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Author Topic: (req) soft start for 3 phase AC induction motor  (Read 3569 times)
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marcosz67
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« on: January 29, 2010, 10:29:36 10:29 »

Hi all,
i want to build a soft start for a 3 phase 380Vac induction motor using triacs and a microcontroller.
Anyone can help me?
Thx in advance.
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oldvan
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« Reply #1 on: January 29, 2010, 10:38:31 10:38 »

There are members of the DSPIC family specialized for motor control, Microchip's app notes might be a good place to start.
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marcosz67
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« Reply #2 on: January 29, 2010, 04:14:55 16:14 »

Not useful for me..
They use inverter solution, i need to use only triac with a small 8 bit microcontroller.
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oldvan
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« Reply #3 on: January 29, 2010, 11:40:24 23:40 »

Sounds like you want a three-phase version of a lamp dimmer.

Microcontroller needs to know zero crossing.  Can do with opto-isolator.

Initially fire triacs, for small amount of time, perhaps 1 mS, turn off at at their phase’s zero crossing.   Fire each triac  progressively earlier until firing full 360 degrees to apply soft-start.

The harsh switching of the triacs is going to create a very noisy electrical situation, filtering is EXTREMELY important to keep that noise from interfering with the world around you.

------------------------------

As a simplest alternative, correctly sized power resistors in series with the motor,
a triac in parallel with each resistor and the microcontroller turns on the triacs after
motor start-up time.

A relay could replace all 3 triacs.
« Last Edit: January 30, 2010, 01:05:03 01:05 by oldvan » Logged

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Walkura
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« Reply #4 on: January 30, 2010, 09:38:09 09:38 »

The method marcosz67 with the resistors describes ,i use this for softstarting big transformers (upto 3 KW) .
Although i prefer to use a relay to bridge te resistor it turned out to be most reliable .
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borberk
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« Reply #5 on: January 30, 2010, 10:58:26 10:58 »

Standard method is star-delta switch:

http://www.patchn.com/index.php?option=com_content&task=view&id=42&Itemid=74
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« Reply #6 on: January 31, 2010, 01:01:22 01:01 »

Hi. I got a circuit and the triac BT131 is bad. I could not find it in my supplier"s shop. Please, anyone know of good triac to use to replace BT131?. It is just to power a fan coil.
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oldvan
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« Reply #7 on: January 31, 2010, 02:56:55 02:56 »

Hi. I got a circuit and the triac BT131 is bad. I could not find it in my supplier"s shop. Please, anyone know of good triac to use to replace BT131?. It is just to power a fan coil.

BT131 datasheet:  http://www.nxp.com/documents/data_sheet/BT131_SER_D_E.pdf

$0.49 USD for exact replacement:  http://parts.digikey.com/1/parts/991298-triac-logic-600v-1a-5ma-92-bt131-600d-412.html

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