Download DC Motors

DC Motors
©Dr. B. C. Paul 2012
After slides prepared earlier by the author.
The DC Motor
Stator
A direct current voltage
is applied in parallel to
the field poles of the
electromagnet and to
conductors in the
armature
Poles for
Field
Current
Armature or Rotor
The wire loops on the armature are connected through slip rings and brushes
so that current flow in the wire loops keeps reversing forcing the armature to
rotate to line up with the electromagnetic field
Making the Motor Work
The strength of the Magnetic Field
is controlled by the flow
of current to the electro-magnet
Poles for
Field
Current
Armature or Rotor
Since the voltage is fixed
the current through the field
circuit is controlled by the
resistance
Most of the resistance on the field current comes from a variable resistor
that can be adjusted as part of control of the motor
The Armature Current
• A DC voltage is applied to the armature
• the current is not quite Vt / Ra
• a current loop rotating in a magnetic field induces a
back voltage against the applied voltage
• vg = Køη
• K is a constant for the machine
• ø is the magnetic flux from the poles of the electromagnet
• η is the rpms turned by the armature
• Thus
ia = (Vt - vg )/ Ra
The Speed of a DC Motor
• The equations for armature current can be subbed into
each other and solved for armature speed
• η = (Vt - iaRa) / (KØ)
• But Ø = ƒ (if)
• And if is determined by changing the resistance on a variable resistor
We now have a motor that we can
speed control by adjusting a resistor
Uses of the DC Motor
• Because speed control is almost trivial the motor
is popular where ever speed control is needed
• Mine Hoists
• Most of the motions on large shovels and draglines
are candidates
• DC motor is an almost constant torque motor
• It can start at full torque (theoretically)
• It draws steady state current proportional to the
torque of the assigned load.6
What does Almost Constant
Torque Mean?
• When a DC motor starts there can be a rush of
current 20 or 30 times greater than steady state
operation
• This allows the motor full starting torque (provided the
power supply system can supply the starting current)
• Once going the current draw is proportional to the
load on the motor
• Since heating is a linear effect of current draw the
ultimate motor rating is based on heat.