How to use VSD in existing HVAC installations T able of contents

Transcription

How to use VSD in existing HVAC installations
■ Comparison
Traditional AHU controlled by IGV ........................... 3
AHU controlled by VLT 6000 HVAC ........................ 3
■ Considerations
Steps to be taken to retrofit by drives .................... 4
The electrical installations ........................................ 4
Sizing the drive ........................................................ 4
Connecting the control system ............................... 5
Before choosing a drive .......................................... 6
Cons ider motor derating ......................................... 8
Consider short-circuit capabilities of the drive ......... 8
Consider capabilities of switching on the output ..... 8
Consider user friendliness ........................................ 9
Consider galvanic isolation ...................................... 9
Consider other important features ........................ 10
Zero inrush current ................................................ 10
Catches spinning motor without tripping .............. 10
Acoustic noise ....................................................... 11
Serial communication .......................................... 11
MC.60.E1.02 - VLT is registered Danfoss trademark
1
Table of contents
■ Introduction
Where can drives be applied ................................... 2
Function and mounting place ................................. 2
Before choosing a drive .......................................... 2
Introduction
■
Where can drives be applied
For energy saving the great advantage lies in
installing variable speed drives on applications using
centrifugal fans and pumps.
Such typical installations in buildings are:
❖ Air handling units and ventilating plants, cooling
towers used in most buildings and extract fans
(extract fans are widely used in kitchens,
production areas or underground car parks,
tunnels etc.).
❖ Water pumps for cooling towers, chilled water
pumps or pumps for potable water.
■
Function and mounting place
When using a drive the flow of air or water will be
controlled by varying the speed of the fan or pump.
Hence the function of damper, guide vane, by pass
or motorised valves will no longer be necessary.
These parts should either be removed or blocked in
the fully open position.
Existing BMS signals, previously used for controlling
guide vanes, motorised valves or contactors, may all
be used as input (e.g. feedback and start/stop for
the drive(s).
The best place to mount a drive is next to the motor it
is controlling. Choosing a drive in IP54 enclosure
makes it possible to use the drive in dusty or wet
areas. For panel mounting an IP20/IP21 version
should be chosen.
2
Standard low-cost sensors/transmitters for temperature, air or pressure will be needed to make the
drive increase or decrease speed in order to
maintain a given setpoint. These transmitters,
however, may typically already exist in the installation.
■ Before choosing a drive
If the drives are correctly retrofitted, the building will
both be more energy efficient and become a more
comfortable place to work in. To obtain that,
however, the correct selection of drives is important.
The most critical issues are discussed in the
following.
■ Installation
Retrofit installation of drives should be let to professional control companies or HVAC contractors, in
order to ensure correct installation.
Fig. 1 shows a traditional VAV-Airconditioning plant
using Inlet Guide Vane, IGV, to control the airflow as
well as three port valves to control the heating/
cooling liquid.
Fig. 2 shows a VLT 6000 HVAC frequency converter
controlled plant.
For a comparison between the two systems, see
overleaf.
MC.60.E1.02 - VLT is a registered Danfoss trademark
Comparison
Fig. 1.Traditional AHU controlled by IGV
Fig. 2. AHU controlled by VLT 6000 HVAC
3
Considerations
■ Steps to be taken to retrofit the installation
See Figs. 1 and 2
❖ The motor only needs a three-wire cable for
connection to the VLT 6000 HVAC drive (3
wires of existing cables can be used).
The existing star/delta starter is no longer
required and can therefore be removed.
❖ Bypass loops incl. 3 port valves on pump systems are not required. If dificult to take out, the
valves can be blocked in fully open forward position. If necessary a NRV (Non-Return-Valve) can
be applied instead of the existing 3-port valves.
❖ The Inlet Guide Vane (IGV) is no longer required,
as flow in controlled directly by changing fan
speed. If too troublesome to remove, it should be
blocked in fully open position. The IGV actuator
signal and motor signal are no longer required,
but the signal can be re-used as a control signal
for the drive.
