Variable Frequency Drives (VFD) are motor starters primarily used for dynamically changing motor RPM during operation. They ensure a smooth motor start without jerking the load and limit the starting current to 1.1 to 1.5 times the motor's rated current. This contrasts with the starting current of 6 times in DOL Motor Starters and 3 times in Star-Delta and Soft Starters.
Types of VFD in General Applications
a) Variable Torque (VT) Drive
b) Constant Torque (CT) Drive
Variable Torque (VT) Drives, also known as low overload capability drives, offer an overload capability of 1.1 times the rated output current for 60 seconds in a 300-second cycle time, or 1.4 times the rated output current for 3 seconds in a 300-second cycle time. These drives are suitable for applications such as blowers and pumps, where full torque is only required at the rated speed (fmax). They do not demand high overload capabilities due to limited starting torques and load surges.
Constant Torque (CT) Drives, also referred to as high overload capability drives, provide an overload capability of 1.5 times the rated output current for 60 seconds in a 300-second cycle time, or 2 times the rated output current for 3 seconds in a 300-second cycle time. They are suitable for applications with a load characteristic represented by M = constant, such as positive displacement pumps, screw pumps, and lobe blowers. These applications require full torque across the entire speed range and often experience increased starting torques and load surges, necessitating a higher overload capability in the drive inverter.
By understanding the different types of VFDs and their applications, you can select the appropriate drive for your specific motor control needs. Consider the load characteristics and the required overload capabilities to ensure optimal performance and reliability.