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Introduction to Variable Frequency DrivesLouisiana User Group August 2018

Our presentation will begin at 12:00 pm CDT

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Today’s Topic:

Introduction toVariable Frequency Drives (VFDs)

Guest Presenter:Rob McLinAutomation SpecialistThe Reynolds Company

What is a VFD?

• Variable Frequency Drive (VFD)• A VFD can be used to control both the speed and

torque of a standard induction AC electric motor. • It varies both the frequency and amps of the AC

waveform being delivered to the motor saving money in electricity.

• Basic components of a VFD: • Input section, draws AC electric power from the utility,

Rectifier section, converts the AC into DC power. • Inverter section, converts DC back into a controllable AC

waveform.

Purpose of a VFD

• Control the speed and/or the torque of a motor• VFDs help to limit demand and electrical

consumption of motors by reducing the amount of energy they consume.

• Standard motors are constant speed and when they are energized they run at a 100% no matter the load.

• Soft Start• Only use energy you need

Schematic of a VFD

How Does A Variable Frequency Drive Work ?

• AC is rectified to a constant value of DC• The inverter IGBTs are switched creating an AC voltage and current

waveform that an AC motor can use.• The technique gives us control of the frequency which can be varied.

Note: Output current is greater than input current.

VFD Waveform

Potential Problems with a VFD

• Improper Circuit Design• Harmonic Distortion• Safety • 3 phase generation• Microcontroller programming

Why Use Variable Frequency Drives In Lieu of Other Variable Speed Devices ?

• Energy Savings• Running Efficiency• Reduced Demand Charge• Reduced Power Factor Charges

• Improved Process Control• Integration of control into a process• Reduced Water Hammer Effects• Reduced Pump Cavitation Problems

• High speed interface/response to PLCs and Networks• Over speed Capability• Lower system Maintenance• Easily Bypassed• Multi-motor Control

Load Types

Normal Duty DriveOverload – 115% for one minute

• Centrifugal pumps• Fans• Centrifugal compressors

Heavy Duty DriveOverload – 150% for one minute

• Reciprocating pumps• Gear pumps• Mixers• Extruders• Conveyors

Centrifugal Loads Constant Torque Loads

Reflected Wave

Reflected Wave Background

• Voltage doubling effect due to pulse reflections can occur at cable lengths greater than 30-50 ft, with typical IGBT rise times of 100-300 ns

Reflected Wave Background

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