Electric Motor Types: Series, Shunt, Compound & Losses

Electric Motor Fundamentals

The current electric motor converts electrical energy (DC) into mechanical energy through the rotation of its parts.

Key Components

• Stator: The fixed part, generating the magnetic field with coils (spools) around electromagnet poles.
• Rotor: The mobile part, with induced coils wound on an iron core. Coil ends are welded to copper plates (Delgado) forming the collector, all mounted on a shaft.
• Brushes: Mounted in brush holders, they maintain contact with the collector, supplying current to the induced coils.

The counter-electromotive force (cemf) appears when the motor operates. Motors absorb a net emf, rotating within the magnetic field, inducing an opposing emf. At startup, this cemf is zero.

Motor Types

Series Motor

High startup torque, with torque increasing with current. Speed varies with load, maintaining almost constant power, making it self-regulated. It cannot operate without a load due to the risk of runaway. Used in trams, etc. If armature current approaches zero, speed increases significantly.

Shunt Motor

Speed remains constant regardless of load, suitable for machines requiring constant speed, like machine tools and lifting machines. Torque is proportional to current. It’s a self-adjusting motor; speed decreases with increasing torque and vice versa.

Compound Motor

Combines characteristics of series and shunt motors. Versatile for applications needing consistent speed and high torque.

Losses and Efficiency

Power and Efficiency

Power is work done per unit time. Useful power is produced by the machine, while nominal power is absorbed. Rated power means the machine is fully loaded. Useful power depends on resistance mechanisms.

Output power is always less than absorbed power due to losses:

• Losses in Conductors: Joule effect from electron collisions with metal ions, dissipating heat.
• Losses in Iron: Iron concentrates magnetic force lines. Closed currents form by induction in solid conductors, causing energy loss. Laminated iron cores minimize these currents. Hysteresis heat from cyclic magnetization also contributes to losses.
• Mechanical Losses: Friction in moving parts like bearings and brushes.

Stability

A stable machine corrects speed variations automatically. An unstable machine reinforces speed alterations. For example, if speed increases, a stable motor reduces torque to reach equilibrium, while an unstable motor increases torque, further increasing speed.