Hi,
I need to make a simple on/off automation for a motor.
Will it be sufficient with a single relay?
If so, should I wire the relay on "line" or "neutral"?
I live in Europe so the plug can go in either way in wall socket.
Thank you so much!
All good
Normally you switch the "hot" side (light switches, etc.) but if it might be connected either way I'd go with a double-pole relay. DPDT relays are super-common.
Hello,
You don't give the power of the motor so how can we guess if the relay is good for this job or no?
We either don't know how you wanna control it, once in a while? Every 5min? Every second?
Usually, we cut the line. Like this if you touch by accident the cable you have more chance to stay alive (touching the neutral is way less dangerous than touching the line)
Thank you. I have a couple of these single pole relays so I can use two of them in parallell. That should be OK right?
If one of them is on the line and the other on the neutral, yes you will cut both of the cable. But still, we cannot say that it's the good way to do it since we don't know what's the power of the motor, the voltage, the current...
I am assuming your AC will be 230 VAC 50 Hz. We do not know the size (HP) of the motor you are controlling.
Here in the States we usually work with nominal 120 VAC 60 Hz, but we also use 230/240 VAC single phase for heavy equipment such as central air conditioning.
Motors draw high current during start-up and generally we use a specialized kind of relay referred to as an AC contactor. Not only is a contactor two-pole, one for each leg, which is advisable for your application, but each side of the switching portion uses two contacts each, which are bridged when the contactor pulls in. This spreads the current surge to double contacts and reduced sparking.
Look up "AC motor contactors" to learn more.
Ok so the power is very low, not even 2 Amps so with a relay 230V/10A you can do what you want
However, if the state changes every 30sec, I'm afraid that it won't last very long... Relay are mechanical so it can quickly get off service.
You should look for a SSR 
Thank you, I will try to describe the project here so it is clear what I am doing.
The current machine looks like this:
-it pumps around goo (slime made of potato starch and color pigment) in a loop. It is practically a prototype for a much larger installation.
I control the speed with this (or similar product):
The motor looks like this:
-they used to sit in a sculpture to make it vibrate. That is why they have objects taped to them. They ran DC power in that project, 20v, for safety reasons. For my purpose that is too weak.
The relays I have:
-The installation will be for a week six hours per day. So if the relays last that long it is ok. I also run the motors on lowers speed possible as long as the torque is high enough to lift the mud.
Should I put the relay between the speed control and the motor?
You notice that the relay does not have a value for motor controls? That is on purpose! A relay qualified for AC motor control will tell you that and give the maximum horse power the relay is rated for.
So if I put the relay between the mains (the power switch) and the speed controller?
Also what is the risk? The relay breaks? I have the speed control very low, like 1/10 (its an analog control pot)
The risk is the tiny, hemispherical relay contact weld together and when you think the circuit should be open, it is permanently closed and there is continuous power to the motor.
For the mechanical degrade they should last that easily.
That relay is not rated for motors, but for current that is only 1/10 of max I expect it will survive your temporary setup. Put the relay between mains and the "dimmer" input.
Maybe overheat and catch fire!
That relay is not rated for inductive loads at 220V AC.
Use at your own risk.
There have be several complaints on the forum about that particular relay, it very poor quality.
I guess I could use the speed controller potentiometer connection for automation instead. But that is more hacky and I am not sure of the outcome of that.
Thank you so much for your input, it really helped me.
Now, the final question. Safety. Please bear with me so I can get this right.
Why is there no ground cable coming out from the motor?
Should the motor be mounted totally isolated?
Should you make sure ground is connected to the case, in some way.
When the power is connected and I touch the metal case and the floor at the same time I can feel a very slight tingle. This I would like to get rid of.
As you stated you only have two prong power plugs, they have no ground.
So where where would you ground it?
Because the laundry machine "body" is earth grounded. There's no need to bring the earth wire to the motor.
or you could buy the proper relays.
I should buy the proper relays however the exhibition goes live on Thursday. I do not have time to order and then integrate new electronics. That is how I feel now.
For safety I make sure the motor is isolated.
The right way to do it would be to make sure the case of the motor and the whole construction is connected to ground? This I need an electrician for.
