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
- 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
The DC motor, 2D force diagram
DC machines concept map
DC Motors & Generators
DC motors & generators quiz
DC Motors & Generators, quiz
1. Which rule gives the direction of force on a current-carrying conductor in a motor?
Fleming's left-hand ruleFleming's right-hand ruleLenz's law2. A conductor 0.5 m long carries 10 A at right angles to a 0.8 T field. The force on it is:
4 N0.4 N40 N3. The purpose of a commutator in a DC motor is to:
Reverse the armature current every half revolutionConvert AC supply to DCRegulate the field strength4. A DC motor draws a very large current at the instant of starting because:
There is no back-EMF at zero speedThe field is at its weakestThe commutator is open-circuit5. Back-EMF in a DC motor is greatest:
At full speedAt the moment of startingWhen the motor is stalled6. Fleming's right-hand rule applies to:
GeneratorsMotorsTransformers7. Which DC motor type gives the highest starting torque and is used for engine starters?
Series-woundShunt-woundSeparately excited8. The force on a current-carrying conductor in a magnetic field is given by:
F = BILF = ½LI²F = V/R9. A shunt-wound DC motor is characterised by:
Near-constant speed under varying loadVery high starting torqueBeing unable to run without a load10. In a DC generator, the commutator:
Converts the internally generated AC into DC at the terminalsIncreases the output voltageReduces armature reaction