Electric Motor Fundamentals
Electric Motors
Motor Classification by Current Type
Electric motors are categorized based on the type of current they use:
- DC motors
- AC motors
- Universal motors (compatible with both AC and DC)
DC Motor Classification by Excitation
DC motors are further classified by their excitation method:
- Independent excitation
- Series excitation
- Shunt excitation
- Compound excitation
- Permanent magnet excitation
AC Motor Classification
AC motors are classified based on several factors:
- Speed: Synchronous or asynchronous
- Rotor type: Wound rotor, shorted rotor, or squirrel cage rotor
- Phases: Single-phase (universal and auxiliary winding, capacitor) or three-phase
DC Motors
Structure of a DC Motor
DC motors operate on the principles of electromagnetic force and induced electromotive force. They consist of two main components:
- Inductor (Stator): Creates the magnetic field. It’s the stationary part of the motor and houses coils of copper wire wrapped around a ferromagnetic core. It can also utilize permanent magnets.
- Armature (Rotor): Creates opposing magnetic fields. It’s the rotating part of the motor and consists of copper conductors arranged in coils. These coils are housed in grooves within a cylindrical ferromagnetic core. The coil ends are connected to a commutator, a segmented copper ring. Brushes, typically made of graphite, make contact with the commutator segments, connecting the internal circuit to the external power source.
Types of DC Motors
Different types of DC motors are created based on how the inductor and armature circuits are connected:
- Separately Excited Motor: The armature and field windings are powered by independent voltage sources.
- Shunt Motor: The field winding is connected in parallel with the armature winding, sharing the same voltage source.
- Series Motor: The field winding is connected in series with the armature winding.
- Compound Motor: Combines features of series and shunt motors.
Characteristics of DC Motors
The performance of a DC motor depends on its excitation type. Key characteristics include:
- Speed (n) as a function of current (I)
- Torque (M) as a function of current (I)
- Mechanical characteristic (M) as a function of speed (n)
Shunt Motor Characteristics
- Speed (n) vs. Armature Current (Ii)
- Torque (Mi) vs. Armature Current (Ii)
- Mechanical Characteristic: Torque (M) vs. Speed (n)
Series Motor Characteristics
- Speed (n) vs. Armature Current (Ii)
- Torque (M) vs. Armature Current (Ii)
- Mechanical Characteristic: Torque (M) vs. Speed (n)
Starting DC Motors
Starting a DC motor involves controlling the voltage or armature resistance. A starting rheostat is often used to limit the initial current. As the motor accelerates, the back EMF increases, reducing the current, and the starting resistance is gradually removed.
Speed Control
Speed control maintains a desired motor speed. This can be achieved by adjusting the voltage or the field flux:
- Resistance Control: Inserting a resistor in series with the armature.
- Voltage Control: Varying the applied voltage.
- Field Flux Control: Adjusting the field current using a rheostat.
Reversing Rotation
Changing the direction of rotation is done by reversing the connections of either the inductor or the armature. If the motor is stopped, reversing the armature connections is recommended. If the motor is running, reversing the armature connections is preferred to avoid loss of excitation.
Braking DC Motors
Braking involves using the motor as a generator to create a reverse torque. Two methods are common:
- Rheostatic Braking: Dissipating generated energy in braking resistors.
- Regenerative Braking: Returning generated energy to the power line.