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Old 11-29-2007, 06:48 AM   #16
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Making 2rpm motor spin slower?


The dimmer I had tried I got from Home Depot and it was a "Lamp" dimmer which came with a receptacle in which you could plug directly into it and that plugs into the wall, tell me this, would it make a difference if I were to purchase a more commercial dimmer cut the plug off of the motor wire and solder the wires directly to the dimmer switch? I don't want to cut off the motor wire plug but I will do so if it can work?

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Old 11-29-2007, 11:56 AM   #17
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Making 2rpm motor spin slower?


If memory serves me correctly, (once in a blue moon) the one my parents had, had a motor in a box wit a small gear turning a larger gear reducing the rpms
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Old 11-29-2007, 12:02 PM   #18
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Making 2rpm motor spin slower?


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The dimmer you had was probably a cheap dimmer that uses resistors (or something) to reduce the voltage to the motor. If it's a 100v motor, it needs close to that to operate. If you drop the voltage to 90v, it might spin slower but at a certain point there won't be enough voltage to make the motor spin. So 50v might seem like it will reduce the speed by 50% but in practice it won't spin at all.

A fan control switch probably uses PWM (pulse width modulation) and will be more expensive. PWM supplies the same voltage but it does it in pulses. So instead of always having electricity, it will switch the voltage on and off really fast. It does it very quickly so that the motor turns on and off many, many times a second. It gives you the full voltage so the motor will spin but the motor won't get to full speed because it doesn't have the voltage on long enough.

If you are really interested in how it works, read this: http://en.wikipedia.org/wiki/Pulse-width_modulation.

Long post short -- buy a more expensive dimmer and it will probably work.
Unlike incandescant lights, ceiling fans have an ac supply with a wound rotor motor and brushes (generally). This is similar to DC as the the rotor voltage determins the speed. When you install a dimmer all it does is act like variac. Lowers and increases voltage. The cycles (HZ) do not change, just the voltage.
PWM is not incorporated in a dimmer switch. Only 0-120 volts + or -whatever.
PWM is the adjustment of voltage and HZ proportionately, by creating a simulated signwave..
It is designed for 3 phase motors. As you reduce or increase voltage the Hz is changed relative to the voltage.

Example: 1800 rpm, 460 volt, 60 Hz motor 3 phase.
So to make a motor run at 50% (900rpm) you would need 240 volt at 30 Hz.

If the motor in question is wound for 120 volt, has no wound rotor or brushes. It needs 120 volts to operate. If this motor for the ball is designed like a ceiling fan motor a dimmer will work. If not, it will not work.
bjones....PWM is designed for VFD's and You are correct regarding the switching of the transistor bank. However, do a little more checking on this and you will find much more interesting aspects of PWM.
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Old 11-30-2007, 07:37 AM   #19
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Making 2rpm motor spin slower?


My comments in blue.

Quote:
Originally Posted by J. V. View Post
Unlike incandescant lights, ceiling fans have an ac supply with a wound rotor motor and brushes (generally). This is similar to DC as the the rotor voltage determins the speed. When you install a dimmer all it does is act like variac. Lowers and increases voltage. The cycles (HZ) do not change, just the voltage.
PWM is not incorporated in a dimmer switch. Only 0-120 volts + or -whatever.
PWM is the adjustment of voltage and HZ proportionately, by creating a simulated signwave..
It is designed for 3 phase motors. As you reduce or increase voltage the Hz is changed relative to the voltage.

Virtually all "dimmer" controls now sold in Australia are electronic. There are primarily 2 types - "leading edge" & "trailing edge". Essentially, the dimmer controls the speed of the appliance by limiting the average power that can be delivered to the appliance. It is almost impossible to buy a resistive/inductive dimmer in Australia, although these items are now considered to be "specialist" items & therefore only available for "special" circumstances. These "leading/trailing edge" dimmers do not use PWM technology but instead control the phase angle of the voltage waveform. They are nothing like PWM drives etc.

Example: 1800 rpm, 460 volt, 60 Hz motor 3 phase.
So to make a motor run at 50% (900rpm) you would need 240 volt at 30 Hz.

If the motor in question is wound for 120 volt, has no wound rotor or brushes. It needs 120 volts to operate. If this motor for the ball is designed like a ceiling fan motor a dimmer will work. If not, it will not work.
bjones....PWM is designed for VFD's and You are correct regarding the switching of the transistor bank. However, do a little more checking on this and you will find much more interesting aspects of PWM.
I would find it hard to believe that phase angle controlling "dimmers" are not available in the USA.
As I said before & provided the Op does not have a synchronous motor in his disco ball, it can be easily controlled by a "leading edge" dimmer.

BTW, 100% of the ceiling fans in Australia do not have a wire wound rotor (expensive & unnecessary for such an application). They are usually split phase capacitor start/run motors, which are easily speed controlled by a "leading edge" dimmer.
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Old 11-30-2007, 12:10 PM   #20
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Making 2rpm motor spin slower?


Elk,
I thought about my post after I already posted it. You are correct. Thanks for the clarification.....John
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Old 11-30-2007, 12:18 PM   #21
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Making 2rpm motor spin slower?


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Elk,
I thought about my post after I already posted it. You are correct. Thanks for the clarification.....John
Thanks John . I am unaware of what type of "dimmers" are available in the US but I'm pretty certain that some sort of commercially available device should be able to do what the OP wants. What do you think?
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Old 12-01-2007, 11:03 AM   #22
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Making 2rpm motor spin slower?


go to home depot and for around 15-20 bucks buy a rotisserie motor used on grills. that's about the speed your looking for. Not sure how you would mount it .
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Old 12-01-2007, 01:50 PM   #23
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Making 2rpm motor spin slower?


Elk,
Without knowing exactly what type of motor it is, I have no idea. Maybe the poster can open it up and give us more info.......John
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Old 12-01-2007, 02:25 PM   #24
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Making 2rpm motor spin slower?


The DC motor from the site listed is only 8.95.. That should be a lot easier to control the speed with a variable resistor.
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Old 12-03-2007, 11:23 AM   #25
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Making 2rpm motor spin slower?


Just called the supplier, he said they get those from overseas so they have no idea which type of motor is in it.

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