Starting Torque, Under-Volting, Fan Wear, and Stall Issues
What is starting torque?
Most electric motors require more power to start spinning than they do to keep spinning. This resistance to movement is caused by friction in the motor bearings, the magnetic forces in the hub, the inertia of the hub itself, and whatever external loading is on the motor. Once these initial forces have been overcome, the motor can continue to spin using far less energy.
What is motor stall?
If you were to apply, say, 4 volts to a 12v fan, it may just sit there unable to get the rotor turning. But if you were to hand-start it, it may be able continue turning on its own once started. This inability to get moving is what is called motor stall. You can run a fan in the stall region, but only after it has been boosted up to speed using a higher voltage.
When running in the stall region, the motor can be very sensitive to small disturbances such as changing airflow patterns in the case or simple thermal expansion and contraction. If run slowly enough, the motor may eventually stall all by itself because the resistance to rotation is just ever so slightly above the power being applied to turn the rotor.
Power requirements, motor aging, and stall voltage
For a given motor, there will be a range where there is insufficient voltage to get moving or stay moving, a stall area where the voltage is just high enough to overcome rotational resistance but not enough to start the motor, and a voltage range which will always start and run the motor.
These ranges are not fixed over the life of the motor. Generally, more power will be needed to start the motor as it ages and dust buildup begins on the rotor blades and in the bearings.
Therefore you need to be careful when running a fan at lower than rated voltage that you are well above the stalling speed of the motor, or it may eventually stop working due to dust buildup and slowly increasing wear in the bearings.
Why are manufacturer specifications so conservative?
When a fan manufacturer is writing their fan specifications, they need to take all these wear factors plus the many various fan mounting positions into account. Therefore the manufacturer's minimum rated starting voltage for the fan has to be fairly conservative because the manufacturer wants to specify a voltage rating they know will start the fan in all normal operating conditions, not just the absolute best, brand-new conditions.
Although you may be able to start a fan with a lower than rated voltage, this is likely because the fan is brand new or may be mounted in the optimal position that creates the least bearing friction. If you set the fan to use a lower than specified starting voltage and close the case thinking nothing more of it, six months later the fan may no longer start up due to progessive dust buildup and bearing wear.
Making sure an undervolted fan starts and keeps turning
If you intend to run a fan at a voltage lower than the manufacturer's ratings, there are a number of ways to make sure the fan properly gets up to speed and keeps turning in the event of a stall.
If you use a simple rheostat, you could temporarily turn up the speed when you turn on the computer, then turn the speed back down in a few seconds. However, this is a fully manual process and you must do this everytime you turn on the computer. And if the fan happens to stall for some reason, it will not be able start turning again unless you are around to notice the problem and increase power to the fan.
A slightly better option is a timed starting circuit that applies full voltage to the fan for a few seconds, then drops to running voltage after the fan has started. This way the fan gets the power it needs to get spinning, then switches to reduced power once it is turning. But this cannot deal with fan stalling at all, after the timer has completed its one-shot startup timing run.
Using a smart fan controller that can monitor either fan speed or system temperature is the best way to make sure an undervolted fan always starts and will start spinning again in the event of a stall. If something happens that causes the fan to stop turning, the fan controller can sense either the lack of fan speed or rising temperatures, and automatically temporarily increase power to the fan to get it turning again.
Even better smart fan controllers will also include a fan-speed passthrough connector that plugs into the motherboard, so that if the fan stalls and stays stalled for some reason (such as hub lockup from dust), the system can perform an emergency shutdown to save your equipment from damage.
Tips for temperature sensing fan controllers
For best operation, a temperature sensing fan controller should have the temperature probe touching the fan-cooled chips or heatsink. If the probe is floating out in the middle of the case and the fan on the video card stalls, the probe may not detect the temperature rise several inches away from the card until after the video chips have been damaged.
For the most reliable temperature monitoring, the probe should also be out of the fan's airflow path, because the airflow over the probe may cool the probe faster than the chips/heatsink, and lead to falsely low temperature readings and slower fan speeds that lead to chip overheating.
Starting Torque, Under-Volting, Fan Wear, and Stall Issues
Moderators: NeilBlanchard, Ralf Hutter, sthayashi, Lawrence Lee
Panaflo 3rd wire: Locked Rotor sensing vs tachometer
Why does Panaflo offer fans with a locked rotor sensor? Because for industrial applications it allows for a less complicated device design, compared to having to read a tachometer signal. Measuring the speed of the fan based on a tachometer signal requires complicated time-based analysis circuitry.
But if the fan simply reports a locked rotor signal, for industrial purposes that may be all that is needed. If you're building an industrial/manufacturing system, all you may care about is that the cooling fans in your device keep spinning. Running slowly is not an issue, as long as SOME air is flowing.
With locked-rotor signalling, the cooling fans merely need to be wired in parallel so that the locked rotor signal on one or more fans will flash a warning light on the front panel of the device, to warn the operator or maintenance personnel to replace the locked-rotor fan(s). It makes for a quick, easy, and simple fan monitoring system.
-Scalar
Why does Panaflo offer fans with a locked rotor sensor? Because for industrial applications it allows for a less complicated device design, compared to having to read a tachometer signal. Measuring the speed of the fan based on a tachometer signal requires complicated time-based analysis circuitry.
But if the fan simply reports a locked rotor signal, for industrial purposes that may be all that is needed. If you're building an industrial/manufacturing system, all you may care about is that the cooling fans in your device keep spinning. Running slowly is not an issue, as long as SOME air is flowing.
With locked-rotor signalling, the cooling fans merely need to be wired in parallel so that the locked rotor signal on one or more fans will flash a warning light on the front panel of the device, to warn the operator or maintenance personnel to replace the locked-rotor fan(s). It makes for a quick, easy, and simple fan monitoring system.
-Scalar