shunt motor is different from the series motor in that the field
winding is connected in parallel with the armature instead of in series.
You should remember from basic electrical theory that a parallel
circuit is often referred to as a shunt. Since the field winding is
placed in parallel with the armature, it is called a shunt winding and
the motor is called a shunt motor. Figure 12-13 shows a diagram of a
shunt motor. Notice that the field terminals are marked Fl and F2, and
the armature terminals are marked Al andA2. You should notice in this
diagram that the shunt field is represented with multiple turns using a
shunt winding is made of small-gauge wire with many turns on the coil.
Since the wire is so small, the coil can have thousands of turns and
still fit in the slots. The small-gauge wire cannot handle as much
current as the heavy-gauge wire in the series field, but since this coil
has many more turns of wire, it can still produce a very strong
magnetic field. .A
shunt motor has slightly different operating characteristics than a
series motor. Since the shunt field coil is made of fine wire, it cannot
produce the large current for starting like the series field. This
means that the shunt motor has very low starting torque, which requires
that the shaft load be rather small.The
direction of rotation of a DC shunt motor can be reversed by changing
the polarity of either the armature coil or the field coil. In this
application the armature coil is usually changed, as was the case with
the series motor. Figure 12-16 shows the electrical diagram of a DC
shunt motor connected to a forward and reversing motor starter. You
should notice that the Fl and F2 terminals of the shunt field are
connected directly to the power supply, and the Al and A2 terminals of
the armature winding are connected to the reversing starter.
voltage is applied to the motor, the high resistance of the shunt coil
keeps the overall current flow low. The armature for the shunt motor is
similar to the series motor and it will draw current to produce a
magnetic field strong enough to cause the armature shaft and load to
start turning. Like the series motor, when the armature begins to turn,
it will produce back EMF. The back EMF will cause the current in the
armature to begin to diminish to a very small level. The amount of
current the armature will draw is directly related to the size of the
load when the motor reaches full speed. Since the load is generally
small, the armature current will be small. When the motor reaches full
rpm, its speed will remain fairly constant.
the shunt motor reaches full rpm, its speed will remain fairly
constant. The reason the speed remains constant is due to the load
characteristics of the armature and shunt coil. You should remember that
the speed of a series motor could not be controlled since it was
totally dependent on the size of the load in comparison to the size of
the motor. If the load was very large for the motor size, the speed of
the armature would be very slow. If the load was light compared to the
motor, the armature shaft speed would be much faster, and if no load was
present on the shaft, the motor could run away.
shunt motor's speed can be controlled. The ability of the motor to
maintain a set rpm at high speed when the load changes is due to the
characteristic of the shunt field and armature. Since the armature
begins to produce back EMF as soon as it starts to rotate, it will use
the back EMF to maintain its rpm at high speed. If the load increases
slightly and causes the armature shaft to slow down, less back EMF will
be produced. This will allow the difference between the back EMF and
applied voltage to become larger, which will cause more current to flow.
The extra current provides the motor with the extra torque required to
regain its rpm when this load is increased slightly.
shunt motor's speed can be varied in two different ways. These include
varying the amount of current supplied to the shunt field and
controlling the amount of current supplied to the armature. Controlling
the current to the shunt field allows the rpm to be changed 10-20% when
the motor is at full rpm.
type of speed control regulation is accomplished by slightly increasing
or decreasing the voltage applied to the field. The armature continues
to have full voltage applied to it while the current to the shunt field
is regulated by a rheostat that is connected in series with the shunt
field. When the shunt field's current is decreased, the motor's rpm will
increase slightly. When the shunt field's current is reduced, the
armature must rotate faster to produce the same amount of back EMF to
keep the load turning. If the shunt field current is increased slightly,
the armature can rotate at a slower rpm and maintain the amount of back
EMF to produce the armature current to drive the load. The field
current can be adjusted with a field rheostat or an SCR current control.
shunt motor's rpm can also be controlled by regulating the voltage that
is applied to the motor armature. This means that if the motor is
operated on less voltage than is shown on its data plate rating, it will
run at less than full rpm. You must remember that the shunt motor's
efficiency will drop off drastically when it is operated below its rated
voltage. The motor will tend to overheat when it is operated below full
voltage, so motor ventilation must be provided. You should also be
aware that the motor's torque is reduced when it is operated below the
full voltage level.
the armature draws more current than the shunt field, the control
resistors were much larger than those used for the field rheostat.
During the 1950s and 1960s SCRs were used for this type of current
control. The SCR was able to control the armature current since it was
capable of controlling several hundred amperes.The
armature's torque increases as the motor gains speed due to the fact
that the shunt motor's torque is directly proportional to the armature
current. When the motor is starting and speed is very low, the motor has
very little torque. After the motor reaches full rpm, its torque is at
its fullest potential. In fact, if the shunt field current is reduced
slightly when the motor is at full rpm, the rpm will increase slightly
and the motor's torque will also in-crease slightly. This type of
automatic control makes the shunt motor a good choice for applications
where constant speed is required, even though the torque will vary
slightly due to changes in the load. Figure 12-15 shows the torque/speed
curve for the shunt motor. From this diagram you can see that the speed
of the shunt motor stays fairly constant throughout its load range and
drops slightly when it is drawing the largest current.
the FMS is energized, its contacts connect the Al lead to the positive
power supply terminal and the A2 lead to the negative power supply
terminal. The Fl motor lead is connected directly to the positive
terminal of the power supply and the F2 lead is connected to the
negative terminal. When the motor is wired in this configuration, it
will begin to run in the forward direction.
the RMS is energized, its contacts reverse the armature wires so that
the Al lead is connected to the negative power supply terminal and the
A2 lead is connected to the positive power supply terminal. The field
leads are connected directly to the power supply, so their polarity is
not changed. Since the field's polarity has remained the same and the
armature's polarity has reversed, the motor will begin to rotate in the
reverse direction. The control part of the diagram shows that when the
FMS coil is energized, the RMS coil is locked out.