AN1857 Freescale Semiconductor / Motorola, AN1857 Datasheet - Page 11

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

Speed Sensing
AN1857
MOTOROLA
TACHOMETER INPUT
1
2
GNDA
Adjusting the set-point of the potentiometer allows the user to vary the
acceptable system bus current and bus voltage thresholds for fault
generation. Approximately 20 mV of hysteresis is included in the circuit
to aid with noise immunity.
One method used to establish the speed of a motor is to use the analog
ac output signal from a tachometer generator that is connected to the
shaft of the motor. A 16-pole tachometer is coupled to the shaft of the
motor as shown in
into a timer interrupt on the MR32. The period between interrupts is used
to calculate motor shaft speed.
The circuit in
tachometer output into a digital signal acceptable to the timer interrupt
input. The input of this circuit has a threshold of approximately 180 mV.
Its input hysteresis is set at approximately 20 mV to aid with noise
immunity.
3.3 k
Freescale Semiconductor, Inc.
For More Information On This Product,
Figure 5. Tachometer Input Circuit
3.3 k
V_sense
I_sense/
GNDA
Input
Go to: www.freescale.com
Figure 5
220 nF/100 V
15 k
2.2 k
68 pF
Figure 4. Fault Generation Circuit
MMSZ5230BT1
Figure
GNDA
is used to “square” the ac signal from the
GNDA
+5V_A_ref
19. The conditioned signal can then be carried
15 k
+5V_A_ref
10 k
I_DCB_ref/V_DCB_ref
15 k
GNDA
560
7
6
+
-
1M
5
4
12
3
LM339D
330 k
+
–
+15V_A_ref
LM339D
GND
2
+5V_D
10 k
+5V_D
10 k
System Hardware
Application Note
OVERVOLTAGE
TO TIMER
INTERRUPT
11