Acronym/Term Definition
expansion valves. It contains four relays for the solenoid valves, compressor status and
compressor run status respectively.
SDT Saturated Discharge Temperature.
SMT Service Monitor Tools a PC program provided byDanfoss LLC. A user friendly way of
displaying compressor data to the user and offer adjustment of predetermined
parameters. The user interface adjusts itself according to the active access level at the
compressor.
Soft-Start Board / Soft-
Starter
The Soft-Start Board limits in-rush current by progressively increasing the conduction
angle of the SCRs. This technique is used at compressor startup while the DC capacitors
are charging up. The Soft-Start Board takes as input a 3-phase voltage source at 50/60Hz
from the input terminal and a DC voltage signal from the SCR output. In turn, it outputs
pulses to the SCR and provides power to the High-Voltage (HV) DC-DC Converter. All
voltages from the Soft-Start Board are with respect to the positive DC bus and not the
compressor ground.
SST Saturated Suction Temperature.
Surge The condition at which the compressor cannot sustain the discharge pressure, allowing
refrigerant to temporarily and rapidly re-enter the compressor fluid path, creating a
cavitating effect. This is an undesirable situation that should be avoided.
Ton The basic unit for measuring the rate of heat transfer (12,000 BTU/H; 3.516 kw/H).
Touchdown Bearings Carbon races or ball bearing for the purpose of preventing mechanical interference
between the shaft and the magnetic bearings should they lose power or fail.
TT Twin Turbine.
Two-Stage Centrifugal
compressor
Type of centrifugal compressor having two impellers. The first-stage impeller raises the
pressure of the refrigerant vapor approximately halfway from the cooler pressure to the
condenser pressure, and the second-stage impeller raises the pressure the rest of the
way. With a two-stage compressor, an interstage economizer may be used to improve the
refrigeration cycle efficiency.
UL Underwriters Laboratories (www.ul.com).
VAC Volts Alternating Current.
Vaned Diffuser An assembly of plates with curved vanes that serve to slow, compress, and reduce
refrigerant rotation as it enters the second-stage impeller.
Vaneless Diffuser Similar to a Vaned Diffuser, except that it does not possess any de-swirl vanes.
M-AP-001-EN Rev. S-9/8/2021 Page 17 of 136
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