soft start circuit for  3 phase AC motor - Page 1
Forum section: electronics / projects

author: agmmalah
author_rank: Newbie
timestamp: April 10, 2017, 09:37:13 am
content: Hi; I have a triac circuit which controls a 120v, 1hp AC motor which works OK. But for a 3 phase, 5hp motor the triacs would suffer spikes.Any simple soft start circuit or controller chip around? Thanks, Alberto at Caracas, Venezuela.

author: ohdsp
author_rank: Contributor
timestamp: April 10, 2017, 10:06:33 am
content: Maybe you could use a line reactor to take out some of the spikes or peak current?No sure if it would work.

author: Benta
author_rank: Super Contributor
timestamp: April 10, 2017, 02:01:07 pm
content: No spikes from a squirrel cage 3-phase motor, they are very well behaved.Starting current, however, will be 6...7 times the full load current. A soft starter will normally not solve this issue, but just prolong the time the motor draws excessive current. DOL start is in most cases the best solution.An option could be wye start, delta run, if the motor supports this.The only real soft start option is a VFD.

author: SeanB
author_rank: Super Contributor
timestamp: April 10, 2017, 07:19:35 pm
content: 3 phase soft starter reduces the peak current, and allows you to start a large inertial load slowly so there is a lot less stress on the motor and load, and you do not have massive line load during the run up stage so do not need massively overrated cabling, switchgear and supply protection ratings. With these you do not need a star delta switch, and can do a DOL start easily. I use one on a 8kW motor, it does a lot better than the star delta ever did, and ramps the unit up to speed slowly ( deliberately to reduce snatch) over a period of 30 seconds, and also has a setting for start torque.It only has to control current in 2 phases, and just measures the current in the third for reference.Myself, for a 5kW motor I would go look for a 30A unit, they are expensive, but very reliable and well worth it, and will last for quite a long time, at least as long as the motor life.

author: james_s
author_rank: Super Contributor
timestamp: April 10, 2017, 07:19:59 pm
content: A VFD gives you lots of other advantages too, and they are cheap these days.

author: Benta
author_rank: Super Contributor
timestamp: April 10, 2017, 08:44:34 pm
content: Well, as long as we don't know the load or the application, everything is pure guesswork.I do have a feeling though, that the OP thinks he can control a 3-phase motor through triac phase control...

author: agmmalah
author_rank: Newbie
timestamp: April 12, 2017, 04:37:09 am
content: Sorry, I should have been more explicit. First of all, I am not into power electronics and triacs are somewhat confusing.Still, I want triac control instead of contactors due to lower pricing, no noise, more compact and less weight.The best I know are ST BTA24-800 BW and even they are “ snubberless “ I need guarantee that they wont blow up.I understand the need for varistors and heat sinking.The gates will be microprocessor controlled by a DC signal, optically isolated triac drivers.First question: which would be better, random phase or zero crossing?I`ll try to upload a simplified schematic tomorrow; I still have to check line reactor, VFD.Please understand that internet is slow, sometimes null at my place; thanks again.