Understanding DC Motors: Principles, Types, and Applications
Principle of Operation (Generator)
The principle of operation of a generator can be explained by basic electrical theory. Rotating a wire ring within a magnetic field induces an electrical current in the wire.
Establishment of a Dynamo
A dynamo consists of several key components:
- Magnetic circuit (fixed and mobile)
- Electrical circuit (inductor, induced, collector, brushes)
Circuit Magnet
The circuit magnet is typically of the salient pole type and is made of mild steel sheets 0.8 to 1.5 mm thick. Ancillary or switching poles may also be present.
Mobile Magnetic Circuit
The mobile magnetic circuit is made of soft iron plates 0.5 mm thick, insulated from each other by varnish to reduce eddy current losses.
Inductor Electric Circuit
The inductor electric circuit consists of a series of coils electrically isolated by a special varnish. These coils are wound around the magnetic poles and create a fixed magnetic field.
Induced Electric Circuit
The induced electric circuit consists of a series of coils wound around the rotor’s magnetic circuit. These coils are connected in series, forming a closed circuit. As the rotor rotates within the magnetic field, a voltage is induced in the coils. This voltage is transmitted through the commutator bars and brushes for external use.
Collector
The collector transforms the sinusoidal alternating EMF induced in the rotating coils into a continuous EMF, which is then passed through the brushes to the external circuit.
Operation (DC Motor)
Inductor or Stator
The inductor, or stator, is an electromagnet with an even number of poles. Coils wrapped around the poles produce the magnetic field.
Armature Rotor
The armature rotor is a rotating piece with a magnetic core. The armature winding, wrapped around the core, interacts with the magnetic field, causing the rotor to turn.
Collector
The collector is a ring of thin, bare copper segments arranged on the rotor shaft. It connects the armature coils to the external circuit through the brushes.
Brushes
Brushes, typically made of graphite, maintain electrical contact between the rotating commutator bars and the external circuit. As the rotor spins, the brushes slide against the commutator bars, connecting the appropriate armature coil to the external circuit.
DC Motor Operation
A DC motor operates based on the force produced on a current-carrying conductor within a magnetic field. When a current flows through the armature winding, it interacts with the stator’s magnetic field, generating a force that rotates the rotor.
Starting Resistance
When a DC motor starts, the initial current is very high. To limit this current and prevent damage, a series resistance is often connected to the rotor during startup.
Types of DC Motors
Separately Excited DC Motors
In separately excited DC motors, the rotor and stator receive power from two independent voltage sources. The stator field is constant and independent of the motor load. This type of motor offers precise speed control but is rarely used due to the need for an external power source for the field winding.
Shunt Excited DC Motors
In shunt excited DC motors, the armature windings are connected in parallel with the field winding and fed by a common voltage source. An increase in the induced voltage increases the motor’s speed. These motors are well-suited for applications requiring constant speed.
Series Excited DC Motors
In series excited DC motors, the armature windings and the field winding are connected in series and powered by the same voltage source. The motor torque and speed are dependent on each other. As the excitation current increases, the motor speed decreases. These motors are suitable for high-torque applications.
Compound Excited DC Motors
Compound excited DC motors combine features of both series and shunt excitation. The field winding is partially connected in series and partially in parallel with the armature winding. There are two types of compound excitation:
- Cumulative Compound: The current flows in the same direction through the series and parallel windings, resulting in an additive effect on the field strength.
- Differential Compound: The current flows in opposite directions through the series and parallel windings, resulting in a subtractive effect on the field strength.
Advantages of DC Motors
- Variable speed control
- Ease of control
- Flexibility in speed range
- High performance
- Ease of reversing
Applications of DC Motors
- Reversible rolling mills
- Trains
- Shears in hot rolling mills
- Paper industry
- Machine tools
- Mining machinery
- Elevators
- Cranes
Preventive Maintenance of a DC Machine
Planning and Monitoring
- Establish a regular maintenance schedule.
- Monitor motor performance parameters such as temperature, vibration, and noise.
Performance Testing
- Periodically perform insulation resistance tests.
- Check brush wear and condition.
- Inspect the commutator surface for wear and damage.
- Verify rotor balance.
- Check terminal board connections.
Maintenance Tasks
- Clean the interior and exterior of the motor.
- Check and clean ventilation paths.
- Lubricate bearings and other mechanical components.
- Inspect and tighten all connections.
Machine in Operation
Monitoring
- Monitor operating temperature.
- Listen for unusual noises, vibrations, or friction.
- Measure voltage and current.
Cleaning
- Clean the exterior of the motor regularly.
- Ensure adequate ventilation.
Generation of Motion in a Conductor
When a current-carrying conductor is placed within a magnetic field, a force is exerted on the conductor. The direction of the force depends on the direction of the current and the direction of the magnetic field. This principle is fundamental to the operation of electric motors.