Electronic – Can PWM affect a brushless DC fan given sufficient time

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tl;dr – Is driving brushless DC fans using PWM unhealthy for the fan compared to variable but steady DC voltage? If yes, why and how?

The super simple input [PWM] -> MOSFET driver [PWM] -> DC Fan to adjust the speed of DC fans is well known. The DC fan receives a PWM of the same frequency as the input, with sufficient juice from the MOSFET at higher voltage. Ignoring everything else, assume the fan gets a high-current PWM signal swinging between 0-12V at some duty cycle to vary the fan speed, and that the 0-12V levels are clean (no spikes etc).

Say I have a bunch of 0.5A, 12V brushless DC fans. These are not PWM fans (i.e. they only have 2 wires). I now drive them using the above PWM circuit to vary their speed. Assume the PWM frequency is around 25 kHz, and that the MOSFET can easily switch at that frequency.

I've read that adjusting the speed of DC fans using a variable DC level voltage is "more healthy" for the fan than using PWM like above, but didn't give details.

For the experts out there:

Are there dangers to driving DC fans like the above using PWM (pulsed) instead of steady voltage levels? If yes, what are they, exactly, how do they manifest? How important is the PWM frequency for the fan (assuming the MOSFET is fast)?

p.s. I can build a PWM-to-DC circuit (e.g. source follower, etc) but here I'm interested to understand the dangers of driving DC fans with PWM.

p.p.s. My personal experience (edit: excellent explanation for this by Tony EE below)

I'm pretty sure PWM may affect them (though here I'm asking a general question). I've heard clicks coming from all my DC fans when driving them with PWM, particularly at low-duty cycles. More importantly, after a little less than 1 year under continuous operation at various duty cycles (50% being the most prevalent), some of the fans no longer respond to low duty-cycle PWM. Specifically, they still spin at 100% speed when the duty-cycle is 100% (practically 12V steady) but any lower duty cycle results in the fan spinning at a very slow speed, regardless of the actual duty cycle value — as if the fan had 2 speeds only: full and very-low. All these fans responded well to PWM before. They still spin freely when pushed by hand, I feel no extra resistance compared to the other fans (so it's not the rotor).

Best Answer

All DC motors are actually AC motors with some sort of commutation from DC with some DC coil resistance, DCR.

The BLDC draws excitation current from the coil resistance. (I=V+/DCR). As average voltage increases and it overcomes stiction, it starts spinning and the commutated coil impedance now is added to the DCR.

HOWEVER, an internal filter capacitor is needed by the fan to reduce incoming and outgoing voltage ripple, caused by current commutation in the poles.

  • If you pulse that capacitor, with a non-PWM controlled fan, you may be pulsing that capacitor with more ripple current than the fan, and it may fail prematurely according to Irms and its rating, or cause weird aliasing noises.
  • Nevertheless, you are imposing a new spectrum of current interference to the commutation for the motor and thermal feedback to regulate fans may have audible aliasing winding noise, from these ideal quiet fans.

Conclusion: Use the right fan or design a DC-DC regulator to control it, that guarantees no stalling from low start voltage by setting thresholds for voltage and temperature control range.

Anecdotal experience from early 1980s to now

  • Fans generate acoustic eddy currents from the blade turbulence near a fixed grill. The solution to this is make the fan inline to a plenum. The design trick is to make this as short as possible to remove all the heat outside and not add much noise.

  • For SMPS forced air cooling, it is this same eddy current turbulent velocity that reduces the thermal resistance of air cooling ferrite transformers and power transistors.

    • So maximize air velocity over the hot spots, with a plenum or folded cover rather than simple panel-mounted fan that pushes or pulls a certain volume of air.
    • The secret is that the air just blows over the top of the hot spots thus ineffective or weak thermal coupling, instead of turbulent air over the part at 1 to 3 m/s. In my case design it reduced full load hotspots in a restricted space from 70 °C to 10 °C temperature rise. I verified my design with cigarette smoke, thermocouples and mylar folded plenums. (This also helped me survive UL's safety "coke-spill and sledge-hammer tests" to my 19" 1U rack design.)
    • The motor senses rotation position by a precision Hall effect pole sensors which commutates the FET bridge.
    • All electrolytic components (batteries included) have a capacitance that degrades due to a chemical reaction accelerated by current and heat that affects the ageing rate.
    • This is defined by Arrhenius' law, which is approximately -50% MTBF for every 10 °C rise above ambient.

  • The coil impedance rises above stall speed so the current now increases much slower due to wind loading.

  • The stall speed might be around 25% starting and 15% stopping so that if stalled with no cooling the coil heats up the magnet and that could degrade the magnet over time if above its rating.

  • Back in the 1980s Toshiba, Fujitsu using some higher power muffin fans would have a thermistor with self heating by applied power and self-cooling by the fan's air velocity to detect a fan fail and create a halt alert.

  • The fan not only cools the host, but the internal parts as well, except when it is stalled. For small fans well designed, no problem, but if the coils heat up the rare earth magnets unusual failures can occur (dead-spots in starting fan, etc.). This usually only happens on one of four resting positions.

  • For this reason for the last 35 years, any high volume production of systems I have been involved with, an automated start-stop incoming test was performed, testing for this fault. The design/process is a critical tradeoff between efficiency near magnetic neutral and offset so the circuit knows which direction to start commutating. This slim margin exists in every PM BLDC motor. Where the fan is not dead, it's just dithering back and forth, or motionless until you nudge it.
    • Even the famous Nidec Japanese brand fans occasionally had 1% of 100 fans with dead spots, so corrective feedback to OEM eliminated their problem by forcing them to fix it.
  • I also gave our distributor my design for a simple test jig design that did this, Start <= 0.1 s stop <1 s repeat for 100 cycles, 100 fans in parallel.
    • Then we had perfect yields - no failures, three shipments in a row, so then I halted those incoming tests. Problem solved.
  • Often these problems occur when the factory moves or some other minor process change.

p.p.s. reply

You probably blew the internal electrolytic cap from external PWM resulting in an abrupt change in source impedance with PWM now going open/close in parallel with fan's load including a capacitor with ESR, to give a low source impedance to the bridge. I suspected the capacitor ESR should be worn out with much higher ESR and lower C, so the fan has a load high regulation error due to a much higher source impedance. This explains your dramatic losses below 100% PWM from full speed to slow speed.

The capacitor ESR must be << 10% of the coil DCR for good voltage control RPM regulation or better, so use the proper fan or a linear design. A cheap fix may be to add a series R to transfer capacitor losses to series or RL to improve maximum speed with low Q for a field fix. Or do get it right the first time and listen to the wisdom of experience.

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