how to slow 3 speed fan

Discussion in 'General Electronics Chat' started by fixit45, Jan 7, 2014.

  1. fixit45

    Thread Starter New Member

    Jan 5, 2014
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    Hi, I have an evaporative cooler with a 3 speed fan. The speed switch has 6 speed positions, 3 speeds with the water pump running , 3 speeds without the pump. Occasionally I find the low fan speed still too high and would like to lower it by about 50%.
    attached is the circuit diagram.
    The motor specs are :
    Fasco model 808555KVA A19
    240V , 50Hz phase 1 (I am in Australia)
    Watts 140
    Amps 1.6
    RPM 1300/3sp Type: CSR
    μf/VS nil
    μf/VR 1/440

    I am tempted to simply place a 50 watt or 100 watt light globe in series with the motor with a switch to enable this to be selected as required. The actual light globe can be hidden and seems to be a very simple solution for me to implement and cheap. I am not worried about the small waste of energy as it is only likely to be used 2 or three hours at a time on maybe 10 times per year.
    Ï do not need a very sophisticated solution as long as it did not damage the motor.
    I t would not matter if I had to start it on a higher speed first to let the motor get up to speed.
    If a thing such as a variable automatic transformer , or an additional capacitor would do the job I could possibly do that but would not know exactly what to get. With light globes I can just keep experimenting until I get the right value.
    Thanks in advance ...
     
  2. debe

    Well-Known Member

    Sep 21, 2010
    946
    184
    I purchased 6 of these on Ebay recently for $3.43 ea freight free from Hong Kong. used them to control the speed of a motor circuit very similar to yours (it will be trial & error) but it worked for me. Listed as Adjustable Voltage Regulated PWM AC motor speed controller 50V-220V AC, 2000Watt, 10Amp. Be very surprised if the light globe idea works!!!
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Google as "dimmer circuit diagram".
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You may not have any luck with that motor using a Dimmer/triac type controller, the 'resistors' shown in the diagram are the windings of a 1ph split phase/cap motor, these often do not take to dimmer type control as do shaded pole motors.
    This is called a consequent pole motor where the winding sequence is changed by the switch for speed control.
    Max.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    The light bulb idea will not work on that type of motor.

    How about just changing the motor sheave (pulley) to a smaller diameter. That will change all three speed settings downward simultaneously. You likely are not using the highest speed anyway, and this will make the motor draw less current, and run cooler...
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    632
    I tried the lamp dimmer approach (use an RC snubber circuit or a "snubberless" triac) but found that the fan sang at harmonics of the power line because of the phase control. What worked for me was to use a Variac (an adjustable autotransformer).
    http://lmgtfy.com/?q=variac
     
  7. crutschow

    Expert

    Mar 14, 2008
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    I had the same problem with "singing" using a dimmer switch to a 12" cooling fan I wanted to slow so I used a non-polarized capacitor in series, which worked well. You'll have to experimentally determine the capacitor size to get the speed you want. You could buy a number of 1 to 2 μf 500V (or greater) film capacitors and connect them in parallel as needed to adjust the speed. I would think you would need no more than 10μF total.
     
  8. fixit45

    Thread Starter New Member

    Jan 5, 2014
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    Thanks everybody....
    Debe, that speed controller looks interesting. Looks like it works by reducing the voltage. ( I have used the light globe before to slow down a simple bathroom exhaust fan but not sure about this type of motor.
    I have heard of problems before with ordinary light dimmers. Can someone tell me is this motor an inductive load?.
    I have seen a controller that says it is suitable for an inductive load.
    Have also heard of using a capacitor somehow so I will go and read up on how capacitors work.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The motor is a induction motor so yes 'inductive'!
    The capacitor is needed to phase shift the start/run winding otherwise it would just sit and 'hum'.
    You have to be very careful of speed controlling a split phase motor, some will drop out of run if the rpm is low with a load applied.
    Max.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    The capacitor I'm referring to is not the start capacitor, it's in series with the motor power to reduce the voltage without loss due to the capacitive reactance. It acts more or less like a lossless resistor.
     
  11. fixit45

    Thread Starter New Member

    Jan 5, 2014
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    Thanks Max... and crutschow...
    I will be carefull with low speed issue , I can easily keep a check on motor temp as I start fiddling. (strap a thermometer probe onto the motor .)
    I might start using the lightglobe as a test for what size resistance I want and then try and find out what capacitance would give me a similar result.
    Thanks for reminding me about adding the capacitors in parallel to increase the capacitance.( I think i did this in school about 1964 .. hasn't changed yet!)
    Thanks for you help.
     
  12. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Be careful using that method. I innocently tried it once on a pump motor. Although speed was reduced the motor had a short life. Post-mortem inspection showed blackening around the motor terminations. I ran a Spice simulation for the motor inductance/capacitance combination and was surprised to find voltages >1kV occurring. Obviously due to a resonance effect. So the capacitor value must be chosen to avoid resonance at any harmonic of the mains frequency (something which hadn't occurred to me previously!).
     
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  13. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Why was the obvious solution of changing the sheave rejected?
     
  14. #12

    Expert

    Nov 30, 2010
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    Nobody said it has a pulley.
     
  15. Brownout

    Well-Known Member

    Jan 10, 2012
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    I was thinking the same thing, but those pullys are already quite small. One might use an intermediate set up pullys to mulitply the motor/blower rotation ratio.
     
  16. Brownout

    Well-Known Member

    Jan 10, 2012
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    He said it was an evaporative cooler. They have pullys.
     
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  17. #12

    Expert

    Nov 30, 2010
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    Thanks. The machines I work on don't have pulleys. I assumed that is why they have 3 speed motors. I learn something almost every day on this site.
     
  18. fixit45

    Thread Starter New Member

    Jan 5, 2014
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    The fan is mounted directly on motor shaft.. no pulleys. This is a "Bonaire Integra".. small through the wall /window cooler.
    Thanks for warning Alec t, I might give the capacitor idea a miss if there is any doubt. I might stick with just adding some resistance , eg light globes or the variable voltage device suggested by debe . Listed as Adjustable Voltage Regulated PWM AC motor speed controller 50V-220V AC, 2000Watt, 10Amp.
    Thanks for help everybody
     
  19. inwo

    Well-Known Member

    Nov 7, 2013
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    Can you restrict airflow?

    Some types of fans will operate at lower load as airflow is restricted.

    Restrict the flow to the maximum that can be tolerated on the low speed setting.

    The flow will then not reach maximum, on the high speed setting however.
     
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  20. #12

    Expert

    Nov 30, 2010
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    You're right inwo. A squirrel cage fan has a range of modulation with an intake restriction. A propeller fan, not so much. They tend to lose their grip and go through velocity surges. (They get on the wrong side of the horsepower/rpm curve.)

    Do not mistake higher RPM for more air flow with a squirrel cage. The "run free" without enough air to slow them down.
     
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