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2002 Microchip Technology Inc.

Preliminary

DS30485A-page 113

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:

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

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

Speed

F 120 P

() Slip

–

where: V is applied voltage

I is motor current

φ is stator flux

f is input frequency

φω

∝

πfφ

∝

---

∝∝

(1-1)

(1-2)

(1-3)

or:

PIC18FXX39

IGBT

Driver

Voltage

Monitor

Current

Monitor

IGBT

H-Bridge

Motor

PWM1

PWM2

+15V

+5V

GND

Single Phase

AC Input

Gate

Drives

Power

Rectifier

MOV

A/D

I/OPorts

Supply

I/O Interface

Digital

Analog

A/D

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