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© 2017 WEG Electric Corporation. All rights reserved.

WEG Variable Frequency Drives Training

Dave Mintzlaff – Product Line Manager, LV Drives and Soft Starters

July 2017

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Agenda

1. VFD Terminology

2. VFD System Architecture

3. Typical VFD Features

4. Induction Motors

5. Speed Range

6. VFD System Installation Considerations

7. VFD and Motor Cabling and Grounding

8. Summary of Major Points

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Terms and Definitions

Variable Frequency Drive – “An electronic device used for controlling

the rotational speed of an alternating current (AC) electric motor by

controlling the frequency and voltage of the electrical power supplied

to the motor.”

Other names you may have heard:

• Variable Frequency Drive (VFD)

• Variable Speed Drive (VSD)

• Adjustable Frequency Drive (AFD)

• Adjustable Speed Drive (ASD)

• Freq. Drive (Frequency Drive)

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Terms and Definitions

A Variable Frequency Drive consists of:

• Converter or Rectifier: Changes the AC Supply Power to DC Voltage

• DC Bus or DC Link: Capacitors that Filter and Store the DC Voltage

• Inverter: A Group of Transistors that Change the DC Bus Voltage to a

Variable AC Voltage and Frequency to Control the AC Motor

• Controller: Typically a Microprocessor and Circuitry that Manages

the Operation of the System

Utility

AC Power

DC Bus Rectifier Inverter

Controller

Logic I/O

Communication

Analog I/O

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4 Basic Parts of a VFD

Rectifier, Capacitors, Inverter, and CPU

Inverts

DC to AC

Capacitors Rectifier Inverter

Converts

AC to DC

Filters

DC Power

Controller

Utility

(AC Power)

Motor

(VFD Power)

Logic I/O

Communication

Analog I/O

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Rectifier and Capacitors • At the supply to a typical VFD is a full wave diode

bridge rectifier module.

• The purpose of this module is to convert AC voltage

into DC voltage (to rectify).

Example of 480 Vac Converted to DC by a Six-Pulse Rectifier

480 Vac

3-phase 60Hz

Supply

D1 D3 D5

D6 D4 D2

L1

L2

L3

DC –

DC +

+/– 650 Vdc

(DC Bus)

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Inverter

• By utilizing six transistors, this inverter example is able to convert DC

voltage into a simulated sinusoidal output waveform. This method of

power conversion is called Pulse Width Modulation (PWM). There are 2

transistors per phase, one for the positive switching and one for the

negative.

• The frequency of these pulses is significantly higher than the frequency

of the simulated sinusoidal output, and is known as the carrier

frequency.

Pulse Width Modulation Output Basic IGBT configuration in an Inverter

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Basic Power Sections

Reactors

(design specific)

Input

• Fixed Voltage

• Fixed Frequency

Output

• Variable Voltage

• Variable Frequency

Diode Rectifier Inverter DC Bus Induction Motor

Reactors

(design specific)

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AC Induction Motors

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Conduit Box (F3 Position)

Fan Shroud

End Bell (Drive End)

Lubrication Point

Stator Windings

Stator Laminations

Rotor Bars & Laminations

Bearing (Drive End)

Motor Frame

Motor Shaft

Drive End Non-Drive End

(Opposite Drive End)

Motor Mounting Feet

Air Gap

Bearing

(Non-Drive End)

AC Motor Nomenclature

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Motor Operating Speed • Variable frequency drives operate on the principle that the

synchronous speed of an AC motor is determined by the frequency

of the AC supply and the number of poles in the motor.

𝑅𝑃𝑀 = 120 × (𝐻𝑧)

# 𝑃𝑜𝑙𝑒𝑠 𝑖𝑛 𝑀𝑜𝑡𝑜𝑟

• Synchronous RPM = The speed of the rotating magnetic field

produced in the motor stator windings.

• Full Load RPM of rotor is slower due to SLIP

• Through a process called induction, the

rotor bars (conductors) become energized

and create a magnetic field of their own.

• The motor speed is determined by number

of magnetic poles which is typically fixed by

the motor design and can be calculated from

the motor nameplate data.

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Motor Speeds: 60 Hz • Below are typical speeds vs. number of poles for

60Hz rated motors:

Full Load

Speed

Synchronous

Speed

Number

of Poles

3555 3600 2

1771 1800 4

1179 1200 6

886 900 8

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Motor Speeds: 50 Hz • Below are typical speeds vs. number of poles for

50Hz rated motors:

Full Load

Speed

Synchronous

Speed

Number

of Poles

2962 3000 2

1476 1500 4

983 1000 6

738 800 8

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VFD Environmental Considerations

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• Consideration must be given to the environmental conditions where the VFD will

be installed.

• Ambient temperature range and moisture levels are most often identified as

potential problems, but high altitude may also be an issue.

• VFD’s should be located in clean, dry, well ventilated areas.

• Heat is the enemy for electrical equipment. Be sure the installation site has

sufficient cooling air available.

• Most drives installed inside buildings may only need a NEMA Type 1 enclosure

for protection. There may be situations where more protection is needed.

• Be sure to identify any airborne contaminants and vapors that may damage

the VFD electrical components. These can be present wherever solvents or

other chemicals are in use such as with water treatment systems.

• Combustion systems can emit corrosive vapors as well as particulate matter.

• WEG includes a conformal coating for all VFD circuit boards to help protect

against these contaminants.

