# Turning a ac water pump on and off

Discussion in 'The Projects Forum' started by jonnsl, Aug 29, 2015.

1. ### jonnsl Thread Starter New Member

Aug 18, 2015
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I want to turn a 220v ac (380w, according to the manufacturer's site) water pump on and off based on a timer, using a Arduino. From what I have read, this could be accomplished by using a TRIAC.
I found this pre-built circuit here:

schematics

Is this triac suitably for this kind of load?

Please keep in mind that I have a pretty limited knowledge about electricity and circuits. thanks.

May 21, 2014
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3. ### tcmtech Distinguished Member

Nov 4, 2013
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Unfortunately induction motors have a rather high inrush current when starting so running them with a Triac requires one with a working capacity considerably higher than the motors running current so keep that in mind.

4. ### jonnsl Thread Starter New Member

Aug 18, 2015
2
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the pump manufacturer's site doesn't say anything about how much the inrush current is, how do I measure this? or if it is to difficult to do so, how many times the sustained current should the relay/triac be able to support to work properly?
Considering that the nominal power is 380w the sustained current would be approximately 1.7amp (380w/220v), is this correct?
What would you recommend for my case, the relay or the triac? the relay board that ISB123 mentioned is considerably cheaper.
does the relay board or the mentioned triac have a snubber? should I worry about that?

thank you for the help.

5. ### tsan Member

Sep 6, 2014
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Sustained current can be quite much higher than 1.7 A. Has manufacturer only specified motor power and not voltage and current? I assume 380 W is shaft power. 380W/220V calculation is for resistive load and assumes that there is no losses. Just to estimate nominal/sustained input current I use 0.75 as power factor and 70 % efficiency. If 380 W is shaft power and efficiency is 70% then electrical power to motor is 380 W/0.7=543 W. Current with 220V would be 543 W/(220 V * 0.75) = 3.3 A.

If the motor is capacitor start type motor starting current is about 4-7 times nominal current. With factor 7 we get 23 A starting current. Peak value would be 1,41 * 23 A which is about 33 A. Split phase motor has higher starting current than capacitor start motor.

Properly selected relay and triac should both work but triac is technically more suitable if you switch ON/OFF frequently. On quick look triac BTA08-600 on your schematic looks suitable when looking current ratings only. Nominal RMS current is 8 A and one-cycle surge current is 80 A peak. Snubberless version of the BTA-08 is recommended for inductive load.

The relay (Songle SRD-05VDC-SL-C) ISB213 linked is underrated but perhaps a relay (board) with bigger rating is still a good choice. The relay in question is rated for 240 VAC with resistive load only! With inductive load rating is 120 VAC and current rating is reduced to 3 A (when using contact type "form C").