AN889

AN889
VF Control of 3-Phase Induction Motors
Using PIC16F7X7 Microcontrollers
Author:
Rakesh Parekh
Microchip Technology Inc.
INTRODUCTION
An induction motor can run only at its rated speed when
it is connected directly to the main supply. However,
many applications need variable speed operations.
This is felt the most in applications where input power
is directly proportional to the cube of motor speed. In
applications like the induction motor-based centrifugal
pump, a speed reduction of 20% results in an energy
savings of approximately 50%.
Driving and controlling the induction motor efficiently
are prime concerns in today’s energy conscious world.
With the advancement in the semiconductor fabrication
technology, both the size and the price of semiconductors have gone down drastically. This means that the
motor user can replace an energy inefficient mechanical motor drive and control system with a Variable
Frequency Drive (VFD). The VFD not only controls the
motor speed, but can improve the motor’s dynamic and
steady state characteristics as well. In addition, the
VFD can reduce the system’s average energy
consumption.
Although various induction motor control techniques
are in practice today, the most popular control technique is by generating variable frequency supply, which
has constant voltage to frequency ratio. This technique
is popularly known as VF control. Generally used for
open-loop systems, VF control caters to a large number of applications where the basic need is to vary the
motor speed and control the motor efficiently. It is also
simple to implement and cost effective.
The PIC16F7X7 series of microcontrollers have three
on-chip hardware PWM modules, making them
suitable for 3-phase motor control applications. This
application note explains how these microcontrollers
can be used for 3-phase AC induction motor control.
 2004 Microchip Technology Inc.
VF CONTROL
A discussion of induction motor control theory is
beyond the scope of this document. We will mention
here only the salient points of VF control.
The base speed of the induction motor is directly
proportional to the supply frequency and the number of
poles of the motor. Since the number of poles is fixed
by design, the best way to vary the speed of the
induction motor is by varying the supply frequency.
The torque developed by the induction motor is directly
proportional to the ratio of the applied voltage and the
frequency of supply. By varying the voltage and the frequency, but keeping their ratio constant, the torque
developed can be kept constant throughout the speed
range. This is exactly what VF control tries to achieve.
Figure 1 shows the typical torque-speed characteristics
of the induction motor, supplied directly from the main
supply. Figure 2 shows the torque-speed characteristics
of the induction motor with VF control.
Other than the variation in speed, the torque-speed
characteristics of the VF control reveal the following:
• The starting current requirement is lower.
• The stable operating region of the motor is
increased. Instead of simply running at its base
rated speed (NB), the motor can be run typically
from 5% of the synchronous speed (NS) up to the
base speed. The torque generated by the motor
can be kept constant throughout this region.
• At base speed, the voltage and frequency reach
the rated values. We can drive the motor beyond
the base speed by increasing the frequency
further. However, the applied voltage cannot be
increased beyond the rated voltage. Therefore,
only the frequency can be increased, which
results in the reduction of torque. Above the base
speed, the factors governing torque become
complex.
• The acceleration and deceleration of the motor
can be controlled by controlling the change of the
supply frequency to the motor with respect to
time.
DS00889B-page 1
AN889
FIGURE 1:
TORQUE-SPEED CHARACTERISTICS OF INDUCTION MOTOR
Breakdown Torque
Torque
Current
Full Load Torque
Locked Rotor Torque
Torque and
Current
TRATED
IRATED
Pull-up Torque
NB
Slip
FIGURE 2:
NS
Speed
TORQUE-SPEED CHARACTERISTICS OF INDUCTION MOTOR WITH VF CONTROL
Torque
Voltage
VRATED
Torque and
Voltage
VMIN
fmin
frated
fmax
(Base Speed)
Frequency
DS00889B-page 2
 2004 Microchip Technology Inc.
AN889
MOTOR DRIVE
The 3-phase induction motor is connected to a 3-phase
inverter bridge as shown in Figure 3.The power inverter
has 6 switches that are controlled in order to generate
3-phase AC output from the DC bus. PWM signals,
generated from the microcontroller, control these 6
switches. Switches IGBTH1 through IGBTH3, which
are connected to DC+, are called upper switches.
Switches IGBTL1 through IGBTL3, connected to DC-,
are called lower switches.
The amplitude of phase voltage is determined by the
duty cycle of the PWM signals. While the motor is running, three out of six switches will be on at any given
time; either one upper and two lower switches or one
lower and two upper switches. The switching produces
a rectangular shaped output waveform that is rich in
FIGURE 3:
harmonics. The inductive nature of the motor’s stator
windings filters this supplied current to produce a
3-phase sine wave with negligible harmonics. When
switches are turned off, the inductive nature of the
windings oppose any sudden change in direction of
flow of the current until all of the energy stored in the
windings is dissipated. To facilitate this, fast recovery
diodes are provided across each switch. These diodes
are known as freewheeling diodes.
