Toshiba RELIABILITY IN MOTION 1700 - Manual (Page 30)

TOSHIBA
30
Appendix B: Fan Speed Control
The fans in the 1700 Series will operate at any of four different speeds
depending on the environment and system conditions. In the first stage
the fans are off. As the load and or the temperature increase the fans
will subsequently go to low, medium or high speed as required.
The temperature reference given on the graph above refers to the
temperature inside the UPS. There will typically be a 5°C difference
between the internal and external temperature. The 10% hysteresis
shown is to ensure that the unit will not oscillate between fan speeds.
When the output power reaches a level that requires the fan speed to
increase the load will have to be reduced 10% below that level before the
fan speed will return to the lower speed.