DC Machine (Motor / Generator)
DC Machine
The classic brushed machine with field and armature windings — works as a motor or a generator. Series, shunt and compound connections give it the famous family of torque-speed characteristics every EE student memorises.
Schematic symbol
Motor
How dc machine (motor / generator) appears in a circuit diagram.
Types & variants
Key specs
Rated voltage
V (DC)Armature supply voltage — lab machines commonly 220 V DC.
Power
kW / HPShaft output as motor, electrical output as generator.
Speed
RPMN ∝ (V − IaRa)/φ: raise armature voltage to speed up, strengthen field to slow down — and never open a running shunt field (it overspeeds dangerously).
Back EMF
VEb = V − IaRa. It's why starting current is huge (no back-EMF at standstill) and why a 3-point starter exists.
Armature resistance
ΩSmall (fraction of an ohm to a few ohms); sets losses and the IaRa drop.
Markings
Terminal box marks A1/A2 (armature), F1/F2 (shunt field), S1/S2 (series field) per IEC, or AA/A, ZZ/Z, YY/Y in older Indian convention. Nameplate gives V, A, kW, RPM and excitation type.
Standard values
Lab machines: 220 V, 0.5–5 HP, 1500 RPM with 3-point or 4-point starters. Small PMDC motors: 6/12/24 V in toys, wipers, e-bikes (also covered under DC Motor in electromechanical).
How to choose
1) Series motor for huge starting torque (traction, cranes) — never run it unloaded (it overspeeds). 2) Shunt for near-constant speed (lathes, fans in old plants). 3) Compound for a compromise (presses, lifts). 4) Today, choose DC mainly for cheap fine speed control at small scale or legacy replacement; new designs mostly use inverter-fed AC or BLDC.
Pinout & package
Stator carries the field poles; rotor is the wound armature with a commutator and carbon brushes in rocker-mounted holders. Terminal box brings out armature and field pairs separately so any connection (shunt/series/separately excited) can be made externally.
Example circuits
- Speed control below base speed by armature voltage, above base speed by field weakening
- Swinburne's test: predict efficiency from a no-load run
- DC shunt generator self-excitation building up from residual magnetism
- Ward-Leonard set: motor-generator pair giving smooth wide-range speed control
Common failures
Worn brushes and dirty/grooved commutator (sparking, black streaks), open field winding (runaway or no torque), shorted armature coils (hot spots, low torque), and failure of a generator to build up voltage (lost residual magnetism, wrong rotation, or field resistance above critical).
How to test
Check brush length and spring pressure; measure armature resistance between opposite commutator segments (should be uniform — a bar-to-bar jump means an open coil); megger windings to frame; for a generator that won't excite, flash the field briefly from a battery to restore residual magnetism.
Substitutes
PMDC replaces small shunt machines (no field supply needed). For new variable-speed drives, a VFD + induction motor or a BLDC drive replaces most brushed DC applications with far less maintenance.