PowerTec 2000C Instruction Manual page 12

1.3 A BRUSHLESS
DC PRIMER
© copyright 1995 by
Powertec Industrial Motors
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 "bus"), 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 bus (the
number of capacitors) 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 motor is turned on, the capacitors
will charge up to this peak voltage. When the motor is started, it uses
Figure 4: Simplified drawing of input power section
of a Brushless DC motor control.
power from the bus to perform the work required. The effect of this is
to partially discharge the capacitors, lowering the bus 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 the AC line. The result of this is
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