PowerTec 2000C Instruction Manual page 16

© copyright 1995 by
Powertec Industrial Motors
governs the switching of the lower transistors 4, 5, and 6. The ar-
rangement is such that each of the upper transistors may be operated
with either of the lower transistors 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
bus 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 7 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 6 schematically and in their relative positions
around the rotor in Figure 7). 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 8),
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 bus. The currents will
be as indicated in Figure 8, 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, in effect, 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
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. The next step will occur
as the 5 transistor turns off, and the 6 transistor turns on (Figure 9).
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