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Topic: Quick question on using a triac (Read 1 time) previous topic - next topic

aoeud

Aug 05, 2013, 09:42 pm Last Edit: Aug 05, 2013, 09:54 pm by aoeud Reason: 1
So I've never used a triac before so I just want to ask... http://www.digikey.ca/product-detail/en/BTA140-800,127/568-3686-5-ND/1154798 is the triac I'm considering.  I'm trying to control a 14.2 amp, 110 VAC motor with this off of a digital pin's PWM.  Will it work?

jackrae

I don't think so, a triac's gate is connected to the AC mains system which is not a safe concept with a Dc powered arduino.  Also a triac needs to be switched in synchoronous with the AC mains.   

Do you want to turn the AC motor On/Off or are you attempting to control its speed via variation of the supply voltage.  Is the motor a squirrel cage type unit or a universal motor (with brushes).

More details required of exactly what it is you are trying to accomplish.

Grumpy_Mike

Quote
Will it work?

No.
Once a triac is on it will stay on until the AC cycle crosses zero.
So feeding it with PWM will switch it on for a continuously varying random time.

You need a zero crossing detector and once you see the zero crossing then delay a short time and then fire the triac. The delay controls the phase angle and thus the motor speed.
You need an opto isolator in the triac's gate and another for the zero crossing detector.

aoeud

I'm trying to control its speed.

Here's the motor I'm looking at.  I don't know any more than what's on that page.  I can choose another motor if it would allow me to control it.

I'm open to any suggestions, triacs just seemed to be what I was looking for.

Grumpy_Mike

Quote
I can choose another motor if it would allow me to control it.

No better or worse that that.

Mind you that triac is way too small for that motor. Look at the power dissipation ratings.

aoeud

Oh, I sort of assumed since it was rated for more volts and amps than I was using it'd work out.  I didn't check the power rating.  I don't think I can find a big enough one.

So how do I control this large motor?

Grumpy_Mike

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So how do I control this large motor?

You might be better off buying a ready built speed controller such a large motor is not easy to design for.

The power rating of the triac is not the power rating of the motor.
You see what the on voltage is for the triac. That is the voltage developed across the triac  when it is on and the full current is flowing through it. You will find that in the data sheet. Then multiply that by the current to find the power the triac has to dissipate. Then you need to look at the thermal resistance between the chip and case and finally work out what size your heat sink has to be.
http://www.thebox.myzen.co.uk/Tutorial/Power.html

aoeud

#7
Aug 06, 2013, 12:11 am Last Edit: Aug 06, 2013, 12:28 am by aoeud Reason: 1
The motor controllers I found on eBay seemed to all be hand-adjusted.  Are there any that can be controlled by the arduino/where would I find them?

I'm fairly ready to give up, it's not critical that I do this.

jackrae

That motor data-sheet states it is capacitor start.  That means its probably a squirrel cage design and as such cannot be speed controlled by reduction of supply voltage.  If you want real speed control then go for a 3-phase motor and a single-to-3phase inverter unit.  These can be controlled by external DC inputs such as an arduino.

aoeud

Well, thanks for noticing that for me.  I'm new to big electric motors, didn't realize some required capacitors.  Now the question is, which capacitors?..  I'm trying to find a real data sheet on that motor but I can't find one... I'll investigate some other motors that match my specs.

We're stuck at 110 volts residentially over here, so I'd need both a transformer and a phase converter.  Speed control isn't that critical, but it would have been nice.

PedroS23

Hi, if i were you i don't use Triac.
Use a Relay. It's isolate better the circuit for not damege the Arduino
like this:
http://www.glacialwanderer.com/_blog/blog2008/04_April/hb_relay5.jpg

I use a 1n4001 diode, and a BD645 transistor, my Relay is 12V 30A.

Good luck!!


regards

dc42


Well, thanks for noticing that for me.  I'm new to big electric motors, didn't realize some required capacitors.  Now the question is, which capacitors?..  I'm trying to find a real data sheet on that motor but I can't find one... I'll investigate some other motors that match my specs.

We're stuck at 110 volts residentially over here, so I'd need both a transformer and a phase converter.  Speed control isn't that critical, but it would have been nice.


You don't need to add capacitors, "Capacitor start" means the motor is a single-phase induction motor that has a built-in capacitor to start it. Induction motors run at speeds defined by the supply frequency and load, so you can't speed-control them unless you use an inverter to vary the supply frequency.

To switch the motor on and off from an Arduino, use either a mechanical relay driven by a transistor, or a random-fire SSR. You can make an SSR from a triac, opto triac and a few resistors, but it's better to buy a ready-made one.
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