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Have 12v AC, converted to 12v DC, motor slow

  1. Aug 30, 2014 #1
    Hello All,

    I have a stater that puts out AC voltage, at max RPM it produces 40 volts. I have a voltage regulator installed parallel and it controls volts to max 14+v AC.

    I need to run an 80 watt 12V DC cooling fan so I installed a little block that has 4 diodes inside. Possibly this is called a rectifier, Im not sure. I was told how to install with AC on two posts and DC load on the two posts that are labeled "+" and "-".

    After install and test I have found that the fan is spinning slower than if I powered the fan from a 12V battery. ( I would like not to run a battery )

    Would a DC motor run slower with a 12V AC converted to DC than just wiring it to a 12V DC car battery?

    Please explain in simple terms as this is not my field of expertise.

    This is a snowmobile motor with a 230 watt stater plate and I am cooling it with a radiator and fan similar to a car setup. the fan is 12v DC 80 watt.

    Thank you in advance for any input to help with original fan speed to gain more airflow to cool motor.
  2. jcsd
  3. Aug 30, 2014 #2


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    Welcome to PF.
    How much slower?

    The assumed “bridge rectifier” you are using will have silicon diodes that will drop about 1 volt each when conducting. That reduces your 14V to 12V or less.

    If you have wired it wrongly then you might be getting 12V for only half the time.

    AC is measured as RMS. So the peak voltage of AC is √2 = 1.4142 times the equivalent DC.

    Does your regulator set the peak at 14V for battery charging? You may need a small battery to maintain the 14V all the time.
  4. Aug 30, 2014 #3
    There's two main reasons I can think of that would affect your motor:

    1. The output voltage of a rectified signal is not a flat DC voltage, it is actually just a sin wave except the negative part of the wave is reversed to be positive. The average voltage of this wave is lower than your RMS voltage. This picture represents that well (taken from isaac42's post on HarmonyCentral Forums). Just pretend the 24V RMS is your 14V and the ratio of 24V to 15.3V still stands.:

      Source: http://www.harmonycentral.com/forum/forum/Forums_General/acapella-94/293518-

    2. There is a voltage drop across diodes, usually about 0.7 V. This will also reduce your effective DC voltage.
  5. Aug 30, 2014 #4
    the outflow of air is noticeably less using the converted AC power than just using the batter power.

    Knowing VERY little on this subject .... could a cap be installed to help in this problem?

    (I only know that a cap is like a very small battery. it stores power to be discharged later when primary voltage drops.)

    I would like to eliminate a battery for weight saving, can it be done in a different way? maybe I just need to run a battery.

    What is the best way (and LIGHTEST) to run a 80 watt DC motor (fan) the fastest for cooling with a given 12-14 Volt AC supply rated for 230 watts?

    ( tomorrow I will meter the AC volt at idle and then after the gismo measure the "DC" voltage and see what drop there is.)
  6. Aug 30, 2014 #5
    here is a picture of how my "gismo" was explained to me. It is the diamond shape in the middle.

    would guess the left image is a transformer which I don't have because my stater is already outputting 12-14V AC.

    In the picture, on the right there is a straight line over a curve (frown) line. this I think is a cap. If this is true, hw would I size one if it would help the fan spin faster?

    If one is installed ... is there a cycle/service life for a cap?

    Attached Files:

  7. Aug 30, 2014 #6


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    Does the stator produce 1, 2 or 3 phase output?
    Is it wired as a 3 phase star with the centre connected to the chassis, or maybe a floating delta?

    If your 14V regulator is for battery charging it will be regulating the peak AC, not the RMS

    To use a capacitor instead of a battery;
    An 80W fan on 12V will draw a current of 80W/12V = 6.67A
    What is the frequency of the AC? If 100Hz then period T = 10msec.
    C = I * T / dV; For a 1.0V droop in the capacitor during the cycle;
    C needed will be C= 6.67A * 0.01sec / 1.0V = 0.0667F = 66.7mF, rated at 15V or better.

    Maybe buy ten x 10,000 uF capacitors. US$2 each.
    Wire them in parallel to give 0.1F that will handle the current at the stator output voltage peaks.
  8. Aug 30, 2014 #7

    jim hardy

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    Are you sure that regulator gives 14volts AC not DC ?
    I'm guessing that your snowmobile regulator is similar to the motorcycle regulators i've seen. Their operation verges on brutal.
    What they do is first convert to DC then measure the voltage across the battery and figure out how much of the sinewave needs to be let through to keep about 14.5 volts there.
    At some point in the sinewave the regulator throws a dead short across the alternator which removes the remainder of the alternator's ac sine wave.
    In Kavik's sketch above, the regulator would chop off the wave as required to maintain battery voltage about 14.5 .

    With no battery present to absorb current i'd be at a loss to predict how your regulator would work. It'd probably chop the wave early which would give you lower average voltage across the motor.
    In Kavik's sketch, draw a line straight down from each sinewave peak and erase the right half of the sine wave.

    That's how the regulators that i've seen work. That'd give you lower average voltage at the fan.

    You might try a large capacitor across your fan.

    80 watts at 12 volts is 6.67 amps.
    Around 5000 microfarads of capacitor should make the regulator think there's at least a tiny battery out there. The more capacitance the better. You could try a few of these.

    EDIT: Ahh i see Baluncore already answered. We're thinking pretty much alike.
    Last edited by a moderator: May 6, 2017
  9. Aug 31, 2014 #8
    I would say that the regulator is outputting AC because the fan would just twitch when turned on, it would not spin until I installed the square block and I have the meter set on AC to read the voltage output.

    The regulator is wired parallel from the factory if that bit of information helps.

    I am clueless on Caps ... I like Jims post showing me what to buy, I like Baluncore's post telling me exactly what I need.

    Just looking at the numbers, they are not the same. Am I wrong here? I like to ask ten questions and get this correct the first time than try once and fry some wiring.

    I think I can test for frequency, I do not know if the stator is single or 3 phase. If you describe a test method, I can report back. I know that the stator is a 6 pulse and there is 2 wires coming out, Positive and ground.
    Last edited: Aug 31, 2014
  10. Aug 31, 2014 #9

    jim hardy

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    Balun and i assumed different frequencies, he used 100 hz and i guessed at 1000.

    He used mf for millifarads, i used microfarads which also starts with "m".

    We're not so far apart.

    You just need plenty of capacitance. British motorcycle racers in the late fifties used a large capacitor to replace their battery. Manufacturers thoughtfully oriented the rotor magnets to provide a pulse of current just when the ignition coil needed it. If you ever kick-started a 500cc single you really appreciate that !

    Anyhow - a few tens of thousands of microfarad should do the job. More won't hurt.

    You might think of a capacitor as a tank that stores electric charge.
    The unit of capacitance is the farad.
    A farad is a lot of capacitance. We usually work in millionths of a farad, microfarads.
    Think of a farad as a 55 gallon drum compared to a microfarad , which holds just a thimblefull.

    Do you have a friend who'd loan you a car stereo capacitor? Walmart sells a 1.5 farad (1.5 million microfarads) for $25.

    here's some 10,000 microfarad like Baluncore suggested


    these are 25 volt rated . 16 volt would probably be okay but i like a margin of safety.
    Ten of them in parallel would be 1/10th of a farad.

    capacitor goes on DC side of your rectifier - right across fan wires. Observe polarity : + to +
  11. Aug 31, 2014 #10
    I dialed meter to Hz and hooked probs to AC regulated wires. The voltage at idle (1500) is 12.3, the meter on Hz read about 150. When motor was reved to say 4000 the meter switched range to "K" and meter read ".5xx".

    I than moved the probes to the "DC" output and it was 9.3V DC, no AC measurable.

    DO the caps, when installed, boost voltage average like the average line in pic of post #3?
  12. Aug 31, 2014 #11


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    You have two terminals from the stator, that is single phase. If the chassis wire remains connected to the chassis, then the bridge rectifier will produce +ve and –ve to the capacitors and fan. Both the +ve and –ve will be swinging together in voltage relative to the chassis connection.

    High RPM gives about 500 Hz so that will give a ripple frequency of 1kHz, T = 1 msec.
    C = I * T / V = 6.67A * .001sec / 1.0V = 0.006670F = 6670 uF
    Voltage will be lower at idle when cooling may be not needed so much.

    For the same fan voltage at idle,
    Idling, at 150Hz, will give a full wave rectified ripple frequency of 300 Hz. T = 3.3msec
    C = I * T / V = 6.67A * .0033sec / 1.0V = 0.022011 F = 22000 uF.

