A Brushless Dc Primer - PowerTec 2000C Instruction Manual

1.3

A BRUSHLESS DC PRIMER

Three phase AC plant power is converted to DC by
the input side of a Brushless DC motor control to charge up
a "bank" of storage capacitors (called the "buss"), whose
function is to store energy and supply DC power to the
power transistors in the output bridge as power is required
by the motor. The size of the buss (the number of capaci-
tors) varies with the size of the motor.
This rectification is accomplished by six diodes,
which may be in a single package or in several modules.
The diodes are protected by the input fuses, which are
chosen for their speed and interrupting capacity. An input
choke in the DC leg of the diode bridge protects against line
transients and limits the rate at which input current may
increase or decrease.
This input section is self regulating. The highest
voltage level possible on the capacitors is 1.4 times the
line-to line voltage (the peak voltage). Initially, before the
Figure 3: Simplified drawing of input power section
of a Brushless DC motor control.
motor is turned on, the capacitors will charge up to this
peak voltage. When the motor is started, it uses power from
the buss to perform the work required. The effect of this is
to partially discharge the capacitors, lowering the buss
voltage. With three phase input power, there are six periods
in each cycle of AC when the line-to line voltage is greater
than the capacitor voltage. The capacitors will only draw
current from the power lines when the capacitor voltage is
lower than the instantaneous line-to-line voltage, and then
it will only draw enough power to replenish the energy used
by the motor since the last time the line-to line voltage was
greater than the capacitor voltage.
Torque in a motor is a function of current. Power is
a function of speed AND torque. Even though the current
required by the motor to develop the torque may be large,
the actual power used is small at low speeds. Because the
energy drawn from the capacitors is the actual power used
by the motor, the energy drawn from the input power lines
is the actual power supplied to the motor. The Brushless
DC motor control is capable of running at very low speeds
at very high torques while drawing very little current from
© copyright 1992, 1996 by Powertec Industrial
Motors
the AC line. The result of this is that the RMS current at the
input of the Brushless DC motor control is directly propor-
tional to the output power of the motor, rather than being
proportional to the motor's load.
Figure 4: The power output bridge consists of six
power transistors and associated snubber components.
A Brushless DC motor is wound like an AC induc-
tion motor, but it uses permanent magnets on the rotor
instead of shorted rotor bars. There are three power carry-
ing wires going to the motor. Each of these wires has to be,
at synchronized times, connected to either side of the DC
buss. This is accomplished with a six transistor power
output bridge configuration (figure 4).
Applying power to the motor requires turning on one
transistor connected to the positive side of the buss and one
transistor connected to the negative side of the buss, but
never the two transistors in the same leg of the output.
When two transistors are turned on, the entire buss 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 buss) until
the transistors are turned off. Due to the inductive nature of
the motor windings, the current will not cease immediately
when the transistors 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
heavy load will be quickly picked up. This accounts for the
high efficiency of Brushless DC.
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