How it works
Torque developed by the armature is Ta = (φ × Ia × Z × P)/(2πA), simplified to Ta = KaφIa, where Ka is the machine constant. In a shunt motor, flux φ is nearly constant (field winding across supply), so torque is proportional to Ia. Speed N = K(V − IaRa)/φ; reducing flux φ increases speed above base speed, while reducing terminal voltage reduces it below base. A series motor has φ ∝ Ia, giving T ∝ Ia², which explains the enormous starting torque — useful for traction applications like Mumbai local trains.
Key points to remember
Never run a DC series motor on no load — flux collapses as Ia → 0 and speed theoretically becomes infinite, destroying the machine. The speed regulation of a shunt motor is excellent (3–8%), making it suitable for machine tools. Starting current without a starter = V/Ra; for a 220 V motor with Ra = 0.4 Ω this is 550 A, about 25 times full-load current — which is why a face-plate starter or electronic drive is essential. Speed can be controlled by armature voltage, field flux, or armature resistance, with armature voltage control being the most efficient.
Exam tip
The examiner always asks why a series motor should never be started on no load and what happens to speed — explain it in terms of flux collapse and the speed equation N ∝ 1/φ.