To prevent the DC bus supply from being shorted, the
upper and lower switches of the same half bridge
should not be switched on at the same time. A dead
time is given between switching off one switch and
switching on the other. This ensures that both switches
are not conductive at the same time as each one
changes states.
3-PHASE INVERTER BRIDGE
DC+
IGBTH1
IGBTH2
IGBTH3
3-PH
Induction
Motor
IGBTL1
IGBTL2
IGBTL3
DC-
 2004 Microchip Technology Inc.
DS00889B-page 3
AN889
Control
half bridge of the inverter: one to the upper switch, the
other to the lower switch. The driver also adds a fixed
dead time between the two PWM signals.
Members of the PIC16F7X7 family of microcontrollers
have three 10-bit PWMs implemented in hardware. The
duty cycle of each PWM can be varied individually to
generate a 3-phase AC waveform as shown in
Figure 4. The upper eight bits of the PWM’s duty cycle
is set using the register CCPRxL, while the lower two
bits are set in bits 4 and 5 of the CCPxCON register.
The PWM frequency is set using the Timer2 Period register (PR2). Because all of the PWMs use Timer2 as
their time base for setting the switching frequency and
duty cycle, all will have the same switching frequency.
3-Phase Sine Waveform Synthesis
Along with the three PWM modules, the 16-bit Timer1
hardware module of PIC16F7X7 is used to generate
the control signals to the 3-phase inverter.
This is done by using a sine table, stored in the
program memory with the application code and
transferred to the data memory upon initialization.
Loading the table this way minimizes access time
during the run time of the motor. Three registers are
used as the offset to the table. Each of these registers
will point to one of the values in the table, such that they
will always have a 120-degree phase shift relative to
each other (Figure 4). This forms three sine waves with
120 degrees phase shift to each other.
To derive a varying 3-phase AC voltage from the DC
bus, the PWM outputs are required to control the six
switches of the power inverter. This is done by connecting the PWM outputs to three IGBT drivers (IR2109).
Each driver takes one PWM signal as input and
produces two PWM outputs, one being complementary
to the other. These two signals are used to drive one
FIGURE 4:
SYNTHESIS OF 3-PHASE SINE WAVEFORM FROM A SINE TABLE
Sine Table Value + Offset 1
Sine Table Value + Offset 2
Sine Table Value + Offset 3
DC+
Voltage
DC-
Time
DS00889B-page 4
 2004 Microchip Technology Inc.
AN889
A potentiometer connected to a 10-bit ADC channel
(AN1) determines the motor frequency. The microcontroller uses the ADC results to calculate the PWM
duty cycle and thus, the frequency and the amplitude of
the supply to the motor. For smooth frequency
transitions, the channel AN1 is converted at every 4 ms.
loaded to the corresponding PWM duty cycle register.
Subsequently, the offset registers are updated for next
access. If the motor direction key is pressed, then
PWM1, PWM2 and PWM3 duty cycle values are
loaded to PWM2, PWM1 and PWM3 duty cycle
registers, respectively.
The Timer1 reload value is based on the ADC result
(AN1), the main clock frequency (FOSC) and the number of sine table entries (36 in the present application).
After every Timer1 overflow, the value pointed to by the
offset register on the sine table is read. The value read
from the sine table is scaled based on the motor frequency input. The sine table value is multiplied with the
frequency input to find the PWM duty cycle and is
The new PWM duty cycle values will take effect at the
next Timer2 overflow. Also, the duty cycle will remain
the same until the next Timer1 overflow occurs, as
shown in Figure 5. The frequency of the new PWM duty
cycle update determines the motor frequency, while the
value loaded in the duty cycle register determines the
amplitude of the motor supply.
FIGURE 5:
TIMER1 OVERFLOW, PWM DUTY CYCLE AND OUTPUT VOLTAGE
1
1
1
1
1
Timer
Events
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
1
Timer1 Interrupt
Timer2 to PR2
Match Interrupt
Instantaneous
Voltage
2
Average Voltage
Output
Voltage
Time
The equation used to calculate the Timer1 reload value
is given in Equation 1. In the present application, the
Timer1 prescaler is 1:8. PR2 is set to generate a
20 kHz PWM frequency with FOSC of 20 MHz.