Environments and Enclosures

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Environment: Temperature • WEG VFD’s are rated for a range of ambient temperature and altitude.

• Common temperature ranges are between -10°C (14°F) to 50°C (122°F).

• In some applications higher temperatures may be allowed provided the VFD can

be de-rated per the guidelines in the user manual.

Example of a temperature de-rating scenario:

• A common derating formula for temperatures above the VFDs rated ambient

temperature is 2% decrease in current per degree C.

• For example, a 480V VFD rated 55 kW (75 HP) has a current rating of 88 Amps at

50°C (122°F).

• In a 60°C (140°F) environment the VFD would require a decreased maximum

current per the following:

60°C (140°F) - 50°C (122°F) = 10°C (18°F)

10°C (18°F) x 2% per degree C = 20% i.e.{(60-50) x 2% = 20%}

Or: Derate 88 Amps by 20% = 70.4 Amps maximum

NOTE: Always consult with WEG Drives Application Engineers when derating a VFD

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Environment: Temperature (cont’d.)

The graph below shows that as the temperature increases (horizontal axis) the

available drive current is reduced.

At 50°C the current rating is at 100% of the drive’s rating but at 60°C as described

in the prior slide, the drive’s rating has to be reduced to 80% (derated by 20%).

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Environment: Altitude • WEG VFDs are rated for operation at altitudes up to 1000 meters (3300 feet)

without de-rating.

• In some applications the installation site may be at a higher altitude.

• Higher altitudes may be allowed provided the VFD can be de-rated per the

manufacturer’s recommendations.

Example of a high altitude de-rating scenario:

• A common derating formula for altitude above the VFDs rating is 1% decrease in

current per 100 meters (330 feet) additional altitude.

• For example, a 480V VFD rated 110 kW (150 HP) has a current rating of 180 Amps

at 1000 meters (3300 feet).

• At 3000 meters (9900 feet) altitude, the VFD has a current de-rating of 1% per

100 meters which means 20% decrease in maximum current

i.e. {(3000-1000)/100 = 20%}.

Or: 180 Amps – 20% derating (36 Amps) = 144 Amps maximum

NOTE: Always consult with WEG Drives Application Engineers when derating a VFD

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Environment: Altitude (cont’d.) The graph below shows that as the altitude increases (horizontal axis) the available

drive current is reduced.

At 1000 meters the current rating is at 100% of the drive’s rating but at 3000

meters described in the prior slide, the drive’s rating has to be reduced to 80%

(derated by 20%).

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Installation Considerations

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Drive Installation: Grounding

Improper grounding is a common VFD installation issue

• VFDs must be properly grounded before they can function as designed.

• Follow recommended grounding practice as in the example below:

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Improper grounding is a common VFD installation issue

• VFDs must be properly grounded before they can function as designed.

• Follow recommended grounding practice as in the example below:

NO! Enclosure

backpanel

Yes Enclosure

backpanel

Drive Installation: Grounding

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PVC - Inner &

Outer Sheath

Armor Cable Example

Filler

Cable

Armor

B

G

C

A

G

G

Three

Ground

Conductors

Typical VFD Grounding Scheme

Example of Wiring for VFD and Armor Cable

Drive Installation: Motor Cables

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Drive Installation: LV Cables

• Proper low voltage cable routing practices should be followed to ensure

optimal performance for the VFD and connected system devices.

• Electromagnetic interference (EMI) can be a very important issue with

high power devices such as VFDs.

• Most VFD manufacturers offer an RFI filter built into the VFD to help

mitigate electrical interference problems.

Variable Frequency Drive

+

AI-AO+AO– DI1 DI2 DI3 DI4+24vGND

Basic VFD Low Voltage Wiring Example

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Drive Installation: LV Cables

Some common wiring recommendations:

• Control wiring should never be run in the same conduit that

contains power wiring of any kind.

• For 120 Vac signals, a standard 600 volt stranded wire or single

conductor can be used.

• For 24 Vdc signals, it is best to use multi-conductor twisted pair

control cable. An overall shield is recommended but not required.

• For analog signals such as 0-10 Vdc, 0-20mA or 4-20mA, a twisted

pair cable with a shield should be used.

• A recommended separation of 200 mm (8 inches) should be

maintained between power and control wiring when run along side

each other.

• If power and control cables need to cross each other, it should be

done at a 90 degree angle.

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Summary and Key Points

• Variable Frequency Drives are widely used today to vary the speed of the AC motors in pumping applications

• Care must be taken in selection and installation of the drive and motor to ensure trouble free operation and long service life.

• Ensure the Variable Frequency Drive and AC motor are suited for the environment and load characteristics.

• Know the ‘worst case’ operating environment and evaluate enclosure designs.

• Understand the potential issues of cable length between VFD and motor and don’t exceed the manufactures recommendations.

• Make sure that motors used with VFD’s are VFD suitable. VFD operation can impact cooling, insulation and bearings.

• Be sure to consult the VFD and motor manufacturer(s) for additional guidelines and recommendations for long service life.

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It’s HERE!

The new 2017 Automation Catalog:

• Focused on Low Voltage Drives,

Soft Starters & Engineered Panels

• New layout, easier to follow

• Updated Quick Selection Guides

with current products

• Updated pricing

• Available in hard copy, or download

the PDF at: http://www.weg.net/us

• The online version will be

maintained regularly

• Just updated January 2017!

Low Voltage Drives and Soft Starters

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WEG Variable Frequency Drives Training

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Thank you!

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