ENB205 Lecture Electrical and Computer Engineering
Permanent Magnet DC Motors
Can only generate small amount of torque because the field relies upon ‘residual’ magnetic flux in the permanent
magnets. Current in the machine tends to cause a reaction in the field, demagnetising the field poles. Only good for
Current in the rotor works to increase or decrease flux density at trailing/leading edges of the magnet. This reaction
works to weaken the magnets in a PMDC motor.
By increasing the air-gap between rotor and poles at the edges of the pole faces, the magnetic field lines are less
inclined to ‘bunch up’ reducing the armature reaction.
1 Week 3 Tuesday, 27 August 2013 ENB205 Lecture Electrical and Computer Engineering
B-H Curve De-Magnetisation
Ferromagnetic B-H curves are not the same going up, as
down. PM Material needs to have a high residual flux
density and a high magnetic intensity to demagnetise.
PM Material Choice
Neodymium-iron-boron (rare earth magnets) have both a very high H, and high maximum residual flux density.
NdFeB good choice for PM motors.
Straight line –ignoring armature reaction
Armature reaction works to weaken flux, lower flux
means lower induced EMF and hence the motor can spin
faster for the same terminal voltage.
Speed Control (PMDC)
Since terminal Voltage is proportional to speed of the motor, assuming the same torque is applied:
Decrease in terminal voltage, decrease in speed
Increase in terminal voltage, increase in speed.
Wound Field DC Machine
Wound Field Poles
A much higher flux density can be achieved in the same material, if magnetising force is applied. Wound field poles
have the capacity to generate higher flux density and higher torque. Cost is they require constant current to
maintain magnetic field intensity, losses in field windings. Advantage is they have a controllable field density, will not
demagnetise and can be wound to negative the effect of armature reaction.
Shunt Field Winding
Shunt machine: Field winding is placed across same Separately Excited: Field winding is placed across a different
supply as the rotor (parallel). supply and controlled independently.
Note that the winding has an inductance, and resistance.
Remember an inductor is simply a coil of wire, and that
inductors in DC act as a dead short.
If the voltage for the two windings is identical, there is no difference between Shunt and Separately Excited
2 Week 3 Tuesday, 27 August 2013 ENB205 Lecture Electrical and Computer Engineering
Straight line, still suffers from armature reaction, however can be countered by compensating windings. Speed does
not decrease significantly with increased load, and hence is said to have good ‘speed regulation’
Small difference in speed, good speed regulation.
Speed regulation is the difference between the ‘no-load’ speed and the ‘full load’ speed, compared to the full load
No-load speed is the theoretical maximum speed.
Full-load speed is the speed when the armature current is at its rated value.
Speed Control (Shunt)
Controlling field current
Decreases field current
Decreases flux per pole
Allows more current to flow in
armature, allowing the motor to
accelerate to a new equilibrium
Increasing the field resistance has an
effect on the speed-torque curve,
making it steeper.
At very high loads the decrease in flux
will not cause sufficiently high enough
increase in current and the torque
output will actually decrease, slowing
3 Week 3 Tuesday, 27 August 2013 ENB205 Lecture Electrical and Computer Engineering
Always provided at ‘rated’ or ‘full-load’ speed Changing the field current, does not translate to a linear
change in speed for the Shunt DC Motor. Due to
saturation of the magnetic material used to create the
field poles, the B-H curve is not linear, and hence the
back-EMF generated due to the field current is not
linear. A magnetisation curve is often used to predict