EQUATION 1:
The method of accessing and scaling of the PWM duty
cycle is shown in an excerpt from the application code
in Example 1.
TIMER1 RELOAD VALUE CALCULATION
FOSC


4

Timer1 Reload Value = FFFFh – 2 x 
 Sine Samples per Cycle x Timer1 Prescaler x Value of AN1 
 2004 Microchip Technology Inc.
DS00889B-page 5
AN889
EXAMPLE 1:
SINE TABLE UPDATE
;*********************************************************************************************
;This routine will update the PWM duty cycle on CCPx according to the offset to the table with
;0-120-240 degrees.
;This routine scales the PWM value from the table based on the frequency to keep VF
;constant and loads them in appropriate CCPx register depending on setting of FWD/REV flag
;*********************************************************************************************
UPDATE_PWM_DUTYCYCLES
MOVLW
LOW(SINE_TABLE_RAM)
MOVWF
FSR
;Base address of sine table in RAM is loaded to FSR
BANKSEL
TABLE_OFFSET1
MOVF
TABLE_OFFSET1,W
;Table_offset1 is copied To WREG
ADDWF
FSR,F
;Address to be read = sine table base adress + Table_offset1
BANKISEL
SINE_TABLE_RAM
MOVF
INDF,W
;Copy sine table value pointed to by FSR to WREG
BTFSC
STATUS,Z
;Check is value read zero?
GOTO
PWM1_IS_0
;Yes, goto PWM1_IS_0
MOVWF
NO_1_LSB
;No, sine table value x Set_freq to scale table value
;based on frequency setting
CALL
MUL_8X8
;Call routine for unsigned 8x8 bit multiplication
MOVF
RESULT_MSB,W
;8 MSB of 16 bit result is stored at TEMP_LOC MOVWF
TEMP_LOC
;this represent PWM Duty Cycle value for phase 1
GOTO
UPDATE_PWM2
;Go for updating PWM Duty Cycle for 2nd phase
PWM1_IS_0
CLRF
TEMP_LOC
;Clear PWM Duty Cycle value for phase 1
UPDATE_PWM2
MOVLW
LOW (SINE_TABLE_RAM)
MOVWF
FSR
;Base address of sine table in RAM is loaded to FSR
BANKSEL
TABLE_OFFSET2
MOVF
TABLE_OFFSET2,W
;Table_offset2 is copied to WREG
ADDWF
FSR,F
;Address to be read = Sine table base adress + Table_offset2
BANKISEL
SINE_TABLE_RAM
MOVF
INDF,W
;Copy sine table value pointed to by FSR to WREG
BTFSC
STATUS,Z
;Check is value read zero?
GOTO
PWM2_IS_0
;Yes, go to PWM2_IS_0
MOVWF
NO_1_LSB
;No, sine table value x set_freq to scale table value
;based on frequency setting
CALL
MUL_8X8
;Call routine for unsigned 8x8 bit multiplication
MOVF
RESULT_MSB,W
;8 MSB of 16 bit result is stored at TEMP_LOC_1 MOVWF
TEMP_LOC_1
;this represent PWM Duty Cycle value for phase 2
GOTO
UPDATE_PWM3
;Go for updating PWM Duty Cycle for 3rd phase
PWM2_IS_0
CLRF
TEMP_LOC_1
;Clear PWM Duty Cycle value for phase 2
UPDATE_PWM3
MOVLW
LOW(SINE_TABLE_RAM)
MOVWF
FSR
;Base address of sine table in RAM is loaded to FSR
BANKSEL
TABLE_OFFSET3
MOVF
TABLE_OFFSET3,W
;Table_offset3 is copied to WREG
ADDWF
FSR,F
;Address to be read=Sine table base address + Table_offset3
BANKISEL
SINE_TABLE_RAM
MOVF
INDF,W
;Copy sine table value pointed by FSR to WREG
BTFSC
STATUS,Z
;Check is value read zero?
