Introduction
A DC series motor behaves very differently from a DC shunt motor. Because its field winding is connected in series with the armature, the flux changes with load — giving the series motor its unique characteristics: extremely high starting torque and a speed that drops sharply as load increases.
Understanding the characteristics of a DC series motor explains why it's the preferred choice for electric traction (trains, trams), cranes, and hoists — applications that demand high torque at startup and variable speed under load.
Table of Contents
- Key Equations
- Speed–Current Characteristics
- Torque–Current Characteristics
- Speed–Torque Characteristics
- Comparison with DC Shunt Motor
- Applications
- FAQs
- Conclusion
Key Equations
Before analyzing the characteristics, recall the fundamental relationships for a DC motor:
The critical difference in a series motor: Φ ∝ Ia (until saturation), because the field winding carries the armature current. This single fact shapes all three characteristics.
Speed–Current Characteristics
From the speed equation:
Since Ra is small, the IaRa drop is negligible compared to V. So approximately:
This gives a rectangular hyperbola — speed is inversely proportional to armature current.
Key Observations
- At low current (light load), speed is dangerously high
- At high current (heavy load), speed drops significantly
- At no load, Ia → very small, so N → very large (theoretically infinite)
Critical safety rule: A DC series motor must NEVER be run without load. At no-load, the motor can accelerate to a speed that mechanically destroys the armature. This is why series motors are always directly coupled to their load (never belt-driven, since a belt can slip off).
Torque–Current Characteristics
From the torque equation:
Since Φ ∝ Ia in a series motor (before saturation):
This is a parabolic relationship — torque increases as the square of armature current. This gives the series motor its famous high starting torque.
After Saturation
Once the magnetic core saturates, flux becomes constant (Φ = constant). Now:
The curve transitions from parabolic to linear after saturation. In practice, the motor operates in the linear region at heavy loads.
Why Is Starting Torque So High?
At startup, the motor draws high current (since back EMF is zero). In a series motor, this high current also creates high flux. Since T ∝ Ia², doubling the current quadruples the torque. No other DC motor type can match this starting torque capability.
Speed–Torque Characteristics
This is the most practically useful characteristic. It's derived by combining the speed–current and torque–current curves:
Since T ∝ Ia² and N ∝ 1/Ia, eliminating Ia:
What This Means Practically
- High torque at low speed — perfect for starting heavy loads
- Low torque at high speed — motor speeds up as load decreases
- Automatic speed adjustment — the motor naturally slows down when load increases and speeds up when load decreases
This "self-adjusting" behavior is exactly what traction applications need — high torque to start a train, then higher speed once it's moving and the load reduces.
Comparison with DC Shunt Motor
For a detailed study of shunt motor behavior, see characteristics of DC shunt motor.
Applications of DC Series Motor
The high starting torque and variable speed characteristics make the DC series motor ideal for:
- Electric traction — trains, trams, metro systems (high torque to start, speed increases as train accelerates)
- Cranes and hoists — heavy loads need high torque at low speed for lifting
- Electric vehicles — starter motors in automobiles
- Conveyors — where load varies and high starting torque is needed
- Winches — pulling heavy loads from rest
FAQs
Why should a DC series motor never run without load?
Because at no-load, armature current is very small, making flux very small. Since N ∝ 1/Φ, the speed increases to dangerously high values that can mechanically destroy the armature due to centrifugal forces.
Why does a DC series motor have higher starting torque than a shunt motor?
In a series motor, T ∝ Ia² (torque increases as square of current). In a shunt motor, T ∝ Ia (linear relationship since flux is constant). For the same starting current, the series motor produces much more torque.
What happens to speed after magnetic saturation?
After saturation, flux becomes constant. The speed equation becomes N ∝ (V − IaRa), which decreases linearly with increasing current rather than following the hyperbolic curve.
Can a DC series motor be used for constant-speed applications?
No. The speed of a series motor varies significantly with load, making it unsuitable for applications requiring constant speed. A DC shunt motor or an induction motor would be better choices for constant-speed applications.
Why are series motors preferred for traction?
Traction requires high torque at low speed (to start the vehicle) and high speed at low torque (once moving). The series motor's speed-torque characteristic naturally provides this — it automatically adjusts speed based on load without any external control.
Conclusion
The characteristics of a DC series motor are fundamentally shaped by one fact: flux is proportional to armature current. This gives it a hyperbolic speed-current curve, a parabolic torque-current curve, and the famous high-torque-at-low-speed behavior that makes it indispensable for traction and heavy-duty starting applications. The trade-off is poor speed regulation and the danger of runaway at no-load.