"dimmers" for inductive motor loads - diyAudio
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Old 18th August 2007, 02:57 PM   #1
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Location: UK
Default "dimmers" for inductive motor loads

No, I don't want to make my fan go dark, just control its speed!

240v AC, a few 10s of watts, classic induction motor I suppose.

Are there special triac circuits for inductive loads like this or can a standard one (ie filament lamp dimmer) be used?

The only one I have to hand is potted so I cannot see the circuit, and marked "not for inductive loads"

(It does say "Anything Else" at the top

TIA

Cliff
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Old 20th August 2007, 03:45 PM   #2
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I spend most of my work day working with (mostly) leading edge dimmers and I'm not aware of any standard type dimmer being made in the lighting industry for inductive loads.

I have seen it done with small motors but you do run the risk of cooking the dimmer, perhaps spectacularly. Standard dimmers tend to cut the AC sine wave pretty nastily too , I can't see that being good for the motor. The cheaper the dimmer the quicker the rise time - or harshness of the cut, if you like, that's why you find cheap dimmers kill light bulbs more quickly.

You might find something triac or SCR based made specifically for inductive loads but I'm not aware of anything made for lighting that could be used for both. Or at least, not reliably.

Hope that helps

Peter.
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Old 20th August 2007, 05:07 PM   #3
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Thanks for replying, BUT

The previous owner fitted a speed control to my kitchen cooker hood!

The motor is a 230V caged induction motor (I assume it is - no cap or brushes) and the speed controller is in a standard lighting patress, with a fan symbol on the control knob.

It works fine fron almost idling to full speed - unfortunately the innards are potted in black gunk so I cannot find out the circuit without destroying it!

I am not to sure about your statement on "cut sine-waves".

All the lamp dimmers I have seen use a simple phase-angle (R-C-diac) fired triac with no attempt at slope tuning.
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Old 20th August 2007, 05:26 PM   #4
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Go to your local "Home improvement" type store.
Ask them for a ceiling fan speed controller, one that is made to be mounted like a light switch.

Install it like the instructions show, or make it fit your custom installation.

Enjoy.

At least, that's what I did here in the States.

Tall Shadow
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Old 20th August 2007, 08:31 PM   #5
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Never seen a domestic ceiling fan in the UK - not hot enough!

The point is, though, I want to know how they work and how different are they to resistive triac dimmers?
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Old 21st August 2007, 02:19 AM   #6
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I have seen ceiling fans that change speed with a "dimmer" but i do not think they are the same thing as a light.

The light just gets a bit l of the current cycle turned off by a triac. but most fans use a synchronous motor (AC brushless) ithink it might get a bit slower and then just stop. (if the inductance of the motor did not kill it first)

240V cieling fan controller might be available in warmer countries that use 240v?
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Old 21st August 2007, 02:31 AM   #7
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A little more info..........
http://www.fairchildsemi.com/an/AN/AN-7511.pdf

Chapter 3 in this text....
http://www.nxp.com/acrobat_download/...es/APPCHP3.pdf

Looks to be a Triac AC motor control....
http://www.spelektroniikka.fi/kuvat/triacd.pdf

I hope this helps!

Tall Shadow
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Old 21st August 2007, 09:50 AM   #8
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Quote:
Originally posted by Tall Shadow
A little more info..........
http://www.fairchildsemi.com/an/AN/AN-7511.pdf

Chapter 3 in this text....
http://www.nxp.com/acrobat_download/...es/APPCHP3.pdf

Looks to be a Triac AC motor control....
http://www.spelektroniikka.fi/kuvat/triacd.pdf

I hope this helps!

Tall Shadow
Thanks again!

The third item is interesting (the first two deal with PWM control).

Is that Hungarian or Finish?

Anyway, the chip used (U208B) has a Telefunken data sheet showing this circuit and clearly states:

Because the current lags the voltage with inductive (motor) loads, there is a built-in current detector which prevents false triggering when the load current is > 0.

I don't find any of this very intuitive - a life-long fear of coils etc!

Anyway, thanks for your help!
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Old 23rd August 2007, 05:37 AM   #9
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TDA1185 is a phase controller IC used for firing triacs.
Small modules are available from RS for this purpose.

Gajanan Phadte
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Old 23rd August 2007, 11:56 PM   #10
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Use a variac. You will be happier with the results and no extra noise will be generated.
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