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4.14 Soft Start
There are two (2) different variants of compressor Soft Starts. All Revision G and earlier compressors had a factory
installed Closed-Top Soft Start. Beginning with Revision H compressors, the Open-Top Soft Start was introduced.
There is a significant visual difference between the two (2) variants, refer to Figure 4-94 Soft Start Variants for the
visual differences.
The Open-Top Soft Start can handle all input voltages, whereas there are distinct versions of the Closed-Top Soft
Starts to handle specific voltages. If a Closed-Top Soft Start must be replaced, Danfoss LLC offers conversion kits to
allow the use of the Open-Top Soft Start. Refer to the Spare Part Selection Guide for TTS/TGS compressors for kit
details.
Figure 4-94 Soft Start Variants
The Soft Start limits in-rush current when power is applied to the compressor by progressively increasing the
conduction angle of the voltage through the SCRs to charge the DC capacitors. It uses a 3-phase voltage input at
50/60Hz, between 380-575VAC, and a DC voltage signal from the SCR output to generate output pulses of 0-12VDC
to the SCR gates for the in-rush current control signal.
For Closed-Top Soft Starts, the 3-phase AC voltage is passed through 1/4A fast-acting fuses to two (2) onboard
transformers that reduce the primary voltage to a secondary 15VAC. Both transformers pass the secondary voltage
through separate Nano fuses. These transformers power the Soft Start and provide the trigger signal voltage to the
Potted DC-DC when DC bus voltage levels reach the minimum level.
The Closed-Top Soft Start also uses a 2A 1000VDC fuse to pass high-voltage DC to the DC-DC Converter.
For Open-Top Soft Starts, the Mains AC voltage is passed through resettable fuses to an on-board isolated DC-DC
converter during the initial startup stage. After DC bus voltage reaches minimum level, the Open-Frame DC-DC
converter begins providing voltage to the service side without assistance from the Soft Start.
In addition to monitoring AC mains voltage and the HV DC bus, the Open-Top Soft Start also provides an on-board
temperature sensor, varistors, and EMI filters for system protection.
All DC voltages from the Soft Start are with respect to the positive DC bus, not the compressor ground. Refer to
Figure 4-95 Closed-Top Soft Start Connections on page 114. and Table 4-22 Closed-Top Soft Start Connection
Identification on page 114. for the Soft Start connector locations and identifications.
M-SV-001-EN Rev. H-1/23/2023 Page 113 of 294
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