GOTO
PWM3_IS_0
;Yes, goto PWM3_IS_0
MOVWF
NO_1_LSB
;No, sine table value x set_freq to scale table value
;based on frequency setting
CALL
MUL_8X8
;Call routine for unsigned 8x8 bit multiplication
MOVF
RESULT_MSB,W
;8 MSB of 16 bit result is stored at TEMP_LOC_2 MOVWF
TEMP_LOC_2
;this represents PWM duty cycle value for phase 3
GOTO
SET_PWM12
;Go for checking direction of motor rotation reequired
PWM3_IS_0
CLRF
TEMP_LOC_2
;Clear PWM duty cycle value for phase 3
DS00889B-page 6
 2004 Microchip Technology Inc.
AN889
EXAMPLE 1:
SET_PWM12
BANKSEL
BTFSS
GOTO
MOVF
MOVWF
MOVF
MOVWF
BSF
MOVF
MOVWF
BCF
BSF
SINE TABLE UPDATE (CONTINUED)
CCPR1L
FLAGS,MOTOR_DIRECTION
ROTATE_REVERSE
TEMP_LOC,W
CCPR1L
TEMP_LOC_1,W
CCPR2L
STATUS,RP0
TEMP_LOC_2,W
CCPR3L
STATUS,RP0
LED_PORT,FWD_REV_LED
;Is MOTOR_DIRECTION flag set for forward rotation?
;No - Go for reverse rotation
;Copy TEMP_LOC and TEMP_LOC_1 values to
;CCPR1L and CCPR2L respectively for
;forward rotation of motor
;Copy TEMP_LOC_2 to CCPR3L
;Turn on FWD_REV_LED to indicate
;forward rotation of motor
RETURN
ROTATE_REVERSE
MOVF
MOVWF
MOVF
MOVWF
BSF
MOVF
MOVWF
BCF
BCF
TEMP_LOC_1,W
CCPR1L
TEMP_LOC,W
CCPR2L
STATUS,RP0
TEMP_LOC_2,W
CCPR3L
STATUS,RP0
LED_PORT,FWD_REV_LED
;Copy TEMP_LOC_1 and TEMP_LOC values to
;CCPR1L and CCPR2L respectively for
;reverse rotation of motor
;Copy TEMP_LOC_2 to CCPR3L
;Turn off FWD_REV_LED to indicate
;reverse rotation of motor
RETURN
 2004 Microchip Technology Inc.
DS00889B-page 7
AN889
OVERVIEW OF SYSTEM HARDWARE
Overcurrent Protection
Figure 6 shows the overall block diagram of the power
and control circuit for the motor control demo board.
The main single phase supply is rectified by using a
diode bridge rectifier. The ripple on the DC bus is filtered by using an electrolytic capacitor. The filtered DC
bus is connected to the IGBT-based 3-phase inverter,
which is controlled by the PIC16F7X7. The inverter
output is a 3-phase, variable frequency supply with a
constant voltage-to-frequency ratio.
A non-inductive resistor is connected between the
common source point of the inverter and the power
ground. Voltage drop across this resistor is linearly proportional to the current flowing through the motor. This
voltage drop is compared against the reference voltage
signal, through an optoisolator (linear optocoupler),
which represents overcurrent limit. There are three
possible ways to compare these voltage signals:
• Using an external comparator
• Using the PIC16F7X7 on-chip comparator
• In software, by reading the voltage drop across
the resistor through one of the ADC channels
A potentiometer connected to AN1 sets the motor
frequency. Push button keys are interfaced for issuing
commands, like Run/Stop and Fwd/Rev, to the
microcontroller. Acceleration and deceleration features
are implemented to change the motor frequency
smoothly. Time for both of these features are user
selectable and can be set during compile time. LEDs
are provided for Status/Fault indications like Run/Stop,
Forward/Reverse, Undervoltage, Overvoltage, etc.
The design discussed in this application note
implements an external comparator. It’s output drives the
shutdown signal of the driver through an optoisolator
(optocoupler). At the same time, this signal is provided
to RB4. By using the PORTB interrupt-on-change
feature, the microcontroller responds to Fault detection
and stops the motor.
The PWM outputs are generated by on-chip hardware
modules on the PIC16F7X7. These are used to drive
the IGBT drivers through optoisolators. Each IGBT
driver, in turn, generates complementary signals for
driving the upper or lower halves of the 3-phase
inverter. It also adds a dead time of 540 ns between the
respective higher and lower switch driving signals.
Overvoltage and Undervoltage Protection
To implement voltage protection, the DC bus voltage is
attenuated by a potential divider. The resulting signal is
fed to AN2 through an optoisolator (linear optocoupler).
The application monitors the voltage via periodic A/D
conversions of the value on RA2; if the voltage falls
outside of a preset range, the motor is stopped.
The IGBT driver has a shutdown signal (SD) which is
controlled by an overcurrent protection circuit. The
driver also has its own on-chip Fault monitoring circuit
for driver power supply undervoltage conditions. Upon
any overcurrent or undervoltage event, the outputs are
driven low and remain low until the time the Fault
condition is removed.
Note:
Refer to Appendix B: “Motor Control
Schematics” for schematics of the motor
control demo board.
