Variable Frequency Drives
Variable frequency drives (VFDs) are used to control the speed of AC induction motors. They reduce inrush current when starting a motor, provide controlled acceleration and deceleration and help extend motor life. VFDs also save energy by running a motor at an optimal speed for a given application.
VFDs are commonly found in a wide range of industrial applications including pumps, conveyor belt systems, metal processing machines, air conditioning and ventilation systems in large buildings, hoists and cranes, etc. VFDs can also be found in traction applications like train locomotives or electric motors in elevators and lifts.
Inside a VFD, incoming AC power is run through a rectifier which converts it to DC power. Then it is fed into capacitors which smooth out the electrical waveform. The DC power then goes to the inverter which changes it back into AC power at a designated frequency. This is then outputted to the motor.
Most modern drives use a PWM (pulse width modulation) method to generate the voltage and frequency needed for VFD control. This technology is the most common and offers many advantages over older drive designs. Older drives used current source inverters or thyristors to control the motor.
Variable Frequency Drives
A VFD has the ability to vary the frequency and voltage of a motor at a high speed (RPMs). This allows the drive to meet the operating requirements of a specific application, even if the motor is running at full load or underloaded.
Using a VFD on an electric motor can prevent excessive wear and tear on the shaft, bearings and gears. In addition, it can reduce costs by minimizing energy consumption and downtime due to unscheduled stops. VFDs can also help protect the motor from issues like electro thermal overloads, phase faults, over and under voltage conditions, etc.
When a variable frequency drives is used to start a large motor, it greatly reduces the inrush current. This helps protect the motor from winding stress and overheating as well as any potential damage to the connected power bus. VFDs also offer optimal voltage and frequency control which increases motor life.
VFD’s are also used to increase the efficiency of pumps, compressors and fans by matching the volume or pressure to system demand. This can result in a significant reduction in energy costs, as well as improved functionality of the equipment. For example, a fan with a VFD running at low speed will consume less electricity than when it is at full speed, since the internal cooling fans are not spinning as fast.