The same considerations should be made when
using a damper.
❖ No Power Factor Correction (PFC) equipment
required, as it will have no influence on the
power factor of drives.
NB:
The use of some brands of VSDs, with low
power factor, may damage PFC capacitors.
Dedicated drives provide a power factor better
than >0.9/cosϕ ~ 1.0.
■ Considerations on electrical installations
The existing motor cable can be cut where most
convenient and the VLT 6000 HVAC drive
connected directly to the cable ends. The VLT 6000
HVAC drive comes in an IP54 enclosure and can
therefore be mounted close to the motor or where
best suited depending on space. If the drive is to be
mounted in a panel, it can also be supplied in an
IP00 or an IP20/21 enclosure.
The Danfoss VLT 6000 HVAC drives are supplied
with built-in filter and are fully EMC tested and
certified to qualify for the CE marking. For more
detailed description, see below.
Drives not including galvanic isolation to VDE 0160
(PELV) have to be fitted with isolators, according to
standards, between BMS system and drive. VLT
6000 has fully galvanic isolation according to PELV
requirements.
■ Sizing the drive
In retrofit applications determining the size of a drive
to a given motor should cause no problems.
However, some drive brands require the motor output power to be derated: Drives using standard sine
wave controlled PWM principle cannot give full voltage to the motor at 50 Hz, unless a third harmonic
voltage is introduced. This results in a non-sinusoidal
output waveform from the drive, which will cause
additional heat loss in the motor, resulting in higher
motor temperature and shortened motor life.
Do not forget to check the drive manufacturers’
recommendations on this.
For use of the motor’s full 100% output power, it is
necessary either to choose a motor one frame size
larger, to get more surface and material to
compensate for the additional heat loss, or to use a
motor rated for temperature class F.
For retrofit applications this is often not realistic, as
the motor is already installed.
When using a Danfoss VLT 6000 HVAC drive the
motor temperature will not significantly exceed that
of a motor connected directly to the mains (<+1°C).
That is because Danfoss’ specially developed VVC
(Voltage Vector Control) principle ensures full voltage
at 50 Hz, without any distortion. Besides it provides
fully optimised magnetising voltage for the motor at
any lower frequencies (speeds).
No motor derating is required with VLT 6000 HVAC
drives.
Danfoss has this certified by third party (Western
Electric Australia PTY. Ltd.)
EMC (Electro Magnetic Compatibility) is an important
issue in order not to interfere with computers and
BMS systems. Please carefully follow the
instructions given by the manufacturer on EMC
correct installation of control and motor cables.
4
Read the nominal full load motor current on the
motor name plate and choose the Danfoss drive
that meets or exceeds that value.
Should you want Danfoss to match the drive for
your motor and applications, just give us a call.
■ Connecting to the control system
The drive can be easily connected to an existing
BMS control system or it can be self-contained. For
most fan and pump applications the speed control is
based on temperature or pressure feedback signals.
If connected to a BMS system, the BMS will provide
the speed control and the start/stop signals for the
drive. Either analogue or digital signals can be used.
If self-contained, the Danfoss VLT 6000 HVAC drive
will take a signal (e.g. 0/4 - 20 mA) directly from a
temperature/pressure transmitter, or differential
pressure transmitter, and control the fan or pump
application using the built-in PID controller
*) (IEC Publ. 34-1. (IInternational Electrotechnical
C ommission).
Smooth Control
No need for any neutral zone
known from ON/OFF control.
Temperature or pressure will be kept constant.
Fig. 3
5
Considerations
As no motor derating is required with VLT 6000
HVAC drives it is possible to use standard IEC*)
specified AC induction motors of class B insulation
together with any Danfoss VLT 6000 HVAC drive.