FIGURE 6:
BLOCK DIAGRAM OF THE MOTOR CONTROL DEMO BOARD
Single-Phase
AC Input
Rectifier
Voltage
Current
Attenuator Comparator
1
Speed Set
Fwd/Rev
AN1
AN2
RB4
PWM1
PWM2
PWM3
2
1
2
2
Run/Stop
PIC16F7X7
Optoisolators
SD
HIN1
HIN2
HIN3
HOut1
IGBTH1
HOut2
IGBTH2
HOut3
IGBTH3
LOut1
IGBTL1
LOut2
IGBTL2
IGBT
Drivers LOut3
3-Phase
Induction
Motor
3-Phase
IGBTL3 Inverter
Status/Fault Indicators
DS00889B-page 8
 2004 Microchip Technology Inc.
AN889
Isolation
The use of optoisolators ensures that power ground
(P_GND) and control ground (D_GND) are separated.
This means that development tools, such as MPLAB®
ICD 2 and MPLAB® ICE can be safely connected to the
system while it is connected to the AC supply. This
simplifies the task of debugging a live system.
The isolation components are often removed when a
design goes for production. To remove isolation:
• Remove the PWM drive optoisolators
(U6 through U9).
• Remove the power isolation optoisolators
(U17 and U18).
• Disconnect the voltage followers for U17 and U18
(U13B, U13C, U16A and U16B). DO NOT
physically remove U13 and U16, since U13A and
U16C are still used by the system.
• Remove all other components associated with the
power isolation system (capacitors C41/42/43 and
resistors R81/82/83/84/91/92/93/96).
• Make all grounds common by shorting P_GND to
D_GND.
VF CONTROL FIRMWARE
While the PIC16F7X7 microcontroller makes 3-phase
motor control possible, it is the firmware that makes VF
control straightforward. In addition to maintaining the
sine table and driving the PWM modules to produce the
AC output (previously described in the “3-Phase Sine
Waveform Synthesis” section), the firmware interprets control inputs and system status to sense and act
on Fault conditions. It also manages other features of
motor control, such as direction, acceleration and
deceleration (as described below).
 2004 Microchip Technology Inc.
The VF control firmware uses a set of defined routines
and parameters for operation. Users can change these
parameters as needed for their applications. The firmware can also be incorporated as the motor control
core of a larger application, using the parameters to
pass information between sections of the code. An
overview of the firmware’s logic flow is provided in
Figure 7 and Figure 8. A complete list of parameters
and defined functions is provided in Tables 1 through 4.
Users are encouraged to download the complete
source code of the firmware from the Microchip web
site (www.microchip.com) and examine the application
in more detail.
Acceleration and Deceleration
Acceleration and deceleration time can be specified
during compile time. The actual motor frequency
(SET_FREQ) and the required user frequency
(NEW_FREQ), set through the potentiometer, is compared at 4 ms intervals. If the SET_FREQ and the
NEW_FREQ are different, then the SET_FREQ is changed
step by step (each step size is 0.25 Hz) until it reaches
the NEW_FREQ.
The time to change the SET_FREQ by one step is calculated in software, depending upon the difference
between the SET_FREQ and the NEW_FREQ, as well as
the acceleration and deceleration parameters entered
during compile time. If the NEW_FREQ is changed during
the acceleration and deceleration process, then the
time to change each step is recalculated.
DS00889B-page 9
AN889
FIGURE 7:
MOTOR CONTROL FLOW CHART (MAIN AND ADC ROUTINES)
Main Routine
START
Initialization of Motor Parameters
and On-Chip Peripherals
Has Timer1
overflowed?
Yes
Update PWM Duty Cycle
by Reading Sine Table
No
Update Sine
Table Offset<1:3>
Call SET_ADC_GO
Key Scan to Read Run/Stop
and Fwd/Rev Switch Status
ADC Routine
SET_ADC_GO
No
Yes
Is 4 ms
interval over?
No
Configure and Start ADC
for Converting DC bus
Voltage Level Signal
Calculate Time Step
Required for Unit
Change in SET_FREQ
Is
SET_FREQ =
NEW_FREQ?
Yes
Calculate Timer1
Reload Value (X)
Configure and Start ADC for
Converting Potentiometer Set
Reference Signal
Return
DS00889B-page 10
 2004 Microchip Technology Inc.
AN889
FIGURE 8:
MOTOR CONTROL FLOW CHART (INTERRUPT SERVICE ROUTINE)
ISR
Yes
Run/Stop the Motor
per Status of PORTB<5>
Is RBIF = 1?
(overcurrent protection)
No
Yes
Timer1 = X
Is TMR1IF = 1?
