-out relative to the poles, and, as a result, an uneven air gap and additional sparking; timely bearing replacement and lubrication checks are among the cheapest preventive measures available.
Can be caused by overload, poor ventilation, contaminated windings, an interturn short, or the wrong duty cycle; prolonged overheating accelerates insulation aging and eventually leads to breakdown.
The most common causes are armature imbalance, poor alignment with the driven machine, worn bearings, commutator run-out, or a damaged fan; vibration, in turn, worsens brush-to-commutator contact and increases sparking.
Regular attention to the brush-commutator assembly and the mechanical parts substantially extends a DC machine’s service life and reduces the risk of an unplanned shutdown.
In a DC motor, the magnetic flux and the armature current can be controlled independently, and the commutator mechanically switches the current in the armature coils; an induction motor is fed directly from an AC supply, its rotating field is produced by the stator’s own polyphase winding, and it has no commutator at all.
This motor’s flux depends on the armature current; when the mechanical load is lost, current and torque fall, the flux weakens, and speed can climb to dangerous levels — the so-called runaway condition, which can destroy the armature winding and commutator.
Armature voltage control varies speed from zero to rated with almost no loss of torque and is therefore the primary method; field weakening is used in addition to reach speeds above rated, at the cost of a lower available torque.
In most cases repair is technically and economically sound: the armature, commutator, field coils and mechanical parts can be restored to the original winding data; replacement is usually the better option only when the laminated core or the frame itself is damaged, not the windings or commutator.
The interval depends on duty and brush grade, but in practice the brushes and commutator are inspected at every scheduled maintenance visit, and out of schedule the moment sparking, unusual noise or a burning smell appears.
Over time the commutator surface wears unevenly, developing run-out, grooves and burn marks; turning restores the correct cylindrical shape, after which undercutting — recessing the inter-segment insulation below the copper — is always required.
Yes: the exposed brush-commutator assembly is more sensitive to dust, moisture and aggressive environments than the fully enclosed winding of an induction motor, so DC machines more often need a protected or special enclosure and more frequent maintenance.
Elektropromremont, LLC performs diagnostics, repair and manufacturing of DC machine components — for traction, crane and rolling-mill motors, generators and exciters.
The scope of work includes:
A DC motor is an electrical machine in which the commutator and brushes turn the armature’s rotation into a continuous reversal of current direction in its winding, so that the electromagnetic torque consistently acts in one direction.
How the field winding is connected to the armature determines the motor’s torque-speed characteristic — from the soft curve and high starting torque of a series-wound machine to the stiff, near-constant speed of a shunt- or separately excited machine.
A machine’s service life depends not only on the armature winding but also on the condition of the commutator, the brush assembly, the field coils, the bearings and the ventilation system — so a repair is best treated as restoring one electromechanical system, not a set of separate parts.
This article is for general informational purposes. The choice of motor type, excitation scheme, speed-control method, winding data and repair technology for a specific machine should be based on the manufacturer’s documentation, the nameplate, and the results of professional diagnostics.
This material is for informational purposes. The values, diagnostic methods, scope of work and recommendations given here are general and do not replace the manufacturer’s technical documentation. The final decision for a specific machine is made from its own diagnostics and inspection, taking into account its type, power, design, duty, operating history and applicable standards.
We will inspect the armature, commutator, brush assembly and magnetic system, determine the scope of repair, and restore the machine to its original factory characteristics.