VSD frequently asked questions
Why doesn’t the measured input amps of the OSCD agree with the rated FLA?
The input current to the OSCD may be considerably lower, compared to the output current. This is
due to the power factor at the input to the OSCD being greater than 0.95, and nearly unity when
the Harmonic Filter option is included. Chiller FLA must be measured at the motor terminals, where
the power factor is the normal motor power factor. Use a true RMS reading meter to make these
measurements.
Is a Condenser Water Strainer used with the shell and tube heat exchanger?
Since the shell and tube heat exchanger can be cleaned with a rifle brush, no extra precautions are
needed to keep the heat exchanger cleaned. No strainer is provided with this OSCD. The intent is to
have the heat exchanger cleaned annually. Gaskets are available (refer to the service parts list).
What is the timing of the Anti-Recycle when an OSCD is applied?
The anti-recycle time is much quicker with an OSCD than with a starter. The reason is the
elimination of inrush current on start-up. The OptiSpeed compressor drive slowly accelerates the
compressor motor so that the motor does not consume more than 100% of the motor’s nameplate
full load amps. The anti-recycle time is five starts in succession, followed by a ten minute wait. After
ten minutes, the OSCD can be started five more successive starts. This is permitted on OSCD units
only, due to the low current draw and reduced motor heating during startup.
Should the customer install isolation between the Power Conduits and the OptiSpeed
Compressor Drive?
We no longer require a section of non-metallic conduit at the entrance and exit of the OSCD as we
did on previous products. If any customer or installer wishes to continue to follow this practice, we
have no objections as long as the OSCD is properly grounded.
When is a Booster Pump required on a Retrofit OptiSpeed Compressor Drive?
Detailed information is supplied in
Form160.05-N4
. In general, the OSCD requires 8 ft of head for
proper water flow to the OSCD heat exchanger. If this amount of head is not available, then a
booster pump is required.
Can I apply an OptiSpeed Compressor Drive to a generator?
Yes, the OSCD can be applied to a generator. No modifications are required for a generator
application. We have several OSCD installations running on generator power without difficulty. It
is necessary that the generator’s output voltage and frequency be maintained within the specified
range for that particular OSCD. This is usually not a problem for most generators, since motor
current at startup is limited to less than 1X the Full Load Amps (FLA).
My chiller will not slow down, why?
The OSCD will not reduce the motor speed until the leaving chilled water temperature is below 0.5
degree from setpoint. Once in this window, the speed still cannot be reduced until the operation
is deemed to be stable, based upon the vanes are not continually moving open and closed to
maintain temperature. This hunting effect is normally due to one of the following:
•
Chilled water and Condenser water flows are not at design GPMs. The rate of change in flow
maybe too fast for the chiller to be determined as stable.
•
Return water temperature is varying due to 3-way valves or other system configuration, and
the chiller is simply following changes in load.
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Liquid-Cooled OptiSpeed Compressor Speed Drive
