PIC18FXX39 Datasheet, PDF (113/322 Pages) Microchip Technology – Enhanced FLASH Microcontrollers with Single Phase Induction Motor Control Kernel

PIC18FXX39
14.0 SINGLE PHASE INDUCTION
MOTOR CONTROL KERNEL
The Motor Control kernel of the PIC18FXX39 family
uses Programmable Motor Processor Technology
(ProMPT) to control the speed of a single phase induc-
tion motor, with variable frequency technology. The
controller’s two PWM modules are used to synthesize
a sine wave current through the motor windings. The
kernel provides open loop control for a continuous
frequency range of 15 Hz to 127 Hz.
14.1 Theory of Operation
The speed of an induction motor is a function of fre-
quency, slip and the number of poles in the motor. They
are related by the equation:
Speed = F  120  P – Slip
where Speed and Slip are in RPM, F is the frequency of
the input voltage (in Hertz), and P represents the number
of motor poles (for this equation, either 2, 4, 6 or 8).
For the purpose of this discussion, slip is assumed to
be constant across the motor’s useful operating range.
Since the rated speed is based on the number of poles
(which is fixed at the time of manufacture), this leaves
changing the frequency of the supplied voltage as the
only way to vary the motor’s speed. When the fre-
quency controlling a motor is reduced, however, its
impedance is also reduced, resulting in a higher motor
current draw.
It can be shown that the voltage applied to the motor is
proportional to both the frequency and the current
(Equation 14-1). So to keep the current constant at, or
below the Full Load Amp rating, the RMS voltage to the
motor must be reduced as the frequency is reduced. By
varying the supply voltage and frequency at a constant
ratio, the motor’s speed can be varied with constant
current. Maintaining this constant ratio is the function of
the Motor Control kernel.
EQUATION 14-1: KEY RELATIONSHIPS IN
SINGLE PHASE MOTORS
V
(1-1)
or: V  2f
I    ^V---
f
(1-2)
(1-3)
where: V is applied voltage
I is motor current
is stator flux
f is input frequency
14.2 Typical Hardware Interface
A block diagram for a recommended single phase
induction motor control using the PIC18FXX39 is
shown in Figure 14-1.
The single phase AC supply is rectified, using a diode
bridge and filtered, using a capacitor. The PWM out-
puts from the PIC18FXX39 synthesize the AC to drive
the motor from this DC bus by switching Insulated Gate
Bipolar Transistors (IGBTs) on and off. The IGBT gate
driver converts the TTL level of PWMs to the required
IGBT gate voltage level, and supplies the gate charging
current when the IGBT turns on.
The I/O ports of the microcontroller can be used for the
external logic controls. The A/D channels can be used
for monitoring the DC bus voltage and motor current; a
potentiometer can also be connected to one of these
channels to provide a variable frequency reference for
the motor.
FIGURE 14-1:
TYPICAL MOTOR CONTROL SYSTEM USING THE PIC18FXX39
L
Single Phase
AC Input ^N
G
Rectifier ⁺
-
MOV
Power
Supply
+15V
+5V
GND
Digital
I/O Interface
Analog
Voltage
Monitor
A/D
Gate
I/OPorts
PWM1
Drives
M1
PWM2
PIC18FXX39
IGBT
M2
Driver
A/D
IGBT
G
A/D
H-Bridge
Motor
Current
Monitor
 2002-2013 Microchip Technology Inc.
Preliminary
DS30485B-page 113