Auto-Transformer Starters; Fault Finding - Danfoss VLT MCD 500 Pocket Manual

Auto-Transformer Starters: How does soft start
compare to auto-transformer starting?
Soft starters are much more flexible than auto-transformer starters
and provide a much smoother start, generally at a lower cost.
Auto-transformer starters cannot accommodate varying load
conditions (e.g. loaded or unloaded starts) and the start torque
cannot be freely adjusted to match motor and load characteristics.
Damaging torque and current transients still occur at the steps
between voltages, and auto-transformer starters are not capable
of providing soft stop. Auto-transformer starters are large and
expensive, especially if high start frequency is required.
Fault finding: What are the key questions?
To assist your service engineer, they require the following
information:
• Model and serial number of the soft starter
• Motor kW and FLC
• Main supply voltage and frequency
• Control voltage
• Application (e.g. pump, compressor)
• Time installed before failure
• Details of other soft starters on the supply bus. Are these failing?
• If the soft starter trips, details of the code and mode of operation
• The installation's power and control schematic diagram
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