And it is easy to choose the right drive for retrofit
applications:
Considerations
■ Before choosing a drive
The reason for choosing a drive in HVAC applications
is to gain energy savings and reduce maintenance
costs. None of this may be achieved if the wrong type
of drive is chosen. (Many industrial drives are not
recommended for HVAC use).
Danfoss VLT 6000 HVAC drives are equipped with
integrated factory tested RFI filters as standard (see
fig. 5) and when following the instructions in the manual, RFI suppression to EN 55011 is achieved.
Danfoss has this certified by third party:
TELEDANMARK, The tele communication
authorities of Denmark.
The drive must comply with your specifications.
A drive’s immunity to interference from electrical
disturbances from mains or other equipment
connected to the same mains phases is specified in
EN 50082, - 1 & 2
2.. (See also paragraph “Input
surge protection” page 7 and “Galvanic isolation”,
page 9).
All Danfoss drives comply with this norm as the
tests are carried out in accordance with the standards IEC 1000-4-2 / 5 and VDE 0160.
Can your supplier prove that his product does meet
some minimum requirements?
Some of the most important issues are:
Electro Magnetic Compatibility) covering
1) EMC (E
RFI (R
Radio F
Frequency Interference) filtering,
Immunity and low frequency harmonics.
■
2) Derating of the Motor (should not be required)
See paragraph “Sizing the drive”, page 4.
Harmonics Interference (on supply transformer)
3) Short Circuit Capabilities of the Drive
An electric motor connected to mains draws sine
wave current and is considered a linear load. No
harmonics arise from that.
A VSD does not draw sine wave current because of
the rectifier in the mains input and is therefore considered a non-linear load. Fig. 4 shows the
conventional PWM frequency converter.
4) User Friendliness
5) Galvanic Isolation
6) Other Important Features
■ EMC (Electro Magnetic Compatibility)
The term EMC covers the ability of an electrical device
to keep high frequency disturbances inside the device
and to withstand disturbances from the outside,
preventing malfunction or damage to other equipment
or the device itself. The degree of compatibility is now
specified in a new set of European Norms, EN (e.g. EN
55011 Group 1, Class A and B, EN 50081, 50082
etc.)
Harmonics are unavoidable and comprise multiple
integers of the fundamental frequency of 50 Hz or
60 Hz. 5th, 7th and 11th harmonics of 50 Hz will
thus be 250Hz, 350 Hz and 550 Hz, respectively.
The harmonics will cause an increase in the total
current consumption IRMS of the VSD, but will not
add to the useful torque on the motor shaft. If the
amount of harmonics gets too high, cables, the
supply transformer as well as other connected
equipment may overheat and eventually be
damaged.
All VSDs produce a lot of above mentioned disturbances called RFI (Radio Frequency Interference). In
order to comply with the new norms, often referred to
in customers’ specifications, the drive must be
equipped with a suitable “filter” preventing this noise
from disturbing the environment.
Most manufacturers can provide this filter as an
external option. Besides, many drive manuals are not
very informative on EMC correct installation, so
whether sufficient RFI suppression is achieved or not
may be accidental.
Fig. 4 Conventional PWM frequency converter
6
AC reactors have the disadvantage that they can
cause a voltage drop across them of up to 5% of
the mains voltage, causing the output voltage to
drop by 5%. Most drives will tolerate up to a ±10%
deviation of the mains voltage level designed for.
Outside of this range they often trip.
When using AC reactors half of this range then
cannot be used and these drives become much
more sensitive to mains sags or “brown outs” (dip in
mains voltage).
Danfoss drives have DC link coils integrated as standard. When using DC link coils, there is no
significant voltage drop.
Danfoss VLT 6000 HVAC drives ensure (guarantee)
full rated motor voltage and torque within ±10% deviation of the mains voltage and will tolerate
deviations of up to ±20% (or more, depending on
drive size), before a trip occurs.
