PowerTec 2000C Instruction Manual page 26

Figure 7: Current flows in the stator of the motor
after the control is turned on in the position shown.
buss. The currents will be as indicated in figure 7, setting
up magnetic poles in the stator as shown (North poles at T1-
T4 and T11-T8, and South poles at T7-T10 and T2-T5).
The North poles in the stator attract the South pole of the
rotor and repel the North pole of the rotor, and the South
poles in the stator attract the North pole of the rotor and
repel the South pole. Rotation will occur in the indicated
direction as torque is developed in the motor.
Since the load is heavily inductive, this current
would build in a nearly linear way if the transistors were on
continuously, but the switching of the lower transistors is
pulse-width modulated, i.e., switched on and off at a
relatively high frequency. The width of each pulse is
determined by the torque required to turn the motor, which
is under speed and, effectively, position control. The
amount of torque required is determined by the load on the
motor shaft. The lighter the load, the narrower the pulse
width. As the load gets heavier, the pulses will get wider
until they reach their maximum width.
With each pulse the current will build up a little until
the end of the period of time that the two transistors are on,
which will occur when the motor has turned far enough to
turn off the 5 transistor. In this example, that will occur
when the rotor has turned 60 degrees (in the actual motor,
that will occur in 60 electrical degrees, not in 60 physical
degrees as in the illustration. There may be as many as 1440
electrical degrees per revolution in a motor). Then the next
step will occur as the 5 transistor turns off, and the 6
transistor turns on (figure 8).
Between the pulses and between stages of operation,
the inductive current will want to continue to flow (since
current cannot be created nor destroyed instantaneously in
an inductive 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 transistors. In this
© copyright 1992, 1996 by Powertec Industrial
Motors
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 buss, 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 de-
grees from the position shown in figure 7, 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. Then the
current flow 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 DC brush-type DC motor, which can only
fire its SCR pairs every 2.6 milliseconds (.0026 second) at
best, the brushless DC motor control can operate its tran-
sistors during the entire commutation cycle. A 2 kilohertz
PWM frequency allows the operation of a transistor every
500 microseconds (.0005 seconds). Frequencies from 2
kilohertz to 20 kilohertz or more are common. There is also
Figure 8: When the motor rotates 60 degrees the encoder
switches the output transistors to rotate the field.
no need to wait for the power line conditions to shut off the
transistor; it may be cut off at any time.
As the field continues to move around the stator, the
rotor follows it. Unlike the induction A.C. motor, in which
all windings are continuously excited, the brushless DC
stator is a DC excited field, and it moves because the
windings are continuously switched in response to the
movement of the rotor. The non-excited windings are
carrying no current, since the windings in a brushless DC
motor are either on or off. They do not go through the slow
transitions that the AC motor windings goes through. If
winding switching were to stop, the motor would come to
a halt.
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