04-04-2010, 01:26 AM
VF Control of 3-Phase Induction Motors Using PIC16F7X7 Microcontrollers
An induction motor can run only at its rated speed when
it is connected directly to the main supply.But many applications need variable speed operations. the
motor user can replace an energy inefficient mechanical
motor drive and control system with a Variable
Frequency Drive (VFD).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.
The PIC16F7X7 series of microcontrollers have three
on-chip hardware PWM modules, making them
suitable for 3-phase motor control applications.
VF CONTROL:
The base speed of the induction motor is directly
proportional to the supply frequency and the number of
poles of the motor.by varying the supply frequency , we can vary the vary the speed of the induction motor.
MOTOR DRIVE:
The 3-phase induction motor is connected to a 3-phase
inverter bridge. The power inverter
has 6 switches that are controlled in order to generate
3-phase AC output from the DC bus. These switches are controlled by the PWM signals,
generated from the microcontroller. The amplitude of phase voltage is determined by the
duty cycle of the PWM signals. Three of the six switches will be on at any time. The switching produces
a rectangular shaped output waveform that is rich in harmonics. A 3-phase sine wave with negligible harmonics is produced from this by the filtering effect of the motor's windings. freewheeling diodes are provided to facilitate the dissipation of energy stored in the windings.
For more details, refer this pdf:
An induction motor can run only at its rated speed when
it is connected directly to the main supply.But many applications need variable speed operations. the
motor user can replace an energy inefficient mechanical
motor drive and control system with a Variable
Frequency Drive (VFD).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.
The PIC16F7X7 series of microcontrollers have three
on-chip hardware PWM modules, making them
suitable for 3-phase motor control applications.
VF CONTROL:
The base speed of the induction motor is directly
proportional to the supply frequency and the number of
poles of the motor.by varying the supply frequency , we can vary the vary the speed of the induction motor.
MOTOR DRIVE:
The 3-phase induction motor is connected to a 3-phase
inverter bridge. The power inverter
has 6 switches that are controlled in order to generate
3-phase AC output from the DC bus. These switches are controlled by the PWM signals,
generated from the microcontroller. The amplitude of phase voltage is determined by the
duty cycle of the PWM signals. Three of the six switches will be on at any time. The switching produces
a rectangular shaped output waveform that is rich in harmonics. A 3-phase sine wave with negligible harmonics is produced from this by the filtering effect of the motor's windings. freewheeling diodes are provided to facilitate the dissipation of energy stored in the windings.
For more details, refer this pdf:
