How can a 12 V computer fan run on 6 V from a PWM controller, but not with 6 V from batteries?

Question

I have this 7A PWM motor speed controller:
http://www.canakit.com/bi-directional-dc-motor-speed-controller-pwm.html

I've connected a 12 V battery pack as input, with the output going to an 80mm 12V computer fan. Using a multimeter to measure the voltage output, I can turn the knob so the controller is outputting about 6 VDC, which the fan will operate on.
If, however, I connect a battery pack with 4 AAA batteries to provide 6 V, the fan will not spin.
What is the difference between these two setups that the AAA batteries won't power the fan?

Answer 1

Your observations have nothing to do with the current capabilities of either the batteries or controller.



A typical 80 mm fan only draws about 150 ma when run at 12 volts and most of them will start and run at reduced speed when supplied with as little as 7 or 8 volts of continuous DC.



The output of a PWM controller isn't continuous DC.



A Pulse Width Modulated controller supplies the full 12 volts to the motor, but only part of the time. You could do the same thing with a switch that you turn on and off for varying amounts of time. Suppose that you turned the switch on for one second and off for one second repeatedly and your meter averaged the resulting voltage supplied to the motor. It would read 6 volts while your motor was actually receiving 12 volt pulses. It would speed up and slow down but the average speed would be somewhat less than full speed. The only real difference is that the PWM controller turns on and off 100's or 1000's of times each second and the ratio of on/off times can be varied.



No real magic here. Just a clever little circuit and a misinterpretation of what the meter was telling you.



Don

Answer 2

A V6 has a block that's angled at ninety° or 60° and has three cylinders on every part. The straight 6 has one long block w/ the cylinders aligned in a single straight line. The straight 6 is an extraordinarily delicate going for walks engine, but I rarely see one, I saw one on an ancient Mustang and not too long ago on a BMW. The smoothest of the V6 engines is the 60° but I believe the ninety° is extra normal. I think it simply has anything to do w/ the ease of manufacturing.

Answer 3

Current. The amount of current needed is inversely proportional to the load so if the impedance of the load is to low there will be a Voltage drop, hence why the controller could run the fan and not the Batteries.

Answer 4

Current! Your PWM is rated at 7 Amps and the AAA's only at mA's. They don't have the current to run it.

Answer 5

The PWM is supplying 12V, but pulsing it on and off.