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Variable Frequency DrivesVFD Basics

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What is a VFD?

A Variable Frequency Drive is like the throttle on a car -• It adjusts the speed of an HVAC fan or pump motor, based on

demand, to save energy and prolong motor and mechanical component life

• Without a VFD, an HVAC fan or pump motor is either 100% ‘on’ or 100% ‘off’

• A VFD eliminates the initial power surge and mechanical shock of switching the motor from ‘off’ to 100% ‘on’

• A VFD conserves energy when an HVAC control systems senses that a fan or pump motor can meet heating or cooling needs by running at less than 100% power

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Electric Motor Facts

• Electric motors account for more than two-thirds of all energy used in industry

• 63% of the energy consumed is linked to air & liquid flow • A typical 50HP industrial motor, with an efficiency of 90%,

costs over $25,000 to operate continuously over a year.

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Other17%

Ventilation& Cooling

52% Lighting31%

Energy Usage in Office Buildings

• More than half the energy consumed by AC motors powers air conditioning equipment.

• Electrical energy costs are often the largest expenditure for building owners.

• Reducing energy demand and improving the working environment of a building are main objectives of facility managers.

• The principle reason for using VFDs is to reduce energy costs.

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Why Use VFDs for HVAC Applications?

Improved• Energy savings• Simplified motor control panel design• Temperature regulation & comfort• Monitoring

Reduction in• Mechanical component stress• Damper commissioning • Cabling• Main power supply capacity• Peak demand• System components • Noise

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Energy Savings Comparison

Potential energy savings using a VFD vs. modulating flow with a damper

Flow Rate (%)

Inp

ut

Po

wer

(%

)

10

10 20 30 40 50 60 70 80 90 100

20

30

40

50

60

70

80

90

100

110

120

130

140

Damper

SED2 VFD

Ideal Fan Control

Energy Saving Potential

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How Does A VFD Work?

• It changes the speed of an AC HVAC fan or pump motor by adjusting or varying the frequency and voltage. For comparison, a DC motor speed is adjusted by only varying voltage.

• Typical frequency adjustment range in HVAC applications is from 10 - 60Hz AC. A VFD is wired in series between main power and motor.

• A Bypass is added to “Bypass” the VFD and run on main line power so HVAC operation is not interrupted if the VFD needs servicing

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How Does A VFD Vary the Frequency?

Switches (IGBT’s)

VFD Block Diagram VFD Block Diagram

Motor

Main Power

Rectifier Capacitor Motor

• A VFD, AC inverter, or electronic speed controller for AC motors converts the AC supply to DC using a rectifier (diode), then converts it back to a variable frequency by using a inverter bridge.

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VFD Specifying

Standard Features vs. The Options Game• When specifying VFDs –• Don’t play the "Options Game" – make sure you get the most

standard features for your money! • Make sure VFD installers are certified

by the manufacturer to get the most out of the drives capabilities.

• Make sure the drives are specified with advanced features, such as an integral harmonics reduction capability.

• Get more. Specify Siemens.

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• Low harmonics design• Small enclosure size• Many standard

features• Reduces operating

cost• Low power losses/high

efficiency• Minimal effect on

customer power• Wide power range –

from ½ to 125 HP

Siemens SED2 VFD Advantages

For more info, see the Features and Benefits menu on this CD