    Yes, the caps raise the average voltage. The caps are charged only during the voltage peak from the rectified AC, they must store enough energy to run the fan through the rest of the half cycle. If the current flows for half the time, the current into the cap will be twice the average current out. If the current flows for 1/5 of the time, the current into the cap will be five times the average current out. The peak current could be maybe 35 amp. That peak current must be handled by the bridge. The cap(s) will need to be low ESR. I prefer multiple capacitors to distribute the peak current amongst several smaller units.
  13. Aug 31, 2014 #12


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    Can the regulator handle a 35 amp charge current? Is it a permanent magnet alternator designed only to charge the battery?

    If the alternator has no controlled field winding then you might consider removing the series regulator for battery charging, then use a single diode to the fan.

    Also consider rectifying a chassis referenced AC, but half wave, with only two diodes into separate positive and negative rails, each with it's own capacitor(s). That will double the fan voltage.
  14. Aug 31, 2014 #13
    the motor will max out in the 8500 RPM range, I only rev the motor on the stand to said 4000 rpm.

    The stator is stationary, there is a flywheel that rotates around the stator with permanent magnets attached to flywheel.

    The regulator is standard factory issue and installed as it comes from a stock factory install. that being said, it was not designed to run a DC motor. Just a few light bulbs ( about 150 watts total ) and heated hand grips that could be 45 watts.

    They sell a 3 wire regulator that is designed to charge a battery if sled was ordered with electric start.

    I would like to run the fan in my application as fast as possible ( w/o burning it out ) for max cooling with out having the "heavy" battery. I'm trying to lighten the chassis as much as possible.
    One of the two wires coming out of the stator goes to ground. I also have a ground wire from engine block to regulator body.

    So with the new information of the motor max RPM in the 8300 rpm range and not testing it but measured at idle the Hz was 150, and mid 400 RPM Hz want to 500 .... DO I need to measure the Hz at top RPM to size the caps correctly?

    the square block is a Motorola MDA 980-2, 8723 W Bridge Rectifier. I see a 2V drop on the DC side vs. the AC side.
    Last edited: Aug 31, 2014
  15. Aug 31, 2014 #14

    jim hardy

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    MDA980 is a 1970's 12 amp rectifier.

    Since the original alternator only needed to run lights and heaters there was no need to turn it into DC. So they might not have.

    On motorcycles one disconnects the regulator and measures stator AC voltage with nothing but a meter connected to it. That's called "open circuit stator test".
    That way you have nothing but magnets and wire active, no electronics to fool you.
    Around twelve volts idle, forty or fifty at midrange rpm . Voltage should be linear with RPM.
    Don't know what voltage to expect on yours. But if open circuit voltage doesn't go up with rpm it sounds like stator troubles.

    If it voltage goes up nicely with rpm, doubling when rpm doubles, then stator and magnets are probably fine. Reconnect regulator and continue testing.

    old jim
  16. Aug 31, 2014 #15

    jim hardy

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    ps i see no need for voltage measurement at top rpm. I dont like to run engines that fast unloaded.
  17. Aug 31, 2014 #16
    here is my wire schematic. the only part that is confusing is the stator plate is labeled engine ground and the regulator is chassis ground. should this be separate? I don't see a closed circuit if the two grounds are not connected.

    I was asking If Hz measurments are needed at top RPM to size the caps correct. just guessing.

    Attached Files:

  18. Aug 31, 2014 #17
    I have tested open voltage and it climbs with RPM to a may of 40V AC. the book says up to 45V AC is good.
  19. Aug 31, 2014 #18
    I did all testing with book specs to stator as open voltage and wire to wire resistance. All very close to book. book also says 20% error is ok and testing should be at 68 degrees F.
  20. Aug 31, 2014 #19
    Is this "big enough" for running a 12V 80 watt DC motor (rad fan) continuous for 20 minutes at a time in under hood heat of say 200 degrees F?
  21. Aug 31, 2014 #20

    jim hardy

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    T to Brn must be the lighting coils? That's where you'd make the open circuit stator test.

    I'd wager that W-WR is trigger coil for CDI and G-R-BR is power for CDI.

    Surely engine is bolted to chassis so grounds are common - is there not a braided link like in modern cars?
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