Fig. 5: VLT 6000 HVAC Frequency Converter
There is no risk of damaging power factor correction
capacitors, PFC capacitors or other electrical/
electronic equipment, or of overheating the supply
transformer when using Danfoss VLT 6000 HVAC
drives.
Many drive brands will damage power factor
correction capacitors and other electrical /electronic
equipment due to low power factor (0.5 - 0.6),
caused by harmonics.
Power factor correction capacitors cannot compensate for a low drive power factor.
Danfoss VLT 6000 HVAC drives have a full rated DC
link filter built in as standard (See fig. 5).
Suggested limits for harmonics disturbance are set
in the recommendations G5/3 and IEEE 519. Norm
VDE 0160 specifies approximately the same levels.
The Danfoss drives observe the normed VDE 0160
specifications.
This ensures good suppression of harmonics and
improves the power factor to above 0.9.
Please bear in mind, that power factor and Cos ϕ
are not the same thing, when drives with bridge
rectifiers in the input are used. In order to find the
true power factor, multiply the fundamental current
I1 (1st harmonic) by Cosϕ1 and divide by the total
rms current, IRMS . Usually the rms current is difficult
to establish as it demands more sophisticated instruments than an ordinary ammeter.
■ Input surge protection
All Danfoss VLT 6000 drives are input surge
protected according to IEC 1000-4-4 and -5 as
well as protected against other disturbances
according to IEC 1000-4-2 and -3.
7
Considerations
To suppress harmonics and at the same time
improve the power factor, most manufacturers
supply AC line reactors (coils to mount in the AC
supply lines) or DC coils (coils to mount in the DC
link circuit), as optional extras to be mounted outside the drive cabinet.
Considerations
■ Motor derating
This issue is already discussed in “Sizing of Drive”
page 6, but there are a few more words to it:
Danfoss VLT 6000 HVAC drives can run a motor
without derating, because of the advanced VVC
(Voltage Vector Control) principle developed by
Danfoss. It is based on a 60° switching pattern
controlled by the software and an ASIC, to give optimum magnetising of the motor under any operating
condition, taking load and speed into consideration.
Used in the VLT 6000 HVAC drives, the software
control ensures close resemblance with the centrifugal characteristic of fans and pumps.
■ Short circuit capabilities of the drive
What precautions are made to prevent damages
because of short-circuits between motor phases or
motor phase to earth?
*
The quality of motors and cable used for motors
may vary. Low quality plastic insulation may
become crisp and eventually cause short
circuits.
*
Accidents may happen causing damage to motor cable, which then short circuits.
*
By accident water can get into electrical systems
and that may cause short-circuits.
*
The application may block the rotor of the motor.
If so, the motor will take up much current and
eventually overheat.
*
The environment may cause even good products
to fail and eventually to short circuit.
■ Switching directly on the drive output
*
In many applications a service switch, mounted
between the drive and the motor, is desirable. In
this way any repair work in the installation can be
carried out safely, even if voltage is connected.
*
It may be relevant to connect more than one
motor in parallel to a drive and then switch the
motors in and out during operation as required.
The situations mentioned may cause an open circuit
between the motor and the drive or rapid load
changes, when motors are switched in and out.
Most drives will not tolerate the motor to be
switched on or off between the drive and the motor
(service switch or parallel operation of motors) during
operation. The drive and motor should always be
switched off/brought to standstill to avoid damages
to the drive. Shock loads may also damage the
drive.
Make your supplier prove that his drive can
withstand such situations during operation, before
you make your choice.
With a Danfoss VLT 6000 HVAC drive, unlimited ON/
OFF switching of the motor, between the drive and
the motor is allowed. The drive may trip but it will
not be damaged.
The VLT 6000 drives can run without a motor
connected and still not be damaged.
The VLT 6000 HVAC drives have extremely fast
current sensors built into all three motor phases.
The problems mentioned are just a few of many
possibilities of short circuits between motor phases,
shorts to ground (earth) or over-current situations
that a drive will have to deal with.
