Different Methods to Control Fan Speeds
Introduction
At first glance, you may think, "It's just a fan". Why complicate
something so simple. After all, it's just another component to deal with
and anything extra adds additional cost to the system. This statement may
be true for most typical applications, but applications that are sensitive
to noise and power, this option adds value!
Fan noise and power are directly related to the output performance
of the fan. If additional CFM is needed, an increase in noise
and power will also be the result. In this world there is no
free lunch. But what if we can reduce the amount of CFM during
times where system loads are minimal. During these moments,
we can reduce the CFM and consequently lower the emitted noise
and power requirements. If the load increases, we can increase
the CFM to meet the cooling requirements. The result is a win-win
situation for today's thermal cooling needs.
There are many ways to control the speed of a fan. Controlling
the speed of the fan can be as simple as regulating the input
voltage to the fan to using more complicated digital microprocessor
inputs. These methods are further addressed below and we offer
guidelines to follow when considering each option for your
application.
Voltage Regulation
The voltage regulation option
is designed for applications where the input power may fluctuate
at different voltage levels. An example would be a backup generator,
or outdoor telecommunications equipment that periodically undergoes
battery regeneration. In these applications, a change in input
voltage would result in a change in the fan's rpm and thus,
noise and power.
To avoid unwanted fluctuations, Sofasco fans can be equipped
with an internal regulating circuit that maintains a constant
voltage source to the fan windings; regardless of how the input
voltage changes. This is accomplished by using a voltage regulator
and clamping the adjust leg with a zener diode. See Figure
1. Everything is built into the fan. There are no extra components
or extra wires to deal with in your system. The Sofasco solution
is simple and easy.
Programmable
A programmable fan allows you
to control the speed of the fan to optimize cooling performance.
This option can create new ideas in thermal management by developing
simple circuits that can either pulse width modulate, vary
the voltage, or vary the resistance. The amount of flexibility
this option can provide is extensive.
Because brushless DC fans rely on an electronic circuit to
rapidly switch the magnetic fields, circuitry can be added
to enhance the fan's ability and add additional value to your
system. The programmable option uses an internal voltage regulator
to set the speed of the fan by means of a third lead wire tied
to the adjust leg of the regulator. See Figure 2. The speed
can be controlled two ways - controlling the voltage to the
adjust lead or controlling the resistance between the adjust
lead and the return. Either pulse width modulation or a variable
voltage/resistance can accomplish either method.
For pulse width modulation (PWM), The pulse signal should be
applied to the program lead (yellow wire) and referenced to
the return lead. For PWM, by means of applying voltage on and
off, the amplitude should be equal to the nominal voltage of
the fan, the frequency should be held constant (20kHz recommended)
and the duty cycle allowed to vary between 0 and 100%. The
voltage must be current limited so that there is no more than
5mA to the program lead. For PWM by means of controlling resistance,
the program lead should be cycled between an open and a short
to the circuit return, with the frequency held constant.
Controlling the speed by varying the resistance or voltage
is accomplished the same way as PWM. The difference is that
the voltage/resistance is varied linearly instead of varying
the duty cycle. Applying a direct voltage to the adjust leg
will achieve the same results as varying the resistance. As
you will notice in Figure 2, the resistors tied to the adjust
leg create a voltage divider. By varying the external resistor,
a voltage divider supplies the adjust leg with a varying voltage.
Each fan is different in the range of input voltage to fan
speed. The rule of thumb is a zero voltage input will result
in a minimum speed condition and an input equal to 50% of the
fans nominal voltage will result in a maximum speed condition.
For instance, a 24 Vdc fan will operate at full speed when
12 Vdc is applied to the program lead.
Unfortunately, the voltage range is preset. If your requirements
call for a linear change from 0 to 10 volts, an op-amp can
be used to customize the programmability input. This option
is not available on all models and minimums do apply.
For controlling the speed by resistance, a closed circuit between
the program lead and the return will result the fan operating
at half speed. An open circuit will result in a full speed
condition. Different resistance values will yield different
fan speeds.
Thermal Speed Control
The thermal speed control option
varies the speed similar to the programmable option, except
it does not need any external input. This option uses a thermistor
to monitor the temperature and regulate the speed accordingly.
The thermistor's metal properties allow it to change its resistance
at different temperatures, thus creating a variable voltage
divider circuit at the adjust leg of the voltage regulator.
The fan will automatically adjust its speed to optimize the
airflow to the surrounding temperature. See Figure 3.
For conditions where power inputs are at a minimum, the fan
operates at half speed, providing a minimal amount of CFM to
those components still in need of cooling. This reduction in
speed results in a significant reduction in noise and power
consumption. As the power requirements increase, so does the
internal temperature. The fan senses this increase and compensates
for it. The fan will continue to increase in speed until it
reaches full speed for maximum cooling. The temperature range
is 25°C to 45°C. Any temperature below 25°C the
fan will operate at half speed. Between 25°C and 45°C,
the fan RPM changes linearly with temperature. Over 45°C,
the fan will maintain full speed operation.
Fans equipped with this option use a thermistor tied to the
adjust leg of the voltage regulator and the power return. The
thermistor can be located on a zip cord and placed on hot spots
such as processors, heatsinks, transformers, etc., or can be
placed in the hub of the fan to monitor the air traveling through
the fan. Mounting the thermistor in the hub of the fan is only
recommended for applications where the fan will be exhausting
the air from the system.
External Controllers
Sometimes more complicated cooling methods are needed. This
may include using multiple temperature sensing, different set
speeds per fan, or other combinations. For cases where complicated
mapping functions are needed, the only choice is to use a microprocessor.
This option is often expensive and may need additional mounting
space. Each application needs to be dealt with on an individual
basis. Please contact our Application Engineering department
for more information on this option.
Conclusion
There are many ways to control the speed of a fan. After all,
it is a simple component. But there are some considerations
that must be addressed. For instance, pulse width modulating
the power lead of a standard fan will also PWM the fragile
ASIC circuitry that makes the fan work. This circuitry is not
designed for rapid switching of power on and off. Sofasco's
speed control fans are. They offer the flexibility and reliability
that our customers have come to trust for over 25 years.
