Q1Electric Drives
Question
Q.1. Explain the torque-speed characteristics of DC motors and their impact on drive applications.
Answer
DC motor torque-speed characteristics vary by motor type — separately/shunt excited motors give nearly constant speed with load (well-suited to constant-speed applications), series motors give high starting torque with speed dropping sharply as load increases (well-suited to traction), and compound motors offer an intermediate characteristic combining both drooping speed and reasonably high starting torque — with the specific characteristic shape directly determining the motor's suitability for a given drive application's speed-regulation and starting-torque requirements.
DC motors are classified by their field excitation arrangement, and each type exhibits a distinctly different torque-speed characteristic, directly determining its suitability for different classes of drive applications.
Separately Excited / Shunt Motor
As discussed in an earlier answer, the separately excited (or shunt-connected) DC motor maintains an essentially constant field flux, giving a torque-speed characteristic that is a nearly horizontal straight line with a small negative slope — speed remains close to its no-load value across a wide range of load torque (T=Kφ·Ia, with speed N=(V-IaRa)/(Kφ) drooping only slightly as Ia/torque increases). This nearly constant-speed characteristic makes shunt/separately-excited motors well suited to applications demanding good speed regulation despite varying load torque, such as machine tools, fans, blowers, and conveyor drives, where consistent speed is more important than very high starting torque.
Series Motor
In a series motor, the field winding carries the same current as the armature, so flux φ is approximately proportional to armature current Ia (before saturation) — since torque T=Kφ·Ia ∝ Ia², torque rises very rapidly (with the square of current) at low speed/high current, giving the series motor an extremely high starting torque, while speed varies inversely (approximately) with the square root of torque, causing speed to drop sharply as load torque increases and to rise to potentially dangerously high values if the load torque is removed entirely (a series motor must never be run unloaded/without a coupled load, since its speed could rise uncontrolled). This characteristic — very high starting torque combined with speed that naturally falls as load increases — makes series motors particularly well suited to traction applications (electric trains, trams, cranes, and hoists) where high starting torque under heavy load is the primary requirement and the load is virtually never fully removed during operation.
Compound Motor
A compound motor combines both a series field winding and a shunt (separately-excited-like) field winding, giving a torque-speed characteristic intermediate between the pure shunt and pure series characteristics — cumulative compound motors (where the two field windings' MMFs add) provide higher starting torque than a pure shunt motor while retaining better speed regulation (less severe speed droop with load) than a pure series motor, making compound motors suitable for applications requiring a combination of reasonably high starting torque and reasonably good speed regulation, such as some elevator, punch press, and rolling mill drive applications.
Impact on drive applications: the specific choice among these DC motor types for a given drive application is governed directly by matching the motor's inherent torque-speed characteristic to the load's own torque-speed demand profile — constant-speed, moderate-starting-torque loads (fans, pumps, machine tools) favor shunt/separately-excited motors; high-starting-torque, variable-speed loads that are always coupled to a substantial load (traction, cranes) favor series motors; and applications requiring an intermediate combination of these properties favor compound motors — making a correct understanding of each motor type's torque-speed characteristic essential to appropriate drive selection for any given industrial or transportation application.