Download Elec467 Power Machines & Transformers

Elec467 Power Machines &
Transformers
Electric Machines by Hubert, Chapter 11
Topic: Characteristics of Shunt,
Compound and Series DC motors
Compound DC motors
Compound motors have an
extra winding in series with
the armature and positioned
so its flux field is additive with
the shunt field winding. Thus
two windings, shunt and
series, combine to form a
single mmf seen by the
armature.
Shunt motor were covered in Chapter 10
Pole showing shunt and series coils
Series DC motors
A series motor eliminates the shunt winding. The series
winding suffices to operate the motor. It consists of a
heavy winding to handle high current flow. If the load on
a Series motor is lost, it will accelerate to destructive
levels because TD > Tload
Characteristics of motor types
This graph is actually two
graphs that share the x-axis
in common. Speed and
torque characteristics of the
three DC motor types are
compared. A Series motor is
the most non-linear but are
capable of extremely high
torque at low speeds (with
high current). Whatever a
shunt motor can do…a
compound motor can do
better (slightly). In common
are the crossing points for
Series, Compound, and
Shunt motors at 100% speed
and 100% torque. These
crossings occur at 100%
armature current. At these
two points all three motors
are essentially the same.
Adjustable voltage drive system
Diesel Electric
The advantage of a diesel
electric drive is the diesel
can be set for optimum
performance as its load is
constant. The connection
between the generator
and the motor is by
electrical cable. This can
eliminating a transmission
box on the output and
long shafts. Ships and
trains now use this type
of power train.
Dynamic braking
In (a) contacts M1 & M2 are closed thru
while M3 is open. This is the normal
motor operating position and the current
flow thru the armature is the source for
the mechanical rotation of the motor in
the CW direction. When braking if the
current flow is reversed the motor will
rotate in the opposite direction. By
opening M1 & M2, closing M3 in (b) and
not changing the field current the rotation
of the motor will now generate a current
in the opposite direction. This means the
motor new rotation will oppose the
momentum that is creating the opposing
mechanical torque.
Plugging operation
Plugging is a little different
than the dynamic breaking in
the last slide. Here the
voltage supply is not
disconnected letting the motor
run on its own but actually
reconnected with the voltage
reversed causing the current
to force the motor to turn in
the opposite direction. This
stresses the mechanical
equipment attached to the
output shaft because of the
high torque differential. In Fig.
11.13, you have to supply the
diagonal open/closed marking
for the contacts.
How to change direction of
rotation for a Compound motor
How to change direction of
rotation for a Series motor
NEMA terminal markings
Terminal markings on Shunt and Compound
motors to enable selection the rotation direction.
Magnetization curves
•
•
•
•
•
The shunt field coils produce the flux
seen by the armature. This flux is a
product of mmf (N*I).
Since the turns are set, flux becomes
a variable of the field current.
The flux created affects the counter
emf created in the armature which
opposes the incoming armature
current.
Magnet saturation creates a nonlinear relationship between the field
current and flux density.
The higher the flux density will result
in a lower armature current and is
the best choice for motor efficiency.