Microchip Technology DM183021 User Manual
PICDEM™ MC LV Development Board
DS51554B-page 28
© 2006 Microchip Technology Inc.
There are 6 sectors, each 60 degrees wide, which accumulate to give one 360 degree
electrical revolution of the motor. In each sector, two windings are excited: one with a
high voltage and the other with a low voltage. The third winding is not excited. As the
rotor rotates from one sector to another, a new set of windings is excited. The sequence
of excitation in each sector is provided by the motor manufacturer. The winding in each
sector that is not excited will be influenced by the Back EMF voltage. This voltage is
not high or low, but a falling or rising voltage level, going symmetrically from a
high-to-low or a low-to-high. It crosses the center, or star point voltage, at about
30 degrees before the next commutation point of the rotor. This center, or star point
voltage, is also referred to as the zero-crossing voltage. Its value is exactly half the volt-
age applied to the excited windings of the motor. The dsPIC device uses its fast ADC
to sense the zero-cross point. Having sensed the zero-cross point, it can predict the
time required for the next commutation phase. The zero-cross sensing and drive of the
motor is shown in Figure 7-2.
electrical revolution of the motor. In each sector, two windings are excited: one with a
high voltage and the other with a low voltage. The third winding is not excited. As the
rotor rotates from one sector to another, a new set of windings is excited. The sequence
of excitation in each sector is provided by the motor manufacturer. The winding in each
sector that is not excited will be influenced by the Back EMF voltage. This voltage is
not high or low, but a falling or rising voltage level, going symmetrically from a
high-to-low or a low-to-high. It crosses the center, or star point voltage, at about
30 degrees before the next commutation point of the rotor. This center, or star point
voltage, is also referred to as the zero-crossing voltage. Its value is exactly half the volt-
age applied to the excited windings of the motor. The dsPIC device uses its fast ADC
to sense the zero-cross point. Having sensed the zero-cross point, it can predict the
time required for the next commutation phase. The zero-cross sensing and drive of the
motor is shown in Figure 7-2.
FIGURE 7-2:
HARDWARE BLOCK DIAGRAM
The PWM signals drive three MOSFET drivers (IR2101s), which in turn, drive the
3-phase bridge inverter connected to the 3 motor windings (see Appendix A. “Circuit
Schematics of the Board”). The motor windings are driven with 24V. This voltage is
scaled down to about 1.8V full scale when sensed by the ADC inputs of the dsPIC
device. The scaling for each winding is done by resistor pairs, R34/R36, R41/R44 and
R49/R52 (see Appendix A. “Circuit Schematics of the Board”). The bus voltage is
sensed and scaled down by resistor pair, R63/R64.
3-phase bridge inverter connected to the 3 motor windings (see Appendix A. “Circuit
Schematics of the Board”). The motor windings are driven with 24V. This voltage is
scaled down to about 1.8V full scale when sensed by the ADC inputs of the dsPIC
device. The scaling for each winding is done by resistor pairs, R34/R36, R41/R44 and
R49/R52 (see Appendix A. “Circuit Schematics of the Board”). The bus voltage is
sensed and scaled down by resistor pair, R63/R64.
3-Phase
Inverter
AN1
PWM3H
PWM3L
PWM2H
PWM2L
PWM1H
PWM1L
FLTA
Fault
BLDC
dsPIC30F2010
AN3
AN4
AN5
AN0
V
DC
I
BUS
AN2
Demand
Phase Terminal Voltage Feedback
R49
R41
R34
R36
R44
R52
V
BUS
V
DC
R63
R64
Note:
All the resistor pairs should have the same value for a given motor voltage.
The resistor pairs used on the PICDEM MC LV Development Board give a
full-scale value of about 2.4V and so, the zero-cross voltage is about 1.2V.
This is based on a motor voltage of 24V.
The resistor pairs used on the PICDEM MC LV Development Board give a
full-scale value of about 2.4V and so, the zero-cross voltage is about 1.2V.
This is based on a motor voltage of 24V.