Page 4
Model 2000C / 3000C Manual
Figure 5 is a schematic representation of the wind-
ings of the Brushless DC motor. The connection shown in
the drawing is a single-wye. There are three other ways the
motor windings can be connected which will change the
speed and horse-
power (the torque
remains constant in
any given motor as
the connections are
changed ), but
these other connec-
tions are not made
for different volt-
ages as is the case
with the AC induc-
tion motor. The
standard connec-
tion conventions of
the AC motor are
followed in the as-
signment of markings on the motor leads, however. For
further information on motor connections see the
POWERTEC motor manual.
Shown in Figure 6 are the major parts of the
POWERTEC Brushless DC motor in the off state. The
stator windings are connected as in figure 5 and the motor
is operated from the power bridge shown in figure 4. This
drawing is very simplified, showing only a two pole motor,
for simplicity. Most POWERTEC Brushless DC motors
are 4 pole or 8 pole motors.
Current is developed in the windings, producing
torque by the interaction of magnetic fields, produced by
the stator windings (with the power supplied from the
control) and the fields of the permanent magnets mounted
on the rotor.
The Brushless DC motor control has an "electronic
commutator", fed by an integral encoder mounted on the
Figure 6: Simplified drawing of the major parts of
the Brushless DC motor.
Figure 5: Single-wye connections
for a Brushless DC motor.
motor. This encoder tells the motor control which transis-
tors should be turned on to obtain the maximum torque
from the motor at whatever position the motor shaft hap-
pens to be in at that point in time. This establishes a com
munication between the motor and its control which is not
present in AC motors and inverters, and which is not a part
of the DC brush-type motor and its SCR control. The
Brushless DC motor control always knows where the
motor shaft is in its rotation because the motor encoder is
constantly monitoring it.
The control's power output bridge (figure 4) consists
of three "legs". Each leg has a power transistor from the
positive side of the DC power buss to the output terminal
(generally referred to as an "upper" transistor), and another
transistor from the output terminal to the negative side of
the DC power buss (herein referred to as a "lower" transis-
tor). Each time a transistor turns on, it connects an output
terminal to one of the sides of the DC power buss. Each
output terminal also has a "free-wheeling" diode con-
nected to each side of the buss to carry currents which the
transistors cannot conduct.
Again, at whatever position the rotor happens to be,
the encoder tells the drive which transistors should be
turned on to deliver maximum torque from the motor.
While this is actually done in an EPROM (an Electrically
Programmable Read Only Memory integrated circuit), the
simplified representation as a switch shown in figure 6 will
suffice for our purpose. Note that the longer arrow on the
switch in the diagram governs the switching of the upper
transistors, which are numbered 1, 2, and 3. The shorter
arrow governs the switching of the lower transistors 4, 5,
and 6. The arrangement is such that each of the upper
transistors may be operated with either of the lower transis-
tors in the other two output legs, but an upper transistor may
never be operated with the lower transistor in the same leg.
To do so would produce a short circuit across the DC power
buss and blow out the fuses, if not the transistors involved.
The driver circuits are also logically interlocked to prevent
the accidental turning on of opposing transistors.
The rotor in figure 6 has two sets of magnets on it, a
North Pole and a South pole, which will interact with the
electro-magnetic fields and poles which are produced by
the current in the stator windings (shown in figure 5
schematically and in their relative positions around the
rotor in figure 6). Remember that the rotor is free to turn,
but the stator windings are stationary.
When the control is turned on in the position shown
(Figure 7), transistors 1 and 5 will turn on, allowing current
to flow from the positive side of the bus through the number
1 transistor out of the control terminal T1 into the T1
winding of the motor and through T4 to T7 and through
T10 to the center of the motor connection. Since the
number 5 transistor is on, the current will flow through T11
to T8 to T5 and T2 in the motor to T2 on the motor control
through the number 5 transistor to the negative side of the
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