how to prevent an ac motor to drop the voltage too low ??? - Page 1
Forum section: electronics / projects

author: kripton2035
author_rank: Super Contributor
timestamp: September 15, 2013, 06:38:03 pm
content: Hi to all,I recently bought a small humidity digital controller WH8040 , to detect the humidity in the air and start an air extractor faster if it's too wet.the output of th controller is a small 12v relay, and it goes to a bigger relay 220v 10A, which them switch the 160w ac motor to the high speed.the problem is that when then relay gets on, the motor seems to drop the ac power supply, so that the digital controller gets off and switches off the relay, then start on and again some 10-20 times then after only the high speed is started.there is no problem when the relay goes from on to off to the low speed again.is there any way to solve this problem in a simple way ?thanks.

author: bilko
author_rank: Frequent Contributor
timestamp: September 15, 2013, 06:42:17 pm
content: Your cable is too thin

author: kripton2035
author_rank: Super Contributor
timestamp: September 15, 2013, 07:18:37 pm
content: the cables are 1.6mm2 copper, the motor is 160w so less than 1A is drawn from the 220v acare you sure they are too thinhow many amps can you draw on a 1.6mm2 wire, on a 2.5mm2 wire ?

author: somlioy
author_rank: Regular Contributor
timestamp: September 15, 2013, 07:39:39 pm
content: It also depends on the lenght of the cable.

author: darko31
author_rank: Regular Contributor
timestamp: September 15, 2013, 07:50:52 pm
content: Maybe the problem is with the humidity controller itself, bad electrolyte or something.

author: bingo600
author_rank: Super Contributor
timestamp: September 15, 2013, 07:58:02 pm
content: If i read this correctly , the controller turns off ... when the motor relay turns on ... Correct ?I'd say it's the psu in the humidiy controller that is lousy.1: Try to feed the motor (230v relay) from another outlet than the controller psu.Or at least use a separate set of wires.2: Put a beefy cap inside the controller psu on the low voltage side/Bingo

author: bilko
author_rank: Frequent Contributor
timestamp: September 15, 2013, 07:58:36 pm
content: Quote from: kripton2035 on September 15, 2013, 07:18:37 pmThe starting current can be more than 6x FLC, will it hurt to try a thicker cable ?

author: bilko
author_rank: Frequent Contributor
timestamp: September 15, 2013, 08:04:23 pm
content: Where are you getting the power for the 10A relay ?

author: kripton2035
author_rank: Super Contributor
timestamp: September 15, 2013, 08:24:43 pm
content: Quote from: bilko on September 15, 2013, 07:58:36 pmthe air extractor already has plugs at his input that accept 1.6mm2, but not the 2.5mm2 copper wires they are too big and won't fit.so may you want me to put thinner wires to power the humidity controller ?

author: kripton2035
author_rank: Super Contributor
timestamp: September 15, 2013, 08:27:35 pm
content: Quote from: bingo600 on September 15, 2013, 07:58:02 pm1/ quite not possible, the air extractor is far from any outlet, there is only one long wire 1.6mm2 that goes to it and that powers the controller and the motor.2/ the filter cap inside is a 1000uF 25v, there is a 220v to 12v transformer, and a 7805 regulator (smd version) - I soldered a second 1000uF in parallel and the problem is the same.

author: bilko
author_rank: Frequent Contributor
timestamp: September 15, 2013, 08:27:59 pm
content: Quote from: kripton2035 on September 15, 2013, 08:24:43 pmMeasure the voltage at the extractor terminals when you power up.If there is more than 6% voltage drop, you have a problem with the supply.Measure again at the utility outlet. Check if you have a voltage drop there.

author: bilko
author_rank: Frequent Contributor
timestamp: September 15, 2013, 08:30:14 pm
content: Quote from: kripton2035 on September 15, 2013, 08:27:35 pmPower the controller direct from the utility outlet, not the extension cable

author: kripton2035
author_rank: Super Contributor
timestamp: September 15, 2013, 08:31:43 pm
content: I will post pictures of the system tomorrow.

author: kripton2035
author_rank: Super Contributor
timestamp: September 15, 2013, 08:33:06 pm
content: Quote from: bilko on September 15, 2013, 08:30:14 pmthe controller is at the side of the air extractor, because it measures the humidity that the air extractor is exhausting !so they are all at the end of the 10 meters wires, and the 10A relay is also there.

