Variable frequency drives control AC motor speed, reducing utility expenses and increasing equipment life. They change voltage and frequency, providing flow control without valves. They’re used in industries with narrow temperature limits and high variability in water flow. Soft start capability reduces stress and increases equipment life, but cooling loss and noise levels can be disadvantages.
A variable frequency drive is used on AC motors to control the speed of the motor. While more expensive in initial cost than a fixed speed motor, this type of system can provide cost savings by reducing utility expenses and increasing equipment life. Variable frequency drive systems change the voltage and frequency of the motor to increase or decrease pump flows.
This system contains an alternating current (AC) to direct current (DC) converter, a DC processing circuit, and a DC to AC inverter. The result is an AC power similar in character to utility-supplied AC, but with the ability to step up and down in voltage and frequency. When connected to a pump, a variable frequency drive motor can provide flow control without the use of control valve systems.
The benefit of variable frequency drive systems comes from the reduced power required to operate the pump at slower speeds. It is not efficient to run a pump at one speed and reduce flows with valves. Utility companies charge customers for lower power factors or electrical efficiency, and it can be an advantage to use variable frequency drive systems with higher power factors.
These systems are used in a wide variety of industries. Some chemical processes have narrow acceptable temperature limits for product quality. Variation of product flow rate can provide faster temperature regulation than variation of heat or cooling source.
Water cooling towers have a high variability in water flow for process cooling. During colder climates, less water flow is required and a variable speed system can maintain required temperatures at much lower cost. The connection of the motor control to the thermoregulators of the system allows the increase of the water flow required for cooling.
Another benefit of variable speed motors is the soft start capability. Motors connected to a constant voltage start quickly and the motor experiences stress from sudden changes in pressure. Starting a motor with variable frequency control allows the motor to slowly increase speed and lower stresses are placed on the system. Less stress can result in longer equipment life and lower costs.
Most electric motors are cooled with fans attached to the motor shaft. A disadvantage of variable frequency drive systems is the cooling loss of the fan when running at lower speeds. This type of system can also cause additional heating of the motor windings. In hot climates it may be necessary to install engine temperature sensors or auxiliary cooling fans to prevent the engine from overheating.
Variable frequency drive motors can have higher noise levels than a constant speed motor. The frequency has a slightly different AC waveform than the one supplied by the utility. This can cause vibration of the motor windings resulting in noise which can increase with lower motor frequencies and speeds.
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