PowerTec 2000C Instruction Manual page 17

Page 1-10
Between the
pulses and be-
tween stages of
operation, the
inductive current
will want to
continue to flow
(since current
cannot be created
nor destroyed
instantaneously in
Figure 9: When the motor rotates 60 degrees the encoder
an inductive
switches the output transistors to rotate the field.
circuit) if the
energy is not used by the turning of the load, and the continued flow
of current is allowed through the free-wheeling diodes which are in
inverse parallel connection with each output transistor. In this case
the current which is entering the motor at T1 and exiting at T2, will
be forced to flow through the diodes around transistors 2 and 4. This
current will decay rapidly, and it has the effect of slowing the motor
down by loading it. It also has the effect of charging up the bus, but
unless the motor has a large inertia on its shaft and is turning at a
very high speed, it will have little effect.
After the motor has turned about 60 electrical degrees from the
position shown in Figure 8, the 5 transistor will turn off and the
current in the T11-T8-T5-T2 leg will die out; the 6 transistor will
pick up the operation. The current flow then will be through the T1-
T4-T7-T10 leg to the
center of the wye, and
out through the T12-
T9-T6-T3 leg. The
stator magnetic poles
will have shifted 60
degrees, causing the
rotor to continue to
move in that direction.
Unlike the brush-
type DC motor, which
can only fire its SCR
pairs every 2.6 milli-
seconds (.0026 second)
at best, the Brushless
DC motor control can
operate its transistors
during the entire
commutation cycle. A 2 kilohertz PWM frequency allows the
operation of a transistor every 500 microseconds (.0005 seconds).
Figure 10: The encoder assembly - four magnets
indicate position; outer magnets indicate speed
and direction.
Model 2000C/3000C Instruction Manual