PowerTec 2000C Instruction Manual page 13

Page 1-6
Figure 5: The power output bridge consists of six power
transistors and associated snubber components.
that the RMS current at the input of the Brushless DC motor control
is directly proportional to the output power of the motor, rather than
proportional to motor load.
A Brushless DC motor is wound like an AC induction motor,
but it uses permanent magnets on the rotor instead of shorted rotor
bars. There are three power carrying wires going to the motor.
Each of these wires has to be, at synchronized times, connected
to either side of the DC bus. This is accomplished with a six transis-
tor bridge configuration (Figure 5).
Applying power to the motor requires turning on one transistor
connected to the positive side of the bus and one transistor con-
nected to the negative side of the bus, but never the two transistors
in the same leg of the output. When two transistors are turned on,
the entire bus voltage is applied to the windings of the motor
through the two wires connected to those transistors and current will
flow (if the CEMF of the motor is not greater than the bus) until the
transistors are turned off. Due to the inductive nature of the motor
windings, the current will not cease immediately when the transis-
tors are turned off. It will decay quickly, but voltages in the bridge
would rise dangerously if the snubber network were not present to
prevent this from happening.
If two transistors were turned on and left on for any length of
time, the current would build to very high levels too quickly, so the
transistors are turned on for only brief intervals at a time. If the
motor is lightly loaded, it will not take a lot of current (torque) to get
it going. If the motor is heavily loaded, each turn-on interval will
cause the current to build up until there is sufficient torque to turn
the load. Once the motor has started turning, the current supplied
will be, in either case, just enough to keep the motor and load
turning. If a heavy load is taken off the motor, the current will
quickly drop to the new level, and an applied load will be quickly
picked up. This accounts for the high efficiency of Brushless DC.
Model 2000C/3000C Instruction Manual