Funct Vari Freq Drive

8/11/2019 Funct Vari Freq Drive

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Pre-Test

Answer the following application questions based on this diagram.

Multiple Choice

1. Variable Frequency Drives work with which of the following motors?

A. Single-Phase AC MotorsB. Three-Phase AC Motors

C. DC Motors

D. B and C are correct but not A.

E. A, B and C are all correct.

2. The setting for the speed of a VFD is known as which of the following:

A. Setpoint

B. Control Point

C. Reference

D. Speed Point

E. All of the above are common names for the speed setting.

See notes for questions 3-5.

3. Assuming no friction losses, if a VFD is operating a fan at 30Hz, the full speed, which

of the following is the correct power consumption?

A. 50% power consumption

B. 33% power consumption

C. 25% power consumption

D. 12.5% power consumption

E. 6.7% power consumption

4. Limits are very important for a VFD operating a fan. Which of the following are the most

commonly used limits for a fan?

A. 18Hz Minimum speed; 60Hz Maximum speed

B. 6Hz Minimum speed; 60Hz Maximum speed

C. 18Hz Minimum speed; 90Hz Maximum speed

D. 6Hz Minimum speed; 90Hz Maximum speed

E. 0Hz Minimum speed; 120Hz Maximum speed

5. Inside the VFD, acceleration, is also known as which of the following:

A. Increase Speed

B. Increase Setpoint

C. Speed UP

D. Run-Up

E. Ramp-Up


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How did you do?

Answers:1. B. Three-Phase AC Motors

2. C. Reference

3. D. 12.5% power consumption

4. B. 6Hz Minimum speed; 60Hz Maximum speed

5. E. Ramp-Up

If you got 4 or 5 right skip this lesson and go to lesson 2

If you got 3 or less right, please review this lesson.


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10/2/2003

VFD 101

Lesson 1

Functions of an Variable Frequency Drive (VFD)

This lesson covers the basic functions of a Variable

Frequency Drive (VFD) as it applies to fans.

Here is the basics outline for this lesson.

Outline:

A. 3-phase AC Motor

B. Functions of an VFD

1. Start/Stop

2. Change Speed

3. Constant Speed

4. Limits

5. Ramping

6. Forward/Reverse

7. Save Energy


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VFD 101

Lesson 1

This lesson covers the basic functions of an Variable Frequency

Drive (VFD) on a 3-phase AC (alternating current) motor.

Pictured above is an AC motor.

The 3-phase motor pictured above is for commercial use, but in your home, AC motors

are used as well. A vacuum cleaner uses an AC motor to clean the carpet; a blender uses

an AC motor to process food; and the clothes dryer uses an AC motor to dry clothes. In

each of these examples, how is the AC motor controlled?

When controlling motors in the home we control them by applying AC power, andremoving it, usually through a switch. Obviously when power, 120 or 240 VAC, is applied to

the motor it runs. With no power, the motor stops.

With the use of a Variable Frequency Drive (VFD) not only can the AC motor be

started and stopped as in the home, but more sophisticated controls are accomplished. A

VFD can send a modulating signal to the motor, which allows a variety of speeds to be

delivered not just an ON/OFF signal. This variety of speeds can be used to match the motor

to a particular task. There are a number of functions that the VFD accomplishes with

commercial 3-phase AC motors, which are covered in the pages that follow.

Motors in the home are almost always single-phase motors which require additional

electric parts to rotate the magnetic field. Because of these extra parts, single-phase motors

do NOT operate correctly with a VFD.


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To understand the functions of an VFD better,

an example of cooling tower fans is used.

What must the fans do?

The cooling tower in the picture above must maintain a certain temperature perhaps

30C (85F) for the condenser water temperature. Looking at this example, see if you can

identify some of the functions that must be performed by the VFD, AC motor and fans? In

other words, what must the fans be able to do? Take a couple of minutes to jot down the

functions.

