3.1 DC Motors: Construction, Types & Speed Control

DC Motors: Construction, Types, and Speed Control

Although alternating current (AC) induction motors dominate modern industrial installations, direct current (DC) motors remain important for applications requiring precise speed control, high starting torque, or simple variable-speed operation — elevators, cranes, hoists, traction systems, and some rolling-mill drives. Republic Act (RA) 7920, Section 19, lists "electrical machines" as an official subject of the Registered Master Electrician (RME) licensure examination, and DC motor theory is foundational to understanding all rotating electrical machinery.

Basic Construction

Every DC motor has four essential parts:

• Armature — the rotating part of the motor, consisting of a laminated iron core wound with coils of insulated copper wire. The armature carries the current that interacts with the magnetic field to produce torque.
• Field — the stationary structure (wound-field or, in small motors, permanent-magnet) that establishes the magnetic field the armature rotates within. In wound-field machines, the field winding is energized by DC current to create magnetic poles.
• Commutator — a segmented copper ring mounted on the armature shaft. As the armature rotates, the commutator mechanically reverses the direction of current in each armature coil at the correct instant, keeping the resulting torque acting in one consistent rotational direction.
• Brushes — carbon or carbon-graphite blocks that ride against the rotating commutator, carrying current from the stationary external circuit into the rotating armature windings.

The Motor Principle

A DC motor operates on a single physical law: a current-carrying conductor placed in a magnetic field experiences a mechanical force. The magnitude of that force is F = BIL, where B is the magnetic flux density, I is the current in the conductor, and L is the conductor's length within the field. Because the armature contains many such conductors arranged around the rotor, and the field surrounds it, the sum of these individual forces produces a rotating torque on the shaft. Torque in a DC motor is directly proportional to both the armature current and the field flux.

Back-EMF and Current Draw

As soon as the armature begins to rotate, it behaves like a generator as well as a motor: its windings cut through the field's magnetic flux and generate an internal voltage that opposes the applied supply voltage. This is called back-EMF (counter-electromotive force), Eb. The armature current is governed by the difference between the applied voltage and this back-EMF, divided by the armature resistance:

Ia = (V - Eb) / Ra

At the instant a motor is switched on, the armature is not yet turning, so Eb = 0. The only thing limiting current is the very low armature resistance Ra, which is why DC motors draw an enormous inrush current — often 10 to 20 times full-load current — at the moment of starting. This is exactly why starting rheostats or electronic starters are used to limit armature current until the motor builds speed. As the armature accelerates, Eb rises toward the supply voltage, and the net current falls automatically to whatever level is needed to supply the load torque. This self-regulating behavior is central to understanding both motor starting and speed control.

Series, Shunt, and Compound DC Motors

DC motors are classified by how the field winding is connected relative to the armature.

Series Motor

In a series motor, the field winding is connected in series with the armature, so the same current flows through both. Because field strength depends directly on load current, a series motor develops very high starting torque — torque increases roughly with the square of current in the unsaturated region — making it well suited to starting heavy mechanical loads such as cranes, hoists, and traction (rail) drives. However, speed varies enormously with load: as load increases, current increases, strengthening the field and slowing the motor; as load decreases, the field weakens sharply and speed rises. If a series motor loses its mechanical load entirely (a broken belt or disconnected coupling), the field collapses toward near-zero and the motor can accelerate to dangerously destructive speeds — a condition called "running away." For this reason, series motors must never be operated unloaded or through a belt drive without a rigid, positive mechanical connection to the load.

Shunt Motor

In a shunt motor, the field winding is connected in parallel (shunt) across the armature, so it sees a fairly constant voltage and therefore produces a nearly constant flux regardless of armature current. Because speed depends on back-EMF and flux, and flux stays nearly constant, a shunt motor's speed drops only slightly as load (and armature current) increases. This makes shunt motors well suited to applications requiring fairly constant speed under varying load — lathes, blowers, fans, and conveyors with moderate speed-regulation requirements.

Compound Motor

A compound motor combines both a series field winding and a shunt field winding on the same poles, blending the characteristics of both types. In the common cumulative compound arrangement, the series field flux adds to the shunt field flux as load increases, giving the motor higher starting torque than a pure shunt motor while retaining better speed regulation than a pure series motor. Compound motors are used where both firm starting torque and reasonable speed stability are needed, such as elevators, punch presses, and shears.

Table 3.1 — DC Motor Type Comparison

Speed Control Methods

Two basic methods are used to control DC motor speed:

• Armature voltage control — Reducing the voltage applied to the armature (via a series rheostat or, more commonly today, an electronic variable-voltage drive) reduces speed below the motor's rated ("base") speed. This method maintains full available torque at reduced speed but wastes energy as heat when a resistor is used, and it is generally limited to speeds at or below base speed.
• Field control (field weakening) — Inserting a rheostat in the shunt field circuit reduces field current and weakens the flux. Because back-EMF must still balance against the applied voltage, the armature must spin faster to generate the same back-EMF with less flux, so weakening the field raises speed above base speed. Field control extends the speed range above rated speed but reduces the torque the motor can deliver at those higher speeds (approximately constant-horsepower operation).

Applying This to the RME Exam

Understanding armature/field/commutator/brush construction, the back-EMF relationship, and the distinct torque-speed personalities of series, shunt, and compound motors gives you the foundation to evaluate any DC motor application question — especially questions asking which motor type suits high-starting-torque loads versus constant-speed loads, or why a series motor must never be run unloaded.