AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 8

• Manufacturer Part Number: AN1857
• Description: A 3-Phase ac Induction Motor Control System Based on the MC68HC908MR32
• Manufacturer: Freescale Semiconductor / Motorola

Application Note
Motor Phase
Current Polarity
Sensing
System Hardware
8
WARNING:
Inserting dead-time to protect the top/bottom inverter transistors in a
motor drive system is almost always a necessity for ac induction motors.
However, inserting this dead-time does not come without a price. For
instance, when dead-time is inserted, the motor voltage is allowed to
float momentarily during the dead-time interval, creating distortion in the
motor’s current waveform. This distortion can be aggravated by
dissimilar turn-on and turn-off delays in the top and bottom transistors.
Three current sensing input pins on the MR32 are labeled IS1–IS3.
These inputs are sampled during the dead-time period. The user needs
to provide current direction sensing hardware and feed the sensing
hardware outputs into the IS1–IS3 inputs. The software then will
compute compensated PWM values and place the two values in an
odd/even PWM register pair. From the current direction sensing
information, the PWM module automatically selects either the odd or
even numbered PWM value register to be used by the PWM generator,
thus greatly improving upon the sine wave output quality from the power
stage.
The system consists of several printed circuit boards, power supplies
and a 3-phase induction motor. This section contains a brief description
of the individual printed circuit boards (PCB) that comprise the system.
The motor control system described in this application note is
capable of operating at dangerous voltages and is capable of
supplying high amounts of power to rotating machines. Power
transistors, PFC coil, and the motor can reach temperatures hot
enough to cause burns. To facilitate safe operation, the high-
voltage input power to the power board should come from a
current-limited dc laboratory power supply. Before moving test or
oscilloscope probes, making connections, etc., it is generally
advisable to power down the high-voltage power supply. When
high voltage is applied, using only one hand for operating the test
setup minimizes the possibility of electrical shock. Operation in
laboratory setups that may have grounded tables and/or chairs
should be avoided. Wearing safety glasses, avoiding ties and
jewelry, and using shields also are advisable.
Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
MOTOROLA
AN1857