Video: What causes USF Undervoltage on an ATS22 Soft Starter?


14 June 2022

Why is USF displayed on Altistart 22?

Product Line:
Altistart 22 ATS22

All models, All serial numbers.

USF is being triggered when the starter is supplied with proper power.

USF is an Undervoltage Fault.  Basically, the incoming supply voltage is too low. 

The following parameters affect the USF fault:
-In the ConF menu,
UIn should be set to the nominal incoming 3 phase voltage.
The factory default is 480V for S6 and S6U ranges, and 400V for Q range.
-In the PrO menu,
USd ( Under Voltage Threshold ) sets the threshold as a percentage of parameter UIn.
USt  (Under Voltage Time Delay ) sets the amount of time delay in seconds until a USF trip.  The voltage must remain under the threshold for the entire delay to activate the fault.

Note that the under voltage protection feature becomes operational only after a run command has been issued.  When voltage drops to zero, the soft starter will trip immediately, overriding the time delay.
An isolation contactor that is not pulling in with run command applied can also cause USF as the main power to the soft start is not made.

Uln= 480V, USd = 70%, USt = 5 seconds.   So, if after a run command is issued, the incoming voltage drops to less than 336 volts (480V * 70%)  and stays there for more than 5 seconds, USF will be triggered.