| The
table on the
Design & Installations Considerations page is
a guide to selecting Control Temperature.
Further specific recommendations are given below. Recommended
Control Temperatures based on equipment
temperature rise are given in the following table. Temperature
rise is measured from equipment air inlet to the
point where the sensor is located, often near the exhaust,
with air movers running at full speed. These Control
Temperatures will result in minimum noise under normal
conditions of room temperature, altitude, and system
resistance while maintaining lowest operating temperatures
under all conditions.
| For
this design temperature rise from air inlet to air
sensor with air movers running at full speed |
3-6°C |
6-8°C |
8-12°C |
| Choose
this Control Temperature |
35°C |
40°C |
45°C |
Recommended
Control Temperatures
Recommended Control Temperatures based on equipment
application are given in the following table.
| 35°C
Control Temperature
|
| 40°C
Control Temperature
-
Good
choice for many purposes
-
Equipment
of moderate size intended for installation
in
open offices, hospitals
|
| 45°C
Control Temperature
-
Smaller
equipment
-
Equipment
to be installed in private offices and homes
-
Equipment
that will be used in proximity to an
operator (e.g., desk-mounted)
|
How
SmartFan® Regulates Component Temperatures
in Electronic Equipment
A
3° to 4°C temperature change causes a closed-loop SmartFan
controller to change speed from idle
to maximum. A unit with a 40°C Control Temperature regulates
temperature within a range from about
36° to 40°C. In principle, a controller could be made
to regulate air temperature more precisely. It would
have very high sensitivity such that at a fraction of
one degree below 40°C, fans would idle and at 40°C
they would reach full speed. There are two reasons why
SmartFan sensitivity is controlled at a moderate
level. First, very high sensitivity could result in
instability (sometimes called hunting) with fans having
difficulty finding a stable speed. Secondly, the temperature
that we really want to control is that of
semiconductor junctions, not air. As the air velocity
over a typical semiconductor device (e.g., DIP
or similar) changes by two-to-one, as it does when fan
speed changes from full speed to idle, the
difference between the air temperature and junction
temperature changes by a few degrees. Changing
the air temperature by an equal number of degrees cancels
this effect to hold absolute junction temperature
more nearly constant.
Temperature
Compensation -The Open-Loop Alternative
An
alternative to regulating temperature inside equipment
is to sense and compensate for changes in inlet air
(room) temperature as shown in the figure below. A disadvantage
of such an open-loop system is that it is
responsive only to inlet air temperature and cannot
respond to changes in power dissipation, altitude, or
system resistance. As the curve shows, a change in inlet
air temperature from 23°C (73°F) to 35°C (95°F)
causes fan speed to change from 50 to 100%. Open-loop
versions of SmartFan controllers are available
and, like closed-loop units, can reduce noise levels
by 15 dB(A) or more.
 |
AC
CONTROLLER OPERATING PRINCIPLES
SmartFan
AC fan speed controllers use a triac phase control
principle to vary the AC voltage applied to the fan.
As the chart below illustrates, the voltage applied
to the fan is switched off for a period of time, called
Toff and
switched on for a period of time, called Ton during each half of one
line cycle. As a result, the fan RMS voltage
changes proportionately to increases and decreases
in Ton
.
Since fan speed is proportional to the RMS voltage
applied, it will also vary in proportion to changes
in
Ton
.
Many permanently split capacitor (PSC), shaded pole,
and universal single phase motors are compatible with
SmartFan AC controllers. Use of SmartFan controllers
with capacitor start motors, where a capacitor is
switched in and out of the motor windings, should
be avoided.
Some PSC and shaded pole fans are not voltage controllable.
When used with a SmartFan controller at idle
speed, and with a nominal pressure load, such fans
will idle at a speed far above or far below the intended
50% of full speed. Except in the smallest sizes, a
PSC-type fan is preferred over a shaded pole-type
fan
for the following reasons:
-
Speeds
are less sensitive to power line voltage changes
-
When
used with SmartFan, idle speed is more closely regulated
-
PSC
fans draw lower electrical current than equivalent
shaded
pole units
-
PSC
fans have higher starting torque
DC
Controller Operating Principles
The
speed of a DC fan is nearly proportional to the DC voltage
applied. Low power Wisp II and Omni L1A
controllers use a linear dissipative principle to vary
applied voltage. At higher power levels, the Omni SD
uses a high frequency switching principle to achieve
power efficiency of more than 90%.
Both
linear and switching controllers are connected between
the negative fan terminal and the negative
terminal of the fan’s power supply. The positive
fan terminal is connected directly to the positive terminal
of the power supply.
Triac
Phase Control Principle:
Line & Fan Voltage vs. Time
|
