DC Machines: Generators and Motors
One machine, two functions: the same construction acts as a generator when driven mechanically and as a motor when fed electrically. Expect questions on construction, the EMF equation, back EMF and machine types.
Construction
Yoke (frame and flux path), poles with field winding, armature core and winding, commutator and brushes. The commutator is a mechanical rectifier in a generator and a mechanical inverter in a motor.
Generator EMF equation
E_g = PΦNZ/(60A), where P = poles, Φ = flux per pole in Wb, N = speed in rpm, Z = number of armature conductors, A = number of parallel paths. For lap winding A = P; for wave winding A = 2.
Motor action
Back EMF E_b = V − I_aR_a. At starting, N = 0 so E_b = 0, and the current is limited only by R_a - this is why a starter is essential. Torque T ∝ ΦI_a.
Types
Shunt: nearly constant speed, suits lathes and drills. Series: high starting torque, suits cranes and traction, but must never run unloaded. Compound: combines both, used for lifts and presses.
Worked example
A 4-pole wave-wound generator has 400 conductors, flux 0.03 Wb per pole and runs at 1500 rpm. For wave winding A = 2. E_g = (0.03 × 1500 × 400 × 4)/(60 × 2) = 72000/120 = 600 V.
Traps
- Swapping lap (A = P) and wave (A = 2) values.
- Using Fleming's right-hand rule for motor action.
- Forgetting the starter and claiming series motors are safe at no load.
- Omitting the commutator's function when describing construction.