DC Motor & Generator Theory
The motor effect F = BIL and Fleming's rules; construction and operation of DC generators and motors; the commutator; armature reaction; back-EMF; series, shunt and compound machines.
DC motors & generators — summary notes
Free- The motor effect: a current-carrying conductor in a magnetic field experiences a force F = B·I·L.
- Fleming's LEFT-hand rule applies to motors; Fleming's RIGHT-hand rule applies to generators. (Left for Lorry/motion produced, right for generation.)
- A commutator and brushes reverse the armature connections every half revolution, so the torque continues to act in the same direction.
- As a motor speeds up, its armature acts as a generator and produces a back-EMF that opposes the supply and limits armature current.
- Starting current is high because at rest there is no back-EMF — only the small armature resistance limits it.
- Motor types: series (high starting torque, used for starters), shunt (near-constant speed), and compound (a combination).
- ⚠ Exam trap: LEFT = motor, RIGHT = generator. Back-EMF is greatest at full speed and zero at the instant of starting — which is why starting current is so large.
- Force on a conductor
- F = B · I · L (newtons)
- Generated EMF
- EMF = B · L · v
- Back-EMF
- E_b = V − I_a·R_a
A conductor 0.5 m long carries 10 A at right angles to a field of 0.8 T. What force acts on it?
F = B·I·L = 0.8 × 10 × 0.5 = 4 N.
Why does a DC motor draw a very large current at the instant of starting?
Back-EMF is proportional to speed. At rest the speed is zero, so there is no back-EMF and only the low armature resistance limits the current. As the motor accelerates, back-EMF builds and the current falls.
The DC motor (3D) — why the commutator matters
FreeThe DC motor — 2D force diagram
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DC machines concept map
FreeDC Motors & Generators
DC motors & generators quiz
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