Handbook of Electrical Installation Practice - BeKnowledge

402 Handbook of Electrical Installation Practice
Fig. 15.6 A Mawdsley’s square frame industrial d.c. motor rated at 250kW with forced
ventilation.
For constant armature voltage its speed and output power are inversely proportional
to field voltage (flux).The speed range over which the motor operates by field
voltage control is known as the shunt range (Fig. 15.7).
Most d.c. motors used are separately-excited shunt-wound machines with interpoles.
Figure 15.8 shows the connection of such a motor and includes a series compounding
winding which may sometimes be fitted to produce a drooping speed
torque characteristic.
To avoid excessive flux distortion and maximise output the motors have interpoles
as well as main poles. The interpoles carry the armature current and influence
the flux as the load and hence armature current changes to minimise the effect of
armature reaction. They significantly reduce the flux appearing in the regions
between the main poles where the brushes are located to reduce sparking between
brushes and commutator.
Figure 15.9 illustrates the section of a four-pole square frame d.c. motor. Tucking
the interpoles up into the corners gives maximum utilisation of field space and
allows the use of a larger diameter armature than would be the case with a conventional
round frame four-pole motor.
On larger d.c. motors, say above 500kW, it is common to achieve the flux necessary
with minimum distortion by employing a six-pole construction when the square
frame gives way to a conventional round shape.
Certainly on these larger motors, and often at much smaller ratings, pole face
compensating windings will be incorporated. As their name suggests, they are
accommodated in slots in the main pole face and are closed coils. They produce a
flux under the main poles to cancel out the effect of the flux produced by the current