However, much of the time the devices they drive may operate at less than maximum design speed. Slowing a pump or fan in this manner reduces energy consumption much more effectively than allowing the motor to run at constant speed and then restricting or bypassing the flow with a valve or damper. In addition to providing speed control, ASDs provide soft starting , whereby a motor starts slowly and then speeds up. This reduces the mechanical stress on both the motor and equipment driven by the motor. This also reduces the voltage sag that can occur when a large motor starts quickly.
Voltage sags can dim lights and cause other equipment to shut down or restart.
The AC motor speed varies in proportion to the drive output frequency. For example, ASDs can be very cost-effective in retrofit or new construction of HVAC systems. Many HVAC systems were designed with constant flow pumps and fans that are throttled to meet changing operating conditions. ASDs are also useful for loads such as elevators, water and wastewater pumps, boiler fans, cooling towers , cranes and conveyors. The speed range, required precision of speed control, and required torque at lower speeds dictate the type of ASD needed.