Understanding DC Motors: Principles, Operation, and Applications
DC Motor
The DC motor is a machine that converts electrical energy into mechanical energy, primarily through rotational movement. While newer applications utilize electric motors for rotary motion, modifications like linear motors produce traction on a rail.
This versatile machine is widely used in various industries. Its easy control over positioning, stopping, and speed makes it a preferred choice for control applications and process automation. However, the rise of electronics and the affordability of AC motors, particularly asynchronous types, have led to a decline in DC motor usage.
Despite this, DC motors remain essential in high-power applications like trains and trams, as well as precision applications such as machinery and micro motors.
Key Features and Drawbacks
The primary advantage of a DC motor is its ability to regulate speed from no load to full load.
However, a significant drawback is its maintenance, which can be both expensive and labor-intensive.
Components of a DC Motor
A DC machine, whether a generator or motor, comprises two main parts:
- Stator: Provides mechanical support and typically features a cylindrical hole in the center. It also houses the stator poles, which can be permanent magnets or windings of copper wire with an iron core.
- Rotor: Generally cylindrical with windings and a core, receiving current through two brushless contacts.
Specific applications may utilize DC motors with permanent magnets in the rotor.
Operating Principle
The diagram illustrates the operation of a basic two-pole DC motor with a thin coil on the rotor, shown in three positions offset by 90 degrees.
- 1, 2: Brushes
- A, B: Terminals
- a, b: Sides of the coil connected to terminals A and B, respectively
Lorentz Law
According to the Lorentz law, a current-carrying conductor immersed in a magnetic field experiences a force perpendicular to both the magnetic field and current direction, following the right-hand rule. The force’s magnitude is given by:
- F: Force (Newtons)
- I: Current (Amps)
- l: Conductor length (Meters)
- B: Magnetic field strength (Tesla)
The rotor has multiple conductors distributed along its periphery. As it rotates, current is activated in the appropriate conductor.
Typically, an opposing current is applied at the opposite end of the rotor to balance the net force and extend the operational time.
Induced EMF in a Motor
This voltage is generated in the motor’s conductors due to the cutting of magnetic field lines, also known as the generator effect.
The polarity of this induced voltage opposes the applied voltage at the motor terminals.
The high initial current draw of a motor at startup is due to the absence of back EMF, causing the winding to act as a pure resistance in the circuit.
Number and Position of Brushes
Brushes short-circuit the coils located in the neutral zone. A two-pole machine has two neutral zones, requiring two brushes. The number of brushes generally equals the number of poles.
The brushes are positioned to coincide with the neutral lines of the poles.
Direction of Rotation
The direction of rotation depends on the relative direction of current flow in the inductor and armature windings.
Reversing the rotation can be achieved by reversing either the magnetic field or the armature current. However, reversing both maintains the same rotation direction.
Polarity changes in both windings are managed within the motor’s terminal box, considering the combined effect of the rotor’s magnetomotive force (MMF).
The right-hand rule can determine the direction of force and rotation: thumb indicates current direction, index finger points towards the magnetic field direction, and the middle finger shows the resulting force and rotation direction.
Reversibility
DC motors and generators share similar components, differing primarily in their application.
Reversibility implies that rotating the rotor induces an electromotive force, which can be converted into energy for the load circuit.
Conversely, applying a DC voltage to the armature winding through the commutator transforms the machine into a motor, converting electrical energy into mechanical energy.
In both scenarios, the armature interacts with the main exciter field.