26-12-2012, 05:29 PM
VARIABLE FREQUENCY DRIVES
[attachment=45334]
WHAT IS A VFD?
System for controlling the rotational speed of an AC motor by controlling the frequency of the electrical power supplied to the motor.
It’s a specific type of adjustable speed drive.
Also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, micro drives or inverter drives.
Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.
VFD TYPES
All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them only on or off.
Drives can be classified as:
1. Constant voltage
2. Constant current
3. Cycloconverters.
Constant voltage converter: Intermediate DC link voltage remains approximately constant during each output cycle.
Constant current drives: Large inductor is placed between the input rectifier and the output bridge, so the current delivered is nearly constant.
Cycloconverter: No input rectifier or DC link and instead connects each output terminal to the appropriate input phase.
VFD SYSTEM DESCRIPTION
A variable frequency drive system generally consists of the following:
1.An AC motor
2. A controller
3.An operator interface.
VFD motor
Usually a three-phase induction motor.
Some types of single-phase motors can be used, but three-phase motors are usually preferred.
Various types of synchronous motors offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice.
Motors that are designed for fixed-speed operation are often used.
Certain enhancements to the standard motor designs offer higher reliability and better VFD performance, such as MG-31 rated motors.
VFD controller
They are solid state electronic power conversion devices.
First converts AC input power to DC intermediate power using a rectifier or converter bridge.
Rectifier: usually 3-phase, full wave diode bridge.
DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit.
VFD operator interface
Provides a means for an operator to start and stop the motor and adjust the operating speed.
Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal.
VFD operation
When an induction motor is connected to a full voltage supply, it draws several times (up to about 6 times) its rated current.
As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.
When a VFD starts a motor, it initially applies a low frequency and voltage to the motor.
Benefits of VFDs
Energy savings
Better process control
Control of process temperature, pressure or flow without the use of a separate controller.
Low maintenance costs.
No need for soft starter for the motor.
Controlled ramp-up speed in a liquid system can eliminate water hammer problems.
The ability of a VFD to limit torque to a user-selected level can protect driven equipment that cannot tolerate excessive torque.
[attachment=45334]
WHAT IS A VFD?
System for controlling the rotational speed of an AC motor by controlling the frequency of the electrical power supplied to the motor.
It’s a specific type of adjustable speed drive.
Also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, micro drives or inverter drives.
Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.
VFD TYPES
All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them only on or off.
Drives can be classified as:
1. Constant voltage
2. Constant current
3. Cycloconverters.
Constant voltage converter: Intermediate DC link voltage remains approximately constant during each output cycle.
Constant current drives: Large inductor is placed between the input rectifier and the output bridge, so the current delivered is nearly constant.
Cycloconverter: No input rectifier or DC link and instead connects each output terminal to the appropriate input phase.
VFD SYSTEM DESCRIPTION
A variable frequency drive system generally consists of the following:
1.An AC motor
2. A controller
3.An operator interface.
VFD motor
Usually a three-phase induction motor.
Some types of single-phase motors can be used, but three-phase motors are usually preferred.
Various types of synchronous motors offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice.
Motors that are designed for fixed-speed operation are often used.
Certain enhancements to the standard motor designs offer higher reliability and better VFD performance, such as MG-31 rated motors.
VFD controller
They are solid state electronic power conversion devices.
First converts AC input power to DC intermediate power using a rectifier or converter bridge.
Rectifier: usually 3-phase, full wave diode bridge.
DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit.
VFD operator interface
Provides a means for an operator to start and stop the motor and adjust the operating speed.
Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal.
VFD operation
When an induction motor is connected to a full voltage supply, it draws several times (up to about 6 times) its rated current.
As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.
When a VFD starts a motor, it initially applies a low frequency and voltage to the motor.
Benefits of VFDs
Energy savings
Better process control
Control of process temperature, pressure or flow without the use of a separate controller.
Low maintenance costs.
No need for soft starter for the motor.
Controlled ramp-up speed in a liquid system can eliminate water hammer problems.
The ability of a VFD to limit torque to a user-selected level can protect driven equipment that cannot tolerate excessive torque.