Variable Frequency Drive:

A Variable Frequency Drive (VFD) is an electric motor control system or device that safeguards and regulates the speed of AC induction motors.

Key Points:

• VFDs control motor speed during startup, shutdown, and operation cycles.
• Also known as Adjustable Frequency Drives (AFDs).
• Minimize voltage drops typically associated with motor startups.

VFDs are utilized in applications that require the following:

• Precise speed control.
• Energy efficiency as a goal.
• Customized control requirements.
• Reduced starting current while maintaining high starting torque.

Working Principles of VFDs:

The motor speed for AC motors is determined by the formula: N = 120*F/P
For example, a three-pole motor operates at 60 Hz. Inserting these values into the formula yields:
N = 120 x 60 / 3=2400(rpm)

Components and Functions:

• AC Power Supply: Sourced from the facility grid.
• Isolation Transformer: Reduces line harmonics to meet voltage and current distortion standards per IEEE 519 (optional for medium voltage VFDs).
• Rectifier: Converts grid AC to DC.
• Filters and DC Bus: Smoothen rectified DC and provide clean, low-ripple DC power to the inverter.
• Inverter: Utilizes DC from the DC bus and filters, employing Pulse Width Modulation (PWM) technology to invert into analog sine waves.

Benefits of VFDs:

• Energy savings.
• Peak energy demand reduction.
• Fully adjustable speeds for pumps, conveyors, and fans.
• Control over startup, stop, and acceleration.
• Dynamic torque control.
• Smooth motion for applications like elevators and escalators.
• Ideal choice for industrial equipment like manufacturing machinery, mixers, grinders, and crushers.

Energy Efficiency:

VFDs offer maximum energy savings in fans and pumps. According to the Affinity Law, flow varies directly with speed, while power decreases with the cube root of speed. For instance, reducing a fan’s speed by half results in only 1/8 of the power/energy demand at full speed. By powering a 500-horsepower fan load primarily between 30% and 80% speed via a VFD, over $100,000 in energy savings can be achieved annually.

Choosing The Right Motor Control: Soft Starter Vs. Variable Frequency Drive (Part 1)–About Soft Start vs Variable Frequency Drive

Choosing the Right Motor Control: Soft Starter vs. Variable Frequency Drive (Part 2)–About Soft Starter