PowerTec 2000C Instruction Manual page 15

Page 1-8
Shown in Figure 7 are the major parts of the POWERTEC
Brushless DC motor in the off state. The stator windings are con-
nected as in figure 6 and the motor is operated from the power
bridge shown in Figure 5. 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 commuta-
tor" fed by an integral encoder mounted on the motor. This encoder
tells the motor control which transistors should be turned on to
obtain the maximum torque from the motor at whatever position the
motor shaft happens to be in at that point in time. This establishes a
communication 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 be-
cause the motor encoder is constantly monitoring it.
The control's power output bridge (Figure 5) consists of three
"legs." Each leg has a power transistor from the positive side of the
DC power bus 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 bus (herein referred to as a
"lower" transistor). Each time a transistor turns on, it connects an
output terminal to one of the sides of the DC power bus. Each output
terminal also has a "free-wheeling" diode connected to each side of
the bus 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 Erasable Programmable Read Only Memory
integrated circuit), the
simplified representa-
tion as a switch
shown in Figure 7
will suffice for our
purpose. Note that the
longer arrow on the
switch in the diagram
governs the switching
of the upper transis-
tors, which are
numbered 1, 2, and 3.
The shorter arrow
Figure 8: Current flows in the stator of the motor after
the control is turned on in the position shown.
Model 2000C/3000C Instruction Manual