(motor frequency decider)
No
Is ADIF = 1?
(motor frequency reading
and UV/OV protection)
Yes
Is
ADC set for reading
DC bus voltage?
No
No
Yes
No
Yes
No
Read Potentiometer
Setting (motor frequency)
Is TMR2IF = 1?
(acceleration/deceleration)
Is 4 ms
interval over?
Is
DC bus voltage outside
set limit of UV/OV?
No
Yes
Yes
No
Is
it the time to change
SET_FREQ ?
Stop the Motor and Indicate
Appropriate Fault Condition
Yes
Change SET_FREQ
by Unit Step
RETFIE
 2004 Microchip Technology Inc.
DS00889B-page 11
AN889
TABLE 1:
USER DEFINED PARAMETERS IN SOFTWARE
Name
Description
OSC_FREQ
Defines the oscillator frequency. In the present application, this is set to 20 MHz.
TIMER1_PRESCALE
Defines Timer1 prescaler value. In the present application, it is set to 1:8.
TIMER2_PRESCALE
Defines the Timer2 prescaler value. In the present application, this is set to 1:1.
PWM_FREQUENCY
Defines the PWM switching frequency. In the present application, this is set to 20 kHz.
ACCELERATION_TIME
Defines the user set acceleration time for the motor speed. In the present application, this
is set to 3 seconds.
DECELERATION_TIME
Defines the user set deceleration time for the motor speed. In the present application, this
is set to 3 seconds.
SINE_TABLE_ENTRIES Defines the length of the sine table. In the present application, this is set to 19.
TABLE 2:
CONSTANTS IN SOFTWARE
Name
Description
FREQ_SCALE
Used to calculate Timer1 reload value. It’s value depends on FOSC, Timer1 prescaler and the
number of sine table entries
PR2_VALUE
Defines the Timer2 overflow time period and thus, the PWM switching frequency. It’s value
depends on FOSC, Timer2 prescaler and required PWM switching frequency.
DEC_CON
Used for calculating time required for unit step decrement in SET_FREQ. It’s value is:
Deceleration Time x 250.
ACC_CON
Used for calculating time required for unit step increment in SET_FREQ. It’s value is:
Acceleration Time x 250.
LIMIT_V_LOW
Defines the DC bus voltage limit for undervoltage protection to activate.
LIMIT_V_HIGH
Defines the DC bus voltage limit for overvoltage protection to activate.
TABLE 3:
VARIABLES IN SOFTWARE
Name
Description
SET_FREQ
Actual motor frequency.
NEW_FREQ
Required motor frequency (set through the potentiometer).
TABLE_OFFSET1
Pointer to sine table for phase 1.
TABLE_OFFSET2
Pointer to sine table for phase 2.
TABLE_OFFSET3
Pointer to sine table for phase 3.
DS00889B-page 12
 2004 Microchip Technology Inc.
AN889
TABLE 4:
FUNCTIONS IN SOFTWARE
Name
Description
UPDATE_PWM_DUTYCYCLES
Loads new duty cycle values to CCPRxL for generating the 3-phase sine wave. This
routine also scales the sine table value depending on SET_FREQ.
UPDATE_TABLE_OFFSET
Changes the pointers to the sine table after every access to maintain 120-degree
phase shift between generated sine waves.
SET_ADC_GO
Configures the ADC for reading DC bus voltage or the potentiometer setting for the
required motor frequency. This routine also calculates time needed for unit change in
SET_FREQ for acceleration and deceleration.
KEY_CHECK
Checks the status of Run/Stop and Fwd/Rev keys and acts accordingly.
CHECK_FAULT
Responds to setting of RBIF. This routine responds to the output status of the external
current comparator.
TIMER1_OVERFLOW
Responds to setting of TMR1IF. This routine sets the user-defined flag indicating Timer1
overflow, which in turn is responsible for calling of UPDATE_PWM_DUTYCYCLES in the
main routine.
AD_CONV_COMPLETE
Responds to setting of ADIF. This routine reads the frequency setting from the
potentiometer through ADC. If the frequency is set below 5 Hz or above 60 Hz, it
limits the frequency to 5 Hz or 60 Hz.
This routine also reads the DC bus voltage level through ADC. If the level is outside
the preset limits, it activates undervoltage or overvoltage protection as required.
TMR2_ISR
Responds to setting of TMR2IF. This routine is used for implementing acceleration
and deceleration feature.
