13-12-2012, 06:26 PM
Variable Frequency Drives
Variable Frequency Drives.ppt (Size: 1.91 MB / Downloads: 271)
Electric & Power Basics
Run a machine or process at a desired speed.
Produce adequate torque to handle the load.
Use power efficiently to produce the necessary torque at a given speed.
Effectively monitor the application or process.
AC Induction Motor Theory
How a motor shaft rotates
Torque is produced as the induction motor generates flux in its rotating field.
This flux must remain constant to produce full-load torque.
AC Induction Motor Issues
Starting Induction Motors
The starting current is very high, between 3 to 8 times the full load current. Depending on the size of the motor, this can result in voltage sags in the power system.
The full torque is applied instantly at starting and the mechanical shock can eventually damage the drive system, particularly with materials handling equipment, such as conveyors.
In spite of the high starting current, for some applications the starting torque may be relatively low, only 1.0 to 2.5 times full load torque.
Components of VFD’s
Transistors provide fast switching capability for a relatively low cost.
The general types of transistors are:
The bipolar transistor;
The gate turn off transistor (GTO)
The field-effect transistor (FET);
The insulated gate-field-effect transistor (IGFET);
The insulated gate-bipolar transistor (IGBT).