The fans must

_____________________ ______________________

_____________________ ______________________

A few of the basic functions of an VFD in controlling the AC motor and fans are

covered on the pages that follow.


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Function #1 Start and Stop

The VFD must be able to START and STOP

the cooling tower fans.

Function #1 Start and Stop

START

One function of the VFD is to start the fans. This could be done locally off the keypad

of the drive or remotely from a switch. This remote switch could be a continuous single-

pole-double-throw (SPDT) switch or a momentary (push button) switch.

STOP

In the picture above, the SPDT switch is used to stop the pump. If there are 2

separate push button switches, one to Start and one to Stop, this arrangement is known as a

3-wire Start/Stop. If only one switch, a continuous switch, is used, then it is referred to as a

2-wire Start/Stop. In the picture above, since there is one switch, so this is a 2-wire

Start/Stop.


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Function #2 Change Speed

The VFD must be able to Change the Reference, Hz.

The Reference could also be temperature or PSI if a

transmitter were attached to the VFD.

Function #2 Change Speed

The speed of the cooling tower fans must be variable to allow for a slower speed when

there is little demand for cooling and a higher speed when more cooling is needed. This

allows the operator to match the speed of the fans to a particular demand. The setting of

this speed is known as the Reference. In most examples, reference refers to speed in Hertz(Hz), maximum reference of 60Hz, and minimum reference of 6Hz for fans and 18Hz for

pumps. It could also be used in regards to a pressure setting, maximum reference of 100psi

(690kPa), minimum reference of 40psi (275kPa), if a transmitter were attached to the VFD.

In the picture above, the display of an VFD, a Danfoss VLT 6000, is shown. Speed in

Hz is the reference. The plus (+) key is used to increase the reference making the fans go

faster and the minus (-) is used to decrease the reference point slowing the fans down.


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Function #3 Maintain a Constant

Speed

Light load or heavy, the drive shouldmaintain the same speed.

Function #3 Maintain a Constant Speed

Another function of the VFD is to maintain the speed of the fans regardless of the

temperature and humidity in the air. The VFD automatically compensates the current and

torque to accommodate changes in the load.


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Function #4 Limits

Limits on current, torque, speed, heat andvoltage and others protect the VFD & motor.

Function #4 Limits

It is important that limits be placed on an VFD. Speed limits can be placed in the

program of the VFD so an operator can not go beyond a maximum speed or less than a

minimum speed. The maximum speed of the fans should not exceed 60Hz, due to

excessive power consumption. Because of the possibility of overheating, fans should not berun less than 6Hz. For the same reason as the fans, pumps should not be run more than

60Hz. For lubrication purposes a pump should have a minimum speed of at least 18Hz.

If the fans gets stuck, there are torque limits that the VFD monitors stopping the motor

if they are exceeded. Current limits are also important for protection of the drive and motor.

In the picture above the maximum reference is set to 60 Hz. Notice that in the diagram there

is a minimum reference of 6Hz.


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Function #5 Ramping

To reduce mechanical wear, it is important tocontrol the acceleration, ramp up anddeceleration, ramp down.

Function #5 Ramping

The VFD also ramps the fans up and ramps them down. When the fans starts,

acceleration, it is important that there is no sudden jump to the reference speed, or there can

be stress on the gear boxes. In the example above, a ramp-up slowly increases the speed

from stopped or 0Hz up to the reference, 34Hz, over a certain amount of seconds perhaps10. If this ramp up is too short, the drive can trip on an over current alarm or torque limit. If

the VFD is tripped, the fans stop and it might require an operator to manually reset the VFD.

Many VFDs have an automatic reset setting of 1 time to infinite times. Ramping is very

important for pumps, to avoid water hammer.


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Function #5 Ramping

All ramp times are based on motor speed,

60Hz in Western Hemisphere, 50Hz in theEastern Hemisphere.

A ramp is also present on the stop side. This is referred to as a ramp down or

deceleration. It is important that the fans do NOT stopped abruptly. A ramp-down of 60

seconds might be entered into the program for this application. If the ramp is too short, the

drive can trip on over voltage.