 2004 Microchip Technology Inc.
DS00889B-page 13
AN889
RESOURCE USAGE
CONCLUSION
The VF control application consumes both memory and
hardware resources, as shown in Table 5. Substantial
resources, particularly memory, are still available to
users for development of their own applications.
VF control provides a simple and cost efficient method
for open-loop speed control of 3-phase induction
motors. A low-cost VF solution can be implemented
using the PIC16F7X7 family of devices. With three dedicated PWM modules implemented in hardware, it is
ideal for controlling 3-phase induction motors. Additional on-chip resources, like multiple timers and ADC,
allow users to easily implement safety and control
features, such as current and voltage protection and
configurable acceleration and deceleration time.
TABLE 5:
RESOURCES USED IN THE
MOTOR CONTROL DEMO
BOARD (USING PIC16F777)
Resource Type
Used
Available to User
when PIC16F777
is Used
Program Memory
1041 words
7215 words
49 bytes
319 bytes
Data Memory
CCP Channels
3
0
ADC Channels
2
12
Timers
2
1
External Interrupts
1
4
I/O Lines
15
21
DS00889B-page 14
 2004 Microchip Technology Inc.
AN889
APPENDIX A:
TABLE A-1:
Test #
TEST RESULTS
TEST RESULTS
Set Frequency (Hz)
Set Speed (RPM)
Actual Speed (RPM)
Speed Regulation (%)
1
7.00
420
413
1.67
2
10.50
630
624
0.95
3
12.75
765
763
0.26
4
15.50
930
923
0.75
5
18.75
1125
1124
0.09
6
24.00
1440
1429
0.76
7
29.50
1770
1767
0.17
8
31.50
1890
1900
-0.53
9
36.25
2175
2184
-0.41
10
40.00
2400
2402
-0.08
11
44.50
2670
2670
0.00
12
46.50
2790
2805
-0.54
13
50.00
3000
3017
-0.57
14
54.50
3270
3275
-0.15
60
3600
3560
1.11
15
Note:
Above tests are conducted on a motor with the following specifications: Terminal Voltage = 208-230V,
Frequency = 60 Hz, Horsepower = 1/2 HP, Speed = 3450 RPM at full load, Rated Current = 1.8A,
Test Condition = no load.
 2004 Microchip Technology Inc.
DS00889B-page 15
DS00889B-page 16
VDD
VDA
VCC
470 µF 450V
0.1 µF 600V
470 µF 450V
VDC
0.001 µF .100V
0.1 µF
25 µF 450V
56 pF 500V
22 µF .40V
2.2 nF .400V
220 pF
1000 µF .10V
1000 µF 25V
0.1 µF
6.8 µH 1A
10 µH 1A
1%
1%
470 µF 10V
470 µF 25V
VDD
VCC
0.1 µF > 30V
0.1 µF > 30V
0.1 µF
FIGURE B-1:
470 µF .10V
APPENDIX B:
1000 µF .10V
6.8 µH 1A
AN889
MOTOR CONTROL SCHEMATICS
POWER SUPPLY
 2004 Microchip Technology Inc.
VCC
AN889
SYSTEM CONTROL
 2004 Microchip Technology Inc.
VSS
VSS
PIC16F7X7-DIP40
PIC16F7X7-DIP28
VSS
VSS
VCC
0.1 µF
VCC
VDD
VCC
VDD
VDD
VREF
0.1 µF
FIGURE B-2:
DS00889B-page 17
DS00889B-page 18
100 pF
MCP6004-DIP14
100 pF
MCP6004-DIP14
+VCCT
+VCC2
+VCC2
MCP6004-DIP14
MCP6004-DIP14
VCC
0.1 µF
Note:
MCP6004-DIP14
C45 0.0033 µF
To disable isolation, remove or disconnect all
components within the bounded area.
Connect A_SV directly to ISO_CSV and VDC_B
to ISO_VDC.
0.1 µF
FIGURE B-3:
+VCCT
See Note
VDC
AN889
INVERTER AND FEEDBACK
 2004 Microchip Technology Inc.
 2004 Microchip Technology Inc.
VCC
VCC
VCC
Note:
VCC
VCC
VCC
To disable isolation, remove all components within the bounded area.
Connect the PWM inputs to the IN pins of their respective drivers.
Connect the OC input to the SD pin of each driver.