author: johansen
author_rank: Super Contributor
timestamp: September 15, 2013, 09:31:16 pm
content: the wire area is non standard or its wrong.14 awg is 2.08mm^2 and 1.62mm diameter12 awg is 3.31mm^2 and 2.05mm diameter.10 awg is 5.26mm^2 and 2.58mm diameter.for 14 awg wire you need 54 meters of cable to get 1 ohm of resistance.For a 160 watt fan on a 220 volt supply you would need around 1000 meters of cable for these issues to show up.

author: bilko
author_rank: Frequent Contributor
timestamp: September 15, 2013, 10:01:21 pm
content: I have been supplied cables of 'Chinese Origin' in the past that have been stamped 1.5mm2, yet they were hardly 0.5mm2.This is also fairly common with made up leads with a utility plug one end and IEC connector the other end.

author: IanB
author_rank: Super Contributor
timestamp: September 15, 2013, 10:28:50 pm
content: Quote from: johansen on September 15, 2013, 09:31:16 pmDid you notice the poster is in France? Outside of America the world does not use American Wire Gauge.1.5 mm² is a standard conductor size for mains wiring in Europe and other parts of the world.

author: IanB
author_rank: Super Contributor
timestamp: September 15, 2013, 10:45:42 pm
content: Quote from: kripton2035 on September 15, 2013, 06:38:03 pmIt seems unlikely that the motor could drop the voltage enough to affect the controller, even if the motor had a heavy start up current surge. I think it is more likely you have some interference that is upsetting the controller electronics.You may need to include some kind of noise filtering, like perhaps putting ferrite beads on appropriate wires.

author: dannyf
author_rank: Super Contributor
timestamp: September 15, 2013, 11:11:30 pm
content: QuoteFrom the power supply to the mcu, insert a diode (preferrably a schottky diode). Connect a small electrolytic capacitor (from a few uf to 47uf) from the mcu's power supply to ground. This insures that when the ac voltage goes down, the mcu isn't bled.Having said that, unless you have really bad wiring (160w vac is really nothing), the issue is likely caused by a spike in the supply when the motor turns on / off. That issue can be much tougher to cure.

author: G7PSK
author_rank: Super Contributor
timestamp: September 16, 2013, 08:03:15 am
content: Could it also be a power factor problem with the motor. I have come across similar problems on generators where quite a small motor will cause the AVR to shut down the generator where a bigger motor wont all due to appalling power factor of some cheap electric motors the only cure is a power factor correction capacitor on the motor.

author: richard.cs
author_rank: Super Contributor
timestamp: September 16, 2013, 12:39:20 pm
content: Could you tell us 1) the length of your 1.6 mm^2 cable (are you sure it's not 1.5 mm^2, that's a standard European size) and 2) the DC resistance of your fan motor which will allow a good approximation of motor inrush current and supply voltage drop.I suspect you have a problem where the supply cable is undersized for the motor starting current and the power supply to the electronics drops out.

author: kripton2035
author_rank: Super Contributor
timestamp: September 16, 2013, 04:54:30 pm
content: Quote from: richard.cs on September 16, 2013, 12:39:20 pm1/ you're right the cable is 1.5mm2, it's written on the cable box ...2/ the low speed dc resistance is around 210 ? and around 3.4k? for the high speedwhat I dont understand is that I have another motor that does the air injection, that is (seems) the same as the extractor, and I have 310? for the low speed, and an infinite value for high speed (and the high speed works ...) I must have some bad wiring somewhere ...also the motor is rated 70W for high speed and 25W for low speed.

author: richard.cs
author_rank: Super Contributor
timestamp: September 16, 2013, 05:01:24 pm
content: Those resistances seem very wrong for a motor this size - I would expect tens of Ohms, not hundreds and certainly not kilohms. Maybe there's some semiconductor switch or something in there messing up the readings.Previously you said the motor was 160W, now it's 25/70W or is that a different motor?

author: kripton2035
author_rank: Super Contributor
timestamp: September 16, 2013, 06:40:18 pm
content: Quote from: richard.cs on September 16, 2013, 05:01:24 pmit was a mistake with 160m3/hour ... shame on me.

author: IanB
author_rank: Super Contributor
timestamp: September 16, 2013, 08:08:16 pm
content: Quote from: richard.cs on September 16, 2013, 12:39:20 pmI am not convinced that supply voltage drop could be the cause of the problems. For instance if we look at 10 A through a 10 m run of 1.5 mm² cable the voltage drop would be 1 or 2 V at most. On a 220 V supply this should be insignificant. Even if the switch-on current were 30 A the voltage drop would still only be in the order of 5 V.I suspect that some kind of interference or noise caused by the motor is the more likely culprit.