Correct way to drive AC Inductive load with a Triac? - Page 1
Forum section: electronics / projects

author: Chriss
author_rank: Frequent Contributor
timestamp: October 10, 2022, 04:16:07 pm
content: Hi!I'm designing my first triac project, I have knowladge about main safety and triac's but just on a schoolar and theorethical basis.I repaired many times defective device where triac's are bad, but never designed a solid state driver with a triac.Im using a moc3021 opto and a BT139 triac.I wanna have the ability to drive inductive load.That would mean, I have to design a snoober circuit too.My main problems are:I saw many schematics on the net, where the designer connect the load between A1 of the triac and main plug.Then I also saw ( more times I assume ) where the designer connect the load between A2 and the main plug.If I remember me correctly from my school days, I've learned to connect the load between A2 and the main plug.Like here on this page:https://bestengineeringprojects.com/ac-motor-speed-controller-circuit-using-microcontroller/I see the designer put the motor on the MT2 side.For me this is correct, but what about all the other project, where the motor is sitting on the MT1 side?This is really confusing me.Thanks for any explanatin and help me to understanding this.

author: boB
author_rank: Frequent Contributor
timestamp: October 10, 2022, 05:36:05 pm
content: I would look at application notes for the MOC30xx series of triac drivers.I'm not sure that it matters a lot for this ? I can't remember but I have used triacs many times for inductive loads. What I like about using triacs is that they are guaranteed to turn OFF at zero (or low) current so that the inductance will not create a large flyback voltage.There are MOC drivers (originally from Motorola the ON Semi I believe) that will turn ON at any random time in the cycle and there are ones that turn ON at only zero crossings if you want to just skip half-cycles.Here is one datasheet.https://www.mouser.com/datasheet/2/149/MOC3041M-195885.pdfboB

author: jonpaul
author_rank: Super Contributor
timestamp: October 10, 2022, 06:25:24 pm
content: Depends on motor size HP or KVA.May need snubber, transient protection.Assymetric triac trigger phase angle can cause DC bias, saturating the motor core steelJon

author: Chriss
author_rank: Frequent Contributor
timestamp: October 10, 2022, 07:12:10 pm
content: Let's say I'm most with AC induction motors with a power of max 1kw.But why should the motor characteristics impact the way they are connected to the triac?Most of time ai saw they are connected in series A2 and live ( or main voltage source ).

author: wraper
author_rank: Unknown
timestamp: October 10, 2022, 07:25:33 pm
content: Unless you have 120V mains voltage, don't use MOC302x or MOC304x which are rated for 400V. A common mistake even for commercial products. There is barely any maximum voltage rating margin even without mains voltage transients. Therefore after some time they often degrade and start randomly triggering by themselves. MOC302x is especially susceptible to this failure mode as there is no zero crossing circuit which prevents from triggering at high voltages. And this is with resistive loads, inductive loads will create transients by themselves. Instead use 600V+ rated parts like MOC305x, MOC306x for 230V applications. You can use MOC3021 for testing at 230V, but if it will be some permanent application use higher voltage part.

author: Chriss
author_rank: Frequent Contributor
timestamp: October 10, 2022, 07:29:37 pm
content: Thanks for this valuable advice.I'm on 220VAC.Any schematic if somebody can share pls. which is really valuable to build up I would really appreciate.Maybe something what was tested in reality too.Thanks in advance.

author: wraper
author_rank: Unknown
timestamp: October 10, 2022, 07:37:43 pm
content: Also be vary of optotriac (triac gate) series resistor value. It's really tricky for random-phase optocouplers. I often seen too low value in different devices, which eventually caused failures. The issue is that it must be high enough that peak current does not exceed 1A at peak mains voltage but low enough to reliably trigger the triac at low voltages (most types require >50mA). For zero-crossing type it's easy as they do not trigger at approximately above 20V.

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2022, 08:09:18 pm
content: The motor is inductive, so the TRIAC should be triggered, during the peak mains voltage, not zero.A larger TRIAC might be needed, due to the current surge, especially if the motor is driving a high inertia load.

author: jonpaul
author_rank: Super Contributor
timestamp: October 10, 2022, 09:01:33 pm
content: Line transients due to inductive loads switching on the mains bus induce kilovolts and joules of energy.IEC/UL and other standards specify the transie t protection level and test transients.Most opto/triacs are NOT designed to withstand such transient exposure.Certain regions have poor power quality with wide bus V tolerance and high transientsThe UK employs a "ring bus" configuration capable of bad transients.Jon