Electrical Engine Lecture 3: DC Motors

DC motors are categorized into shunt, series, and compound motors.

This characteristic shows the relationship between torque and armature current in a DC motor, essentially the motor's electrical behavior.

This characteristic links the motor's speed to its armature current, influencing motor selection for various applications.

This characteristic plots the motor's speed against torque, representing the motor's mechanical performance.

The torque of a series motor is proportional to the square of the armature current (prior to saturation), providing high starting torque.

The speed of a series motor is inversely proportional to the armature current.

Series motors are well-suited for applications demanding high starting torque, like hoists and electric trains.

DC motors with both series and shunt field windings; offer high starting torque and relatively consistent speed.

A type of compound motor where the series field weakens the main flux as load increases, leading to potentially unstable speed.

A type of compound motor where the series field adds to the main flux; improving torque and maintaining a fairly constant speed.

At start-up, the armature current is very high because the back EMF (Eb) is zero.

A crucial component to control the high initial current during motor starting.

Limited use for applications requiring precise constant speed under various load conditions (experimental or research).

Suitable for applications demanding high starting torque and relatively constant speed, like shears, rolling mills, etc.

Often used for applications requiring significant starting torque (e.g., electric trains, hoists).

Suited for applications requiring fairly consistent speed and medium torque (e.g., lathes, centrifugal pumps).

DC motors require a starting resistance to limit high initial current.

Series resistance added to the armature circuit during start-up.

A starter that gradually reduces resistance as the motor speeds up.

Direct connection to supply during the start-up process.

Resistance progressively removed from the armature circuit as the motor speeds up.

Voltage generated by the motor that opposes the supply voltage, limiting current once the motor reaches rated speed.

A safety mechanism that stops the motor if the current exceeds a preset value.

An electromagnet that holds the starter lever at its final position once the motor reaches the correct speed.

Disconnects the motor if the current exceeds a predetermined level, protecting against damage.

Should always start with a mechanical load to avoid excessive speed and damage from centrifugal forces; it's a variable-speed motor.

High starting torque, but speed changes significantly with load.

Avoid starting on heavy loads; high starting current possible.

Speed is nearly constant, though a slight drop in speed occurs with increasing load.

A DC motor with both series and shunt windings; characteristics between series and shunt motors.

Series field boosting shunt field, resulting in increased torque and relatively constant speed.

Series field weakens shunt field, leading to unstable speed under heavy load.