Most drives will not tolerate to be short circuited
directly between the output phases or between an
output phase and ground (earth) and especially not
when it is running.
Make your supplier prove that his drive can
withstand such situations without any limitation or
damage, before you make your choice.
The Danfoss VLT 6000 HVAC drives tolerate direct,
solid short-circuit of the output phases or phase to
ground under any condition, loaded or unloaded.
8
Programming:
Many drives are difficult to set up or programme and
often they communicate with the user in codes. The
manual briefly shows what to do, but there is often
a lot to do before the drive is running. The drives
can be programmed to many different applications,
situations and operating conditions, which are used
in industrial applications. In HVAC applications,
however, most of the functions are superfluous or
even wrong.
The Danfoss VLT 6000 HVAC drives are dedicated
for HVAC applications including relevant I/Os.
The Danfoss VLT 6000 HVAC drives have a 3-line
alphanumeric display explaining the drive conditions
chosen.
The Danfoss VLT 6000 HVAC drives have Hand/Off/Auto functions that can be selected directly from the
keyboard as well as remotely. That makes it very
easy to control the application e.g. during
commissioning, both on site and from the control
room. Hand function can be disabled, if necessary.
VLT 6000 HVAC is preprogrammed from the factory
with HVAC relevant settings.
VLT 6000 HVAC has a Quick Set-up menu in which
you only need to record motor data: voltage,
current and frequency to make the drive run.
Ten parameters are maximum in the quick set-up
menu, but sufficient to adapt to most applications.
VLT 6000 HVAC have RFI and harmonic filters
integrated in compliance with the standards for
building services.
■ Galvanic isolation
Considerations
■ User friendliness
Control terminals must be safe to touch during installation, commissioning and operation.
Any standard signal source should connect to the
control terminals without the use of signal
isolators.
Many drives do not have proper galvanic isolation
between the high voltage power section and the
low voltage control section. This means that the low
voltage control terminals can be “riding” on a high
voltage level and problems can arise when
connecting signal sources, if they are not “floating
signals” or not referring to the same ground potential
as the drive. Damage can even occur if signal
isolators are not used.
Some drives do have opto-couplers as signal
separators in the control input, but the optocouplers used, are usually not certified to any standards. They must comply with VDE 0884 or UL 508.
the norms may also specify certain demands as to
how the opto-couplers are mounted on the circuit
boards, leakage path etc. and high voltage testing
of the finished assembly.
Make your supplier prove that his drive has true
galvanic isolation in accordance with international
norms, before you choose your frequency converter.
Danfoss VLT 6000 HVAC drives provide full galvanic
isolation between control terminals and power
section, according to the norms VDE 0106 and VDE
0160, PELV (Protective Extra Low Voltage). The isolation is for example provided by transformers. The
norms demand three layers of insulating material.
The lacquer on the copper thread is not regarded as
a layer!
The control terminals of VLT 6000 HVAC are safe to
touch and no external signal isolator will be
necessary between drive and source.
9
Considerations
■ Other important features
Zero inrush current
In pump applications it is important to avoid water
hammer, which will damage pipes and valves.
Water hammer comes from the shock wave that
arises in the pipe system every time a pump is
started or stopped.
DOL (Direct On Line) start and Star/Delta starters
both cause water hammer and high inrush currents
from the mains. The inrush current can be 5-7 times
the rated motor current (name plate current). See
fig. 6.
Soft starters limit the water hammer but the inrush
current on mains is 2.5 to 3 times higher than the
rated motor currrent.
The VLT 6000 HVAC drives ensure that there will be
no inrush current at start, as the motor is ramped
up to desired speed in a smooth and controlled way.
On stop, the motor is ramped down to zero speed
in the same smooth and controlled way.
Together with the variable torque characteristic the
ramp up and down function will prevent any water
hammer.
Catches spinning motor without tripping
In for example ventilating plants, several fans could
be installed, but not all of them are used all the time.
