Side-by-side comparison
| Parameter | Series DC Motor | Shunt DC Motor |
|---|---|---|
| Field Winding Connection | Field in series with armature — same current through both | Field in parallel with armature and supply |
| Field Current | I_f = I_a = I_L (all equal; field varies with load) | I_f = V/R_f (constant; independent of armature load) |
| Torque Characteristic | T ∝ I_a² (before saturation) — high starting torque | T ∝ I_a (field constant) — torque proportional to armature current only |
| Speed Characteristic | N ∝ 1/I_a — speed drops sharply with load | N ≈ constant (2–5% drop from no-load to full load) |
| No-Load Behavior | DANGEROUS — speed becomes theoretically infinite | Safe — runs at slightly above rated speed at no-load |
| Starting Torque | Very high — 2.5 to 3 times rated torque | Moderate — about 1.5 times rated torque |
| Speed Control | By varying series resistance or supply voltage | By field weakening (above rated speed) or armature resistance (below) |
| Application | Electric traction (locomotives, cranes, hoists, auto starter motor) | Lathes, milling machines, fans, conveyors requiring constant speed |
Key differences
Series motor torque T ∝ Φ × I_a, and since Φ ∝ I_a (field is in series), T ∝ I_a² before magnetic saturation — giving it torque that rises with the square of current and making it ideal for heavy starting loads like 2000 A traction motors in electric locomotives. Shunt motor has constant flux (Φ = constant), so T ∝ I_a linearly and speed drops only 3–5% from no-load to full load. A series motor must never be run uncoupled — losing mechanical load removes the speed-limiting back EMF and the motor accelerates to mechanical failure within seconds.
When to use Series DC Motor
Use a series DC motor whenever the load requires high starting torque and the motor will always remain coupled to its load — for example, the 750 V DC traction motors on Delhi Metro coaches or the starter motor of a diesel engine.
When to use Shunt DC Motor
Use a shunt DC motor wherever constant speed is needed regardless of load changes — for example, a 5 kW shunt motor driving a lathe spindle at 1440 rpm with less than 3% speed variation between no-load and full-load cutting.
Recommendation
The most important rule: never run a series DC motor without load. For every other scenario, choose shunt for constant speed applications and series for high starting torque and variable speed loads. That single danger rule about no-load operation of series motors appears in almost every exam.
Exam tip: Examiners ask for the torque-speed characteristic shape — series motor gives a hyperbolic curve (high torque at low speed, speed rises sharply as load drops); shunt motor gives a nearly flat, slightly drooping line — sketch both in exam answers for full marks.
Interview tip: Interviewers at railway or crane manufacturers ask why a series motor is used in traction but not in precision machines — answer that the series motor's T ∝ I_a² gives massive starting torque for heavy loads, but its inability to run safely at no-load makes it unsuitable for lathes or milling machines.