What is the difference between VSD and VFD? It can be seen in terms of speed: a VFD can perform speed control during startup, motor stop and operation. However, motor efficiency is lost due to heat generation due to electrical resistance, magnetic polarity reversal in the stator (hysteresis), and currents in the rotor and stator (eddy currents). VSD is used in situations where the operating speed of the device needs to be changed while maintaining a constant motor speed.
What is the difference between VSD and VFD? It can be seen in terms of torque: depending on the application, the motor may require constant or variable torque. If the motor requires constant torque, the torque load is not considered a function of speed. The load torque will be constant even when increasing and decreasing speed. The motor will also draw higher current at lower speed. Constant torque is used in applications such as conveyors, elevators and mixing equipment. In applications requiring variable torque, lower speeds require lower load torque. Load performance can be measured by reducing the load torque by a value equal to the square of the speed. Variable torque is used in applications such as centrifugal pumps and fans.
VFDs are relevant for variable torque applications. Because torque measures the amount of work done, in equipment where the load torque varies with speed, the VFD can adjust the torque to prevent damage or injury to the machine or operator using the motor.
VSD is typically used for applications with constant torque. In the example of a conveyor, the speed of the conveyor may change, but the torque on the system can remain constant, unaffected by changes in speed, without damaging the equipment or the user.
The difference between a VSD and a VFD can be seen in horsepower: VFDs and VSDs differ in the horsepower they use. VFDs are used where constant horsepower loads are required. These applications include machines such as grinders and lathes. In a constant horsepower load, higher torque is required at low speeds, and lower torque can be used at high speeds. This requires the horsepower produced to remain constant, justifying the use of a VFD.
Where constant horsepower is not required, the VFD can control the speed-dependent HP by varying the frequency. For example, at one frequency, the torque can be held at 100% while the HP is reduced. However, changing the frequency above 60 Hz reduces torque while keeping HP at 100%. Compare the ground truth of the above two usage applications with VSD. In VSD, torque is independent of machine speed. This means horsepower is linear with RPM. Increasing RPM increases HP, while decreasing RPM decreases HP.