All ramp times are based on the motor speed, 60Hz in the Western Hemisphere. Thismeans if the the ramp time is set for 60 seconds as in the picture above, but the reference is

set to 30Hz (1/2 of 60Hz), it takes 30/60 x 60seocnds ( the time) or 30 seconds to ramp up.

In the rest of the world 50Hz is used for the motor speed. Using the same ramp up time (10)

and reference (30), the motor then takes 30/50 X 60seconds or 36 seconds to ramp up to

30Hz. Calculations for the ramp down time would be the same.

A special feature of the Danfoss VLT 6000 is automatic ramping. The VFD

automatically extends the ramp times, during ramp up and ramp down, to avoid tripping of

the drive.


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Change of Direction

Forward to cool water

Function #6 Forward/Reverse

Function #6 Forward/Reverse Operation

FORWARD

One function of the VFD is to operate the motor in a forward direction, to move the

air through the cooling tower and out the top. In its default (factory set) condition the VFD is

only allowed to go forward. Some fans if driven backwards may have problems.


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Reverse to defrost cooling tower

Function #6 Forward/Reverse

REVERSE

In the cooling tower example the fans need to operate in Reverse in order to

complete a defrost cycle, when the outside temperature is very cool. Power going to the

motor must be changed to move the fans backwards (Reverse). If there were no VFD, 2 of

the 3 leads of the 3-phase motor would be switched in order for the motor to change itsdirection and go backwards. This switching of the motor leads is done inside the VFD.


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Function #7 Saving Energy

The most important function for the VFD

with this fan application is to save energy.

Function #7 Saving Energy

In many applications, particularly involving fans and pumps, the major function of the

VFD is to save energy. Before VFDs, cooling tower fans might have been cycled On at full

power, when a temperature setting in condenser water of 32C (90F), was reached. When

the water cooled to 28C (82F) the fans were turned OFF coming back ON when thetemperature rose again to 32C (90F). This arrangement uses a great deal of energy and

the frequent cycling causes a great deal of wear on equipment.

A drive is placed on the fans, which slows the fans down to perhaps 30Hz to

constantly maintain the required condenser water temperature. The fans speed up or slow

down following demands. On the chart above, if the fan is running at 30Hz, half of the full

speed, assuming no friction losses, the energy level is 1/8th the HP at full speed. This same

energy savings is seen on pump applications.

This concludes Lesson 1. There is a Post Test to review this information.


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Review - Matching

1. Change Reference ______

2. Maintain Speed ______

3. Ramp Up ______

4. Direction Change ______

5. Ramp Down ______

6. Limits ______

7. Saves Energy ______

8. 3-Phase AC Motor ______

Letter selection, the definitions, A H are in the notes.

Matching:

A. Slowly starts the motor and increases the speed over a certain amount of time, say

60 seconds, before it achieves the reference speed. This is also known as

acceleration.

B. Slowly stops the motor and decreases the speed over a certain amount of time, say60 seconds, before it stops completely. This is also known as deceleration.

C. Stops the drive when the current or torque is too high. It also prohibits the operation

of the VFD and motor going too fast.

D. Allows the operator to select different speeds for the motor slower or faster.

E. This is the device operated by an Variable Frequency Drive (VFD).

F. This function allows the operator to go both backward and forward.

G. This function continues operating the motor at the same speed, regardless of the

load, heavy or light.

H. This the the major reason for using VFDs on fans and pumps.


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End of Lesson 1

Answers1)D. 2) G. 3) A.

4)F. 5) B. 6) C.

7)H. 8) E.

Objectives

1) The student is able to identify 7 basic functions ofan Variable Frequency Drive (VFD) and give a briefdescription of each function.

2) The student is able to identify the device operatedby an Variable Frequency Drive (VFD).

For more information,please contact the MCU Training Team.

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Funct Vari Freq Drive