0.1 µF
0.1 µF
0.1 µF
4.7 µF
4.7 µF
4.7 µF
0.1 µF
0.1 µF
0.1 µF
FIGURE B-4:
VCC
See Note
VDD
AN889
INVERTER DRIVERS AND OPTOISOLATORS
DS00889B-page 19
AN889
DISPLAY AND COMMUNICATION SECTION
DS00889B-page 20
0.1 µF
VCC
MCP6004-DIP14
VCC
VCC
24LC16
0.1 µF
0.1 µF
0.1 µF
VCC
VCC
0.1 µF
0.1 µF
VCC
VCC
0.1 µF
VCC
MCLR/VPP
FIGURE B-5:
 2004 Microchip Technology Inc.
AN889
APPENDIX C:
SOFTWARE
DISCUSSED IN THIS
APPLICATION NOTE
Because of its overall length, a complete source file
listing of the PIC16F7X7 VF motor control firmware is
not provided here. The complete source code is available as a single WinZip archive file, which may be
downloaded from the Microchip corporate web site at:
www.microchip.com
 2004 Microchip Technology Inc.
DS00889B-page 21
AN889
NOTES:
DS00889B-page 22
 2004 Microchip Technology Inc.
Note the following details of the code protection feature on Microchip devices:
•
Microchip products meet the specification contained in their particular Microchip Data Sheet.
•
Microchip believes that its family of products is one of the most secure families of its kind on the market today, when used in the
intended manner and under normal conditions.
•
There are dishonest and possibly illegal methods used to breach the code protection feature. All of these methods, to our
knowledge, require using the Microchip products in a manner outside the operating specifications contained in Microchip’s Data
Sheets. Most likely, the person doing so is engaged in theft of intellectual property.
•
Microchip is willing to work with the customer who is concerned about the integrity of their code.
•
Neither Microchip nor any other semiconductor manufacturer can guarantee the security of their code. Code protection does not
mean that we are guaranteeing the product as “unbreakable.”
Code protection is constantly evolving. We at Microchip are committed to continuously improving the code protection features of our
products. Attempts to break Microchip’s code protection feature may be a violation of the Digital Millennium Copyright Act. If such acts
allow unauthorized access to your software or other copyrighted work, you may have a right to sue for relief under that Act.
Information contained in this publication regarding device
applications and the like is intended through suggestion only
and may be superseded by updates. It is your responsibility to
ensure that your application meets with your specifications.
No representation or warranty is given and no liability is
assumed by Microchip Technology Incorporated with respect
to the accuracy or use of such information, or infringement of
patents or other intellectual property rights arising from such
use or otherwise. Use of Microchip’s products as critical
components in life support systems is not authorized except
with express written approval by Microchip. No licenses are
conveyed, implicitly or otherwise, under any intellectual
property rights.
Trademarks
The Microchip name and logo, the Microchip logo, Accuron,
dsPIC, KEELOQ, microID, MPLAB, PIC, PICmicro, PICSTART,
PRO MATE, PowerSmart, rfPIC, and SmartShunt are
registered trademarks of Microchip Technology Incorporated
in the U.S.A. and other countries.
AmpLab, FilterLab, MXDEV, MXLAB, PICMASTER, SEEVAL,
SmartSensor and The Embedded Control Solutions Company
are registered trademarks of Microchip Technology
Incorporated in the U.S.A.
Analog-for-the-Digital Age, Application Maestro, dsPICDEM,
dsPICDEM.net, dsPICworks, ECAN, ECONOMONITOR,
FanSense, FlexROM, fuzzyLAB, In-Circuit Serial
Programming, ICSP, ICEPIC, Migratable Memory, MPASM,
MPLIB, MPLINK, MPSIM, PICkit, PICDEM, PICDEM.net,
PICLAB, PICtail, PowerCal, PowerInfo, PowerMate,
PowerTool, rfLAB, rfPICDEM, Select Mode, Smart Serial,
SmartTel and Total Endurance are trademarks of Microchip
Technology Incorporated in the U.S.A. and other countries.
SQTP is a service mark of Microchip Technology Incorporated
in the U.S.A.
All other trademarks mentioned herein are property of their
respective companies.
© 2004, Microchip Technology Incorporated, Printed in the
U.S.A., All Rights Reserved.
Printed on recycled paper.
Microchip received ISO/TS-16949:2002 quality system certification for
its worldwide headquarters, design and wafer fabrication facilities in
Chandler and Tempe, Arizona and Mountain View, California in
October 2003. The Company’s quality system processes and
procedures are for its PICmicro® 8-bit MCUs, KEELOQ® code hopping
devices, Serial EEPROMs, microperipherals, nonvolatile memory and
analog products. In addition, Microchip’s quality system for the design
and manufacture of development systems is ISO 9001:2000 certified.
 2004 Microchip Technology Inc.
DS00889B-page 23
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DS00889B-page 24
 2004 Microchip Technology Inc.
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