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14 ANNEX A: VARIABLE FREQUENCY DRIVE
DWSC and DWDC chillers can be equipped with a variable frequency drive (VFD). A VFD modulates the compressor
speed in response to load and evaporator and condenser pressures, as sensed by the compressor controller. Due to the
outstanding part load efficiency, and despite the minor power penalty attributed to the VFD, the chiller can achieve
outstanding overall efficiency.
VFD really prove their worth when there is reduced load combined with low compressor lift (lower condenser water
temperatures) dominating the operating hours. VFD for large capacity compressors over 1200 tons are disproportionally
expensive. DAE
’s dual compressor units (DWDC) with two
half size
compressors allow VFDs to become a reasonable cost
alternative on large chillers compared to the very expensive large capacity drives required for competitors
’ large single
compressor chillers.
The traditional method of controlling centrifugal compressor capacity is by variable inlet guide vanes. Capacity can also be
reduced by slowing the compressor speed, reducing the impeller tip speed,
providing
sufficient tip speed is retained to
meet the discharge pressure (lift) requirements
.
This method is more efficient than guide vanes by themselves. In actual practice, a combination of the two techniques is
used. The microprocessor slows the compressor (to a fixed minimum percent of full load speed) as much as possible,
considering the need for sufficient tip speed to make the required compressor lift. Guide vanes take over to make up the
difference in required capacity reduction. This methodology provides the optimum efficiency under any operating condition.
14.1 Acceptance of the Product
When the VFD is delivered to the place of installation, it must immediately be inspected for damage. All components
described in the delivery note must be carefully inspected and checked; any damage must be reported to the carrier. Before
unloading the machine, check that the model and the voltage shown on the identity plate correspond to your order. The
manufacturer shall accept no liability for any damage which comes to light after acceptance of the machine.
14.1.1 Checks
For your own protection in case the machine is incomplete (missing parts) or has been damaged during transport, carry
out the following checks upon receipt of the machine:
a) Before accepting the VFD, check every single component of the supply. Check for any damage.
b) If the VFD has been damaged, do not remove the damaged parts. A series of pictures can help to verify
responsibility.
c) Immediately report the extent of the damage to the transport company and request them to inspect the machine.
d) Immediately inform the dealer of the extent of the damage to allow the latter to arrange for the necessary repairs.
Under no circumstances must the damage be repaired before being inspected by a representative of the transport
company.
14.2 ABBREVIATIONS
EMI
Electromagnetic Interference
IEC
International Electrotechnical Commission
RCD
Residual Current Device
STO
Safe Torque Off (Functional Safety)
CT
Current Transducer
VFD
Variable Frequency Drive
14.3 VFD Nomenclature For C Series