They are cut in or out as required. When cutting in a
fan, it will probably be rotating in the opposite direction as intended, because of the existing air flow
(wind-milling).
A fan in an Air Handling Unit may be swtiched off
intentionally or the mains may simply disappear.
When power is switched on again, or mains reappears, the fan may still be rotating due to inertia.
Drives connected to fans in such situations should
be able to detect the direction and actual speed of
the fan and bring the fan back to desired speed and
rotational direction, normally without the use of
injected DC voltage. However, wind-milling fans may
have to be brought to standstill by means of DC
injection before some drives will connect without
tripping.
The VLT 6000 HVAC drives can detect the speed of
a disconnected, but already/still spinning motor,
catch it and control it back to desired speed, no
matter in which direction it was spinning. Unidirectional fans can thus be brought back to correct
direction, if they should be windmilling, without tripping.
1 = VLT 6000 HVAC
2 = Star-Delta
3 = Soft Starter
4 = Direct-on-line
Fig. 6
10
Serial communication
Making the motor completely noiseless over the full
frequency range may be desirable, but recent
investigations have revealed that this cannot be
achieved without cost:
An installation with many drives require a lot of
control cables and wires to be connected between
PLCs, Building Management Systems, transmitters,
transducers etc.
Controlling the drives via serial communication
requires only two wires and a PC software program.
The motor bearings will usually erode because of
circulation currents in the motor frame, caused by
high switching frequencies or high du/dt. (Note that
du/dt is the rate of voltage rise over a period of time,
usually defined as the change in voltage in one
microsecond). This can be overcome by replacing
the normal bearings by insulated ones, that will
prevent the flash-over from the outer bearing ring to
the inner bearing ring, via the rollers. But this is an
expensive operation, which should not be
necessary.
The VLT 6000 HVAC drives operate completely
noiseless in the lower output freqeuncy range (up to
25 Hz). Then , as fan or pump noise increases, the
drive will allow the switch frequency to come down
to the preset value, which is reached at 50 Hz output frequency. This is ensured with ASFM
(Automatic Switch Frequency Modulation).
If necessary, drive and motor can be made
noiseless up to 50 Hz output frequency, by running
at high switch frequency all the time. However, that
requires the output power to be somewhat
reduced.
Because of the Danfoss VVC control system, no
damage is made to the motor bearings, when
running high switch fr
equency with a VL
T 6000.
frequency
VLT
Further energy savings
Most HVAC applications only require some
monitoring and not so much controlling. Such
applications can be run with RS 485 serial
communication.
More demanding applications would require faster
communication systems which can handle higher
data rates.
The VLT 6000 HVAC drives have RS 485 serial
communication built in as standard.
Up to 99 drives can be addressed via a PC and
repeaters for programming, monitoring and control,
by means of the communication software VLSTM
Dialog 2.
Should more intelligent serial communication be of
interest, Danfoss can offer solutions with
PROFIBUS, Modbus+, LONWorks & Metasys N2.
Contact Danfoss for further technical information
covering Design Guide, References, Application and
Feature Notes.
The booklet Why
Why... use VSD in existing HVAC installations MA.60.D1.02, Retrofit Guideline Part I,
describes the advantages of fitting VSDs in existing
installations. It deals with the benefits, fully described
with energy saving and pay back time calculations.
When drives are running a HVAC application and
steady conditions are achieved, it should be possible
to reduce the energy consumption according to
load, by allowing the motor voltage to drop to lowest
possible value and still running the application safely
and reliably.
The VL
T 6000 HV
AC drives have an AEO (Automatic
VLT
HVAC
Energy Optimising) function built in as standard.
When the drive has reached the set speed, the motor voltage is adjusted to an optimum value that will
ensure reliable operation, optimise efficiency and
reduce energy consumption further by 5-10%
depending on the application.
11
Considerations
Acoustic noise
For your notes/calculations
12
13
14
15
16

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