SELECTION CRITERIA OF VSDs
The following are the advantages of VSDs:
Variable Process Speed
Some processes operate at different speeds. For example, an ore grinding will operate at different speeds depending on the type, consistency, and size of the ore. VSDs are most suitable for these applications. Other applications requiring variable speed are conveyors and dynamometers.
Compressors and Pumps
Many applications require pumps and compressors to operate at part load. Conventional constant-speed motors and control valves have been used for these applications. This alter- native is more expensive and causes flow-induced vibrations, cavitation, and erosion in the system. The motor operates at full load continuously regardless of the flow required. For 100-kW motor and 5 percent per kilowatthour utility rate, the operating cost of the motor will be $125.00 per day.
Typical pump characteristics (Fig. 10.21) confirm that the flow is proportional to the speed, while the torque varies with the square of the speed and the power with the cube of the speed. If the system requires 50 percent of the maximum flow rate, $113.00 will be wasted every day.
Motor Starting
Starting large motors can be very stressful on the windings and insulation systems due to high currents and torques. The power supply is also affected adversely due to voltage dips. A much smaller VSD (rated at a few percent of the motor) can be used to increase the speed of the motor gently, so it can be synchronized.